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.
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.
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
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..
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.
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.
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
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.
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.
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.
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.
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.
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 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
Hopefully in the order they appeared in the article:
The last glacial cycle: transient simulations with an AOGCM, Robin Smith & Jonathan Gregory, Climate Dynamics (2012)
Insolation and Glacials, George Kukla (1972)
The role of ocean-atmosphere reorganizations in glacial cycles, Wallace Broecker & George Denton, Quaternary Science Reviews (1990)
Last Interglacial and Early Glacial ENSO, George Kukla, Clement, Cane, Gavin & Zebiak (2002)
Last Interglacial Climates, George Kukla et al, Quaternary Research (2002)
The 41 kyr world: Milankovitch’s other unsolved mystery, Maureen Raymo & Kerim Nisancioglu, Paleoceanography (2003)
In defense of Milankovitch, Gerard Roe, Geophysical Research Letters (2006)
Antarctic temperature at orbital timescales controlled by local summer duration, Huybers & Denton, Nature Geoscience (2008)
Glacial terminations as southern warmings without northern control, E. W. Wolff, H. Fischer & R. Röthlisberger, Nature Geoscience (2009)
What caused Earth’s temperature variations during the last 800,000 years? Data-based evidence on radiative forcing and constraints on climate sensitivity, Peter Köhler, Bintanja, Fischer, Joos, Knutti, Lohmann, & Masson-Delmotte, Quaternary Science Reviews (2010)
Quantitative estimate of the Milankovitch-forced contribution to observed Quaternary climate change, Carl Wunsch, Quaternary Science Reviews (2004)