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Archive for the ‘Climate History’ Category

In Part Seven – GCM I  through Part Ten – GCM IV we looked at GCM simulations of ice ages.

These were mostly attempts at “glacial inception”, that is, starting an ice age. But we also saw a simulation of the last 120 kyrs which attempted to model a complete ice age cycle including the last termination. As we saw, there were lots of limitations..

One condition for glacial inception, “perennial snow cover at high latitudes”, could be produced with a high-resolution coupled atmosphere-ocean GCM (AOGCM), but that model did suffer from the problem of having a cold bias at high latitudes.

The (reasonably accurate) simulation of a whole cycle including inception and termination came by virtue of having the internal feedbacks (ice sheet size & height and CO2 concentration) prescribed.

Just to be clear to new readers, these comments shouldn’t indicate that I’ve uncovered some secret that climate scientists are trying to hide, these points are all out in the open and usually highlighted by the authors of the papers.

In Part Nine – GCM III, one commenter highlighted a 2013 paper by Ayako Abe-Ouchi and co-workers, where the journal in question, Nature, had quite a marketing pitch on the paper. I made brief comment on it in a later article in response to another question, including that I had emailed the lead author asking a question about the modeling work (how was a 120 kyr cycle actually simulated?).

Most recently, in Eighteen – “Probably Nonlinearity” of Unknown Origin, another commented highlighted it, which rekindled my enthusiasm, and I went back and read the paper again. It turns out that my understanding of the paper had been wrong. It wasn’t really a GCM paper at all. It was an ice sheet paper.

There is a whole field of papers on ice sheet models deserving attention.

GCM review

Let’s review GCMs first of all to help us understand where ice sheet models fit in the hierarchy of climate simulations.

GCMs consist of a number of different modules coupled together. The first GCMs were mostly “atmospheric GCMs” = AGCMs, and either they had a “swamp ocean” = a mixed layer of fixed depth, or had prescribed ocean boundary conditions set from an ocean model or from an ocean reconstruction.

Less commonly, unless you worked just with oceans, there were ocean GCMs with prescribed atmospheric boundary conditions (prescribed heat and momentum flux from the atmosphere).

Then coupled atmosphere-ocean GCMs came along = AOGCMs. It was a while before these two parts matched up to the point where there was no “flux drift”, that is, no disappearing heat flux from one part of the model.

Why so difficult to get these two models working together? One important reason comes down to the time-scales involved, which result from the difference in heat capacity and momentum of the two parts of the climate system. The heat capacity and momentum of the ocean is much much higher than that of the atmosphere.

And when we add ice sheets models – ISMs – we have yet another time scale to consider.

  • the atmosphere changes in days, weeks and months
  • the ocean changes in years, decades and centuries
  • the ice sheets changes in centuries, millennia and tens of millenia

This creates a problem for climate scientists who want to apply the fundamental equations of heat, mass & momentum conservation along with parameterizations for “stuff not well understood” and “stuff quite-well-understood but whose parameters are sub-grid”. To run a high resolution AOGCM for a 1,000 years simulation might consume 1 year of supercomputer time and the ice sheet has barely moved during that period.

Ice Sheet Models

Scientists who study ice sheets have a whole bunch of different questions. They want to understand how the ice sheets developed.

What makes them grow, shrink, move, slide, melt.. What parameters are important? What parameters are well understood? What research questions are most deserving of attention? And:

Does our understanding of ice sheet dynamics allow us to model the last glacial cycle?

To answer that question we need a model for ice sheet dynamics, and to that we need to apply some boundary conditions from some other “less interesting” models, like GCMs. As a result, there are a few approaches to setting the boundary conditions so we can do our interesting work of modeling ice sheets.

Before we look at that, let’s look at the dynamics of ice sheets themselves.

Ice Sheet Dynamics

First, in the theme of the last paper, Eighteen – “Probably Nonlinearity” of Unknown Origin, here is Marshall & Clark 2002:

The origin of the dominant 100-kyr ice-volume cycle in the absence of substantive radiation forcing remains one of the most vexing questions in climate dynamics

We can add that to the 34 papers reviewed in that previous article. This paper by Marshall & Clark is definitely a good quick read for people who want to understand ice sheets a little more.

Ice doesn’t conduct a lot of heat – it is a very good insulator. So the important things with ice sheets happen at the top and the bottom.

At the top, ice melts, and the water refreezes, runs off or evaporates. In combination, the loss is called ablation. Then we have precipitation that adds to the ice sheet. So the net effect determines what happens at the top of the ice sheet.

At the bottom, when the ice sheet is very thin, heat can be conducted through from the atmosphere to the base and make it melt – if the atmosphere is warm enough. As the ice sheet gets thicker, very little heat is conducted through. However, there are two important sources of heat for surface heating which results in “basal sliding”. One source is geothermal energy. This is around 0.1 W/m² which is very small unless we are dealing with an insulating material (like ice) and lots of time (like ice sheets). The other source is the shear stress in the ice sheet which can create a lot of heat via the mechanics of deformation.

Once the ice sheet is able to start sliding, the dynamics create a completely different result compared to an ice sheet “cold-pinned” to the rock underneath.

Some comments from Marshall and Clark:

Ice sheet deglaciation involves an amount of energy larger than that provided directly from high-latitude radiation forcing associated with orbital variations. Internal glaciologic, isostatic, and climatic feedbacks are thus essential to explain the deglaciation.

..Moreover, our results suggest that thermal enabling of basal flow does not occur in response to surface warming, which may explain why the timing of the Termination II occurred earlier than predicted by orbital forcing [Gallup et al., 2002].

Results suggest that basal temperature evolution plays an important role in setting the stage for glacial termination. To confirm this hypothesis, model studies need improved basal process physics to incorporate the glaciological mechanisms associated with ice sheet instability (surging, streaming flow).

..Our simulations suggest that a substantial fraction (60% to 80%) of the ice sheet was frozen to the bed for the first 75 kyr of the glacial cycle, thus strongly limiting basal flow. Subsequent doubling of the area of warm-based ice in response to ice sheet thickening and expansion and to the reduction in downward advection of cold ice may have enabled broad increases in geologically- and hydrologically-mediated fast ice flow during the last deglaciation.

Increased dynamical activity of the ice sheet would lead to net thinning of the ice sheet interior and the transport of large amounts of ice into regions of intense ablation both south of the ice sheet and at the marine margins (via calving). This has the potential to provide a strong positive feedback on deglaciation.

The timescale of basal temperature evolution is of the same order as the 100-kyr glacial cycle, suggesting that the establishment of warm-based ice over a large enough area of the ice sheet bed may have influenced the timing of deglaciation. Our results thus reinforce the notion that at a mature point in their life cycle, 100-kyr ice sheets become independent of orbital forcing and affect their own demise through internal feedbacks.

[Emphasis added]

In this article we will focus on a 2007 paper by Ayako Abe-Ouchi, T Segawa & Fuyuki Saito. This paper is essentially the same modeling approach used in Abe-Ouchi’s 2013 Nature paper.

The Ice Model

The ice sheet model has a time step of 2 years, with 1° grid from 30°N to the north pole, 1° longitude and 20 vertical levels.

Equations for the ice sheet include sliding velocity, ice sheet deformation, the heat transfer through the lithosphere, the bedrock elevation and the accumulation rate on the ice sheet.

Note, there is a reference that some of the model is based on work described in Sensitivity of Greenland ice sheet simulation to the numerical procedure employed for ice sheet dynamics, F Saito & A Abe-Ouchi, Ann. Glaciol., (2005) – but I don’t have access to this journal. (If anyone does, please email the paper to me at scienceofdoom – you know what goes here – gmail.com).

How did they calculate the accumulation on the ice sheet? There is an equation:

Acc=Aref×(1+dP)Ts

Ts is the surface temperature, dP is a measure of aridity and Aref is a reference value for accumulation. This is a highly parameterized method of calculating how much thicker or thinner the ice sheet is growing. The authors reference Marshall et al 2002 for this equation, and that paper is very instructive in how poorly understood ice sheet dynamics actually are.

Here is one part of the relevant section in Marshall et al 2002:

..For completeness here, note that we have also experimented with spatial precipitation patterns that are based on present-day distributions.

Under this treatment, local precipitation rates diminish exponentially with local atmospheric cooling, reflecting the increased aridity that can be expected under glacial conditions (Tarasov and Peltier, 1999).

Paleo-precipitation under this parameterization has the form:

P(λ,θ,t) = Pobs(λ,θ)(1+dp)ΔT(λ,θ,t) x exp[βp.max[hs(λ,θ,t)-ht,0]]       (18)

The parameter dP in this equation represents the percentage of drying per 1C; Tarasov and Peltier (1999) choose a value of 3% per °C; dp = 0:03.

[Emphasis added, color added to highlight the relevant part of the equation]

So dp is a parameter that attempts to account for increasing aridity in colder glacial conditions, and in their 2002 paper Marshall et al describe it as 1 of 4 “free parameters” that are investigated to see what effect they have on ice sheet development around the LGM.

Abe-Ouchi and co-authors took a slightly different approach that certainly seems like an improvement over Marshall et al 2002:

Abe-Ouchi-eqn11

So their value of aridity is just a linear function of ice sheet area – from zero to a fixed value, rather than a fixed value no matter the ice sheet size.

How is Ts calculated? That comes, in a way, from the atmospheric GCM, but probably not in a way that readers might expect. So let’s have a look at the GCM then come back to this calculation of Ts.

Atmospheric GCM Simulations

There were three groups of atmospheric GCM simulations, with parameters selected to try and tease out which factors have the most impact.

Group One: high resolution GCM - 1.1º latitude and longitude and 20 atmospheric vertical levels with fixed sea surface temperature. So there is no ocean model, the ocean temperature are prescribed. Within this group, four experiments:

  • A control experiment – modern day values
  • LGM (last glacial maximum) conditions for CO2 (note 1) and orbital parameters with
    • no ice
    • LGM ice extent but zero thickness
    • LGM ice extent and LGM thickness

So the idea is to compare results with and without the actual ice sheet so see how much impact orbital and CO2 values have vs the effect of the ice sheet itself – and then for the ice sheet to see whether the albedo or the elevation has the most impact. Why the elevation? Well, if an ice sheet is 1km thick then the surface temperature will be something like 6ºC colder. (Exactly how much colder is an interesting question because we don’t know what the lapse rate actually was). There will also be an effect on atmospheric circulation – you’ve stuck a “mountain range” in the path of wind so this changes the circulation.

Each of the four simulations was run for 11 or 13 years and the last 10 years’ results used:

From Abe-Ouchi et al 2007

From Abe-Ouchi et al 2007

Figure 1

It’s clear from this simulation that the full result (left graphic) is mostly caused by the ice sheet (right graphic) rather than CO2, orbital parameters and the SSTs (middle graphic). And the next figure in the paper shows the breakdown between the albedo effect and the height of the ice sheet:

From Abe-Ouchi et al 2007

From Abe-Ouchi et al 2007

Figure 2 – same color legend as figure 1

Now a lapse rate of 5K/km was used. What happens if the lapse rate of 9K/km was used instead? There were no simulations done with different lapse rates.

..Other lapse rates could be used which vary depending on the altitude or location, while a lapse rate larger than 7 K/km or smaller than 4 K/km is inconsistent with the overall feature. This is consistent with the finding of Krinner and Genthon (1999), who suggest a lapse rate of 5.5 K/km, but is in contrast with other studies which have conventionally used lapse rates of 8 K/km or 6.5 K/km to drive the ice sheet models..

Group Two – medium resolution GCM 2.8º latitude and longitude and 11 atmospheric vertical levels, with a “slab ocean” – this means the ocean is treated as one temperature through the depth of some fixed layer, like 50m. So it is allowing the ocean to be there as a heat sink/source responding to climate, but no heat transfer through to a deeper ocean.

There were five simulations in this group, one control (modern day everything) and four with CO2 & orbital parameters at the LGM:

  • no ice sheet
  • LGM ice extent, but flat
  • 12 kyrs ago ice extent, but flat
  • 12 kyrs ago ice extent and height

So this group takes a slightly more detailed look at ice sheet impact. Not surprisingly the simulation results give intermediate values for the ice sheet extent at 12 kyrs ago.

Group Three – medium resolution GCM as in group two, and ice sheets either at present day or LGM, with nine simulations covering different orbital values, different CO2 values of present day, 280 or 200 ppm.

There was also some discussion of the impact of different climate models. I found this fascinating because the difference between CCSM and the other models appears to be as great as the difference in figure 2 (above) which identifies the albedo effect as more significant than the lapse rate effect:

From Abe-Ouchi et al 2007

From Abe-Ouchi et al 2007

Figure 3

And this naturally has me wondering about how much significance to put on the GCM simulation results shown in the paper. The authors also comment:

Based on these GCM results we conclude there remains considerable uncertainty over the actual size of the albedo effect.

Given there is also uncertainty over the lapse rate that actually occurred, it seems there is considerable uncertainty over everything.

Now let’s return to the ice sheet model, because so far we haven’t seen any output from the ice sheet model.

GCM Inputs into the Ice Sheet Model

The equation which calculates the change in accumulation on the ice sheet used a fairly arbitrary parameter dp, with (1+dp) raised to the power of Ts.

The ice sheet model has a 2 year time step. The GCM results don’t provide Ts across the surface grid every 2 years, they are snapshots for certain conditions. The ice sheet model uses this calculation for Ts:

Ts = Tref + ΔTice + ΔTco2 + ΔTinsol + ΔTnonlinear

Tref is the reference temperature which is present day climatology. The other ΔT (change in temperature) values are basically a linear interpolation from two values of the GCM simulations. Here is the ΔTCo2 value:

Abe-Ouchi-2007-eqn6

 

So think of it like this – we have found Ts at one value of CO2 higher and one value of CO2 lower from some snapshot GCM simulations. We plot a graph with Co2 on the x-axis and Ts on the y-axis with just two points on the graph from these two experiments and we draw a straight line between the two points.

To calculate Ts at say 50 kyrs ago we look up the CO2 value at 50 kyrs from ice core data, and read the value of TCO2 from the straight line on the graph.

Likewise for the other parameters. Here is ΔTinsol:

Abe-Ouchi-eqn7

 

So the method is extremely basic. Of course the model needs something..

Now, given that we have inputs for accumulation on the ice sheet, the ice sheet model can run. Here are the results. The third graph (3) is the sea level from proxy results so is our best estimate of reality, with (4) providing model outputs for different parameters of d0 (“desertification” or aridity) and lapse rate, and (5) providing outputs for different parameters of albedo and lapse rate:

From Abe-Ouchi et al 2007

From Abe-Ouchi et al 2007

Figure 4

There are three main points of interest.

Firstly, small changes in the parameters cause huge changes in the final results. The idea of aridity over ice sheets as just linear function of ice sheet size is very questionable itself. The idea of a constant lapse rate is extremely questionable. Together, using values that appear realistic, we can model much less ice sheet growth (sea level drop) or many times greater ice sheet growth than actually occurred.

Secondly, notice that the time of maximum ice sheet (lowest sea level) for realistic results show sea level starting to rise around 12 kyrs, rather than the actual 18 kyrs. This might be due to the impact of orbital factors which were at quite a low level (i.e., high latitude summer insolation was at quite a low level) when the last ice age finished, but have quite an impact in the model. Of course, we have covered this “problem” in a few previous articles in this series. In the context of this model it might be that the impact of the southern hemisphere leading the globe out of the last ice age is completely missing.

Thirdly – while this might be clear to some people, but for many new to this kind of model it won’t be obvious – the inputs for the model are some limits of the actual history. The model doesn’t simulate the actual start and end of the last ice age “by itself”. We feed into the GCM model a few CO2 values. We feed into the GCM model a few ice sheet extent and heights that (as best as can be reconstructed) actually occurred. The GCM gives us some temperature values for these snapshot conditions.

In the case of this ice sheet model, every 2 years (each time step of the ice sheet model) we “look up” the actual value of ice sheet extent and atmospheric CO2 and we linearly interpolate the GCM output temperatures for the current year. And then we crudely parameterize these values into some accumulation rate on the ice sheet.

Conclusion

This is our first foray into ice sheet models. It should be clear that the results are interesting but we are at a very early stage in modeling ice sheets.

The problems are:

  • the computational load required to run a GCM coupled with an ice sheet model over 120 kyrs is much too high, so it can’t be done
  • the resulting tradeoff uses a few GCM snapshot values to feed linearly interpolated temperatures into a parameterized accumulation equation
  • the effect of lapse rate on the results is extremely large and the actual value for lapse rate over ice sheets is very unlikely to be a constant and is also not known
  • our understanding of ice sheet fundamental equations are still at an early stage, as readers can see by reviewing the first two papers below, especially the second one

 Articles in this Series

Part One - An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Pop Quiz: End of An Ice Age - a chance for people to test their ideas about whether solar insolation is the factor that ended the last ice age

Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2

Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland

Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II

Fourteen – Concepts & HD Data - getting a conceptual feel for the impacts of obliquity and precession, and some ice age datasets in high resolution

Fifteen – Roe vs Huybers - reviewing In Defence of Milankovitch, by Gerard Roe

Sixteen – Roe vs Huybers II - remapping a deep ocean core dataset and updating the previous article

Seventeen – Proxies under Water I - explaining the isotopic proxies and what they actually measure

Eighteen – “Probably Nonlinearity” of Unknown Origin - what is believed and what is put forward as evidence for the theory that ice age terminations were caused by orbital changes

References

Basal temperature evolution of North American ice sheets and implications for the 100-kyr cycle, SJ Marshall & PU Clark, GRL (2002) – free paper

North American Ice Sheet reconstructions at the Last Glacial Maximum, SJ Marshall, TS James, GKC Clarke, Quaternary Science Reviews (2002) – free paper

Climatic Conditions for modelling the Northern Hemisphere ice sheets throughout the ice age cycle, A Abe-Ouchi, T Segawa, and F Saito, Climate of the Past (2007) – free paper

Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume, Ayako Abe-Ouchi, Fuyuki Saito, Kenji Kawamura, Maureen E. Raymo, Jun’ichi Okuno, Kunio Takahashi & Heinz Blatter, Nature (2013) – paywall paper

Notes

Note 1 – the value of CO2 used in these simulations was 200 ppm, while CO2 at the LGM was actually 180 ppm. Apparently this value of 200 ppm was used in a major inter-comparison project (the PMIP), but I don’t know the reason why. PMIP = Paleoclimate Modelling Intercomparison Project, Joussaume and Taylor, 1995.

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A while ago, in Part Three – Hays, Imbrie & Shackleton we looked at a seminal paper from 1976.

In that paper, the data now stretched back far enough in time for the authors to demonstrate something of great importance. They showed that changes in ice volume recorded by isotopes in deep ocean cores (see Seventeen – Proxies under Water I) had significant signals at the frequencies of obliquity, precession and one of the frequencies of eccentricity.

Obliquity is the changes in the tilt of the earth’s axis, on a period around 40 kyrs. Precession is the change in the closest approach to the sun through the year (right now the closest approach is in NH winter), on a period around 20 kyrs (see Four – Understanding Orbits, Seasons and Stuff).

Both of these involve significant redistributions of solar energy. Obliquity changes the amount of solar insolation received by the poles versus the tropics. Precession changes the amount of solar insolation at high latitudes in summer versus winter. (Neither changes total solar insolation). This was nicely in line with Milankovitch’s theory – for a recap see Part Three.

I’m going to call this part Theory A, and paraphrase it like this:

The waxing and waning of the ice sheets has 40 kyr and 20 kyr periods which is caused by the changing distribution of solar insolation due to obliquity and precession.

The largest signal in ocean cores over the last 800 kyrs has a component of about 100 kyrs (with some variability). That is, the ice ages start and end with a period of about 100 kyrs. Eccentricity varies on time periods of 100 kyrs and 400 kyrs, but with a very small change in total insolation (see Part Four).

Hays et al produced a completely separate theory, which I’m going to call Theory B, and paraphrase it like this:

The start and end of the ice ages has 100 kyr periods which is caused by the changing eccentricity of the earth’s orbit.

Theory A and Theory B are both in the same paper and are both theories that “link ice ages to orbital changes”. In their paper they demonstrated Theory A but did not prove or demonstrate Theory B. Unfortunately, Theory B is the much more important one.

Here is what they said:

The dominant 100,000 year climatic component has an average period close to, and is in phase with, orbital eccentricity. Unlike the correlations between climate and the higher frequency orbital variations (which can be explained on the assumption that the climate system responds linearly to orbital forcing) an explanation of the correlations between climate and eccentricity probably requires an assumption of non-linearity.

[Emphasis added].

The only quibble I have with the above paragraph is the word “probably”. This word should have been removed. There is no doubt. An assumption of non-linearity is required as a minimum.

Now why does it “probably” or “definitely” require an assumption of non-linearity? And what does that mean?

A linearity assumption is one where the output is proportional to the input. For example: double the weight of a vehicle and the acceleration halves. Most things in the real world, and most things in climate are non-linear. So for example, double the temperature (absolute temperature) and the emitted radiation goes up by a factor of 16.

However, there isn’t a principle, an energy balance equation or even a climate model that can take this tiny change in incoming solar insolation over a 100 kyr period and cause the end of an ice age.

In fact, their statement wasn’t so much “an assumption of non-linearity” but “some non-linearity relationship that we are not currently able to model or demonstrate, some non-linearity relationship we have yet to discover”.

There is nothing wrong with their original statement as such (apart from “probably”), but an alternative way of writing from the available evidence could be:

The dominant 100,000 year climatic component has an average period close to, and is in phase with, orbital eccentricity. Unlike the correlations between climate and the higher frequency orbital variations.. an explanation of the correlations between climate and eccentricity is as yet unknown, remains to be demonstrated and there may in fact be no relationship at all.

Unfortunately, because Theory A and Theory B were in the same paper and because Theory A is well demonstrated and because there is no accepted alternative on the cause of the start and end of ice ages (there are alternative hypotheses around natural resonance) Theory B has become “well accepted”.

And because everyone familiar with climate science knows that Theory A is almost certainly true, when you point out that Theory B doesn’t have any evidence, many people are confused and wonder why you are rejecting well-proven theories.

In the series so far, except in occasional comments, I haven’t properly explained the separation between the two theories and this article is an attempt to clear that up.

Now I will produce a sufficient quantity of papers and quote their “summary of the situation so far” to demonstrate that there isn’t any support for Theory B. The only support is the fact that one component frequency of eccentricity is “similar” to the frequency of the ice age terminations/inceptions, plus the safety in numbers support of everyone else believing it.

One other comment on paleoclimate papers attempts to explain the 100 kyr period. It is the norm for published papers to introduce a new hypothesis. That doesn’t make the new hypothesis correct.

So if I produce a paper, and quote the author’s summary of “the state of work up to now” and that paper then introduces their new hypothesis which claims to perhaps solve the mystery, I haven’t quoted the author’s summary out of context.

Let’s take it as read that lots of climate scientists think they have come up with something new. What we are interesting in is their review of the current state of the field and their evidence cited in support of Theory B.

Before producing the papers I also want to explain why I think the idea behind Theory B is so obviously flawed, and not just because 38 years after Hays, Imbrie & Shackleton the mechanism is still a mystery.

Why Theory B is Unsupportable

If a non-linear relationship can be established between a 0.1% change in insolation over a long period, it must also explain why significant temperature fluctuations in high latitude regions during glacials do not cause a termination.

Here are two high resolution examples from a Greenland ice core (NGRIP) during the last glaciation:

From Wolff et al 2010

From Wolff et al 2010

The “non-linearity” hypothesis has more than one hill to climb. This second challenge is even more difficult than the first.

A tiny change in total insolation causes, via a yet to be determined non-linear effect, the end of each ice age, but this same effect does not amplify frequent large temperature changes of long duration to end an ice age (note 1).

Food for thought.

Theory C Family

Many papers which propose orbital reasons for ice age terminations do not propose eccentricity variations as the cause. Instead, they attribute terminations to specific insolation changes at specific latitudes, or various combinations of orbital factors completely unrelated to eccentricity variations. See Part Six – “Hypotheses Abound”.

Of course, one of these might be right. For now I will call them the family, so we remember that Theory C is not one theory, but a whole range of mostly incompatible theories.

But remember where the orbital hypothesis for ice age termination came from – the 100,000 year period of eccentricity variation “matching” (kind of matching) the 100,000 year period of the ice ages.

The Theory C Family does not have that starting point.

Papers

So let’s move onto papers. I started by picking off papers from the right category in my mind map that might have something to say, then I opened up every one of about 300 papers in my ice ages folder (alphabetical by author) and checked to see whether they had something to say on the cause of ice ages in the abstract or introduction. Most papers don’t have a comment because they are about details like d18O proxies, or the CO2 concentration in the Vostok ice core, etc. That’s why there aren’t 300 citations here.

And bold text within a citation is added by me for emphasis.

I looked for their citations (evidence) to back up any claim that orbital variations caused ice age terminations. In some cases I pull up what the citations said.

—–

Last Interglacial Climates, Kukla et al (2002), by a cast of many including the famous Wallace S. Broecker, John Imbrie and Nicholas J. Shackleton:

At the end of the last interglacial period, over 100,000 yr ago, the Earth’s environments, similar to those of today, switched into a profoundly colder glacial mode. Glaciers grew, sea level dropped, and deserts expanded. The same transition occurred many times earlier, linked to periodic shifts of the Earth’s orbit around the Sun. The mechanism of this change, the most important puzzle of climatology, remains unsolved.

Note that “linked to periodic shifts of the Earth’s orbit” is followed by an “unknown mechanism”. Two of the authors were the coauthors of the classic 1976 paper that is most commonly cited as evidence for Theory B.

———

Millennial-scale variability during the last glacial: The ice core record, Wolff, Chappellaz, Blunier, Rasmussen & Svensson (2010)

The most significant climate variability in the Quaternary record is the alternation between glacial and interglacial, occurring at approximately 100 ka periodicity in the most recent 800 ka. This signal is of global scale, and observed in all climate records, including the long Antarctic ice cores (Jouzel et al., 2007a) and marine sediments (Lisiecki and Raymo, 2005). There is a strong consensus that the underlying cause of these changes is orbital (i.e. due to external forcing from changes in the seasonal and latitudinal pattern of insolation), but amplified by a whole range of internal factors (such as changes in greenhouse gas concentration and in ice extent).

Note the lack of citation for the underlying causes being orbital. However, as we will see, there is “strong consensus”. In this specific paper from the words used I believe the authors are supporting the Theory C Family, not Theory B.

———

The last glacial cycle: transient simulations with an AOGCM, Robin Smith & Jonathan Gregory (2012)

It is generally accepted that the timing of glacials is linked to variations in solar insolation that result from the Earth’s orbit around the sun (Hays et al. 1976; Huybers and Wunsch 2005). These solar radiative anomalies must have been amplified by feedback processes within the climate system, including changes in atmospheric greenhouse gas (GHG) concentrations (Archer et al. 2000) and ice-sheet growth (Clark et al. 1999), and whilst hypotheses abound as to the details of these feedbacks, none is without its detractors and we cannot yet claim to know how the Earth system produced the climate we see recorded in numerous proxy records.

I think I will classify this one as “Still a mystery”.

Note that support for “linkage to variations in solar insolation” consists of Hays et al 1976 – Theory B – and Huybers and Wunsch 2005 who propose a contradictory theory (obliquity) – Theory C Family. In this case they absolve themselves by pointing out that all the theories have flaws.

———

The timing of major climate terminations, ME Raymo (1997)

For the past 20 years, the Milankovitch hypothesis, which holds that the Earth’s climate is controlled by variations in incoming solar radiation tied to subtle yet predictable changes in the Earth’s orbit around the Sun [Hays et al., 1976], has been widely accepted by the scientific community. However, the degree to which and the mechanisms by which insolation variations control regional and global climate are poorly understood. In particular, the “100-kyr” climate cycle, the dominant feature of nearly all climate records of the last 900,000 years, has always posed a problem to the Milankovitch hypothesis..

..time interval between terminations is not constant; it varies from 84 kyr between Terminations IV and V to 120 kyr between Terminations III and II.

“Still a mystery”. (Maureen Raymo has written many papers on ice ages, is the coauthor of the LR04 ocean core database and cannot be considered an outlier). Her paper claims she solves the problem:

In conclusion, it is proposed that the interaction between obliquity and the eccentricity-modulation of precession as it controls northern hemisphere summer radiation is responsible for the pattern of ice volume growth and decay observed in the late Quaternary.

Solution was unknown, but new proposed solution is from the Theory C Family.

———

Glacial termination: sensitivity to orbital and CO2 forcing in a coupled climate system model, Yoshimori, Weaver, Marshall & Clarke (2001)

Glaciation (deglaciation) is one of the most extreme and fundamental climatic events in Earth’s history.. As a result, fluctuations in orbital forcing (e.g. Berger 1978; Berger and Loutre 1991) have been widely recognised as the primary triggers responsible for the glacial-interglacial cycles (Berger 1988; Bradley 1999; Broecker and Denton 1990; Crowley and North 1991; Imbrie and Imbrie 1979). At the same time, these studies revealed the complexity of the climate system, and produced several paradoxes which cannot be explained by a simple linear response of the climate system to orbital forcing.

At this point I was interested to find out how well these 4 papers cited (Berger 1988; Bradley 1999; Broecker and Denton 1990; Crowley and North 1991; Imbrie and Imbrie 1979) backed up the evidence for orbital forcing being the primary triggers for glacial cycles.

Broecker & Denton (1990) is in Scientific American which I don’t think counts as a peer-reviewed journal (even though a long time ago I subscribed to it and thought it was a great magazine). I was able to find the abstract only, which coincides with their peer-reviewed paper The Role of Ocean-Atmosphere Reorganization in Glacial Cycles the same year in Quaternary Science Reviews, so I’ll assume they are media hounds promoting their peer-reviewed paper for a wider audience and look at the peer-reviewed paper. After commenting on the problems:

Such a linkage cannot explain synchronous climate changes of similar severity in both polar hemispheres. Also, it cannot account for the rapidity of the transition from full glacial toward full interglacial conditions. If glacial climates are driven by changes in seasonality, then another linkage must exist.

they state:

We propose that Quaternary glacial cycles were dominated by abrupt reorganizations of the ocean- atmosphere system driven by orbitally induced changes in fresh water transports which impact salt structure in the sea. These reorganizations mark switches between stable modes of operation of the ocean-atmosphere system. Although we think that glacial cycles were driven by orbital change, we see no basis for rejecting the possibility that the mode changes are part of a self- sustained internal oscillation that would operate even in the absence of changes in the Earth’s orbital parameters. If so, as pointed out by Saltzman et al. (1984), orbital cycles can merely modulate and pace a self-oscillating climate system.

So this paper is evidence for Theory B or Theory C Family? “..we think that..” “..we see no basis for rejecting the possibility ..self-sustained internal oscillation”. This is evidence for the astronomical theory?

I can’t access Milankovitch theory and climate, Berger 1988 (thanks, Reviews of Geophysics!). If someone has it, please email it to me at scienceofdoom – you know what goes here – gmail.com. The other two references are books, so I can’t access them. Crowley & North 1991 is Paleoclimatology. Vol 16 of Oxford Monograph on Geology and Geophysics, OUP. Imbrie & Imbrie 1979 is Ice Ages: solving the mystery.

———-

Glacial terminations as southern warmings without northern control, E. W. Wolff, H. Fischer and R. Röthlisberger (2009)

However, the reason for the spacing and timing of interglacials, and the sequence of events at major warmings, remains obscure.

“Still a mystery”. This is a little different from Wolff’s comment in the paper above. Elsewhere (see his comments cited in Eleven – End of the Last Ice age) he has stated that ice age terminations are not understood:

Between about 19,000 and 10,000 years ago, Earth emerged from the last glacial period. The whole globe warmed, ice sheets retreated from Northern Hemisphere continents and atmospheric composition changed significantly. Many theories try to explain what triggered and sustained this transformation (known as the glacial termination), but crucial evidence to validate them is lacking.

———-

The Last Glacial Termination, Denton, Anderson, Toggweiler, Edwards, Schaefer & Putnam (2009)

A major puzzle of paleoclimatology is why, after a long interval of cooling climate, each late Quaternary ice age ended with a relatively short warming leg called a termination. We here offer a comprehensive hypothesis of how Earth emerged from the last global ice age..

“Still a mystery”

———–

Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Shakun, Clark, He, Marcott, Mix, Zhengyu Liu, Otto-Bliesner,  Schmittner & Bard (2012)

Understanding the causes of the Pleistocene ice ages has been a significant question in climate dynamics since they were discovered in the mid-nineteenth century. The identification of orbital frequencies in the marine 18O/16O record, a proxy for global ice volume, in the 1970s demonstrated that glacial cycles are ultimately paced by astronomical forcing.

The citation is Hays, Imbrie & Shackleton 1976. Theory B with no support.

————

Northern Hemisphere forcing of Southern Hemisphere climate during the last deglaciation, He, Shakun, Clark, Carlson, Liu, Otto-Bliesner & Kutzbach (2013)

According to the Milankovitch theory, changes in summer insolation in the high-latitude Northern Hemisphere caused glacial cycles through their impact on ice-sheet mass balance. Statistical analyses of long climate records supported this theory, but they also posed a substantial challenge by showing that changes in Southern Hemisphere climate were in phase with or led those in the north.

The citation is Hays, Imbrie & Shackleton 1976. (Many of the same authors in this and the paper above).

————-

Eight glacial cycles from an Antarctic ice core, EPICA Community Members (2004)

The climate of the last 500,000 years (500 kyr) was characterized by extremely strong 100-kyr cyclicity, as seen particularly in ice-core and marine-sediment records. During the earlier part of the Quaternary (before 1 million years ago; 1 Myr BP), cycles of 41 kyr dominated. The period in between shows intermediate behaviour, with marine records showing both frequencies and a lower amplitude of the climate signal. However, the reasons for the dominance of the 100-kyr (eccentricity) over the 41-kyr (obliquity) band in the later part of the record, and the amplifiers that allow small changes in radiation to cause large changes in global climate, are not well understood.

Is this accepting Theory B or not?

————–

Now onto the alphabetical order..

Climatic Conditions for modelling the Northern Hemisphere ice sheets throughout the ice age cycle, Abe-Ouchi, Segawa & Saito (2007)

To explain why the ice sheets in the Northern Hemisphere grew to the size and extent that has been observed, and why they retreated quickly at the termination of each 100 kyr cycle is still a challenge (Tarasov and Peltier, 1997a; Berger et al., 1998; Paillard, 1998; Paillard and Parrenin, 2004). Although it is now broadly accepted that the orbital variations of the Earth influence climate changes (Milankovitch, 1930; Hays et al., 1976; Berger, 1978), the large amplitude of the ice volume changes and the geographical extent need to be reproduced by comprehensive models which include nonlinear mechanisms of ice sheet dynamics (Raymo, 1997; Tarasov and Peltier, 1997b; Paillard, 2001; Raymo et al., 2006).

The papers cited for this broad agreement are Hays et al 1976 once again. And Berger 1978 who says:

It is not the aim of this paper to draw definitive conclusions about the astronomical theory of paleoclimates but simply to provide geologists with accurate theoretical values of the earth’s orbital elements and insolation..

Berger does go on to comment on eccentricity:

Berger 1978

Berger 1978

And this is simply again noting that the period for eccentricity is “similar” to the period for the ice age terminations.

Theory B with no support.

——

Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume, Abe-Ouchi, Saito, Kawamura, Raymo, Okuno, Takahashi & Blatter (2013)

Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles, and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles. Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work. Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere ‘excess ice’, but the physical mechanisms underpinning the 100,000-year cycle remain unclear.

The citations for the statistical tests are Lisiecki 2010 and Huybers 2011.

Huybers 2011 claims that obliquity and precession (not eccentricity) are linked to deglaciations. This is development of his earlier, very interesting 2007 hypothesis (Glacial variability over the last two million years: an extended depth-derived agemodel, continuous obliquity pacing, and the Pleistocene progression - to which we will return) that obliquity is the prime factor (not necessarily the cause) in deglaciations.

Here is what Huybers says in his 2011 paper, Combined obliquity and precession pacing of late Pleistocene deglaciations:

The cause of these massive shifts in climate remains unclear not for lack of models, of which there are now over thirty, but for want of means to choose among them. Previous statistical tests have demonstrated that obliquity paces the 100-kyr glacial cycles [citations are his 2005 paper with Carl Wunsch and his 2007 paper], helping narrow the list of viable mechanisms, but have been inconclusive with respect to precession (that is, P > 0.05) because of small sample sizes and uncertain timing..

In Links between eccentricity forcing and the 100,000-year glacial cycle, (2010), Lisiecki says:

Variations in the eccentricity (100,000 yr), obliquity (41,000 yr) and precession (23,000 yr) of Earth’s orbit have been linked to glacial–interglacial climate cycles. It is generally thought that the 100,000-yr glacial cycles of the past 800,000 yr are a result of orbital eccentricity [1–4] . However, the eccentricity cycle produces negligible 100-kyr power in seasonal or mean annual insolation, although it does modulate the amplitude of the precession cycle.

Alternatively, it has been suggested that the recent glacial cycles are driven purely by the obliquity cycle [5–7]. Here I use statistical analyses of insolation and the climate of the past five million years to characterize the link between eccentricity and the 100,000-yr glacial cycles. Using cross-wavelet phase analysis, I show that the relative phase of eccentricity and glacial cycles has been stable since 1.2 Myr ago, supporting the hypothesis that 100,000-yr glacial cycles are paced [8–10] by eccentricity [4,11]. However, I find that the time-dependent 100,000-yr power of eccentricity has been anticorrelated with that of climate since 5 Myr ago, with strong eccentricity forcing associated with weaker power in the 100,000-yr glacial cycle.

I propose that the anticorrelation arises from the strong precession forcing associated with strong eccentricity forcing, which disrupts the internal climate feedbacks that drive the 100,000-yr glacial cycle. This supports the hypothesis that internally driven climate feedbacks are the source of the 100,000-yr climate variations.

So she accepts that Theory B is generally accepted, although some Theory C Family advocates are out there, but provides a new hybrid solution of her own.

References for the orbital eccentricity hypothesis [1-4] include Hays et al 1976 and Raymo 1997 cited above. However, Raymo didn’t think it had been demonstrated prior to her 1997 paper and in her 1997 paper introduces the hypothesis that is primarily ice sheet size, obliquity and precession modulated by eccentricity.

References for the obliquity hypothesis [5-7] include the Huybers & Wunsch 2005 and Huybers 2007 covered just before this reference.

So in summary – going back to how we dragged up these references – Abe-Ouchi and co-authors provide two citations in support of the statistical link between orbital variations and deglaciation. One citation claims primarily obliquity with maybe a place for precession – no link to eccentricity. Another citation claims a new theory for eccentricity as a phase-locking mechanism to an internal climate process.

These are two mutually exclusive ideas. But at least both papers attempted to prove their (exclusive) ideas.

——

Equatorial insolation: from precession harmonics to eccentricity frequencies, Berger, Loutre, & Mélice (2006):

Since the paper by Hays et al. (1976), spectral analyses of climate proxy records provide substantial evidence that a fraction of the climatic variance is driven by insolation changes in the frequency ranges of obliquity and precession variations. However, it is the variance components centered near 100 kyr which dominate most Upper Pleistocene climatic records, although the amount of insolation perturbation at the eccentricity driven periods close to 100-kyr (mainly the 95 kyr- and 123 kyr-periods) is much too small to cause directly a climate change of ice-age amplitude. Many attempts to find an explanation to this 100-kyr cycle in climatic records have been made over the last decades.

“Still a mystery”.

——

Multistability and hysteresis in the climate-cryosphere system under orbital forcing, Calov & Ganopolski (2005)

In spite of considerable progress in studies of past climate changes, the nature of vigorous climate variations observed during the past several million years remains elusive. A variety of different astronomical theories, among which the Milankovitch theory [Milankovitch, 1941] is the best known, suggest changes in Earth’s orbital parameters as a driver or, at least, a pacemaker of glacial-interglacial climate transitions. However, the mechanisms which translate seasonal and strongly latitude-dependent variations in the insolation into the global-scale climate shifts between glacial and interglacial climate states are the subject of debate.

“Still a mystery”

——

Ice Age Terminations, Cheng, Edwards, Broecker, Denton, Kong, Wang, Zhang, Wang (2009)

The ice-age cycles have been linked to changes in Earth’s orbital geometry (the Milankovitch or Astronomical theory) through spectral analysis of marine oxygen-isotope records (3), which demonstrate power in the ice-age record at the same three spectral periods as orbitally driven changes in insolation. However, explaining the 100 thousand- year (ky)–recurrence period of ice ages has proved to be problematic because although the 100-ky cycle dominates the ice-volume power spectrum, it is small in the insolation spectrum. In order to understand what factors control ice age cycles, we must know the extent to which terminations are systematically linked to insolation and how any such linkage can produce a non- linear response by the climate system at the end of ice ages.

“Still a mystery”. This paper claims (their new work) that terminations are all about high latitude NH insolation. They state, for the hypothesis of the paper:

In all four cases, observations are consistent with a classic Northern Hemisphere summer insolation intensity trigger for an initial retreat of northern ice sheets.

This is similar to Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years, Kawamura et al (2007) - not cited here because they didn’t make a statement about “the problem so far”.

——

Orbital forcing and role of the latitudinal insolation/temperature gradient, Basil Davis & Simon Brewer (2009)

Orbital forcing of the climate system is clearly shown in the Earths record of glacial–interglacial cycles, but the mechanism underlying this forcing is poorly understood.

Not sure whether this is classified as “Still a mystery” or Theory B or Theory C Family.

——

Evidence for Obliquity Forcing of Glacial Termination II, Drysdale, Hellstrom, Zanchetta, Fallick, Sánchez Goñi, Couchoud, McDonald, Maas, Lohmann & Isola (2009)

During the Late Pleistocene, the period of glacial-to-interglacial transitions (or terminations) has increased relative to the Early Pleistocene [~100 thousand years (ky) versus 40 ky]. A coherent explanation for this shift still eludes paleoclimatologists (3). Although many different models have been proposed (4), the most widely accepted one invokes changes in the intensity of high-latitude Northern Hemisphere summer insolation (NHSI). These changes are driven largely by the precession of the equinoxes (5), which produces relatively large seasonal and hemispheric insolation intensity anomalies as the month of perihelion shifts through its ~23-ky cycle.

Their “widely accepted” theory is from the Theory C Family. This is a different theory from the “widely accepted” theory B. Perhaps both are “widely accepted”, hopefully by different groups of scientists.

——

The role of orbital forcing, carbon dioxide and regolith in 100 kyr glacial cycles, Ganopolski & Calov (2011)

The origin of the 100 kyr cyclicity, which dominates ice volume variations and other climate records over the past million years, remains debatable..

..One of the major challenges to the classical Milankovitch theory is the presence of 100 kyr cycles that dominate global ice volume and climate variability over the past million years (Hays et al., 1976; Imbrie et al., 1993; Paillard, 2001).

This periodicity is practically absent in the principal “Milankovitch forcing” – variations of summer insolation at high latitudes of the Northern Hemisphere (NH).

The eccentricity of Earth’s orbit does contain periodicities close to 100 kyr and the robust phase relationship between glacial cycles and 100-kyr eccentricity cycles has been found in the paleoclimate records (Hays et al., 1976; Berger et al., 2005; Lisiecki, 2010). However, the direct effect of the eccentricity on Earth’s global energy balance is very small.

Moreover, eccentricity variations are dominated by a 400 kyr cycle which is also seen in some older geological records (e.g. Zachos et al., 1997), but is practically absent in the frequency spectrum of the ice volume variations for the last million years.

In view of this long-standing problem, it was proposed that the 100 kyr cycles do not originate directly from the orbital forcing but rather represent internal oscillations in the climate-cryosphere (Gildor and Tziperman, 2001) or climate-cryosphere-carbonosphere system (e.g. Saltzman and Maasch, 1988; Paillard and Parrenin, 2004), which can be synchronized (phase locked) to the orbital forcing (Tziperman et al., 2006).

Alternatively, it was proposed that the 100 kyr cycles result from the terminations of ice sheet buildup by each second or third obliquity cycle (Huybers and Wunsch, 2005) or each fourth or fifth precessional cycle (Ridgwell et al., 1999) or they originate directly from a strong, nonlinear, climate-cryosphere system response to a combination of precessional and obliquity components of the orbital forcing (Paillard, 1998).

“Still a mystery”.

——–

Modeling the Climatic Response to Orbital Variations, Imbrie & Imbrie (1980)

This is not to say that all important questions have been answered. In fact, one purpose of this article is to contribute to the solution of one of the remaining major problems: the origin and history of the 100,000-year climatic cycle.

At least over the past 600,000 years, almost all climatic records are dominated by variance components in a narrow frequency band centered near a 100,000-year cycle (5-8, 12, 21, 38). Yet a climatic response at these frequencies is not predicted by the Milankovitch version of the astronomical theory – or any other version that involves a linear response (5, 6).

This paper was worth citing because the first author is the coathor of Hays et al 1976. For interest let’s look at what they attempt to demonstrate in their paper. They take the approach of producing different (simple) models with orbital forcing, to try to reproduce the geological record:

The goal of our modeling effort has been to simulate the climatic response to orbital variations over the past 500 kyrs. The resulting model fails to simulate four important aspects of this record. It fails to produce sufficient 100k power; it produces too much 23K and 19K power; it produces too much 413k power and it loses its match with the record ardoun the time of the last 413k eccentricity minimum..

All of these failures are related to a fundamental shortcoming in the generation of 100k power.. Indeed it is possible that no function will yield a good simulation of the entire 500 kyr record under consideration here, because nonorbitally forced high-frequency fluctuations may have caused the system to flip or flop in an unpredictable fashion. This would be an example of Lorenz’s concept of an almost intransitive system..

..Progress in this direction will indicate what long-term variations need to be explained within the framework of a stochastic model and provide a basis for estimating the degree of unpredictability in climate.

——

On the structure and origin of major glaciation cycles, Imbrie, Boyle, Clemens, Duffy, Howard, Kukla, Kutzbach, Martinson, McIntyre, Mix, Molfino, Morley, Peterson, Pisias, Prell, Raymo, Shackleton & Toggweiler (1992)

It is now widely believed that these astronomical influences, through their control of the seasonal and latitudinal distribution of incident solar radiation, either drive the major climate cycles externally or set the phase of oscillations that are driven internally..

..In this paper we concentrate on the 23-kyr and 41- kyr cycles of glaciation. These prove to be so strongly correlated with large changes in seasonal radiation that we regard them as continuous, essentially linear responses to the Milankovitch forcing. In a subsequent paper we will remove these linearly forced components from each time series and examine the residual response. The residual response is dominated by a 100-kyr cycle, which has twice the amplitude of the 23- and 41-kyr cycles combined. In the band of periods near 100 kyr, variations in radiation correlated with climate are so small, compared with variations correlated with the two shorter climatic cycles, that the strength of the 100-kyr climate cycle must result from the channeling of energy into this band by mechanisms operating within the climate system itself.

In Part 2, Imbrie et al (same authors) 1993 they highlight in more detail the problem of explaining the 100 kyr period:

1. One difficulty in finding a simple Milankovitch explanation is that the amplitudes of all 100-kyr radiation signals are very small [Hays et al., 1976]. As an example, the amplitude of the 100-kyr radiation cycle at June 65N (a signal often used as a forcing in Milankovitch theories) is only 2W/m² (Figure 1). This is 1 order of magnitude smaller than the same insolation signal in the 23- and 41- kyr bands, yet the system’s response in these two bands combined has about half the amplitude observed at 100 kyr.

2. Another fundamental difficulty is that variations in eccentricity are not confined to periods near 100 kyr. In fact, during the late Pleistocene, eccentricity variations at periods near 100 kyr are of the same order of magnitude as those at 413 kyr.. yet the d18O record for this time interval has no corresponding spectral peak near 400 kyr..

3. The high coherency observed between 100 kyr eccentricity and d18O signals is an average that hides significant mismatches, notably about 400 kyrs ago.

Their proposed solution:

In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity-driven modulations in the 23,000-year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher-frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100 kyr oscillations.

This doesn’t really appear to be Theory B.

——

Orbital forcing of Arctic climate: mechanisms of climate response and implications for continental glaciation, Jackson & Broccoli (2003)

The growth and decay of terrestrial ice sheets during the Quaternary ultimately result from the effects of changes in Earth’s orbital geometry on climate system processes. This link is convincingly established by Hays et al. (1976) who find a correlation between variations of terrestrial ice volume and variations in Earth’s orbital eccentricity, obliquity, and longitude of the perihelion.

Hays et al 1976. Theory B with no support.

——

A causality problem for Milankovitch, Karner & Muller (2000)

We can conclude that the standard Milankovitch insolation theory does not account for the terminations of the ice ages. That is a serious and disturbing conclusion by itself. We can conclude that models that attribute the terminations to large insolation peaks (or, equivalently, to peaks in the precession parameter), such as the recent one by Raymo (23), are incompatible with the observations.

I’ll take this as “Still a mystery”.

——

Linear and non-linear response of late Neogene glacial cycles to obliquity forcing and implications for the Milankovitch theory, Lourens, Becker, Bintanja, Hilgen, Tuenter & van de Wal, Ziegler (2010)

Through the spectral analyses of marine oxygen isotope (d18O) records it has been shown that ice-sheets respond both linearly and non-linearly to astronomical forcing.

References in support of this statement include Imbrie et al 1992 & Imbrie et al 1993 that we reviewed above, and Pacemaking the Ice Ages by Frequency Modulation of Earth’s Orbital Eccentricity, JA Rial (1999):

The theory finds support in the fact that the spectra of the d18O records contain some of the same frequencies as the astronomical variations (2– 4), but a satisfactory explanation of how the changes in orbital eccentricity are transformed into the 100-ky quasi-periodic fluctuations in global ice volume indicated by the data has not yet been found (5).

For interest, the claim for the new work in this paper:

Evidence from power spectra of deep-sea oxygen isotope time series suggests that the climate system of Earth responds nonlinearly to astronomical forcing by frequency modulating eccentricity-related variations in insolation. With the help of a simple model, it is shown that frequency modulation of the approximate 100,000-year eccentricity cycles by the 413,000-year component accounts for the variable duration of the ice ages, the multiple-peak character of the time series spectra, and the notorious absence of significant spectral amplitude at the 413,000-year period. The observed spectra are consistent with the classic Milankovitch theories of insolation..

So if we consider the 3 references the provide in support of the “astronomical hypothesis”, the latest one says that a solution to the 100 kyr problem has not yet been found – of course this 1999 paper gives it their own best shot. Rial (1999) clearly doesn’t think that Imbrie et al 1992 / 1993 solved the problem.

And, of course, Rial (1999) proposes a different solution to Imbrie et al 1992/1993.

——

Dynamics between order and chaos in conceptual models of glacial cycles, Takahito Mitsui & Kazuyuki Aihara, Climate Dynamics (2013)

Hays et al. (1976) presented strong evidence for astronomical theories of ice ages. They found the primary frequencies of astronomical forcing in the geological spectra of marine sediment cores. However, the dominant frequency in geological spectra is approximately 1/100 kyr-1, although this frequency component is negligible in the astronomical forcing. This is referred to as the ‘100 kyr problem.’

However, the linear response cannot appropriately account for the 100 kyr periodicity (Hays et al. 1976).

Ghil (1994) explained the appearance of the 100 kyr periodicity as a nonlinear resonance to the combination tone 1/109 kyr-1 between precessional frequencies 1/19 and 1/23 kyr-1. Contrary to the linear resonance, the nonlinear resonance can occur even if the forcing frequencies are far from the internal frequency of the response system.

Benzi et al. (1982) proposed stochastic resonance as a mechanism of the 100 kyr periodicity, where the response to small external forcing is amplified by the effect of noise.

Tziperman et al. (2006) proposed that the timing of deglaciations is set by the astronomical forcing via the phase- locking mechanism.. De Saedeleer et al. (2013) suggested generalized synchronization (GS) to describe the relation between the glacial cycles and the astronomical forcing. GS means that there is a functional relation between the climate state and the state of the astronomical forcing. They also showed that the functional relation may not be unique for a certain model.

However, the nature of the relation remains to be elucidated.

“Still a mystery”.

——

Glacial cycles and orbital inclination, Richard Muller & Gordon MacDonald, Nature (1995)

According to the Milankovitch theory, the 100 kyr glacial cycle is caused by changes in insolation (solar heating) brought about by variations in the eccentricity of the Earth’s orbit. There are serious difficulties with this theory: the insolation variations appear to be too small to drive the cycles and a strong 400 kyr modulation predicted by the theory is not present..

We suggest that a radical solution is necessary to solve these problems, and we propose that the 100 kyr glacial cycle is caused, not by eccentricity, but by a previously ignored parameter: the orbital inclination, the tilt of the Earth’s orbital plane..

“Still a mystery”, with the new solution of a member of the Theory C Family.

——

Terminations VI and VIII (∼ 530 and ∼ 720 kyr BP) tell us the importance of obliquity and precession in the triggering of deglaciations, F. Parrenin & D. Paillard (2012)

The main variations of ice volume of the last million years can be explained from orbital parameters by assuming climate oscillates between two states: glaciations and deglaciations (Parrenin and Paillard, 2003; Imbrie et al., 2011) (or terminations). An additional combination of ice volume and orbital parameters seems to form the trigger of a deglaciation, while only orbital parameters seem to play a role in the triggering of glaciations. Here we present an optimized conceptual model which realistically reproduce ice volume variations during the past million years and in partic- ular the timing of the 11 canonical terminations. We show that our model looses sensitivity to initial conditions only after ∼ 200 kyr at maximum: the ice volume observations form a strong attractor. Both obliquity and precession seem necessary to reproduce all 11 terminations and both seem to play approximately the same role.

Note that eccentricity variations are not cited as the cause.

The support for orbital parameters explaining the ice age glaciation/deglaciation are two papers. First, Parrenin & Paillard: Amplitude and phase of glacial cycles from a conceptual model (2003):

Although we find astronomical frequencies in almost all paleoclimatic records [1,2], it is clear that the climatic system does not respond linearly to insolation variations [3]. The first well-known paradox of the astronomical theory of climate is the ‘100 kyr problem’: the largest variations over the past million years occurred approximately every 100 kyr, but the amplitude of the insolation signal at this frequency is not significant. Although this problem remains puzzling in many respects, multiple equilibria and thresholds in the climate system seem to be key notions to explain this paradoxical frequency.

Their solution:

To explain these paradoxical amplitude and phase modulations, we suggest here that deglaciations started when a combination of insolation and ice volume was large enough. To illustrate this new idea, we present a simple conceptual model that simulates the sea level curve of the past million years with very realistic amplitude modulations, and with good phase modulations.

The other paper cited in support of an astronomical solution is A phase-space model for Pleistocene ice volume, Imbrie, Imbrie-Moore & Lisiecki, Earth and Planetary Science Letters (2011)

Numerous studies have demonstrated that Pleistocene glacial cycles are linked to cyclic changes in Earth’s orbital parameters (Hays et al., 1976; Imbrie et al., 1992; Lisiecki and Raymo, 2007); however, many questions remain about how orbital cycles in insolation produce the observed climate response. The most contentious problem is why late Pleistocene climate records are dominated by 100-kyr cyclicity.

Insolation changes are dominated by 41-kyr obliquity and 23-kyr precession cycles whereas the 100-kyr eccentricity cycle produces negligible 100-kyr power in seasonal or mean annual insolation. Thus, various studies have proposed that 100-kyr glacial cycles are a response to the eccentricity-driven modulation of precession (Raymo, 1997; Lisiecki, 2010b), bundling of obliquity cycles (Huybers and Wunsch, 2005; Liu et al., 2008), and/or internal oscillations (Saltzman et al., 1984; Gildor and Tziperman, 2000; Toggweiler, 2008).

Their new solution:

We present a new, phase-space model of Pleistocene ice volume that generates 100-kyr cycles in the Late Pleistocene as a response to obliquity and precession forcing. Like Parrenin and Paillard, (2003), we use a threshold for glacial terminations. However, ours is a phase-space threshold: a function of ice volume and its rate of change. Our model the first to produce an orbitally driven increase in 100-kyr power during the mid-Pleistocene transition without any change in model parameters.

Theory C Family – two (relatively) new papers (2003 & 2011) with similar theories are presented as support of the astronomical theory causing the ice ages. Note that the theory in Imbrie et al 2013 is not the 100 kyr eccentricity variation proposed by Hays, Imbrie and Shackleton 1976.

——

Coherence resonance and ice ages, Jon D. Pelletier, JGR (2003)

The processes and feedbacks responsible for the 100-kyr cycle of Late Pleistocene global climate change are still being debated. This paper presents a numerical model that integrates (1) long-wavelength outgoing radiation, (2) the ice-albedo feedback, and (3) lithospheric deflection within the simple conceptual framework of coherence resonance. Coherence resonance is a dynamical process that results in the amplification of internally generated variability at particular periods in a system with bistability and delay feedback..

..The 100-kyr cycle is a free oscillation in the model, present even in the absence of external forcing.

“Still a mystery” – with the new solution that is not astronomical forcing.

——

The 41 kyr world: Milankovitch’s other unsolved mystery, Maureen E. Raymo & Kerim Nisancioglu (2003)

All serious students of Earth’s climate history have heard of the ‘‘100 kyr problem’’ of Milankovitch orbital theory, namely the lack of an obvious explanation of the dominant 100 kyr periodicity in climate records of the last 800,000 years.

“Still a mystery” – except that Raymo thinks she has found the solution (see earlier)

——

Is the spectral signature of the 100 kyr glacial cycle consistent with a Milankovitch origin, Ridgwell, Watson & Raymo (1999)

Global ice volume proxy records obtained from deep-sea sediment cores, when analyzed in this way produce a narrow peak corresponding to a period of ~100 kyr that dominates the low frequency part of the spectrum. This contrasts with the spectrum of orbital eccentricity variation, often assumed to be the main candidate to pace the glaciations [Hays et al 1980], which shows two distinct peaks near 100 kyr and substantial power near the 413 kyr period.

Then their solution:

Milankovitch theory seeks to explain the Quaternary glaciations via changes in seasonal insolation caused by periodic changes in the Earth’s obliquity, orbital precession and eccentricity. However, recent high-resolution spectral analysis of d18O proxy climate records have cast doubt on the theory.. Here we show that the spectral signature of d18O records are entirely consistent with Milankovitch mechanisms in which deglaciations are triggered every fourth or fifth precessional cycle. Such mechanisms may involve the buildup of excess ice due to low summertime insolation at the previous precessional high.

So they don’t accept Theory B. They don’t claim the theory has been previously solved and they introduce a Theory C Family.

——

In defense of Milankovitch, Gerard Roe (2006) – we reviewed this paper in Fifteen – Roe vs Huybers:

The Milankovitch hypothesis is widely held to be one of the cornerstones of climate science. Surprisingly, the hypothesis remains not clearly defined despite an extensive body of research on the link between global ice volume and insolation changes arising from variations in the Earth’s orbit.

And despite his interesting efforts at solving the problem he states towards the end of his paper:

The Milankovitch hypothesis as formulated here does not explain the large rapid deglaciations that occurred at the end of some of the ice age cycles.

Was it still a mystery or just not well defined. And from his new work, I’m not sure whether that means he thinks he has solved the reason for some ice age terminations, or that terminations are still a mystery.

——

The 100,000-Year Ice-Age Cycle Identified and Found to Lag Temperature, Carbon Dioxide, and Orbital Eccentricity, Nicholas J. Shackleton (the Shackleton from Hays et al 1976), (2000)

It is generally accepted that this 100-ky cycle represents a major component of the record of changes in total Northern Hemisphere ice volume (3). It is difficult to explain this predominant cycle in terms of orbital eccentricity because “the 100,000-year radiation cycle (arising from eccentricity variations) is much too small in amplitude and too late in phase to produce the corresponding climatic cycle by direct forcing”

So the Hays, Imbrie & Shackleton 1976 Theory B is not correct.

He does state:

Hence, the 100,000-year cycle does not arise from ice sheet dynamics; instead, it is probably the response of the global carbon cycle that generates the eccentricity signal by causing changes in atmospheric carbon dioxide concentration.

Note that this is in opposition to the papers by Imbrie et al (2011) and Parrenin & Paillard (2003) that were cited by Parrenin & Paillard (2012) in support of the astronomical theory of the ice ages.

——

Consequences of pacing the Pleistocene 100 kyr ice ages by nonlinear phase locking to Milankovitch forcing, Tziperman, Raymo, Huybers & Wunsch (2006)

Hays et al. [1976] established that Milankovitch forcing (i.e., variations in orbital parameters and their effect on the insolation at the top of the atmosphere) plays a role in glacial cycle dynamics. However, precisely what that role is, and what is meant by ‘‘Milankovitch theories’’ remains unclear despite decades of work on the subject [e.g., Wunsch, 2004; Rial and Anaclerio, 2000]. Current views vary from the inference that Milankovitch variations in insolation drives the glacial cycle (i.e., the cycles would not exist without Milankovitch variations), to the Milankovitch forcing causing only weak climate perturbations superimposed on the glacial cycles. A further possibility is that the primary influence of the Milankovitch forcing is to set the frequency and phase of the cycles (e.g., controlling the timing of glacial terminations or of glacial inceptions). In the latter case, glacial cycles would exist even in the absence of the insolation changes, but with different timing.

“Still a mystery” – but now solved with a Theory C Family (in their paper).

——

Quantitative estimate of the Milankovitch-forced contribution to observed Quaternary climate change, Carl Wunsch (2004)

The so-called Milankovitch hypothesis, that much of inferred past climate change is a response to near- periodic variations in the earth’s position and orientation relative to the sun, has attracted a great deal of attention. Numerous textbooks (e.g., Bradley, 1999; Wilson et al., 2000; Ruddiman, 2001) of varying levels and sophistication all tell the reader that the insolation changes are a major element controlling climate on time scales beyond about 10,000 years.

A recent paper begins ‘‘It is widely accepted that climate variability on timescales of 10 kyrs to 10 kyrs is driven primarily by orbital, or so-called Milankovitch, forcing.’’ (McDermott et al., 2001). To a large extent, embrace of the Milankovitch hypothesis can be traced to the pioneering work of Hays et al. (1976), who showed, convincingly, that the expected astronomical periods were visible in deep-sea core records..

..The long-standing question of how the slight Milankovitch forcing could possibly force such an enormous glacial–interglacial change is then answered by concluding that it does not do so.

“Still a mystery” – Wunsch does not accept Theory B and in this year didn’t accept Theory C Family (later co-authors a Theory C Family paper with Huybers). I cited this before in Part Six – “Hypotheses Abound”.

——

Individual contribution of insolation and CO2 to the interglacial climates of the past 800,000 years, Qiu Zhen Yin & André Berger (2012)

Climate variations of the last 3 million years are characterized by glacial-interglacial cycles which are generally believed to be driven by astronomically induced insolation changes.

No citation for the claim. Of course I agree that it is “generally believed”. Is this theory B? Or theory C? Or not sure?

——

Summary of the Papers

Out of about 300 papers checked, I found 34 papers (I might have missed a few) with a statement on the major cause of the ice ages separate from what they attempted to prove in their paper. These 34 papers were reviewed, with a further handful of cited papers examined to see what support they offered for the claim of the paper in question.

In respect of “What has been demonstrated up until our paper” – I count:

  • 19 “still a mystery”
  • 9 propose theory B
  • 6 supporting theory C

I have question marks over my own classification of about 10 of these because they lack clarity on what they believe is the situation to date.

Of course, from the point of view of the papers reviewed each believes they have some solution for the mystery. That’s not primarily what I was interested in.

I wanted to see what all papers accept as the story so far, and what evidence they bring for this belief.

I found only one paper claiming theory B that attempted to produce any significant evidence in support.

Conclusion

Hays, Imbrie & Shackleton (1976) did not prove Theory B. They suggested it. Invoking “probably non-linearity” does not constitute proof for an apparent frequency correlation. Specifically, half an apparent frequency correlation – given that eccentricity has a 413 kyr component as well as a 100 kyr component.

Some physical mechanism is necessary. Of course, I’m certain Hays, Imbrie & Shackleton understood this (I’ve read many of their later papers).

Of the papers we reviewed, over half indicate that the solution is still a mystery. That is fine. I agree it is a mystery.

Some papers indicate that the theory is widely believed but not necessarily that they do. That’s probably fine. Although it is confusing for non-specialist readers of their paper.

Some papers cite Hays et al 1976 as support for theory B. This is amazing.

Some papers claim “astronomical forcing” and in support cite Hays et al 1976 plus a paper with a different theory from the Theory C Family. This is also amazing.

Some papers cite support for Theory C Family - an astronomical theory to explain the ice ages with a different theory than Hays et al 1976. Sometimes their cited papers align. However, between papers that accept something in the Theory C Family there is no consensus on which version of Theory C Family, and obviously therefore, on the papers which support it.

How can papers cite Hays et al for support of the astronomical theory of ice age inception/termination?

It is required to put forward citations for just about every claim in a paper even if the entire world has known it from childhood. It seems to be a journal convention/requirement:

The sun rises each day [see Kepler 1596; Newton 1687, Plato 370 BC]

Really? Newton didn’t actually prove it in his paper? Oh, you know what, I just had a quick look at the last few papers in my field and copied their citations so I could get on with putting forward my theory. Come on, we all know the sun rises every day, look out the window (unless you live in England). Anyway, so glad you called, let me explain my new theory, it solves all those other problems, I’ve really got something here..

Well, that might be part of the answer. It isn’t excusable, but introductions don’t have the focus they should have.

Why the Belief in Theory B?

This part I can’t answer. Lots of people have put forward theories, none is generally accepted. The reason for the ice age terminations is unknown. Or known by a few people and not yet accepted by the climate science community.

Is it ok to accept something that everyone else seems to believe even though they all actually have a different theory. Is it ok to accept something as proven that is not really proven because it is from a famous paper with 2500 citations?

Finally, the fact that most papers have some vague words at the start about the “orbital” or “astronomical” theory for the ice ages doesn’t mean that this theory has any support. Being scientific, being skeptical, means asking for evidence and definitely not accepting an idea just because “everyone else” appears to accept it.

I am sure people will take issue with me. In another blog I was told that scientists were just “dotting the i’s and crossing the t’s” and none of this was seriously in doubt. Apparently, I was following creationist tactics of selective and out-of-context quoting..

Well, I will be delighted and no doubt entertained to read these comments, but don’t forget to provide evidence for the astronomical theory of the ice ages.

Articles in this Series

Part One – An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Pop Quiz: End of An Ice Age - a chance for people to test their ideas about whether solar insolation is the factor that ended the last ice age

Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2

Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland

Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II

Fourteen – Concepts & HD Data - getting a conceptual feel for the impacts of obliquity and precession, and some ice age datasets in high resolution

Fifteen – Roe vs Huybers - reviewing In Defence of Milankovitch, by Gerard Roe

Sixteen – Roe vs Huybers II - remapping a deep ocean core dataset and updating the previous article

Seventeen – Proxies under Water I - explaining the isotopic proxies and what they actually measure

Nineteen – Ice Sheet Models I - looking at the state of ice sheet models

Notes

Note 1: The temperature fluctuations measured in Antarctica are a lot smaller than Greenland but still significant and still present for similar periods. There are also some technical challenges with calculating the temperature change in Antarctica (the relationship between d18O and local temperature) that have been better resolved in Greenland.

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In Thirteen – Terminator II we had a cursory look at the different “proxies” for temperature and ice volume/sea level. And we’ve considered some issues around dating of proxies.

There are two main proxies we have used so far to take a look back into the ice ages:

  • δ18O in deep ocean cores in the shells of foraminifera – to measure ice volume
  • δ18O in the ice in ice cores (Greenland and Antarctica) – to measure temperature

Now we want to take a closer look at the proxies themselves. It’s a necessary subject if we want to understand ice ages, because the proxies don’t actually measure what they might be assumed to measure. This is a separate issue from the dating: of ice; of gas trapped in ice; and of sediments in deep ocean cores.

If we take samples of ocean water, H2O, and measure the proportion of the oxygen isotopes, we find (Ferronsky & Polyakov 2012):

  • 16O – 99.757 %
  • 17O –   0.038%
  • 18O –   0.205%

There is another significant water isotope, Deuterium – aka, “heavy hydrogen” – where the water molecule is HDO, also written as 1H2HO – instead of H2O.

The processes that affect ratios of HDO are similar to the processes that affect the ratios of H218O, and consequently either isotope ratio can provide a temperature proxy for ice cores. A value of δD equates, very roughly, to 10x a value of δ18O, so mentally you can use this ratio to convert from δ18O to δD (see note 1).

In Note 2 I’ve included some comments on the Dole effect, which is the relationship between the ocean isotopic composition and the atmospheric oxygen isotopic composition. It isn’t directly relevant to the discussion of proxies here, because the ocean is the massive reservoir of 18O and the amount in the atmosphere is very small in comparison (1/1000). However, it might be of interest to some readers and we will return to the atmospheric value later when looking at dating of Antarctic ice cores.

Terminology and Definitions

The isotope ratio, δ18O, of ocean water = 2.005 ‰, that is, 0.205 %. This is turned into a reference, known as Vienna Standard Mean Ocean Water. So with respect to VSMOW, δ18O, of ocean water = 0. It’s just a definition. The change is shown as δ, the Greek symbol for delta, very commonly used in maths and physics to mean “change”.

The values of isotopes are usually expressed in terms of changes from the norm, that is, from the absolute standard. And because the changes are quite small they are expressed as parts per thousand = per mil = ‰, instead of percent, %.

So as δ18O changes from 0 (ocean water) to -50‰ (typically the lowest value of ice in Antarctica), the proportion of 18O goes from 0.20% (2.0‰) to 0.19% (1.9‰).

If the terminology is confusing think of the above example as a 5% change. What is 5% of 20? Answer is 1; and 20 – 1 = 19. So the above example just says if we reduce the small amount, 2 parts per thousand of 18O by 5% we end up with 1.9 parts per thousand.

Here is a graph that links the values together:

From Hoef 2009

From Hoefs 2009

Figure 1

Fractionation, or Why Ice Sheets are So Light

We’ve seen this graph before – the δ18O (of ice) in Greenland (NGRIP) and Antarctica (EDML) ice sheets against time:

From EPICA 2006

From EPICA 2006

Figure 2

Note that the values of δ18O from Antarctica (EDML – top line) through the last 150 kyrs are from about -40 to -52 ‰. And the values from Greenland (NGRIP – black line in middle section) are from about -32 to -44 ‰.

There are some standard explanations around – like this link - but the I’m not sure the graphic alone quite explains it – unless you understand the subject already..

If we measure the 18O concentration of a body of water, then we measure the 18O concentration of the water vapor above it, we find that the water vapor value has 18O at about -10 ‰ compared with the body of water. We write this as δ18O = -10 ‰. That is, the water vapor is a little lighter, isotopically speaking, than the ocean water.

The processes (fractionation) that cause this are easy to reproduce in the lab:

  • during evaporation, the lighter isotopes evaporate preferentially
  • during precipitation, the heavier isotopes precipitate preferentially

(See note 3).

So let’s consider the journey of a parcel of water vapor evaporated somewhere near the equator. The water vapor is a little reduced in 18O (compared with the ocean) due to the evaporation process. As the parcel of air travels away from the equator it rises and cools and some of the water vapor condenses. The initial rain takes proportionately more 18O than is in the parcel – so the parcel of air gets depleted in 18O. It keeps moving away from the equator, the air gets progressively colder, it keeps raining out, and the further it goes the less the proportion of 18O remains in the parcel of air. By the time precipitation forms in polar regions the water or ice is very light isotopically, that is, δ18O is the most negative it can get.

As a very simplistic idea of water vapor transport, this explains why the ice sheets in Greenland and Antarctica have isotopic values that are very low in 18O. Let’s take a look at some data to see how well such a simplistic idea holds up..

The isotopic composition of precipitation:

From Gat 2010

From Gat 2010

Figure 3 – Click to Enlarge

We can see the broad result represented quite well – the further we are in the direction of the poles the lower the isotopic composition of precipitation.

In contrast, when we look at local results in some detail we don’t see such a tidy picture. Here are some results from Rindsberger et al (1990) from central and northern Israel:

From Rindsberger et al 1990

From Rindsberger et al 1990

Figure 4

From Rindsberger et al 1990

From Rindsberger et al 1990

Figure 5

The authors comment:

It is quite surprising that the seasonally averaged isotopic composition of precipitation converges to a rather well-defined value, in spite of the large differences in the δ value of the individual precipitation events which show a range of 12‰ in δ18O.. At Bet-Dagan.. from which we have a long history.. the amount weighted annual average is δ18O = 5.07 ‰ ± 0.62 ‰ for the 19 year period of 1965-86. Indeed the scatter of ± 0.6‰ in the 19-year long series is to a significant degree the result of a 4-year period with lower δ values, namely the years 1971-75 when the averaged values were δ18O = 5.7 ‰ ± 0.2 ‰. That period was one of worldwide climate anomalies. Evidently the synoptic pattern associated with the precipitation events controls both the mean isotopic values of the precipitation and its variability.

The seminal 1964 paper by Willi Dansgaard is well worth a read for a good overview of the subject:

As pointed out.. one cannot use the composition of the individual rain as a direct measure of the condensation temperature. Nevertheless, it has been possible to show a simple linear correlation between the annual mean values of the surface temperature and the δ18O content in high latitude, non-continental precipitation. The main reason is that the scattering of the individual precipitation compositions, caused by the influence of numerous meteorological parameters, is smoothed out when comparing average compositions at various locations over a sufficiently long period of time (a whole number of years).

The somewhat revised and extended correlation is shown in fig. 3..

From Dansgaard 1964

From Dansgaard 1964

Figure 6

So we appear to have a nice tidy picture when looking at annual means, a little bit like the (article) figure 3 from Gat’s 2010 textbook.

Before “muddying the waters” a little, let’s have a quick look at ocean values.

Ocean δ18O

We can see that the ocean, as we might expect, is much more homogenous, especially the deep ocean. Note that these results are δD (think, about 10x the value of δ18O):

From Ferronsky & Polyakov (2012)

From Ferronsky & Polyakov (2012)

Figure 7 – Click to enlarge

And some surface water values of δD (and also salinity), where we see a lot more variation, again as might expect:

From Ferronsky & Polyakov 2012

From Ferronsky & Polyakov 2012

Figure 8

If we do a quick back of the envelope calculation, using the fact that the sea level change between the last glacial maximum (LGM) and the current interglacial was about 120m, the average ocean depth is 3680m we expect a glacial-interglacial change in the ocean of about 1.5 ‰.

This is why the foraminifera near the bottom of the ocean, capturing 18O from the ocean, are recording ice volume, whereas the ice cores are recording atmospheric temperatures.

Note as well that during the glacial, with more ice locked up in ice sheets, the value of ocean δ18O will be higher. So colder atmospheric temperatures relate to lower values of δ18O in precipitation, but – due to the increase in ice, depleted in 18O - higher values of ocean δ18O.

Muddying the Waters

Hoefs 2009, gives a good summary of the different factors in isotopic precipitation:

The first detailed evaluation of the equilibrium and nonequilibrium factors that determine the isotopic composition of precipitation was published by Dansgaard (1964). He demonstrated that the observed geographic distribution in isotope composition is related to a number of environmental parameters that characterize a given sampling site, such as latitude, altitude, distance to the coast, amount of precipitation, and surface air temperature.

Out of these, two factors are of special significance: temperature and the amount of precipitation. The best temperature correlation is observed in continental regions nearer to the poles, whereas the correlation with amount of rainfall is most pronounced in tropical regions as shown in Fig. 3.15.

The apparent link between local surface air temperature and the isotope composition of precipitation is of special interest mainly because of the potential importance of stable isotopes as palaeoclimatic indicators. The amount effect is ascribed to gradual saturation of air below the cloud, which diminishes any shift to higher δ18O-values caused by evaporation during precipitation.

[Emphasis added]

From Hoefs 2009

From Hoefs 2009

Figure 9

The points that Hoefs make indicate some of the problems relating to using δ18O as the temperature proxy. We have competing influences that depend on the source and journey of the air parcel responsible for the precipitation. What if circulation changes?

For readers who have followed the past discussions here on water vapor (e.g., see Clouds & Water Vapor – Part Two) this is a similar kind of story. With water vapor, there is a very clear relationship between ocean temperature and absolute humidity, so long as we consider the boundary layer. But what happens when the air rises high above that – then the amount of water vapor at any location in the atmosphere is dependent on the past journey of air, and as a result the amount of water vapor in the atmosphere depends on large scale circulation and large scale circulation changes.

The same question arises with isotopes and precipitation.

The ubiquitous Jean Jouzel and his colleagues (including Willi Dansgaard) from their 1997 paper:

In Greenland there are significant differences between temperature records from the East coast and the West coast which are still evident in 30 yr smoothed records. The isotopic records from the interior of Greenland do not appear to follow consistently the temperature variations recorded at either the east coast or the west coast..

This behavior may reflect the alternating modes of the North Atlantic Oscillation..

They [simple models] are, however, limited to the study of idealized clouds and cannot account for the complexity of large convective systems, such as those occurring in tropical and equatorial regions. Despite such limitations, simple isotopic models are appropriate to explain the main characteristics of dD and d18O in precipitation, at least in middle and high latitudes where the precipitation is not predominantly produced by large convective systems.

Indeed, their ability to correctly simulate the present-day temperature-isotope relationships in those regions has been the main justification of the standard practice of using the present day spatial slope to interpret the isotopic data in terms of records of past temperature changes.

Notice that, at least for Antarctica, data and simple models agree only with respect to the temperature of formation of the precipitation, estimated by the temperature just above the inversion layer, and not with respect to the surface temperature, which owing to a strong inversion is much lower..

Thus one can easily see that using the spatial slope as a surrogate of the temporal slope strictly holds true only if the characteristics of the source have remained constant through time.

[Emphases added]

If all the precipitation occurs during warm summer months, for example, the “annual δ18O” will naturally reflect a temperature warmer than Ts [annual mean]..

If major changes in seasonality occur between climates, such as a shift from summer-dominated to winter- dominated precipitation, the impact on the isotope signal could be large..it is the temperature during the precipitation events that is imprinted in the isotopic signal.

Second, the formation of an inversion layer of cold air up to several hundred meters thick over polar ice sheets makes the temperature of formation of precipitation warmer than the temperature at the surface of the ice sheet. Inversion forms under a clear sky.. but even in winter it is destroyed rapidly if thick cloud moves over a site..

As a consequence of precipitation intermittancy and of the existence of an inversion layer, the isotope record is only a discrete and biased sampling of the surface temperature and even of the temperature at the atmospheric level where the precipitation forms. Current interpretation of paleodata implicitly assumes that this bias is not affected by climate change itself.

Now onto the oceans, surely much simpler, given the massive well-mixed reservoir of 18O?

Mix & Ruddiman (1984):

The oxygen-isotopic composition of calcite is dependent on both the temperature and the isotopic composition of the water in which it is precipitated

..Because he [Shackleton] analyzed benthonic, instead of planktonic, species he could assume minimal temperature change (limited by the freezing point of deep-ocean water). Using this constraint, he inferred that most of the oxygen-isotope signal in foraminifera must be caused by changes in the isotopic composition of seawater related to changing ice volume, that temperature changes are a secondary effect, and that the isotopic composition of mean glacier ice must have been about -30 ‰.

This estimate has generally been accepted, although other estimates of the isotopic composition have been made by Craig (-17‰); Eriksson (-25‰), Weyl (-40‰) and Dansgaard & Tauber (≤30‰)

..Although Shackleton’s interpretation of the benthonic isotope record as an ice-volume/sea- level proxy is widely quoted, there is considerable disagreement between ice-volume and sea- level estimates based on δ18O and those based on direct indicators of local sea level. A change in δ18O of 1.6‰ at δ(ice) = – 35‰ suggests a sea-level change of 165 m.

..In addition, the effect of deep-ocean temperature changes on benthonic isotope records is not well constrained. Benthonic δ18O curves with amplitudes up to 2.2 ‰ exist (Shackleton, 1977; Duplessy et al., 1980; Ruddiman and McIntyre, 1981) which seem to require both large ice- volume and temperature effects for their explanation.

Many other heavyweights in the field have explained similar problems.

We will return to both of these questions in the next article.

Conclusion

Understanding the basics of isotopic changes in water and water vapor is essential to understand the main proxies for past temperatures and past ice volumes. Previously we have looked at problems relating to dating of the proxies, in this article we have looked at the proxies themselves.

There is good evidence that current values of isotopes in precipitation and ocean values give us a consistent picture that we can largely understand. The question about the past is more problematic.

I started looking seriously at proxies as a means to perhaps understand the discrepancies for key dates of ice age terminations between radiometric dating and ocean cores (see Thirteen – Terminator II). Sometimes the more you know, the less you understand..

Articles in the Series

Part One - An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Pop Quiz: End of An Ice Age - a chance for people to test their ideas about whether solar insolation is the factor that ended the last ice age

Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2

Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland

Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II

Fourteen – Concepts & HD Data - getting a conceptual feel for the impacts of obliquity and precession, and some ice age datasets in high resolution

Fifteen – Roe vs Huybers - reviewing In Defence of Milankovitch, by Gerard Roe

Sixteen – Roe vs Huybers II - comparing the results if we take the Huybers dataset and tie the last termination to the date implied by various radiometric dating

Eighteen – “Probably Nonlinearity” of Unknown Origin - what is believed and what is put forward as evidence for the theory that ice age terminations were caused by orbital changes

Nineteen – Ice Sheet Models I - looking at the state of ice sheet models

References

Isotopes of the Earth’s Hydrosphere, VI Ferronsky & VA Polyakov, Springer (2012)

Isotope Hydrology – A Study of the Water Cycle, Joel R Gat, Imperial College Press (2010)

Stable Isotope Geochemistry, Jochen Hoefs, Springer (2009)

Patterns of the isotopic composition of precipitation in time and space: data from the Israeli storm water collection program, M Rindsberger, Sh Jaffe, Sh Rahamim and JR Gat, Tellus (1990) – free paper

Stable isotopes in precipitation, Willi Dansgaard, Tellus (1964) – free paper

Validity of the temperature reconstruction from water isotopes in ice cores, J Jouzel, RB Alley, KM Cuffey, W Dansgaard, P Grootes, G Hoffmann, SJ Johnsen, RD Koster, D Peel, CA Shuman, M Stievenard, M Stuiver, J White, Journal of Geophysical Research (1997) – free paper

Oxygen Isotope Analyses and Pleistocene Ice Volumes, Mix & Ruddiman, Quaternary Research (1984)  - free paper

- and on the Dole effect, only covered in Note 2:

The Dole effect and its variations during the last 130,000 years as measured in the Vostok ice core, Michael Bender, Todd Sowers, Laurent Labeyrie, Global Biogeochemical Cycles (1994) – free paper

A model of the Earth’s Dole effect, Georg Hoffmann, Matthias Cuntz, Christine Weber, Philippe Ciais, Pierre Friedlingstein, Martin Heimann, Jean Jouzel, Jörg Kaduk, Ernst Maier-Reimer, Ulrike Seibt & Katharina Six, Global Biogeochemical Cycles (2004) – free paper

The isotopic composition of atmospheric oxygen Boaz Luz & Eugeni Barkan, Global Biogeochemical Cycles (2011) – free paper

Notes

Note 1: There is a relationship between δ18O and δD which is linked to the difference in vapor pressures between H2O and HDO in one case and H216O and H218O in the other case.

δD = 8 δ18O + 10 – known as the Global Meteoric Water Line.

The equation is more of a guide and real values vary sufficiently that I’m not really clear about its value. There are lengthy discussions of it and the variations from it in Ferronsky & Polyakov.

Note 2: The Dole effect

When we measure atmospheric oxygen, we find that the δ18O = 23.5 ‰ with respect to the oceans (VSMOW) – this is the Dole effect

So, oxygen in the atmosphere has a greater proportion of 18O than the ocean

Why?

How do the atmosphere and ocean exchange oxygen? In essence, photosynthesis turns sunlight + water (H2O) + carbon dioxide (CO2) –> sugar + oxygen (O2).

Respiration turns sugar + oxygen –> water + carbon dioxide + energy

The isotopic composition of the water in photosynthesis affects the resulting isotopic composition in the atmospheric oxygen.

The reason the Dole effect exists is well understood, but the reason why the value comes out at 23.5‰ is still under investigation. This is because the result is the global aggregate of lots of different processes. So we might understand the individual processes quite well, but that doesn’t mean the global value can be calculated accurately.

It is also the case that δ18O of atmospheric O2 has varied in the past – as revealed first of all in the Vostok ice core from Antarctica.

Michael Bender and his colleagues had a go at calculating the value from first principles in 1994. As they explain (see below), although it might seem as though their result is quite close to the actual number it is not a very successful result at all. Basically due to the essential process you start at 20‰ and should get to 23.5‰, but they o to 20.8‰.

Bender et al 1994:

The δ18O of O2.. reflects the global responses of the land and marine biospheres to climate change, albeit in a complex manner.. The magnitude of the Dole effect mainly reflects the isotopic composition of O2 produced by marine and terrestrial photosynthesis, as well as the extent to while the heavy isotope is discriminated against during respiration..

..Over the time period of interest here, photosynthesis and respiration are the most important reactions producing and consuming O2. The isotopic composition of O2 in air must therefore be understood in terms of isotope fractionation associated with these reactions.

The δ18O of O2 produced by photosynthesis is similar to that of the source water. The δ18O of O2 produced by marine plants is thus 0‰. The δ18O of O2 produced on the continents has been estimated to lie between +4 and +8‰. These elevated δ18O values are the result of elevated leaf water δ18O values resulting from evapotranspiration.

..The calculated value for the Dole effect is then the productivity-weighted values of the terrestrial and marine Dole effects minus the stratospheric diminution: +20.8‰. This value is considerably less than observed (23.5‰). The difference between the expected value and the observed value reflects errors in our estimates and, conceivably, unrecognized processes.

Then they assess the Vostok record, where the main question is less about why the Dole effect varies apparently with precession (period of about 20 kyrs), than why the variation is so small. After all, if marine and terrestrial biosphere changes are significant from interglacial to glacial then surely those changes would reflect more strongly in the Dole effect:

Why has the Dole effect been so constant? Answering this question is impossible at the present time, but we can probably recognize the key influences..

They conclude:

Our ability to explain the magnitude of the contemporary Dole effect is a measure of our understanding of the global cycles of oxygen and water. A variety of recent studies have improved our understanding of many of the principles governing oxygen isotope fractionation during photosynthesis and respiration.. However, our attempt to quantitively account for the Dole effect in terms of these principles was not very successful.. The agreement is considerably worse than it might appear given the fact that respiratory isotope fractionation alone must account for ~20‰ of the stationary enrichment of the 18O of O2 compared with seawater..

..[On the Vostok record] Our results show that variation in the Dole effect have been relatively small during most of the last glacial-interglacial cycle. These small changes are not consistent with large glacial increases in global oceanic productivity.

[Emphasis added]

Georg Hoffmann and his colleagues had another bash 10 years later and did a fair bit better:

The Earth’s Dole effect describes the isotopic 18O/16O-enrichment of atmospheric oxygen with respect to ocean water, amounting under today’s conditions to 23.5‰. We have developed a model of the Earth’s Dole effect by combining the results of three- dimensional models of the oceanic and terrestrial carbon and oxygen cycles with results of atmospheric general circulation models (AGCMs) with built-in water isotope diagnostics.

We obtain a range from 22.4‰ to 23.3‰ for the isotopic enrichment of atmospheric oxygen. We estimate a stronger contribution to the global Dole effect by the terrestrial relative to the marine biosphere in contrast to previous studies. This is primarily caused by a modeled high leaf water enrichment of 5–6‰. Leaf water enrichment rises by ~1‰ to 6–7‰ when we use it to fit the observed 23.5‰ of the global Dole effect.

Very recently Luz & Barkan (2011), backed up by lots of new experimental work produced a slightly closer estimate with some revisions of the Hoffman et al results:

Based on the new information on the biogeochemical mechanisms involved in the global oxygen cycle, as well as new and more precise experimental data on oxygen isotopic fractionation in various processes obtained over the last 15 years, we have reevaluated the components of the Dole effect.Our new observations on marine oxygen isotope effects, as well as, new findings on photosynthetic fractionation by marine organisms lead to the important conclusion that the marine, terrestrial and the global Dole effects are of similar magnitudes.

This result allows answering a long‐standing unresolved question on why the magnitude of the Dole effect of the last glacial maximum is so similar to the present value despite enormous environmental differences between the two periods. The answer is simple: if DEmar [marine Dole effect] and DEterr [terrestrial Dole effect] are similar, there is no reason to expect considerable variations in the magnitude of the Dole effect as the result of variations in the ratio terrestrial to marine O2 production.

Finally, the widely accepted view that the magnitude of the Dole effect is controlled by the ratio of land‐to‐sea productivity must be changed. Instead of the land‐sea control, past variations in the Dole effect are more likely the result of changes in low‐latitude hydrology and, perhaps, in structure of marine phytoplankton communities.

[Emphasis added]

Note 3:

Jochen Hoefs (2009):

Under equilibrium conditions at 25ºC, the fractionation factors for evaporating water are 1.0092 for 18O and 1.074 for D. However under natural conditions, the actual isotopic composition of water is more negative than the predicted equilibrium values due to kinetic effects.

The discussion of kinetic effects gets a little involved and I don’t think is really necessary to understand – the values of isotopic fractionation during evaporation and condensation are well understood. The confounding factors around what the proxies really measure relate to the journey (i.e. temperature history) and mixing of the various air parcels as well as the temperature of air relating to the precipitation event – is the surface temperature, the inversion temperature, both?

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In the last article – Fifteen – Roe vs Huybers - we had a look at the 2006 paper by Gerard Roe, In defense of Milankovitch.

We compared the rate of change of ice volume – as measured in the Huybers 2007 dataset – with summer insolation at 65ºN. The results were interesting, the results correlated very well for the first 200 kyrs, then drifted out of phase. As a result the (Pearson) correlation over 500 kyrs was very low, but quite decent for the first 200 kyrs.

Without any further data we might assume that the results demonstrated that the dataset without “orbital tuning” – and a lack of objective radiometric dating – was drifting away from reality as time went on, and an “orbitally tuned” dataset was the best approach. We would definitely expect that older dates have more uncertainty, as errors accumulate when we use any kind of model for time vs depth.

However, in an earlier article we looked at more objective dates for Termination II (and also in the comments, at some earlier terminations). These dates were obtained via radiometric dating from a variety of locations and methods.

So I wondered:

What happens if we take a dataset like Huybers 2007 and “remap” it using agemarkers?

This is basically how most of the ice core datasets are constructed, although the methods are more sophisticated (see note 1).

For my rough and ready approach I simply provided a set of termination dates (halfway point of ice volume from peak glacial to peak interglacial) from both Huybers and from Winograd et al 1992. Then I remapped the timebase for the existing Huybers proxy data between each set of agemarkers.

It’s probably easier to show the before and after comparison, rather than explain the method further. Note the low point between 100 and 150 kyrs BP. This corresponds to less ice, it is the interglacial:

Huybers-icevolume-last-270kyrs-remapped

Figure 1

The method is basically a linear remapping. I’m sure there are better ways, but I don’t expect they would have a material impact on the outcome.

One point that’s important (with my very simple method) is the oldest agemarker we consider can cause an inconsistency (as there is nothing to constrain the dates between the last agemarker and the end date), which is why the first set below uses 270 kyrs.

T- III is dated by Winograd 1992 at 253 kyrs. So I picked a date shortly after that.

Here is the comparison of rate of change of ice volume with insolation, with the same conventions as in the last article. We can see that everything is nicely anti-correlated:

Roe-comparison-last-270kyrs-remappedHuybers-499px

Figure 2 – Click to Expand

For comparison, the result (in the last article) from 0-200 kyrs BP without remapping the proxy dataset. We can see that everything is nicely correlated:

Roe-comparison-last-200kyrs

Figure 3 – Click to Expand

For the remapped data: correlation  = -0.30. This is as negatively correlated to the insolation value as LR04 (an “orbitally-tuned” dataset) is positively correlated.

For interest I did the same exercise with a 0 – 200kyr BP timebase. This means everything from 140 kyrs – 200 yrs was not constrained by a revised T-III date. The result: correlation = 0. The interpretation is simple – the older data is not pulled out of alignment due to a later objective T-III date, so there is a better match of insolation with rate of change of ice volume for this older data.

Conclusion

Is there a conclusion? It’s surely staring us in the face so is left as an exercise for the interested student.

I have a headache.

Articles in the Series

Part One - An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Pop Quiz: End of An Ice Age - a chance for people to test their ideas about whether solar insolation is the factor that ended the last ice age

Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2

Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland

Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II

Fourteen – Concepts & HD Data - getting a conceptual feel for the impacts of obliquity and precession, and some ice age datasets in high resolution

Fifteen – Roe vs Huybers - reviewing In Defence of Milankovitch, by Gerard Roe

Seventeen – Proxies under Water I - explaining the isotopic proxies and what they actually measure

Eighteen – “Probably Nonlinearity” of Unknown Origin - what is believed and what is put forward as evidence for the theory that ice age terminations were caused by orbital changes

Nineteen – Ice Sheet Models I - looking at the state of ice sheet models

References

In defense of Milankovitch, Gerard Roe, Geophysical Research Letters (2006) – free paper

Glacial variability over the last two million years: an extended depth-derived agemodel, continuous obliquity pacing, and the Pleistocene progression, Peter Huybers, Quaternary Science Reviews (2007) – free paper

Datasets for Huybers 2007 are here:
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_author/huybers2006/
and
http://www.people.fas.harvard.edu/~phuybers/Progression/

Continuous 500,000-Year Climate Record from Vein Calcite in Devils Hole, Nevada, Winograd, Coplen, Landwehr, Riggs, Ludwig, Szabo, Kolesar & Revesz, Science (1992) – paywall, but might be available with a free Science registration

Insolation data calculated from Jonathan Levine’s MATLAB program

Notes

Note 1:

Here is an extract from Parennin et al 2007, The EDC3 chronology for the EPICA Dome C ice core:

In this article, we present EDC3, the new 800 kyr age scale of the EPICA Dome C ice core, which is generated using a combination of various age markers and a glaciological model. It is constructed in three steps.

First, an age scale is created by applying an ice flow model at Dome C. Independent age markers are used to control several poorly known parameters of this model (such as the conditions at the base of the glacier), through an inverse method.

Second, the age scale is synchronised onto the new Greenlandic GICC05 age scale over three time periods: the last 6 kyr, the last deglaciation, and the Laschamp event (around 41 kyr BP).

Third, the age scale is corrected in the bottom ∼500 m (corresponding to the time period 400–800 kyr BP), where the model is unable to capture the complex ice flow pattern..

From Parennin et al 2007

From Parennin et al 2007

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A few people have asked about the fascinating 2006 paper by Gerard Roe, In defense of Milankovitch.

Roe’s paper appears to show an excellent match between the rate of change of ice volume and insolation at 65°N in June. I’ve been puzzled by the paper for a while, because if this value of insolation does successfully predict changes in ice volume then case closed. Except we struggle to match glacial terminations with insolation (see earlier posts like Part Thirteen, Twelve, Eleven – End of the Last Ice age).

And we should also expect to find a 100 kyr period in the 65°N insolation spectrum. But we don’t.

To be fair to Roe, he does state:

The Milankovitch hypothesis as formulated here does not explain the large rapid deglaciations that occurred at the end of some of the ice age cycles

[Emphasis added].

To be critical, it doesn’t seem like anyone is disputing that ice sheets wax and wane with at least some attachment to 40k (obliquity) and 20k (precession) cycles so what exactly does the paper demonstrate that is new? The missing bit of the puzzle is why ice ages start and end.

On the plus side, Roe points out:

Surprisingly, the [Milankovitch] hypothesis remains not clearly defined..

Which is the same point I made in Ghosts of Climates Past – Part Six – “Hypotheses Abound”.

One of the reasons I’ve spent quite a bit of time collecting and understanding datasets – see Part Fourteen – Concepts & HD Data - was for this kind of problem. Roe’s figure 2 spans half a page but covers 800,000 years. With the thick lines used I can’t actually tell if there is a match, and being poor at real statistics I want to see the data rather than just accept a correlation.

There’s not much point comparing SPECMAP (or LR04) with insolation because both of these datasets are “tuned” to summer 65°N insolation. If we find success then we accept that the producers of the dataset were competent in their objective. If we find lack of success we have to write to them with bad news. No one wants to do that.

Fortunately we have an interesting dataset from Peter Huybers (2007). This is an update of HW04 (Huybers & Wunsch 2004) which created a proxy for global ice volume from deep ocean cores without “orbital tuning”. It’s based on an autocorrelated sedimentation model, requiring that key turning points from many different cores all occur at the same time, and a key dateable event at around 800,000 years ago that shows up in most cores.

Some readers are wondering:

Why not use the ice cores you have been writing about?

Good question. The oxygen isotope (δ18O), or deuterium isotope (δD), in the ice is more a measure of local temperature than anything else (and it’s complicated). So Greenland and Antarctic ice cores provide lots of useful data, but not global ice volume. For that, we need to capture the δ18O stored in deep ocean sediments. The δ18O in deep ocean cores, to a first order, appears to be a measure of the amount of water locked up in global ice sheets. However, we have no easy way to objectively date the ocean cores, so some assumptions are needed.

Fortunately, Roe compared his theory with two datasets, the famous SPECMAP (warning, orbital tuning was used in the creation of this dataset) and HW04:

Roe2006-fig2-499px

Figure 1

I downloaded the updated Huybers 2007 dataset. It is in 1 kyr intervals. I have calculated the insolation at all latitudes and all days for the last 500 kyrs using Jonathan Levine’s MATLAB program. This is also in 1 kyrs intervals. I used the values at 65N and June 21st (day 172 – thanks Climateer, for helping me with the basics of calendar days!).

I calculated change in ice volume in a very simple way – (value at time t+1 – value at time t) divided by time change. I scaled the resulting dataset to the same range as the insolation anomalies – so that they plot nicely. And I plotted insolation anomaly = mean(insolation) – insolation:

Roe-comparison-499px

Figure 2 – Click to Expand

The two sets of data look very similar over the last 500 kyrs. I assume that some minor changes, e.g., at about 370 kyrs, are due to dataset updates. Note that insolation anomaly is effectively inverted to help match trends by eye – high insolation should lead to negative change in ice volume and vice-versa.

For reference, here is my calculation on its own (click to get the large version):

dHuybers2007dt-vs-Insolation-65N-499px

Figure 3 – Click to Expand

I did a Pearson correlation between the two datasets and obtained 0.08. That is, very little correlation. This just tells us what we can see from looking at the graph – the two key values are in phase to begin with then move out of phase and back into phase by the end.

Correlation between 0-100 kyrs:   0.66 (great)
Correlation between 101-200 kyrs:  0.51 (great)
Correlation between 201-300 kyrs:  -0.72 (wrong direction)
Correlation between 301-400 kyrs  -0.27 (wrong direction)
Correlation between 401-500 kyrs:  0.18 (wavering)

I also did a Spearman rank correlation (correlates the rank of the two datasets to make it resistance to outliers) = 0.09, and just because I could, a Kendall correlation as well = 0.07.

I’m a bit of a statistics amateur so comparing datasets except by looking is not my forte. Perhaps a rookie mistake somewhere.

Then I checked lag correlations. The physical reasoning is that deep ocean concentration of 18O will take a few thousand years at least to respond to ice volume changes, simply due to the slow circulation of the major ocean currents. The results show there is a better correlation with a lag of 35,000 years, but there is no physical reason for this, it is probably just a better fit to a dataset with an apparent slow phase drift across the period of record. At a meaningful large ocean current lag of a few thousand years the correlation is worse (anti-correlated):

Roe-comparison-lag-correlation

Figure 4

On the plus side, the first 200 kyrs look quite impressive, including terminations:

Roe-comparison-last-100kyrs

Figure 5

Roe-comparison-last-200kyrs

Figure 6

This has got me wondering.

What do we notice from the data for the first 200 kyrs (figure 6)? Well, the last two terminations (check out the last few posts) are easily identified because the rate of change of ice volume in proportion to insolation is about four times its value when no termination takes place.

Forgetting about the small problem of the Southern Hemisphere lead in the last deglaciation (Part Eleven – End of the Last Ice age), there is something interesting going on here. Almost like a theory that is just missing one easily identified link, one piece of the jigsaw puzzle that just needs to be fitted in, and the new Nature paper is waiting..

Onto some details.. it seems that T-II, if marked by the various radiometric dating values we saw Part Thirteen – Terminator II, would cause the 100k-200k values to move out of phase (the big black dip at about 125 kyrs would move about 15 kyrs to the left). So my next objective (see Sixteen – Roe vs Huybers II) is to set an age marker for Termination II from the radiometric dating values and “slide” the Huybers 2007 dataset to this and the current T1 dating. Also, the ice core proxies recorded in deep ocean cores must lag real ice volume changes by some period like say 1 – 3 kyrs (see note 1). This helps the Roe hypothesis because the black curves move to the left.

Let’s see what happens with these changes.

And hopefully, sharp-eyed readers are going to identify opportunities for improvement in this article, as well as the missing piece of the puzzle that will lead to the coveted Nature paper..

Articles in the Series

Part One - An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Pop Quiz: End of An Ice Age - a chance for people to test their ideas about whether solar insolation is the factor that ended the last ice age

Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2

Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland

Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II

Fourteen – Concepts & HD Data - getting a conceptual feel for the impacts of obliquity and precession, and some ice age datasets in high resolution

Sixteen – Roe vs Huybers II - remapping a deep ocean core dataset and updating the previous article

Seventeen – Proxies under Water I - explaining the isotopic proxies and what they actually measure

Eighteen – “Probably Nonlinearity” of Unknown Origin - what is believed and what is put forward as evidence for the theory that ice age terminations were caused by orbital changes

Nineteen – Ice Sheet Models I - looking at the state of ice sheet models

References

In defense of Milankovitch, Gerard Roe, Geophysical Research Letters (2006) – free paper

Glacial variability over the last two million years: an extended depth-derived agemodel, continuous obliquity pacing, and the Pleistocene progression, Peter Huybers, Quaternary Science Reviews (2007) – free paper

How long to oceanic tracer and proxy equilibrium?, C Wunsch & P Heimbach, Quaternary Science Reviews (2008) – free paper

Datasets for Huybers 2007 are here:
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/contributions_by_author/huybers2006/
and
http://www.people.fas.harvard.edu/~phuybers/Progression/

Insolation data calculated from Jonathan Levine’s MATLAB program (just ask for this data in Excel or MATLAB format)

Notes

Note 1: See, for example, Wunsch & Heimbach 2008:

The various time scales for distribution of tracers and proxies in the global ocean are critical to the interpretation of data from deep- sea cores. To obtain some basic physical insight into their behavior, a global ocean circulation model, forced to least-square consistency with modern data, is used to find lower bounds for the time taken by surface-injected passive tracers to reach equilibrium. Depending upon the geographical scope of the injection, major gradients exist, laterally, between the abyssal North Atlantic and North Pacific, and vertically over much of the ocean, persisting for periods longer than 2000 years and with magnitudes bearing little or no relation to radiocarbon ages. The relative vigor of the North Atlantic convective process means that tracer events originating far from that location at the sea surface will tend to display abyssal signatures there first, possibly leading to misinterpretation of the event location. Ice volume (glacio-eustatic) corrections to deep-sea d18O values, involving fresh water addition or subtraction, regionally at the sea surface, cannot be assumed to be close to instantaneous in the global ocean, and must be determined quantitatively by modelling the flow and by including numerous more complex dynamical interactions.

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In previous posts we have seen – and critiqued – ideas about the causes of ice age inception and ice age termination being due to high latitude insolation. These ideas are known under the banner of “Milankovitch forcing”. Mostly I’ve presented the concept by plotting insolation data at particular latitudes, in one form or another. The insolation at different latitudes depends on obliquity and precession (as well as eccentricity).

Obliquity is the tilt of the earth’s axis – which varies over about 40,000 year cycles. Precession is the movement of point of closest approach (perihelion) and how it coincides with northern hemisphere summer – this varies over about a 20,000 year cycle. The effect of precession is modified by the eccentricity of the earth’s axis – which varies over a 100,000 year cycle.

If the earth’s orbit was a perfect circle (eccentricity = 0) then “precession” would have no effect, because the earth would be a constant distance from the sun. As eccentricity increases the impact of precession gets bigger.

How to understand these ideas better?

Peter Huybers has a nice explanation and presentation of obliquity and precession in his 2007 paper, along with some very interesting ideas that we will follow up in a later article.

The top graph shows the average insolation value by latitude and day of the year (over 2M years). The second graph shows the anomaly compared with the average at times of maximum obliquity. The third graph shows the anomaly compared with the average at times of maximum precession. The graphs to the right show the annual average of these values:

From Huybers (2007)

From Huybers (2007)

Figure 1

We can see immediately that times of maximum precession (bottom graph) have very little impact on annual averages (the right side graph). This is because the increase in, say, the summer/autumn, are cancelled out by the corresponding decreases in the spring.

But we can also see that times of maximum obliquity (middle graph) DO impact on annual averages (right side graph). Total energy is shifted to the poles from the tropics .

I was trying, not very effectively, to explain some of this in (too many graphs) in Part Five – Obliquity & Precession Changes.

Here is another way to look at this concept. For the last 500 kyrs, I plotted out obliquity and precession modified by eccentricity (e sin w) in the top graph, and in the bottom graph the annual anomaly by latitude and through time. WordPress kind of forces everything into 500 pixel wide graphs which doesn’t help too much. So click on it to get the HD version:

Obliquity-Precession-Annual-insolation-anomaly-last-500ka-499px

Figure 2 – Click to Expand

It is easy to see that the 40,000 year obliquity cycles correspond to high latitude (north & south) anomalies, which last for considerable periods. When obliquity is high, the northern and southern high latitude regions have an increase in annual average insolation. When obliquity is low, there is a decrease. If we look at the precession we don’t see a corresponding change in the annual average (because one season’s increase mostly cancels out the other season’s decrease).

Huybers’ paper has a lot more to it than that, and I recommend reading it. He has a 2M yr global proxy database, that isn’t dependent on “orbital tuning” (note 1) and an interesting explanation and demonstration for obliquity as the dominant factor in “pacing” the ice ages. We will come back to his ideas.

In the meantime, I’ve been collecting various data sources. One big challenge in understanding ice ages is that the graphs in the various papers don’t allow you to zoom in on the period of interest. I thought I could help to fix that by providing the data  - and comparing the data – in High Definition instead of snapshots of 800,000 years on half the width of a standard pdf. It’s a work in progress..

The top graph (below) has two versions of temperature proxy. One is Huyber’s global proxy for ice volume (δ18O) from deep ocean cores, while the other is the local proxy for temperature (δD) from Dome C core from Antarctica (75°S). This location is generally known as EDC, i.e., EPICA Dome C. The two datasets are laid out on their own timescales (more on timescales below):

EDC-and-Huybers-Proxy-CO2-CH4-Obliquity-last-500kyrs-499px

Figure 3 – Click to Expand

The middle graph has CO2 and CH4 from Dome C. It’s amazing how tightly CO2 and CH4 are linked to the temperature proxies and to each other. (The CO2 data comes from Lüthi et al 2008, and the CH4 data from Loulergue et al 2008).

The bottom graph has obliquity and annual insolation anomaly area-averaged over 70ºS-90ºS. Because we are looking at annual insolation anomaly this value is completely in phase with obliquity.

Why are the two datasets on the top graph out of alignment? I don’t know the full answer to this yet. Obviously the lag from the atmosphere to the deep ocean is part of the explanation.

Here is a 500 kyr comparison of LR04 (Lisiecki & Raymo 2005) and Huybers’ dataset – both deep ocean cores – but LR04 uses ‘orbital tuning’. The second graph has obliquity & precession (modified by eccentricity). The third graph has EDC from Antarctica:

LR04-Huybers-EDC-Obliquity-Precession-last-500kyrs-499px

Figure 4 – Click to Expand

Now we zoom in on the last 150 kyrs with two Antarctic cores on the top graph and NGRIP (North Greenland) on the bottom graph:

EDC-EDML-NGRIP-Obliquity-Prec-last-150kyrs-499px

Figure 5 – Click to Expand

Here we see EDML (high resolution Antarctic core) compared with NGRIP (Greenland) over the last 150 kyrs (NGRIP only goes back to 123 kyrs) plus CO2 & CH4 from EDC – once again, the tight correspondence of CO2 and CH4 with the temperature records from both polar regions is amazing:

EDML-NGRIP-CO2-CH4-last-150kyrs-499px

Figure 6 – Click to Expand

The comparison and linking of “abrupt climate change” in Greenland and Antarctic has been covered in EPICA 2006 (note the timescale is in the opposite direction to the graphs above):

from EPICA 2006

from EPICA 2006

Figure 7 – Click to Expand

Timescales

As most papers acknowledge, providing data on the most accurate “assumption free” timescales is the Holy Grail of ice age analysis. However, there are no assumption-free timescales. But lots of progress has been made.

Huybers’ timescale is based primarily on a) a sedimentation model, b) tying together the various identified inception & termination points for each of the proxies, c) the independently dated Brunhes- Matuyama reversal at 780,000 years ago.

The EDC (EPICA Dome ‘C’) timescale is based on a variety of age markers:

  • for the first 50 kyrs by tying the data to Greenland (via high resolution CH4 in both records) which can be layer counted because of much higher precipitation
  • volcanic eruptions
  • 10Be events which can be independently dated
  • ice flow models – how ice flows and compresses under pressure
  • finally, “orbital tuning”

EDC2 was the timescale on which the data was presented in the seminal 2004 EPICA paper. This 2004 paper presented the EDC core going back over 800 kyrs (previously the Vostok core was the longest, going back 400 kyrs). The EPICA 2006 paper was the Dronning Maud Land Core (EDML) which covered a shorter time (150 kyrs) but at higher resolution, allowing a better matchup between Antarctica and Greenland. This introduced the improved EDC3 timescale.

In a technical paper on dating, Parannin et al 2007 show the differences between EDC3 and EDC2 and also between EDC3 and LR04.

Parennin-2007-EDC3-timescale-fig3-499px

Figure 8 – Click to Expand

So if you have data, you need to understand the timescale it is plotted on.

I have the EDC3 timescale in terms of depth so next I’ll convert the EDC temperature proxy (δD) on EDC2 to EDC3 time. I also have dust vs depth for the EDC core – another fascinating variable that is about 25 times stronger during peak glacials compared with interglacials – this needs converting to the EDC3 timescale. Other data includes some other atmospheric chemical components. Then I have NGRIP data (North Greenland) going back 123,000 years but on the original 2004 timescale, and it has been relaid onto GICC05 timescale (still to find).

Very recently (mid 2013) a new Antarctic timescale was proposed – AICC2012 – which brings all of the Antarctic ice cores onto one common timescale.  See references below.

Matlab

Calling Matlab gurus – plotting many items onto one graph has some benefits. Matlab is an excellent tool but I haven’t yet figured out how to plot lots of data onto the same graph. If multiple data sources have the same x-series data and a similar y-range there is no problem. If I have two data sources with similar x values (but different x-series data) and completely different y values I can use plotyy. How about if I have five datasources, each with different but similar x-series and different y-values. How do I plot them on one graph, and display the multiple y-axes (easily)?

Conclusion

This article was intended to highlight obliquity and precession in a different and hopefully more useful way. And to begin to present some data in high resolution.

Articles in the Series

Part One – An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Pop Quiz: End of An Ice Age - a chance for people to test their ideas about whether solar insolation is the factor that ended the last ice age

Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2

Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland

Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II

Fifteen – Roe vs Huybers - reviewing In Defence of Milankovitch, by Gerard Roe

Sixteen – Roe vs Huybers II - remapping a deep ocean core dataset and updating the previous article

Seventeen – Proxies under Water I - explaining the isotopic proxies and what they actually measure

Eighteen – “Probably Nonlinearity” of Unknown Origin - what is believed and what is put forward as evidence for the theory that ice age terminations were caused by orbital changes

Nineteen – Ice Sheet Models I - looking at the state of ice sheet models

References

Glacial variability over the last two million years: an extended depth-derived agemodel, continuous obliquity pacing, and the Pleistocene progression, Peter Huybers, Quaternary Science Reviews (2007) – free paper

Eight glacial cycles from an Antarctic ice core, EPICA community members, Nature (2004) – free paper

One-to-one coupling of glacial climate variability in Greenland and Antarctica,  EPICA Community Members, Nature (2006) – free paper

High-resolution carbon dioxide concentration record 650,000–800,000 years before present, Lüthi et al, Nature (2008)

Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years, Loulergue et al, Nature (2008)

A Pliocene-Pleistocene stack of 57 globally distributed benthic D18O records, Lorraine Lisiecki & Maureen E. Raymo, Paleoceanography (2005) – free paper

The EDC3 chronology for the EPICA Dome C ice core, Parennin et al, Climate of the Past (2007) – free paper

An optimized multi-proxy, multi-site Antarctic ice and gas orbital chronology (AICC2012): 120–800 ka, L. Bazin et al, Climate of the Past (2013) – free paper

The Antarctic ice core chronology (AICC2012): an optimized multi-parameter and multi-site dating approach for the last 120 thousand years, D. Veres et al, Climate of the Past (2013) – free paper

Notes

Note 1 – See for example Thirteen – Terminator II, under the heading What is the basis for the SPECMAP dating? 

It is important to understand the assumptions built into every ice age database.

Huybers 2007 continues the work of HW04 (Huybers & Wunsch 2004) which attempts to produce a global proxy datbase (a proxy for global ice volume) without any assumptions relating to the “Milankovitch theory”.

Read Full Post »

In Eleven – End of the Last Ice age we saw the sequence of events that led to the termination of the most recent ice age – Termination I.

The date of this event, the time when the termination began, was about 17.5-18.0 kyrs ago (note 1). We also saw that “rising solar insolation” couldn’t explain this. By way of illustration I produced some plots in Pop Quiz: End of An Ice Age - all from the last 100 yrs and asked readers to identify which one coincided with Termination I.

But this simple graph of insolation at 65ºN on July 1st summarizes the problem for the”classic version” (see Part Six – “Hypotheses Abound”) of the “Milankovitch theory” – in simple terms, if solar insolation at 18 kyrs ago caused the ice age to end, why didn’t the same or higher insolation at 100 kyrs, 80 kyrs, or from 60-30 kyrs cause the last ice age to end earlier:

TOA-insolation-65N-Jul1-last-180kyrs-499px

Figure 1 

And for a more visual demonstration of solar insolation changes in time, take a look at the Hövmoller plots in the comments of Part Eleven.

The other problem for the Milankovitch theory of ice age termination is the fact that southern hemisphere temperatures rose in advance of global temperatures. So the South led the globe out of the ice age. This is hard to explain if the cause of the last termination was melting northern hemisphere ice sheets. Take a look at Eleven – End of the Last Ice age.

Now we’ve quickly reviewed Termination I, let’s take a look at Termination II. This is the end of the penultimate ice age.

The traditional Milankovitch theory says that peak high latitude solar insolation around 127 kyrs BP was the trigger for the massive deglaciation that ended that earlier ice age.

The well-known SPECMAP dating of sea-level/ice-volume vs time has Termination II at 128 ± 3 kyrs BP. All is good.

Or is it?

What is the basis for the SPECMAP dating?

The ice age records that have been most used and best known come from ocean sediments. These were the first long-dated climate records that went back hundreds of thousands of years.

How do they work and what do they measure?

Oxygen exists in the form of a few different stable isotopes. The most common is 16O with 18O the next most common, but much smaller in proportion. Water, aka H2O, also exists with both these isotopes and has the handy behavior of evaporating and precipitating H218O water at different rates to H216O. The measurement is expressed as the ratio (the delta, or δ) as δ18O in parts per thousand.

The journey of water vapor evaporating from the ocean, followed by precipitation, produces a measure of the starting ratio of the isotopes as well as the local precipitation temperature.

The complex end result of these different process rates is that deep sea benthic foraminifera take up 18O out of the deep ocean, and the δ18O ratio is mostly in proportion to the global ice volume.  The resulting deep sea sediments are therefore a time-series record of ice volume. However, sedimentation rates are not an exact science and are not necessarily constant in time.

As a result of lots of careful work by innumerable people over many decades, out popped a paper by Hays, Imbrie & Shackleton in 1976. This demonstrated that a lot of the recorded changes in ice volume happened at orbital frequencies of precession and obliquity (approximately every 20kyrs and every 40 kyrs). But there was an even stronger signal – the start and end of ice ages – at approximately every 100 kyrs. This coincides roughly with changes in eccentricity of the earth’s orbit, not that anyone has a (viable) theory that links this change to the start and end of ice ages.

Now the clear signal of obliquity and precession in the record gives us the option of “tuning up” the record so that peaks in the record match orbital frequencies of precession and obliquity. We discussed the method of tuning in some past comments on a similar, but much later, dataset – the LR04 stack (thanks to Clive Best for highlighting it).

The method isn’t wrong, but we can’t confirm the timing of key events with a dataset where dates have been tuned to a specific theory.

Luckily, some new methods have come along.

Ice Core Dating

It’s been exciting times for the last twenty plus years in climate science for people who want to wear thick warm clothing and “get away from it all”.

Greenland and Antarctica have yielded a number of ice cores. Greenland now has a record that goes back 123,000 years (NGRIP). Antarctica now has a record that goes back 800,000 years (EDC, aka, “Dome C”). Antarctica also has the Voskok ice core that goes back about 400,000 years, Dome Fuji that goes back 340,000 years and Dronning Maud Land (aka DML or EDML) which is higher resolution but only goes back 150,000 years.

What do these ice cores measure and how is the dating done?

The ice cores measure temperature at time of snow deposition via the δ18O measurement discussed above (note 2), which in this case is not a measure of global ice volume but of air temperature. The gas trapped in bubbles in the ice cores gives us CO2 and CH4 concentrations. We also can measure dust deposition and all kinds of other goodies.

The first problem is that the gas is “younger” than the ice because it moves around until the snow compacts enough. So we need a model to calculate this, and there is some uncertainty about the difference in age between the ice and the gas.

The second problem is how to work out the ice age. At the start we can count annual layers. After sufficient time (a few tens of thousands of years) these layers can’t be distinguished any more, instead we can use models of ice flow physics. Then a few handy constraints arrive like 10Be events that occurred about 50 kyrs BP. After ice flow physics and external events are exhausted, the data is constrained by “orbital tuning”, as with deep ocean cores.

Caves, Corals and Accurate Radiometric Dating

Neither deep sea cores, nor ice cores, give us much possibility of radiometric dating. But caves and corals do.

For newcomers to dating methods, if you have substance A that decays into substance B with a “half-life” that is accurately known, and you know exactly how much of substance A and B was there at the start (e.g. no possibility of additional amounts of A or B getting into the thing we want to measure) then you can very accurately calculate the age that the substance was formed.

Basically it’s all down to how to deposition process works. Uranium-Thorium dating has been successfully used to date calcite depositions in rock.

So, take a long section that has been continuously deposited, measure the δ18O (and 13C) at lots of points along the section, and take a number of samples and calculate the age along the section with radiometric dating. The subject of what exactly is being measured in the cores is complicated, but I will greatly over-simplify and say it revolves around two points:

  1. The actual amount of deposition, as not much water is available to create these depositions during extreme glaciation
  2. The variation of δ18O (and 13C), which to a first order depends on local air temperature

For people interested in more detail, I recommend McDermott 2004, some relevant extracts below in note 3).

Corals offer the possibility, via radiometric dating, of getting accurate dates of sea level. The most important variable to know is any depression and uplift of the earth’s crust.

Accurate dating of caves and coral has been a growth industry in the last twenty years with some interesting results.

Termination II

Winograd et al 1992 analyzed Devils Hole in Nevada (DH-11):

The Devils Hole δ18O time curve (fig 2) clearly displays the sawtooth pattern characteristic of marine δ18O records that have been interpreted to be the result of the waxing and waning of Northern Hemisphere ice sheets.. But what caused the δ18O variations in DH-11 shown on fig. 2? ..The δ18O variations in atmospheric precipitation are – to a first approximation – believed to reflect changes in average winter-spring surface temperature..

From Winograd et al 1992

From Winograd et al 1992

Figure 2

Termination II occurs at 140±3 (2σ) ka in the DH-11 record, at 140± 15 ka in the Vostok record (14), and at 128 ± 3 ka in the SPECMAP record (13). (The uncertainty in the DH-11 record is in the 2σ uncertainties on the MS uranium-series dates; other dates and uncertainties are from the sources cited.) Termination III occurred at about 253 +/- 3 (2σ) ka in the DH11 record and at about 244 +/- 3 ka in the SPECMAP record. These differences.. are minimum values..

They compare summer insolation at 65ºN with SPECMAP, Devils Hole and the Vostok ice core on a handy graph:

Winograd et al 1992

Winograd et al 1992

Figure 3

Of course, not everyone was happy with this new information, and who knows what the isotope measurement really was a proxy for?

Slowey, Henderson & Curry 1996

A few years later, in 1996, Slowey, Henderson & Curry (not the famous Judith) made this statement from their research:

Our dates imply a timing and duration for substage 5e in substantial agreement with the orbitally tunes marine chronology. Initial direct U-Th dating of the marine δ18O record supports the theory that orbital variations are a fundamental cause of Pleistocene climate change.

[Emphasis added, likewise with all quotes].

Henderson & Slowey 2000

Then in 2000, the same Henderson & Slowey (sans Curry):

Milankovitch proposed that summer insolation at mid-latitudes in the Northern Hemisphere directly causes the ice-age climate cycles. This would imply that times of ice-sheet collapse should correspond to peaks in Northern Hemisphere June insolation.

But the penultimate deglaciation has proved controversial because June insolation peaks 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15kyr earlier.

There is a clear signature of the penultimate deglaciation in marine oxygen-isotope records. But dating this event, which is significantly before the 14C age range, has not been possible.

Here we date the penultimate deglaciation in a record from the Bahamas using a new U-Th isochron technique. After the necessary corrections for a-recoil mobility of 234U and 230Th and a small age correction for sediment mixing, the midpoint age for the penultimate deglaciation is determined to be 135 +/-2.5 kyr ago. This age is consistent with some coral-based sea-level estimates, but it is difficult to reconcile with June Northern Hemisphere insolation as the trigger for the ice-age cycles.

Zhao, Xia & Collerson (2001)

High-precision 230Th- 238U ages for a stalagmite from Newdegate Cave in southern Tasmania, Australia.. The fastest stalagmite growth occurred between 129.2 ± 1.6 and 122.1 ± 2.0 ka (†61.5 mm/ka), coinciding with a time of prolific coral growth from Western Australia (128-122 ka). This is the first high-resolution continental record in the Southern Hemisphere that can be compared and correlated with the marine record. Such correlation shows that in southern Australia the onset of full interglacial sea level and the initiation of highest precipitation on land were synchronous. The stalagmite growth rate between 129.2 and 142.2 ka (†5.9 mm/ka) was lower than that between 142.2 and 154.5 ka (†18.7 mm/ka), implying drier conditions during the Penultimate Deglaciation, despite rising temperature and sea level.

This asymmetrical precipitation pattern is caused by latitudinal movement of subtropical highs and an associated Westerly circulation, in response to a changing Equator-to-Pole temperature gradient.

Both marine and continental records in Australia strongly suggest that the insolation maximum between 126 and 128 ka at 65°N was directly responsible for the maintenance of full Last Interglacial conditions, although the triggers that initiated Penultimate Deglaciation (at †142 ka) remain unsolved.

From Zhao et al 2001

From Zhao et al 2001

Figure 4

Gallup, Cheng, Taylor & Edwards (2002)

An outcrop within the last interglacial terrace on Barbados contains corals that grew during the penultimate deglaciation, or Termination II. We used combined 230Th and 231Pa dating to determine that they grew 135.8 ± 0.8 thousand years ago, indicating that sea level was 18 ± 3 meters below present sea level at the time. This suggests that sea level had risen to within 20% of its peak last- interglacial value by 136 thousand years ago, in conflict with Milankovitch theory predictions..

Figure 2B summarizes the sea level record suggested by the new data. Most significantly our record includes corals that document sea level directly during Termination II, suggesting that the majority (~80%) of the Termination II sea level rise occurred before 135 ka. This is broadly consistent with early shifts in δ18O recorded in the Bahamas and Devils Hole and with early dates (134 ka) of last interglacial corals from Hawaii, which call into question the timing of Termination II in the SPECMAP record..

From Gallup et al 2002

From Gallup et al 2002

Figure 5 – Click to expand

 Of course, all is not lost for the many-headed Hydra (and see note 4):

..The Milankovitch theory in its simplest form cannot explain Termination II, as it does Termination I. However, it is still plausible that insolation forcing played a role in the timing of Termination II. As deglaciations must begin while Earth is in a glacial state, it is useful to look at factors that could trigger deglaciation during a glacial maximum. These include – (i) sea ice cutting off a moisture source for the ice sheets; – (ii) isostatic depression of continental crust; and – (iii) high Southern Hemisphere summer insolation through effects on the atmospheric CO concentration.

Yuan et al 2004 provide evidence in opposition:

Thorium-230 ages and oxygen isotope ratios of stalagmites from Dongge Cave, China, characterize the Asian Monsoon and low-latitude precipitation over the past 160,000 years. Numerous abrupt changes in 18O/16O values result from changes in tropical and subtropical precipitation driven by insolation and millennial-scale circulation shifts.

The Last Interglacial Monsoon lasted 9.7 +/- 1.1 thousand years, beginning with an abrupt (less than 200 years) drop in 18O/16O values 129.3 ± 0.9 thousand years ago and ending with an abrupt (less than 300 years) rise in 18O/16O values 119.6 ± 0.6 thousand years ago. The start coincides with insolation rise and measures of full interglacial conditions, indicating that insolation triggered the final rise to full interglacial conditions.

But they also comment:

Although the timing of Monsoon Termination II is consistent with Northern Hemisphere insolation forcing, not all evidence of climate change at about this time is consistent with such a mechanism (Fig. 3).

From Yuan et al 2004

From Yuan et al 2004

Figure 6 – Click to expand

Sea level apparently rose to levels as high as –21 m as early as 135 ky before the present (27 & Gallup et al 2002), preceding most of the insolation rise. The half-height of marine oxygen isotope Termination II has been dated at 135 +/- 2.5 ky (Henderson & Slowey 2000).

Speleothem evidence from the Alps indicates temperatures near present values at 135 +/- 1.2 ky (31). The half-height of the d18O rise at Devils Hole (142 +/- 3 ky) also precedes most of the insolation rise (20). Increases in Antarctic temperature and atmospheric CO2 (32) at about the time of Termination II appear to have started at times ranging from a few to several millennia before most of the insolation rise (4, 32, 33).

[Their reference numbers amended to papers where cited in this article]

Drysdale et al 2009

Variations in the intensity of high-latitude Northern Hemisphere summer insolation, driven largely by precession of the equinoxes, are widely thought to control the timing of Late Pleistocene glacial terminations. However, recently it has been suggested that changes in Earth’s obliquity may be a more important mechanism. We present a new speleothem-based North Atlantic marine chronology that shows that the penultimate glacial termination (Termination II) commenced 141,000 ± 2,500 years before the present, too early to be explained by Northern Hemisphere summer insolation but consistent with changes in Earth’s obliquity. Our record reveals that Terminations I and II are separated by three obliquity cycles and that they started at near-identical obliquity phases.

Standard stuff by now, for readers who have made it this far.

But the Drysdale paper is interesting on two fronts - their dating method and their “one result in a row” matching a theory with evidence (I extracted more text from the paper in note 5  for interested readers). Let’s look at the dating method first.

Basically what they did was match up the deep ocean cores that record global ice volume (but have no independent dating) with accurately radiometrically-dated speleothems (cave depositions). How did they do the match up? It’s complicated but relies on the match between the δ18O in both records. The approach of providing absolute dating for existing deep ocean cores will give very interesting results if it proves itself.

From Drysdale et al 2009

From Drysdale et al 2009

Figure 7 – Click to expand

The correspondence between Corchia δ18O and Iberian-margin sea-surface temperatures (SSTs) through T-II (Fig. 2) is remarkable. Although the mechanisms that force speleothem δ18O variations are complex, we believe that Corchia δ18O is driven largely by variations in rainfall amount in response to changes in regional SSTs. Previous studies from Corchia show that speleothem δ18O is sensitive to past changes in North Atlantic circulation at both orbital and millennial time scales, with δ18O increasing during colder (glacial or stadial) phases and the reverse occurring during warmer (inter- glacial or interstadial) phases.

From Drysdale et al 2009

From Drysdale et al 2009

Figure 8 - Click to expand

Now to the hypothesis:

From Drysdale et al 2009

From Drysdale et al 2009

Figure 9

We find that NHSI [NH summer insolation] intensity is unlikely to be the driving force for T-II: Intensity values are close to minimum at the time of the start of T-II, and a lagged response to the previous insolation peak at ~148 ka is unlikely because of its low amplitude (Fig. 3A). This argues against the SPECMAP curve being a reliable age template through T-II, given the age offset of ~8 ky for the T-II midpoint (8) with respect to our record. A much stronger case can be made for obliquity as a forcing mechanism.

On the basis of our results (Fig. 3B), both T-I and T-II commence at the same phase of obliquity, and the period between them is exactly equivalent to three obliquity cycles (~123 ky).

(More of their explanation in note 5).

EPICA 2006

Here is my plot of the Dronning Maud Land ice core (DML) on EDC3 timescale from EPICA 2006 (data downloaded from the Nature website):

Data from EPICA

Data from EPICA

Figure 10

The many Antarctic and Greenland ice cores are still undergoing revisions of dating, and so I haven’t attempted to get the latest work. I just thought it would be good to throw in an ice core.

The value of δ18O here is a proxy for local temperature. On this timescale local temperatures began rising about 138 kyrs BP.

Conclusion

New data on Termination II from the last 20 years of radiometric dating from a number of different sites with different approaches demonstrate that TII started about 140 kyrs BP.

Here is the solar insolation curve at 65ºN over the last 180 kyrs, with the best dates of the two ice age terminations, separated by about 121 – 125 kyrs:

TOA-insolation-65N-Jul1-last-180kyrs-label-T1-T2-499px

Figure – Click to expand

It’s proven that ice ages are terminated by low solar insolation in the high latitudes of the northern hemisphere. The basis for this is that the low solar insolation allows a much quicker build up of the northern hemisphere ice sheets, causing dynamic instability, leading to ice sheet calving, which disrupts ocean currents, outgassing CO2 in large concentrations and thereby creating a positive feedback for temperature rise. The local ice sheet collapses also create positive feedback due to higher solar insolation now absorbed. As the ice sheets continue to melt, the (finally) rising solar insolation in high northern latitudes strengthens the pre-existing conditions and helps to establish the termination. Thus the orbital theory is given strong support from the evidence of the timing of the last two terminations.

I just made that up in a few minutes for fun. It’s not true.

We saw one paper with different evidence for the start of TII – Yuan et al (and see note 6). However, it seems that most lines of evidence, including absolute dating of sea level rise puts TII starting around 140 kyrs BP.

This also means, for maths wizards, that the time between ice age terminations, from one result in a row, is about 122 kyrs.

As an aside, because Winograd et al 1992 calculated their age for TIII “at about 253 kyrs”:

This puts the time between TIII and TII at about 113 kyrs which is exactly the time of five precessional cycles!

I haven’t yet dug into any other dates for TIII so this theory is quite preliminary.

Articles in the Series

Part One – An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Pop Quiz: End of An Ice Age - a chance for people to test their ideas about whether solar insolation is the factor that ended the last ice age

Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2

Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland

Fourteen – Concepts & HD Data - getting a conceptual feel for the impacts of obliquity and precession, and some ice age datasets in high resolution

Fifteen – Roe vs Huybers - reviewing In Defence of Milankovitch, by Gerard Roe

Sixteen – Roe vs Huybers II - remapping a deep ocean core dataset and updating the previous article

Seventeen – Proxies under Water I - explaining the isotopic proxies and what they actually measure

Eighteen – “Probably Nonlinearity” of Unknown Origin - what is believed and what is put forward as evidence for the theory that ice age terminations were caused by orbital changes

Nineteen – Ice Sheet Models I - looking at the state of ice sheet models

References

A Pliocene-Pleistocene stack of 57 globally distributed benthic D18O records, Lorraine E. Lisiecki & Maureen E. Raymo, Paleoceanography (2005) – free paper

Continuous 500,000-Year Climate Record from Vein Calcite in Devils Hole, Nevada, Winograd, Coplen, Landwehr, Riggs, Ludwig, Szabo, Kolesar & Revesz, Science (1992) – paywall, but might be available with a free Science registration

Palaeo-climate reconstruction from stable isotope variations in speleothems: a review, Frank McDermott, Quaternary Science Reviews 23 (2004) – free paper

Direct U-Th dating of marine sediments from the two most recent interglacial periods, NC Slowey, GM Henderson & WB Curry, Nature (1996)

Evidence from U-Th dating against northern hemisphere forcing of the penultimate deglaciation, GM Henderson & NC Slowey, Nature (2000)

Timing and duration of the last interglacial inferred from high resolution U-series chronology of stalagmite growth in Southern Hemisphere, J Zhao, Q Xia & K Collerson, Earth and Planetary Science Letters (2001)

Direct determination of the timing of sea level change during Termination II, CD Gallup, H Cheng, FW Taylor & RL Edwards, Science (2002)

Timing, Duration, and Transitions of the Last Interglacial Asian Monsoon, Yuan, Cheng, Edwards, Dykoski, Kelly, Zhang, Qing, Lin, Wang, Wu, Dorale, An & Cai, Science (2004)

Evidence for Obliquity Forcing of Glacial Termination II, Drysdale, Hellstrom, Zanchetta, Fallick, Sánchez Goñi, Couchoud, McDonald, Maas, Lohmann & Isola, Science (2009)

One-to-one coupling of glacial climate variability in Greenland and Antarctica, EPICA Community Members, Nature (2006)

Millennial- and orbital-scale changes in the East Asian monsoon over the past 224,000 years, Wang, Cheng, Edwards, Kong, Shao, Chen, Wu, Jiang, Wang & An, Nature (2008)

Notes

Note 1 – In common ice age convention, the date of a termination is the midpoint of the sea level rise from the last glacial maximum to the peak interglacial condition. This can be confusing for newcomers.

Note 2 – The alternative method used on some of the ice cores is δD, which works on the same basis – water with the hydrogen isotope Deuterium evaporates and condenses at different rates to “regular” water.

Note 3 – A few interesting highlights from McDermott 2004:

2. Oxygen isotopes in precipitation

As discussed above, d18O in cave drip-waters reflect

(i) the d18O of precipitation (d18Op) and

(ii) in arid/semi- arid regions, evaporative processes that modify d18Op at the surface prior to infiltration and in the upper part of the vadose zone.

The present-day pattern of spatial and seasonal variations in d18Op is well documented (Rozanski et al., 1982, 1993; Gat, 1996) and is a consequence of several so-called ‘‘effects’’ (e.g. latitude, altitude, distance from the sea, amount of precipitation, surface air temperature).

On centennial to millennial timescales, factors other than mean annual air temperature may cause temporal variations in d18Op (e.g. McDermott et al., 1999 for a discussion). These include:

(i) changes in the d18O of the ocean surface due to changes in continental ice volume that accompany glaciations and deglaciations;

(ii) changes in the temperature difference between the ocean surface temperature in the vapour source area and the air temperature at the site of interest;

(iii) long-term shifts in moisture sources or storm tracks;

(iv) changes in the proportion of precipitation which has been derived from non-oceanic sources, i.e. recycled from continental surface waters (Koster et al., 1993); and

(v) the so-called ‘‘amount’’ effect.

As a result of these ambiguities there has been a shift from the expectation that speleothem d18Oct might provide quantitative temperature estimates to the more attainable goal of providing precise chronological control on the timing of major first-order shifts in d18Op, that can be interpreted in terms of changes in atmospheric circulation patterns (e.g. Burns et al., 2001; McDermott et al., 2001; Wang et al., 2001), changes in the d18O of oceanic vapour sources (e.g. Bar Matthews et al., 1999) or first-order climate changes such as D/O events during the last glacial (e.g. Spo.tl and Mangini, 2002; Genty et al., 2003)..

4.1. Isotope stage 6 and the penultimate deglaciation

Speleothem records from Late Pleistocene mid- to high-latitude sites are discussed first, because these are likely to be sensitive to glacial–interglacial transitions, and they illustrate an important feature of speleothems, namely that calcite deposition slows down or ceases during glacials. Fig. 1 is a compilation of approximately 750 TIMS U-series speleothem dates that have been published during the past decade, plotted against the latitude of the relevant cave site.

The absence of speleothem deposition in the mid- to high latitudes of the Northern Hemisphere during isotope stage 2 is striking, consistent with results from previous compilations based on less precise alpha-spectrometric dates (e.g. Gordon et al., 1989; Baker et al., 1993; Hercmann, 2000). By contrast, speleothem deposition appears to have been essentially continuous through the glacial periods at lower latitudes in the Northern Hemisphere (Fig. 1)..

..A comparison of the DH-11 [Devils Hole] record with the Vostok (Antarctica) ice-core deuterium record and the SPEC- MAP record that largely reflects Northern Hemisphere ice volume (Fig. 2) indicates that both clearly record the first-order glacial–interglacial transitions.

Note 4 - Note the reference to Milankovitch theory “explaining” Termination I. This appears to be the point that insolation was at least rising as Termination began, rather than falling. It’s not demonstrated or proven in any way in the paper that Termination I was caused by high latitude northern insolation, it is an illustration of the way the “widely-accepted point of view” usually gets a thumbs up. You can see the same point in the quotation from the Zhao paper. It’s the case with almost every paper.

If it’s impossible to disprove a theory with any counter evidence then it fails the test of being a theory.

Note 5 – More from Drysdale et al 2009:

During the Late Pleistocene, the period of glacial-to-interglacial transitions (or terminations) has increased relative to the Early Pleistocene [~100 thousand years (ky) versus 40 ky]. A coherent explanation for this shift still eludes paleoclimatologists. Although many different models have been proposed, the most widely accepted one invokes changes in the intensity of high-latitude Northern Hemisphere summer insolation (NHSI). These changes are driven largely by the precession of the equinoxes, which produces relatively large seasonal and hemispheric insolation intensity anomalies as the month of perihelion shifts through its ~23-ky cycle.

Recently, a convincing case has been made for obliquity control of Late Pleistocene terminations, which is a feasible hypothesis because of the relatively large and persistent increases in total summer energy reaching the high latitudes of both hemispheres during times of maximum Earth tilt. Indeed, the obliquity period has been found to be an important spectral component in methane (CH4) and paleotemperature records from Antarctic ice cores.

Testing the obliquity and other orbital-forcing models requires precise chronologies through terminations, which are best recorded by oxygen isotope ratios of benthic foraminifera (d18Ob) in deep-sea sediments (1, 8).

Although affected by deep-water temperature (Tdw) and composition (d18Odw) variations triggered by changes in circulation patterns (9), d18Ob signatures remain the most robust measure of global ice-volume changes through terminations. Unfortunately, dating of marine sediment records beyond the limits of radiocarbon methods has long proved difficult, and only Termination I [T-I, ~18 to 9 thousand years ago (ka)] has a reliable independent chronology.

Most marine chronologies for earlier terminations rely on the SPECMAP orbital template (8) with its a priori assumptions of insolation forcing and built-in phase lags between orbital trigger and ice-sheet response. Although SPECMAP and other orbital-based age models serve many important purposes in paleoceanography, their ability to test climate- forcing hypotheses is limited because they are not independent of the hypotheses being tested. Consequently, the inability to accurately date the benthic record of earlier terminations constitutes perhaps the single greatest obstacle to unraveling the problem of Late Pleistocene glaciations..

..

Obliquity is clearly very important during the Early Pleistocene, and recently a compelling argument was advanced that Late Pleistocene terminations are also forced by obliquity but that they bridge multiple obliquity cycles. Under this model, predominantly obliquity-driven total summer energy is considered more important in forcing terminations than the classical precession-based peak summer insolation model, primarily because the length of summer decreases as the Earth moves closer to the Sun. Hence, increased insolation intensity resulting from precession is offset by the shorter summer duration, with virtually no net effect on total summer energy in the high latitudes. By contrast, larger angles of Earth tilt lead to more positive degree days in both hemispheres at high latitudes, which can have a more profound effect on the total summer energy received and can act essentially independently from a given precession phase. The effect of obliquity on total summer energy is more persistent at large tilt angles, lasting up to 10 ky, because of the relatively long period of obliquity. Lastly, in a given year the influence of maximum obliquity persists for the whole summer, whereas at maximum precession early summer positive insolation anomalies are cancelled out by late summer negative anomalies, limiting the effect of precession over the whole summer.

Although the precise three-cycle offset between T-I and T-II in our radiometric chronology and the phase relationships shown in Fig. 3 together argue strongly for obliquity forcing, the question remains whether obliquity changes alone are responsible.

Recent work invoked an “insolation-canon,” whereby terminations are Southern Hemisphere–led but only triggered at times when insolation in both hemispheres is increasing simultaneously, with SHSI approaching maximum and NHSI just beyond a minimum. However, it is not clear that relatively low values of NHSI (at times of high SHSI) should play a role in deglaciation. An alternative is an insolation canon involving SHSI and obliquity.

Note 6 - There are a number of papers based on Dongge and Hulu caves in China that have similar data and conclusions but I am still trying to understand them. They attempt to tease out the relationship between δ18O and the monsoonal conditions and it’s involved. These papers include: Kelly et al 2006, High resolution characterization of the Asian Monsoon between 146,000 and 99,000 years B.P. from Dongge Cave, China and global correlation of events surrounding Termination II; Wang et al 2008, Millennial- and orbital-scale changes in the East Asian monsoon over the past 224,000 years.

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In Part Eleven we looked at the end of the last ice age. We mainly reviewed Shakun et al 2012, who provided some very interesting data on the timing of Southern Hemisphere and Northern Hemisphere temperatures, along with atmospheric CO2 values – in brief, the SH starts to heat up, then CO2 increases very close in time with SH temperatures, providing positive feedback on an initial temperature rise – and global temperatures follow SH the whole way:

From Shakun et al 2012

From Shakun et al 2012

Figure 1

This Nature paper also provided some modeling work which I had some criticism of, but it wasn’t the focus of the paper. Eric Wolff, one of the key EPICA steering committee members, also had similar criticisms of the modeling, which were published in the same Nature edition.

In this article we will look at He et al 2013, published in Nature, which is a modeling study of the same events. One of the co-authors is Jeremy Shakun, the lead author of our earlier paper. The co-authors also include Bette Otto-Bliesner, one of the lead authors of the IPCC AR5 on Paleoclimate.

For new readers, I suggest reading:

He et al 2013

Readers who have followed this series will see that the abstract (cited below) covers some familiar territory:

According to the Milankovitch theory, changes in summer insolation in the high-latitude Northern Hemisphere caused glacial cycles through their impact on ice-sheet mass balance.

Statistical analyses of long climate records supported this theory, but they also posed a substantial challenge by showing that changes in Southern Hemisphere climate were in phase with or led those in the north.

Although an orbitally forced Northern Hemisphere signal may have been transmitted to the Southern Hemisphere, insolation forcing can also directly influence local Southern Hemisphere climate, potentially intensified by sea-ice feedback, suggesting that the hemispheres may have responded independently to different aspects of orbital forcing.

Signal processing of climate records cannot distinguish between these conditions, however, because the proposed insolation forcings share essentially identical variability.

Here we use transient simulations with a coupled atmosphere–ocean general circulation model to identify the impacts of forcing from changes in orbits, atmospheric CO2 concentration, ice sheets and the Atlantic meridional overturning circulation (AMOC) on hemispheric temperatures during the first half of the last deglaciation (22–14.3 kyr BP).

Although based on a single model, our transient simulation with only orbital changes supports the Milankovitch theory in showing that the last deglaciation was initiated by rising insolation during spring and summer in the mid-latitude to high-latitude Northern Hemisphere and by terrestrial snow–albedo feedback.

[Emphasis added]. The abstract continues:

The simulation with all forcings best reproduces the timing and magnitude of surface temperature evolution in the Southern Hemisphere in deglacial proxy records.

This is a similar modeling result to the paper in Part Nine which had the same approach of individual “forcings” and a simulation with all “forcings” combined. I put “forcings” in quotes, because the forcings (ice sheets, GHGs & meltwater fluxes) are actually feedbacks, but GCMs are currently unable to simulate them.

AMOC changes associated with an orbitally induced retreat of Northern Hemisphere ice sheets is the most plausible explanation for the early Southern Hemisphere deglacial warming and its lead over Northern Hemisphere temperature; the ensuing rise in atmospheric CO2 concentration provided the critical feedback on global deglaciation.

In this paper the GCM simulations are:

  • ORB (22–14.3 kyr BP), forced only by transient variations of orbital configuration
  • GHG (22–14.3 kyr BP), forced only by transient variations of atmospheric greenhouse gas concentrations
  • MOC (19–14.3 kyr BP), forced only by transient variations of meltwater fluxes from the Northern Hemisphere (NH) and Antarctic ice sheets
  • ICE (19– 14.3 kyr BP), forced only by quasi-transient variations of ice-sheet orography and extent based on the ICE-5G (VM2) reconstruction.

And then there is an ALL simulation which combines all of these forcings. The GCM used is CCSM3 (we saw CCSM4, an updated version of CCSM3, used in Part Ten – GCM IV).

The idea behind the paper is to answer a few questions, one of which is why, if high latitude Northern Hemisphere (NH) solar insolation changes are the key to understanding ice ages, did the SH warm first? (This question was also addressed in Shakun et al 2012).

Another idea behind the paper is to try and simulate the actual temperature rises in both NH and SH during the last deglaciation.

Let’s take a look..

Their first figure is a little hard to get into but the essence is that blue is the model with only orbital forcing (ORB), red is the model with ALL forcings and black is the proxy reconstruction of temperature (at various locations).

From He et al 2013

From He et al 2013

Figure 2 

We can see that orbital forcing on its own has just about no impact on any of the main temperature metrics, and we can see that Antarctica and Greenland have different temperature histories in this period.

  • We can see that their ALL model did a reasonable job of reconstructing the main temperature trends.
  • We can also see that it did a poor job of capturing the temperature fluctuations on the scale of centuries to 1kyr when they occurred.

For reference here is the Greenland record from NGRIP from 20k – 10kyrs BP:

NGRIP-20k-10k-BP-499px

Figure 3 – NGRIP data

We can see that the main warming in Greenland (at least this location in N. Greenland) took place around 15 kyrs ago, whereas Antarctica (compare figure 1 from Shakun et al) started its significant warming around 18 kyrs ago.

The paper basically demonstrates that they can capture two main temperature trends due to two separate effects:

  1. The “cooling” in Greenland from 19k-17k years with a warming in Antarctica over the same period – due to the MOC
  2. The continued warming from 17k-15k in both regions due to GHGs

Note that my NGRIP data shows a different temperature trend from their GISP data and I don’t know why.

Let’s understand what the model shows – this data is from their Supplementary data found on the Nature website.

First, 19-17 kyrs ago, Antartica (AN) has a significant warming, while Greenland (GI) has a bigger cooling:

He et al 2013-figS25

Figure 4

Note that the MOC (yellow) is the simulation that produces both the GI and AN change. (SUM is the values of the individual runs added together, while ALL is the simulation with all forcings combined).

Second, 17 – 15 kyrs ago, Antartica (AN) continues its warming and Greenland (GI) also warms:

He et al 2013-figS27

Figure 5

Note that GHG (pink) is the primary cause of the temperature rises now.

We can see the temperature trends over time as a better way of viewing it. I added some annotations because the layout wasn’t immediately obvious (to me):

He et al 2013-fig4-annotated-499px

Figure 6 – Red/blue annotations on side, and orange annotations on top

Again, as with figure 1, we can see that the main trends are quite well simulated but the model doesn’t pick shorter period variations.

The MOC in brief

A quick explanation – the MOC brings warmer surface tropical water to the high northern latitudes, warming up the high latitudes. The cold water returns at depth, making a really big circulation. When this circulation is disrupted Antarctica gets more tropical water and warms up (Antarctica has a similar large scale circulation running from the tropics on the surface and back at depth), while the northern polar region cools down.

It’s called the bipolar seesaw.

When your pour an extremely big vat of fresh water into the high latitudes it slows down, or turns off, the MOC. This is because fresh water is not as heavy as salty water, it can’t sink and it slows down the circulation.

So – if you have lots of ice melting in the high northern latitudes it flows into the ocean, slowing down the MOC, cooling the high northern latitudes and warming up Antarctica.

That’s what their model shows.

The available data on the MOC supports the idea, here is the part d from their fig 1 – the black line is the proxy reconstruction:

He et al 2013 fig1d

Figure 7

The units on the left are volume rates of water flowing between the tropics and high northern latitudes.

What Did Orbital Forcing do in their Model?

If we look back at their figure 1 (our figure 2) we see no change to anything as a result of simulation ORB so the abstract might seem a little confusing when their paper indicates that insolation, aka the Milankovitch theory, is what causes the whole chain of events.

In their figure 2 they show a geographical look of polar and high latitude summer temperature changes as a result of simulation ORB.

The initial increase of the mid-latitude to high-latitude NH spring–summer insolation between 22 and 19 kyr BP was about threefold that in the SH (Fig. 2a, b). Furthermore, the decrease in surface albedo from the melting of terrestrial snow cover in the NH results in additional net solar flux absorption in the NH (Supplementary Figs 12–15). Consequently, NH summers in simulation ORB warm by up to 2°C in the Arctic and by up to 4°C over Eurasia, with an area average of 0.9°C warming in mid to high latitudes in the NH (Fig. 2c, e).

In their model this doesn’t affect Greenland (for reasons I don’t understand). They claim:

Our ORB simulation thus supports the Milankovitch theory in showing that substantial summer warming occurs in the NH at the end of the Last Glacial Maximum as a result of the larger increase in high-latitude spring–summer insolation in the NH and greater sensitivity of the land-dominated northern high latitudes to insolation forcing from the snow–albedo feedback.

This orbitally induced warming probably initiated the retreat of NH ice sheets and helped sustain their retreat during the Oldest Dryas.

[Emphasis added].

Analysis

1. If we run the same orbital simulation at 104, 83 or 67 kyrs BP (or quite a few other times) what would we find? Here are the changes in insolation at 60°N from 130 kyrs BP to the present:

TOA-insolation-June21-60N-130k-present-499px

Figure 8

It’s not at all clear what is special about 21-18 kyr BP insolation. It’s no surprise that a GCM produces a local temperature increase when local insolation rises.

2. The meltwater pulse injected in the model is not derived from a calculation of any ice melt as a result of increased summer temperatures over ice sheets, it is an applied forcing. Given that the ice melt slows down the MOC and therefore acts to reduce the high latitude temperature, the MOC should act as a negative feedback on any ice/snow melt.

4. The Smith & Gregory 2012 paper that we look at in Part Nine maybe has different effects from the individual forcings to those found by He et al. Because 20 – 15 kyrs is a little compressed in Smith & Gregory I can’t be sure. Take, for example, the effect on (only) ice sheets during this period. Quite an effect in SG2012 over Greenland, nothing in He et al (see fig 6 above).

From Smith & Gregory 2012

From Smith & Gregory 2012

Figure 9

Conclusion

It’s an interesting paper, showing that changes in the large scale ocean currents between tropics and poles (the MOC) can account for a Greenland cooling and an Antarctic warming roughly in line with the proxy records. Most lines of evidence suggest that large-scale ice melt is the factor that disrupts the MOC.

Perhaps high latitude insolation changes at about 20 kyrs BP caused massive ice melt, which slowed the MOC, which warmed Antarctica, which led (by mechanisms unknown) to large increases in CO2, which created positive feedback on the temperature rise and terminated the last ice age.

Perhaps CO2 increased at the same time as Antarctic temperature (see the brief section on Parrenin et al 2013 in Part Eleven), therefore raising questions about where the cause and effect lies.

To make sense of climate we need to understand why:

a) previous higher insolation in the high latitudes didn’t set off the same chain of events
b) previous temperature changes in Antarctica didn’t set off the same chain of events
c) whether the temperature changes produced in simulation ORB can account for enough ice melt seen in the MOC changes (and what feedback effect that has)

And of course, we need to understand why CO2 increased so sharply at the end of the last ice age.

Articles in the Series

Part One - An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Pop Quiz: End of An Ice Age - a chance for people to test their ideas about whether solar insolation is the factor that ended the last ice age

Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2

Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II

Fourteen – Concepts & HD Data - getting a conceptual feel for the impacts of obliquity and precession, and some ice age datasets in high resolution

Fifteen – Roe vs Huybers - reviewing In Defence of Milankovitch, by Gerard Roe

Sixteen – Roe vs Huybers II - remapping a deep ocean core dataset and updating the previous article

Seventeen – Proxies under Water I - explaining the isotopic proxies and what they actually measure

Eighteen – “Probably Nonlinearity” of Unknown Origin - what is believed and what is put forward as evidence for the theory that ice age terminations were caused by orbital changes

Nineteen – Ice Sheet Models I - looking at the state of ice sheet models

References

Northern Hemisphere forcing of Southern Hemisphere climate during the last deglaciation, He, Shakun, Clark, Carlson, Liu, Otto-Bliesner & Kutzbach, Nature (2013) – free paper (there is considerable supplementary information probably only available on the Nature website)

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In the recent articles we mostly reviewed climate models’ successes or otherwise with simulating the last glacial inception.

Part Seven looked at some early GCM work – late 80′s to mid 90′s. Part Eight reviewed four different studies a decade or so ago. Part Nine was on one study which simulated the last 120 kyrs, and Part Ten reviewed one of the most recent studies of glacial inception 115 kyrs ago with a very latest climate model, CCSM4.

We will return to glacial inception, but in this article we will look at the end of the last ice age, partly because on another blog someone highlighted a particular paper which covered it and I spent some time trying to understand the paper.

The last 20 kyrs now have some excellent records from both polar regions. The EPICA project, initiated almost 20 years ago, has produced ice core data for Antarctica to match up with the Greenland NGRIP ice core data going back almost 800 kyrs. And from other research more proxy temperature data has become available from around the globe.

Shakun et al 2012

This paper is from Shakun et al 2012 (thanks to commenter BBD for highlighting it). As an aside, Bette Otto-Bliesner is one of the co-authors, also for Jochum et al (2012) that we reviewed in Part Ten. She is one of the lead authors of the IPCC AR5 for the section on Paleoclimate.

The Last Glacial Maximum (LGM) was around 22k-18 kyrs ago. Sea level was 120m lower than today as thick ice sheets covered parts of North America and Europe.

Why did it end? How did it end?

The paper really addresses the second question.

The top graph below shows Antarctic temperatures in red, CO2 in yellow dots and global temperatures in blue:

From Shakun et al 2012

From Shakun et al 2012

Figure 1

The second graph shows us the histogram of leads and lags vs CO2 changes for both Antarctica and global temperature.

We can see clearly that the Antarctic temperatures started a sustained increase about 18 kyrs ago and led the global temperatures. We can see that CO2 is slightly preceded by, or in sync with, Antarctic temperatures. This  indicates that the CO2 increases here are providing a positive feedback on an initial Antarctic temperature rise (knowing from basic physics that more CO2 increases radiative forcing in the troposphere – see note 1).

But what caused this initial rise in Antarctic temperatures? One possibility put forward is an earlier rise in temperature in the higher northern latitudes that can be seen in the second graph below:

From Shakun et al 2012

From Shakun et al 2012

Figure 2

..An important exception is the onset of deglaciation, which features about 0.3ºC of global warming before the initial increase in CO2 ,17.5 kyr ago. This finding suggests that CO2 was not the cause of initial warming.

..Substantial temperature change at all latitudes (Fig. 5b), as well as a net global warming of about 0.3ºC (Fig. 2a), precedes the initial increase in CO2 concentration at 17.5 kyr ago, suggesting that CO2 did not initiate deglacial warming. This early global warming occurs in two phases: a gradual increase between 21.5 and 19 kyr ago followed by a somewhat steeper increase between 19 and 17.5 kyr ago (Fig. 2a). The first increase is associated with mean warming of the northern mid to high latitudes, most prominently in Greenland, as there is little change occurring elsewhere at this time (Fig. 5 and Supplementary Fig. 20). The second increase occurs during a pronounced interhemispheric seesaw event (Fig. 5), presumably related to a reduction in AMOC strength, as seen in the Pa/Th record and our modelling (Fig. 4f, g).

..In any event, we suggest that these spatiotemporal patterns of temperature change are consistent with warming at northern mid to high latitudes, leading to a reduction in the AMOC at ~19 kyr ago, being the trigger for the global deglacial warming that followed, although more records will be required to confirm the extent and magnitude of early warming at such latitudes.

The interhemispheric seesaw referred to is important to understand and refers to the relationship between two large scale ocean currents – between the tropics and the high northern latitudes and between the tropics and Antartica. (A good paper to start is Asynchrony of Antarctic and Greenland climate change during the last glacial period, Blunier et al 1998). Perhaps a subject for a later article.

Then a “plausible scenario” is presented for the initial NH warming:

A possible forcing model to explain this sequence of events starts with rising boreal summer insolation driving northern warming. This leads to the observed retreat of Northern Hemisphere ice sheets and the increase in sea level commencing, 19 kyr ago (Fig. 3a, b), with the attendant freshwater forcing causing a reduction in the AMOC that warms the Southern Hemisphere through the bipolar seesaw.

This is a poor section of the paper. I find it strange that someone could write this and not at least point out the obvious flaws in it. Before explaining, two points are worth noting:

  1. That this is described a “possible forcing model” and it’s really not the paper’s subject or apparently supported by any evidence in the paper
  2. Their model runs, fig 4c, don’t support this hypothesis – they show NH temperatures trending down over this critical period. Compare 4b and 4c (b is proxy data, c is the model). However, they don’t break out the higher latitudes so perhaps their model did show this result.
From Shakun et al 2012

From Shakun et al 2012

Figure 3

The obvious criticism of this hypothesis is that insolation (summer, 65ºN) has been a lot higher during earlier periods:

TOA-insolation-June21-65N-125k-present

Figure 4

We saw this in Ghosts of Climates Past – Pop Quiz: End of An Ice Age.

And also earlier periods of significant temperature rises in the high northern latitudes have been recorded during the last glacial period. Why were none of these able to initiate this same sequence of events and initiate an Antarctic temperature rise?

At the time of the LGM, the ice sheets were at their furthest extent, with the consequent positive feedback of the higher albedo. If a small increase in summer insolation in high northern latitudes could initiate a deglaciation, surely the much higher summer insolation at 100 kyrs BP or 82 kyrs BP would have initiated a deglaciation given the additional benefit of the lower albedo at the time.

As I was completing this section of the article I went back to the Nature website to see if there was any supplemental information (Nature papers are short and online material that doesn’t appear in the pdf paper can be very useful).

There was a link to a News & Views article on this paper by Eric Wolff. Eric Wolff is one of the key EPICA contributors, a lead author and co-author of many EPICA papers, so I was quite encouraged to read his perspective on the paper.

Many people seem convinced the Milankovitch theory is without question and not accepting it is absurd, see for example the blog discussion I referred to earlier, so it’s worth quoting extensively from Wolff’s short article:

Between about 19,000 and 10,000 years ago, Earth emerged from the last glacial period. The whole globe warmed, ice sheets retreated from Northern Hemisphere continents and atmospheric composition changed significantly. Many theories try to explain what triggered and sustained this transformation (known as the glacial termination), but crucial evidence to validate them is lacking.

On page 49 of this issue, Shakun et al use a global reconstruction of temperature to show that the transition from the glacial period to the current interglacial consisted of an antiphased temperature response of Earth’s two hemispheres, superimposed on a globally coherent warming. Ocean-circulation changes, controlling the contrasting response in each hemisphere, seem to have been crucial to the glacial termination.

Once again, a key climate scientist notes that we don’t know why the last ice age ended. As we saw in Part Six – “Hypotheses Abound” - the title explains the content..

..Some studies have proposed that changes in ocean heat transport are an essential part of glacial termination. Shakun et al. combine their data with simulations based on an ocean–atmosphere general circulation model to present a plausible sequence of events from about 19,000 years ago onwards. They propose that a reduction in the AMOC (induced in the model by introducing fresh water into the North Atlantic) led to Southern Hemisphere warming, and a net cooling in the Northern Hemisphere. Carbon dioxide concentration began to rise soon afterwards, probably owing to degassing from the deep Southern Ocean; although quite well documented, the exact combination of mechanisms for this rise remains a subject of debate. Both hemispheres then warmed together, largely in response to the rise in carbon dioxide, but with further oscillations in the hemispheric contrast as the strength of the AMOC varied. The model reproduces well both the magnitude and the pattern of global and hemispheric change, with carbon dioxide and changing AMOC as crucial components.

The success of the model used by Shakun and colleagues in reproducing the data is encouraging. But one caveat is that the magnitude of fresh water injected into the Atlantic Ocean in the model was tuned to produce the inferred strength of the AMOC and the magnitude of interhemispheric climate response; the result does not imply that the ocean circulation in the model has the correct sensitivity to the volume of freshwater input.

Shakun and colleagues’ work does provide a firm data-driven basis for a plausible chain of events for most of the last termination. But what drove the reduction in the AMOC 19,000 years ago? The authors point out that there was a significant rise in temperature between 21,500 and 19,000 years ago in the northernmost latitude band (60–90° N). They propose that this may have resulted from a rise in summer insolation (incoming solar energy) at high northern latitudes, driven by well-known cycles in Earth’s orbit around the Sun. They argue that this rise could have caused an initial ice-sheet melt that drove the subsequent reduction in the AMOC.

However, this proposal needs to be treated with caution. First, there are few temperature records in this latitude band: the warming is seen clearly only in Greenland ice cores. Second, there is at least one comparable rise in temperature in the Greenland records, between about 62,000 and 60,000 years ago, which did not result in a termination. Finally, although it is true that northern summer insolation increased from 21,500 to 19,000 years ago, its absolute magnitude remained lower than at any time between 65,000 and 30,000 years ago. It is not clear why an increase in insolation from a low value initiated termination whereas a continuous period of higher insolation did not.

In short, another ingredient is needed to explain the link between insolation and termination, and the triggers for the series of events described so well in Shakun and colleagues’ paper. The see-saw of temperature between north and south throughout the glacial period, most clearly observed in rapid Greenland warmings (Dansgaard–Oeschger events), is often taken as a sign that numerous changes in AMOC strength occurred. However, the AMOC weakening that started 19,000 years ago lasted for much longer than previous ones, allowing a much more substantial rise in southern temperature and in carbon dioxide concentration. Why was it so hard, at that time, to reinvigorate the AMOC and end this weakening?

And what is the missing ingredient that turned the rise in northern insolation around 20,000 years ago into the starting gun for deglaciation, when higher insolation at earlier times failed to do so? It has been proposed that terminations occur only when northern ice-sheet extent is particularly large. If this is indeed the extra ingredient, then the next step in unwinding the causal chain must be to understand what aspect of a large ice sheet controls the onset and persistence of changes in the AMOC that seem to have been key to the last deglaciation.

[Emphasis added].

Thanks, Eric Wolff. My summary on Shakun et al, overall – on its main subject – it’s a very good paper with solid new data, good explanations and graphs.

However, this field is still in flux..

Parrenin et al 2013

Understanding the role of atmospheric CO2 during past climate changes requires clear knowledge of how it varies in time relative to temperature. Antarctic ice cores preserve highly resolved records of atmospheric CO2 and Antarctic temperature for the past 800,000 years.

Here we propose a revised relative age scale for the concentration of atmospheric CO2 and Antarctic temperature for the last deglacial warming, using data from five Antarctic ice cores. We infer the phasing between CO2 concentration and Antarctic temperature at four times when their trends change abruptly.

We find no significant asynchrony between them, indicating that Antarctic temperature did not begin to rise hundreds of years before the concentration of atmospheric CO2, as has been suggested by earlier studies.

[Emphasis added].

Ouch. In a later article we will delve into the complex world of dating ice cores and the air trapped in the ice cores.

WAIS Divide Project Members (2013)

The cause of warming in the Southern Hemisphere during the most recent deglaciation remains a matter of debate.

Hypotheses for a Northern Hemisphere trigger, through oceanic redistributions of heat, are based in part on the abrupt onset of warming seen in East Antarctic ice cores and dated to 18,000 years ago, which is several thousand years after high-latitude Northern Hemisphere summer insolation intensity began increasing from its minimum, approximately 24,000 years ago.

An alternative explanation is that local solar insolation changes cause the Southern Hemisphere to warm independently. Here we present results from a new, annually resolved ice-core record from West Antarctica that reconciles these two views. The records show that 18,000 years ago snow accumulation in West Antarctica began increasing, coincident with increasing carbon dioxide concentrations, warming in East Antarctica and cooling in the Northern Hemisphere associated with an abrupt decrease in Atlantic meridional overturning circulation. However, significant warming in West Antarctica began at least 2,000 years earlier.

Circum-Antarctic sea-ice decline, driven by increasing local insolation, is the likely cause of this warming. The marine-influenced West Antarctic records suggest a more active role for the Southern Ocean in the onset of deglaciation than is inferred from ice cores in the East Antarctic interior, which are largely isolated from sea-ice changes.

[Emphasis added].

We see that “rising solar insolation” in any part of the world from any value can be presented as a hypothesis for ice age termination. Here “local solar insolation” means the solar insolation in the Antarctic region, compare with Shakun et al, where rising insolation (from a very low value) in the high northern latitudes was presented as a hypothesis for northern warming which then initiated a southern warming.

That said, this is a very interesting paper with new data from Antarctica, the West Antarctic Ice Sheet (WAIS) Divide ice core (WDC), where drilling was completed in 2011:

Because the climate of West Antarctica is distinct from that of interior East Antarctica, the exclusion of West Antarctic records may result in an incomplete picture of past Antarctic and Southern Ocean climate change. Interior West Antarctica is lower in elevation and more subject to the influence of marine air masses than interior East Antarctica, which is surrounded by a steep topographic slope. Marine-influenced locations are important because they more directly reflect atmospheric conditions resulting from changes in ocean circulation and sea ice. However, ice-core records from coastal sites are often difficult to interpret because of complicated ice-flow and elevation histories.

The West Antarctic Ice Sheet (WAIS) Divide ice core (WDC), in central West Antarctica, is unique in coming from a location that has experienced minimal elevation change, is strongly influenced by marine conditions and has a relatively high snow-accumulation rate, making it possible to obtain an accurately dated record with high temporal resolution.

WDC paints a slightly different picture from other Antarctic ice cores:

..and significant warming at WDC began by 20 kyr ago, at least 2,000 yr before significant warming at EDML and EDC.

..Both the WDC and the lower-resolution Byrd ice-core records show that warming in West Antarctica began before the decrease in AMOC that has been invoked to explain Southern Hemisphere warming [the references include Shakun et al 2012]. The most significant early warming at WDC occurred between 20 and 18.8 kyr ago, although a period of significant warming also occurred between 22 and 21.5 kyr ago. The magnitude of the warming at WDC before 18 kyr ago is much greater than at EDML or EDC..

From WAIS Divide Project (2013)

From WAIS Divide Project (2013)

Figure 5

We will look at this paper in more detail in a later article.

Conclusion

The termination of the last ice age is a fascinating topic that tests our ability to understand climate change.

One criticism made of climate science on many blogs is that climate scientists are obsessed with running GCMs, instead of doing “real science”, “running real experiments” and “gathering real data”. I can’t say where the balance really is, but in my own journey through climate science I find that there is a welcome and healthy obsession with gathering data, finding new sources of data, analyzing data, comparing datasets and running real experiments. The Greenland and Antarctic ice core projects, like NGRIP, EPICA and WAIS Divide Project are great examples.

On other climate blogs, writers and commenters seem very happy that climate scientists have written a paper that “supports the orbital hypothesis” without any critical examination of what the paper actually supports with evidence.

Returning to the question at hand, explaining the termination of the last ice age – the problem at the moment is less that there is no theory, and more that the wealth of data has not yet settled onto a clear chain of cause and effect. This is obviously essential to come up with a decent theory.

And any theory that explains the termination of the last ice age will need to explain why it didn’t happen earlier. Invoking “rising insolation” seems like lazy journalism to me. Luckily Eric Wolff, at least, agrees with me.

Articles in the Series

Part One - An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Pop Quiz: End of An Ice Age - a chance for people to test their ideas about whether solar insolation is the factor that ended the last ice age

Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland

Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II

Fourteen – Concepts & HD Data - getting a conceptual feel for the impacts of obliquity and precession, and some ice age datasets in high resolution

Fifteen – Roe vs Huybers - reviewing In Defence of Milankovitch, by Gerard Roe

Sixteen – Roe vs Huybers II - remapping a deep ocean core dataset and updating the previous article

Seventeen – Proxies under Water I - explaining the isotopic proxies and what they actually measure

Eighteen – “Probably Nonlinearity” of Unknown Origin - what is believed and what is put forward as evidence for the theory that ice age terminations were caused by orbital changes

Nineteen – Ice Sheet Models I - looking at the state of ice sheet models

References

Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation, Shakun, Clark, He, Marcott, Mix, Liu, Otto-Bliesner, Schmittner & Bard, Nature (2012) – free paper

Climate change: A tale of two hemispheres, Eric W. Wolff, Nature (2012)

Synchronous Change of Atmospheric CO2 and Antarctic Temperature During the Last Deglacial Warming, Parrenin, Masson-Delmotte, Köhler, Raynaud, Paillard, Schwander, Barbante, Landais, Wegner & Jouzel, Science (2013) – free paper, in submission form (rather than published form)

For interest  Valérie Masson-Delmotte is one of the two co-ordinating lead authors for AR5 for the Paleoclimate section, Frédéric Parrenin is a contributing author.

Onset of deglacial warming in West Antarctica driven by local orbital forcing, WAIS Divide Project Members, Nature (2013) – free paper

Notes

Note 1 – see for example, CO2 - An Insignificant Trace Gas? - an 8-part series on CO2, Atmospheric Radiation and the “Greenhouse” Effect - a 12-part series on how radiation interacts with the atmosphere, Visualizing Atmospheric Radiation - a 13-part series to help us get a better grasp of how this works including “does water vapor overwhelm CO2″, “is CO2 saturated” and many other questions.

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In previous articles we have discussed the Milankovitch hypothesis – classically paraphrased as:

Solar insolation at 65ºN in summer determines the start and end of ice ages – with minimum summer insolation preventing snow melt at high latitudes which allows perennial snow cover, positive feedback from reflected solar radiation and the consequent growth of ice sheets.

Conversely maximum solar insolation at high latitudes causes ice sheets to melt and (with the same positive feedback effect) ends the ice age.

And “summer” is usually taken as  the insolation on June 21st even if it is a somewhat arbitrary date (we can also average over a month or the season).

So I produced a few contour plots, showing the insolation anomaly by latitude and day of year compared with the present for 8 different years between the start of the last ice age (about 115 kyrs ago) and today.

The challenge for readers is to identify which graph corresponds to the end of the last ice age. And some kind of reason why you chose that graph.

I made them a little smaller so that they could be more easily compared – just click on each set to expand.

The x-axis (left to right) is day of year, and June 21st is about day 200 (actually it is 172, thanks to Climateer for pointing this out!). The y-axis (bottom to top) is the latitude. The colors represent the same in each graph and the contour lines are 10 W/m² apart.

ice-ages-part11-A-D-499px

Figures A – D – Click for larger view

ice-ages-part11-E-H-499px

Figures E – H – Click for larger view

Articles in the Series

Part One – An introduction

Part Two – Lorenz - one point of view from the exceptional E.N. Lorenz

Part Three – Hays, Imbrie & Shackleton - how everyone got onto the Milankovitch theory

Part Four – Understanding Orbits, Seasons and Stuff - how the wobbles and movements of the earth’s orbit affect incoming solar radiation

Part Five – Obliquity & Precession Changes - and in a bit more detail

Part Six – “Hypotheses Abound” - lots of different theories that confusingly go by the same name

Part Seven – GCM I - early work with climate models to try and get “perennial snow cover” at high latitudes to start an ice age around 116,000 years ago

Part Seven and a Half – Mindmap - my mind map at that time, with many of the papers I have been reviewing and categorizing plus key extracts from those papers

Part Eight – GCM II - more recent work from the “noughties” – GCM results plus EMIC (earth models of intermediate complexity) again trying to produce perennial snow cover

Part Nine – GCM III - very recent work from 2012, a full GCM, with reduced spatial resolution and speeding up external forcings by a factors of 10, modeling the last 120 kyrs

Part Ten – GCM IV - very recent work from 2012, a high resolution GCM called CCSM4, producing glacial inception at 115 kyrs

Eleven – End of the Last Ice age - latest data showing relationship between Southern Hemisphere temperatures, global temperatures and CO2

Twelve – GCM V – Ice Age Termination - very recent work from He et al 2013, using a high resolution GCM (CCSM3) to analyze the end of the last ice age and the complex link between Antarctic and Greenland

Thirteen – Terminator II - looking at the date of Termination II, the end of the penultimate ice age – and implications for the cause of Termination II

Fourteen – Concepts & HD Data - getting a conceptual feel for the impacts of obliquity and precession, and some ice age datasets in high resolution

Fifteen – Roe vs Huybers - reviewing In Defence of Milankovitch, by Gerard Roe

Sixteen – Roe vs Huybers II - remapping a deep ocean core dataset and updating the previous article

Seventeen – Proxies under Water I - explaining the isotopic proxies and what they actually measure

Eighteen – “Probably Nonlinearity” of Unknown Origin - what is believed and what is put forward as evidence for the theory that ice age terminations were caused by orbital changes

Nineteen – Ice Sheet Models I - looking at the state of ice sheet models

Read Full Post »

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