November | 2013 | The Science of Doom

Archive for November, 2013

Ghosts of Climates Past – Part Seven and a Half – Mindmap

Posted in Climate History on November 26, 2013| 6 Comments »

For those interested, I’ve been using a mindmap to try and keep on top of all of the different papers and ideas. It’s a work in progress. The iPad app produces a pdf output but not a scalable graphic (just a blurred one).

Lots of papers and extracts:

Ice Ages-3

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 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

Read Full Post »

Ghosts of Climates Past – Part Seven – GCM I

Posted in Climate History, Climate Models on November 23, 2013| 63 Comments »

In Part Six we looked at some of the different theories that confusingly go by the same name. The “Milankovitch” theories.

The essence of these many theories – even though the changes in “tilt” of the earth’s axis and the time of closest approach to the sun don’t change the total annual solar energy incident on the climate, the changing distribution of energy causes massive climate change over thousands of years.

One of the “classic” hypotheses is increases in July insolation at 65ºN cause the ice sheets to melt. Or conversely, reductions in July insolation at 65ºN cause the ice sheets to grow.

The hypotheses described can sound quite convincing. Well, one at a time can sound quite convincing – when all of the “Milankovitch theories” are all lined up alongside each other they start to sound more like hopeful ideas.

In this article we will start to consider what GCMs can do in falsifying these theories. For some basics on GCMs, take a look at Models On – and Off – the Catwalk.

Many readers of this blog have varying degrees of suspicion about GCMs. But as regular commenter DeWitt Payne often says, “all models are wrong, but some are useful“, that is, none are perfect, but some can shed light on the climate mechanisms we want to understand.

In fact, GCMs are essential to understand many climate mechanisms and essential to understand the interaction between different parts of the climate system.

Digression – Ice Sheets and Positive Feedback

For beginners, a quick digression into ice sheets and positive feedback. Melting and forming of ice & snow is undisputably a positive feedback within the climate system.

Snow reflects around 60-90% of incident solar radiation. Water reflects less than 10% and most ground surfaces reflect less than 25%. If a region heats up sufficiently, ice and snow melt. Which means less solar radiation gets reflected, which means more radiation is absorbed, which means the region heats up some more. The effect “feeds itself”. It’s a positive feedback.

In the annual cycle it doesn’t lead to any kind of thermal runaway or a snowball earth because the solar radiation goes through a much bigger cycle.

Over much longer time periods it’s conceivable that (regional) melting of ice sheets leads to more (regional) solar radiation absorbed, causing more melting of ice sheets which leads to yet more melting. And the converse for growth of ice sheets. The reason it’s conceivable is because it’s just that same mechanism.

Digression over.

Why GCMs ?

The only alternative is to do the calculation in your head or on paper. Take a piece of paper, plot a graph of the incident radiation at all latitudes vs the time period we are interested in – say 150 kyrs ago through to 100 kyrs – now work out by year, decade or century, how much ice melts. Work out the new albedo for each region. Calculate the change in absorbed radiation. Calculate the regional temperature changes. Calculated the new heat transfer from low to high latitudes (lots of heat is exported from the equator to the poles via the atmosphere and the ocean) due to the latitudinal temperature gradient, the water vapor transported, and the rainfall and snowfall. Don’t forget to track ice melt at high latitudes and its impact on the Meridional Overturning Circulation (MOC) which drives a significant part of the heat transfer from the equator to poles. Step to the next year, decade or century and repeat.

How are those calculations coming along?

A GCM uses some fundamental physics equations like energy balance and mass balance. It uses a lot of parameterized equations to calculate things like heat transfer from the surface to the atmosphere dependent on the wind speed, cloud formation, momentum transfer from wind to ocean, etc. Whatever we have in a GCM is better than trying to do it on a sheet of paper (and in the end you will be using the same equations with much less spatial and time granularity).

If we are interested in the “classic” Milankovitch theory mentioned above we need to find out the impact of an increase of 50W/m² (over 10,000 years) in summer at 65ºN – see figure 1 in Ghosts of Climates Past – Part Five – Obliquity & Precession Changes. What effect does the simultaneous spring reduction at 65ºN have. Do these two effects cancel each other out? Is the summer increase more significant than the spring reduction?

How quickly does the circulation lessen the impact? The equator-pole export of heat is driven by the temperature difference – as with all heat transfer. So if the northern polar region is heating up due to ice melting, the ocean and atmospheric circulation will change and less heat will be driven to the poles. What effect does this have?

How quickly does an ice sheet melt and form? Can the increases and reductions in solar radiation absorbed explain the massive ice sheet growth and shrinking?

If the positive feedback is so strong how does an ice age terminate and how does it restart 10,000 years later?

We can only assess all of these with a general circulation model.

There is a problem though. A typical GCM run is a few decades or a century. We need a 10,000 – 50,000 year run with a GCM. So we need 500x the computing power – or we have to reduce the complexity of the model.

Alternatively we can run a model to equilibrium at a particular time in history to see what effect the historical parameters had on the changes we are interested in.

Early Work

Many readers of this blog are frequently mystified by my choosing “old work” to illuminate a topic. Why not pick the most up to date research?

Because the older papers usually explain the problem more clearly and give more detail on the approach to the problem.

The latest papers are written for researchers in the field and assume most of the preceding knowledge – that everyone in that field already has. A good example is the Myhre et al (1998) paper on the “logarithmic formula” for radiative forcing with increasing CO2, cited by the IPCC TAR in 2001. This paper has mystified so many bloggers. I have read many blog articles where the blog authors and commenters throw up their metaphorical hands at the lack of justification for the contents of this paper. However, it is not mystifying if you are familiar with the physics of radiative transfer and the papers from the 70’s through the 90’s calculating radiative imbalance as a result of more “greenhouse” gases.

It’s all about the context.

We’ll take a walk through a few decades of GCMs..

We’ll start with Rind, Peteet & Kukla (1989). They review the classic thinking on the problem:

Kukla et al. [1981] described how the orbital configurations seemed to match up with gross climate variations for the last 150 millennia or so. As a result of these and other geological studies, the consensus exists that orbital variations are responsible for initiating glacial and interglacial climatic regimes. The most obvious difference between these two regimes, the existence of subpolar continental ice sheets, appears related to solar insolation at northern hemisphere high latitudes in summer. For example, solar insolation at these latitudes in August and September was reduced, compared with today’s values, around 116,000 years before the present (116 kyr B.P.), during the time when ice growth apparently began, and it was increased around 10 kyr B.P. during a time of rapid ice sheet retreat [e.g., Berger, 1978] (Figure 1).

And the question of whether basic physics can link the supposed cause and effect:

Are the solar radiation variations themselves sufficient to produce or destroy the continental ice sheets?

The July solar radiation incident at 50ºN and 60ºN over the past 170 kyr is shown in Figure 1, along with August and September values at 50ºN (as shown by the example for July, values at the various latitudes of concern for ice age initiation all have similar insolation fluctuations). The peak variations are of the order of 10%, which if translated with an equal percentage into surface air temperature changes would be of the order of 30ºC. This would certainly be sufficient to allow snow to remain throughout the summer in extreme northern portions of North America, where July surface temperatures today are only about 10ºC above freezing.

However, the direct translation ignores all of the other features which influence surface air temperature during summer, such as cloud cover and albedo variations, long wave radiation, surface flux effects, and advection.

[Emphasis added].

Various energy balance climate models have been used to assess how much cooling would be associated with changed orbital parameters. As the initiation of ice growth will alter the surface albedo and provide feedback to the climate change, the models also have to include crude estimates of how ice cover will change with climate. With the proper tuning of parameters, some of which is justified on observational grounds, the models can be made to simulate the gross glacial/interglacial climate changes.

However, these models do not calculate from first principles all the various influences on surface air temperature noted above, nor do they contain a hydrologic cycle which would allow snow cover to be generated or increase. The actual processes associated with allowing snow cover to remain through the summer will involve complex hydrologic and thermal influences, for which simple models can only provide gross approximations.

They comment then on the practical problems of using GCMs for 10 kyr runs that we noted above. The problem is worked around by using prescribed values for certain parameters and by using a coarse grid – 8° x 10° and 9 vertical layers.

The various GCMs runs are typical of the approach to using GCMs to “figure stuff out” – try different runs with different things changed to see what variations have the most impact and what variations, if any, result in the most realistic answers:

We have thus used the Goddard Institute for Space Studies (GISS) GCM for a series of experiments in which orbital parameters, atmospheric composition, and sea surface temperatures are changed. We examine how the various influences affect snow cover and low-elevation ice sheets in regions of the northern hemisphere where ice existed at the Last Glacial Maximum (LGM). As we show, the GCM is generally incapable of simulating the beginnings of ice sheet growth, or of maintaining low-elevation ice sheets, regardless of the orbital parameters or sea surface temperatures used.

[Emphasis added].

And the result:

The experiments indicate there is a wide discrepancy between the model’s response to Milankovitch perturbations and the geophysical evidence of ice sheet initiation. As the model failed to grow or sustain low-altitude ice during the time of high-latitude maximum solar radiation reduction (120-110 kyrB.P.), it is unlikely it could have done so at any other time within the last several hundred thousand years.

If the model results are correct, it indicates that the growth of ice occurred in an extremely ablative environment, and thus demanded some complicated strategy, or else some other climate forcing occurred in addition to the orbital variation influence (and CO2 reduction), which would imply we do not really understand the cause of the ice ages and the Milankovitch connection. If the model is not nearly sensitive enough to climate forcing, it could have implications for projections of future climate change.

[Emphasis added].

The basic model experiment on the ability of Milankovitch variations by themselves to generate ice sheets in a GCM, experiment 2, shows that in the GISS GCM even exaggerated summer radiation deficits are not sufficient. If widespread ice sheets at 10-m elevation are inserted, CO2 reduced by 70ppm, sea ice increases to full ice age conditions, and sea surface temperatures reduced to CLIMAP 18 kyr BP estimates or below, the model is just barely able keep these ice sheets from melting in restricted regions. How likely are these results to represent the actual state of affairs?

That was 1989 GCM’s.

Phillipps & Held (1994) had basically the same problem. This is the famous Isaac Held, who has written extensively on climate dynamics, water vapor feedback, GCMs and runs an excellent blog that is well-worth reading.

While paleoclimatic records provide considerable evidence in support of the astronomical, or Milankovitch, theory of the ice ages (Hays et al. 1976), the mechanisms by which the orbital changes influence the climate are still poorly understood..

..For this study we utilize the atmosphere-mixed layer ocean model.. In examining this model’s sensitivity to different orbital parameter combinations, we have compared three numerical experiments.

They describe the comparison models:

Our starting point was to choose the two experiments that are likely to generate the largest differences in climate, given the range of the parameter variations computed to have occurred over the past few hundred thousand years. The eccentricity is set equal to 0.04 in both cases. This is considerably larger than the present value of 0.016 but comparable to that which existed from ~90 to 150k BP.

In the first experiment, the perihelion is located at NH summer solstice and the obliquity is set at the high value of 24°.

In the second case, perihelion is at NH winter solstice and the obliquity equals 22°.

The perihelion and obliquity are both favorable for warm northern summers in the first case, and for cool northern summers in the second. These experiments are referred to as WS and CS respectively.

We then performed another calculation to determine how much of the difference between these two integrations is due to the perihelion shift and how much to the change in obliquity. This third model has perihelion at summer solstice, but a low value (22°) of the obliquity. The eccentricity is still set at 0.04. This experiment is referred to as WS22.

Sadly:

We find that the favorable orbital configuration is far from being able to maintain snow cover throughout the summer anywhere in North America..

..Despite the large temperature changes on land the CS experiment does not generate any new regions of permanent snow cover over the NH. All snow cover melts away completely in the summer. Thus, the model as presently constituted is unable to initiate the growth of ice sheets from orbital perturbations alone. This is consistent with the results of Rind with a GCM (Rind et al. 1989)..

In the next article we will look at more favorable results in the 2000’s.

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 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

Can Milankovitch Orbital Variations Initiate the Growth of Ice Sheets in a General Circulation Model?, Rind, Peteet & Kukla, JGR (1989) – behind a paywall, email me if you want to read it, scienceofdoom – you know what goes here – gmail.com

Response to Orbital Perturbations in an Atmospheric Model Coupled to a Slab Ocean, Phillipps & Held, Journal of Climate (1994) – free paper

New estimates of radiative forcing due to well-mixed greenhouse gases, Myhre et al, GRL (1998)

Read Full Post »

Ghosts of Climates Past – Part Six – “Hypotheses Abound”

Posted in Climate History on November 11, 2013| 17 Comments »

It is common to find blogs and articles from what we might call the “consensus climate science” corner that we know what caused the ice ages.

The cause being changes in solar insolation at higher latitudes via the orbital changes described in Part Four and Five. These go under the banner of the “Milankovitch theory”.

While that same perspective is present in climate science papers, the case is presented more clearly. Or perhaps I could say, it’s made clear that the case is far from clear. It’s very very muddy.

Here are Smith & 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.

[Emphasis added].

Still, there are always outliers in every field and one paper doesn’t demonstrate a consensus on anything. So let’s take a walk through the mud..

Wintertime NH High Latitude Insolation

Kukla (1972):

The link between the Milankovitch mechanism and climate remains unclear. Summer half-year insolation curves for 65°N are usually offered on the assumption that the incoming radiation could directly control the retreat or advance of glaciers, thus controlling the global climate.

The validity of this assumption was questioned long ago by Croll (1875) and Ball (1891). Modern satellite measurements fully justify Croll’s concept of climate formation, with ocean currents playing the basic role in distributing heat and moisture to continents. The simplistic model of Koppen and Wegener must be definitely abandoned..

..The principal cold periods are found, within the accuracy limits of radiometric dating, to be precisely parallelled by intervals of decreasing winter insolation income for Northern Hemisphere (glacial insolation regime) and vice versa. Gross climatic changes originate in winters on the continents of the Northern Hemisphere.

Just for interest for history buffs, he also comments:

Two facts are highly probable: (1) in A. D. 2100 the globe will be cooler than today (Bray 1970), and (2) Man-made warming will hardly be noticeable on global scale at that time.

Self-Oscillations of the Climate System

Broecker & Denton (1990):

Although we are convinced that the Earth’s climate responds to orbital cycles in some fashion, we reject the view of a direct linkage between seasonality and ice-sheet size with consequent changes to climate of distant regions. 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 global climates are driven by changes in seasonality, then another linkage must exist.

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..

..Existing data from the Earth’s glacier system thus imply that the last termination began simultaneously and abruptly in both polar hemispheres, despite the fact that summer insolation signals were out of phase at the latitude of the key glacial records..

..Although variations in the Earth’s orbital geometry are very likely the cause of glacial cycles (Hays et al., 1976; Imbrie et al., 1984), the nature of the link between seasonal insolation and global climate remains a major unanswered question..

[Emphasis added].

Strictly speaking this is a “not quite Milankovitch” theory (and there are other flavors of this theory not covered in this article). I put forward this paper because Wallace S. Broecker is a very influential climate scientist on this topic and the subject of the thermohaline circulation (THC) in past climate, has written many papers, and generally appears to stick with a “Milankovitch” flavor to his theories.

Temperature Gradient between Low & High Latitude

George Kukla, Clement, Cane, Gavin & Zebiak (2002):

Although the link between insolation and climate is commonly thought to be in the high northern latitudes in summer, our results show that the start of the last glaciation in marine isotope stage (MIS) 5d was associated with a change of insolation during the transitional seasons in the low latitudes.

A simplified coupled ocean-atmosphere model shows that changes in the seasonal cycle of insolation could have altered El Nino Southern Oscillation (ENSO) variability so that there were almost twice as many warm ENSO events in the early glacial than in the last interglacial. This indicates that ice buildup in the cooled high latitudes could have been accelerated by a warmed tropical Pacific..

..Since the early 1900s, the link between insolation and climate has been seen in the high latitudes of the Northern Hemisphere where summer insolation varies significantly.

Insolation at the top of the atmosphere (TOA) during the summer solstice at 65°N is commonly taken to represent the solar forcing of changing global climate. This is at odds with the results of Berger et al. (1981), who correlated the varying monthly TOA insolation at different latitudes of both hemispheres with the marine oxygen isotope record of Hays et al. (1976). The highest positive correlation (p ≤ 0.01) was found not for June but for September, and not in the high latitudes but in the three latitudinal bands representing the tropics (25°N, 5°N, and 15°S)..

..At first glance the implications of our results appear to be counterintuitive, indicating that the early buildup of glacier ice was associated not with the cooling, but with a relative warming of tropical oceans. Recent analogs suggest that it might even have been accompanied by a temporary increase of globally averaged annual mean temperature. If correct, the main trigger of glaciations would not be the expansion of snow fields in subpolar belts, but rather the increase in temperature gradient between the low and the high latitudes.

[Emphasis added].

A Puzzle

George Kukla et al (2002) – written along with a cast of eminents like Shackleton, Imbrie, Broecker:

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.

[Emphasis added].

Gradient in Insolation from Low to High Latitudes

Maureen Raymo & Kerim Nisancioglu (2003):

Based mainly on climate proxy records of the last 0.5 Ma, a general scientific consensus has emerged that variations in summer insolation at high northern latitudes are the dominant influence on climate over tens of thousands of years. The logic behind nearly a century’s worth of thought on this topic is that times of reduced summer insolation could allow some snow and ice to persist from year to year, lasting through the ‘‘meltback’’ season. A slight increase in accumulation from year to year, enhanced by a positive snow-albedo feedback, would eventually lead to full glacial conditions. At the same time, the cool summers are proposed to be accompanied by mild winters which, through the temperature-moisture feedback, would lead to enhanced winter accumulation of snow. Both effects, reduced spring-to-fall snowmelt and greater winter accumulation, seem to provide a logical and physically sound explanation for the waxing and waning of the ice sheets as high-latitude insolation changes.

Then they point out the problems with this hypothesis and move onto their theory:

We propose that the gradient in insolation between high and low latitudes may, through its influence on the poleward flux of moisture which fuels ice sheet growth, play the dominant role in controlling climate from ~3 to 1 million years ago..

And conclude with an important comment:

..Building a model which can reproduce the first-order features of the Earth’s Ice Age history over the Plio-Pleistocene would be an important step forward in the understanding of the dynamic processes that drive global climate change.

In a later article we will look at the results of GCMs in starting and ending ice ages.

Summertime NH High Latitude Insolation

Roe (2006):

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. In this paper, a specific hypothesis is formulated. Basic physical arguments are used to show that, rather than focusing on the absolute global ice volume, it is much more informative to consider the time rate of change of global ice volume.

This simple and dynamically-logical change in perspective is used to show that the available records support a direct, zero-lag, antiphased relationship between the rate of change of global ice volume and summertime insolation in the northern high latitudes.

[Emphasis added]

And with very nice curve fits of his hypothesis.

Length of Southern Hemisphere Summer

Huybers & Denton (2008):

We conclude that the duration of Southern Hemisphere summer is more likely to control Antarctic climate than the intensity of Northern Hemisphere summer with which it (often misleadingly) covaries. In our view, near interhemispheric climate symmetry at the obliquity and precession timescales arises from a northern response to local summer intensity and a southern response to local summer duration.

And with very nice curve fits of their hypothesis.

Warming in Antarctic Changes Atmospheric CO2

Wolff et al (2009):

The change from a glacial to an interglacial climate is paced by variations in Earth’s orbit.

However, the detailed sequence of events that leads to a glacial termination remains controversial. It is particularly unclear whether the northern or southern hemisphere leads the termination. Here we present a hypothesis for the beginning and continuation of glacial terminations, which relies on the observation that the initial stages of terminations are indistinguishable from the warming stage of events in Antarctica known as Antarctic Isotopic Maxima, which occur frequently during glacial periods. Such warmings in Antarctica generally begin to reverse with the onset of a warm Dansgaard–Oeschger event in the northern hemisphere.

However, in the early stages of a termination, Antarctic warming is not followed by any abrupt warming in the north.

We propose that the lack of an Antarctic climate reversal enables southern warming and the associated atmospheric carbon dioxide rise to reach a point at which full deglaciation becomes inevitable. In our view, glacial terminations, in common with other warmings that do not lead to termination, are led from the southern hemisphere, but only specific conditions in the northern hemisphere enable the climate state to complete its shift to interglacial conditions.

[Emphasis added]

A Puzzle

In a paper on radiative forcing during glacial periods and attempts to calculate climate sensitivity, Köhler et al (2010) state:

Natural climate variations during the Pleistocene are still not fully understood. Neither do we know how much the Earth’s annual mean surface temperature changed in detail, nor which processes were responsible for how much of these temperature variations.

Another Perspective

Final comments from the always fascinating Carl Wunsch:

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..

..The appeal of explaining the glacial/interglacial cycles by way of the Milankovitch forcing is clear: it is a deterministic story..

..Evidence that Milankovitch forcing ‘‘controls’’ the records, in particular the 100 ka glacial/ interglacial, is very thin and somewhat implausible, given that most of the high frequency variability lies elsewhere. These results are not a proof of stochastic control of the Pleistocene glaciations, nor that deterministic elements are not in part a factor. But the stochastic behavior hypothesis should not be set aside arbitrarily—as it has at least as strong a foundation as does that of orbital control. There is a common view in the paleoclimate community that describing a system as ‘‘stochastic’’ is equivalent to ‘‘unexplainable’’.

Nothing could be further from the truth (e.g., Gardiner, 1985): stochastic processes have a rich physics and kinematics which can be described and understood, and even predicted.

Conclusion

This is not an exhaustive list of hypotheses because I have definitely missed some (Wunsch, in another paper, notes there are at least 30 theories).

It’s also possible I have misinterpreted the key point of at least one of the hypotheses above (apologies to any authors of papers if so). Attempting to understand the ice ages, and attempting to survey the ideas of climate science on the ice ages are both daunting tasks.

What should be clear from this small foray into the subject is that there is no “Milankovitch theory”.

There are many theories with a common premise – solar insolation changes via orbital changes “explain” the start and end of ice ages – but then each with a contradictory theory of how this change is effected.

Therefore, a maximum of one of these theories is correct.

And my current perspective – and an obvious one from reading over 50 papers on the causes of the ice ages – is the number of confusingly-named “Milankovitch theories” that are correct is zero.