Guest Post by Willis Eschenbach
Inspired by a random comment by Steve McIntyre over at his marvelous blog Climate Audit, I got to thinking about the ice ages. I’ve long heard that the ice ages are caused by the changes in summer insolation in the northern hemisphere. As the story goes, the Milankovitch cycles of variations in the earths orbit make it so that there is a variation in how strong the summer sun is in the northern hemisphere. When the summer sun is weaker, the ice sheets advance, and eventually the buildup of ice reflects enough solar energy to spiral us into the icebox. Then about every hundred thousand years, the sun gets stronger again, and melts away the ice, and within a few thousand years the great ice sheets melt away and we’re out of the icebox.
So of course, once I’ve had that thought, I was doomed, and so I had to take a look. I got the data, and here is the variation in average northern hemisphere insolation for the months of June, July, and August.
Figure 1. Average insolation during the summer months (J-J-A) at 40° north latitude. DATA SOURCE: NOAA
Now, I found that surprising. I hadn’t realized the size of the swings. The cycles are about 21,000 years long and the swings are quite large, up to 100 W/m2 from trough to peak. So IF the temperature is following the forcing as the current hypothesis claims, a swing of 100 W/m2 is certainly large enough to cause a very large swing in temperatures. The current hypothesis is that at equilibrium we should see a swing of ~3°C for each additional 3.7W/m2 of forcing. However, we’re talking annual swings. Transient climate sensitivity is about 70% of equilibrium sensitivity, so I’ll use 50% to give some cushion. So according to the current thinking, a swing of an additional 100 W/m2 which is maintained for a thousand years should result in an increased annual temperature swing of about 40°C (73°F) … and we don’t see anything in the geological records even half that size.
I also didn’t realize that there is an underlying ~400,000 year cycle, which leads to the larger peaks at about 200,000 and 600,000 years before present (BP), and also leads to the very, very small peak at about 400,000 years BP.
But obviously we don’t see such a swing in geological temperatures. In fact, we don’t see anything even near that. So, scratching my head, I went and got the longest temperature record we have. This is the record from the ice cores at the EPICA dome in Antarctica. Figure 2 shows that record:
Figure 2. Antarctic temperature variations estimated from deuterium data. DATA SOURCE: NOAA
Here, we can see the ~ 100,000 year cyclical nature of the emergence from the ice ages. The swing is generally on the order of about 12°C, and the usual estimate is that because the poles swing more than the tropics, the global swing is half the Antarctic swing, or about 6°C. We can also see that the current interglacial period, the “Holocene”, has lasted quite a while compared to the other interglacials.
Note also the very large and roughly symmetrical peak at about 400,000 years.
So … how does this relate to the Milankovitch cycles? Figure 3 shows the temperature overlaid over the Milankovitch cycles.
Figure 3. Antarctic temperature variations estimated from deuterium data, overlaid on the Milankovitch insolation cycles.
I gotta say I’m just not seeing it. The biggest oddity is that around 400,000 years, the very small insolation peak is correlated with a very large temperature peak. In addition, in general there seems to be very little correlation between the swings in insolation and the swings in temperature. Finally, the most interesting thing is the total lack of any 21,000 year cycle in the temperature.
Now, some authorities say that the crucial factor is not the insolation at 40°N, but the insolation at 60°N. So I checked that … but the difference in the pattern is only trivial. It mainly just affects the size of the swings, which are somewhat smaller at 60°N, but the pattern of large and small swings is essentially unchanged.
Now as might be imagined, I’m not the first one to be puzzled by this. It’s widespread enough that there’s a Wikipedia page entitled “The 100,000-year problem”, which points out that:
The 100,000 year problem is a discrepancy between past temperatures and the amount of incoming solar radiation, or insolation. The latter rises and falls according to the strength of radiation given off by the sun, the distance from the earth to the sun, and the tilt of the Earth’s axis of rotation. However, the recent change between glacial and inter-glacial states that occurs on a circa 100,000 year (100 ka) timescale, does not correlate well with these factors.
Due to variations in the Earth’s orbit, the amount of insolation varies with periods of around 21,000, 40,000, 100,000, and 400,000 years. Variations in the amount of incident solar energy drive changes in the climate of the Earth, and are recognised as a key factor in the timing of initiation and termination of glaciations. Isotope analysis shows the dominant periodicity of the climate response to be around 100,000 years, but the orbital forcing at this period is small.
However, my perplexity seems to be for a different reason than the other folks discussing this, which is that the really large insolation swings occur on a 21,000 year cycle, and there’s no trace of that in the EPICA data. I’m not so much interested in the existence of the 100,000-year cycles in the temperatures, as I am by the lack of any temperature response to the ~100 W/m2 swing in the insolation. Yes, I know that overall for the globe as a whole the swing is small because the hemispheric changes oppose each other, but for each hemisphere the changes are very large. Why do we see no trace of those very large swings?
Anyhow, all comments welcome.
Best wishes to all. It’s one AM, there was a new moon earlier tonight, I’m going outside for some stargazing, and I wish the same level of joy and awe to all of you.
w.
As is my custom, I ask that if you disagree with someone, please QUOTE THE EXACT WORDS YOU DISAGREE WITH so that we can all understand the exact nature of your objection.
Most likely it is not just ONE thing triggering initiation/termination of Glaciation, but a multiple of forces all hitting a certain beat at the same time. This happens quite often in other areas of nature.
Possibly, but it is multiple very slow, very persistent forces in that case. The current ice age has had a gradual onset and has persisted for three million years with brief interglacials punctuating the ice, with the temperature lowering and persistence increasing of the glaciation intervals very smoothly throughout.
That smooth variation is actually remarkably difficult to explain, and yet it is obviously key to understanding the entire phenomenon. The system is clearly bistable, which means that underneath all of this there is a fairly well-known and reasonably well-understood system of differential equations that have an implicit description of the local equilibrium in both the warm and cold phase. A smooth variation in the stable cold phase to colder and colder temperatures is very, very difficult to explain. The closest that I can come to a hypothesis is that it has something to do with the cooling efficiency of the oceans that slowly, slowly alters with continental drift in a very systematic way, but there are so MANY alternative hypotheses — gradual alteration of atmospheric chemistry, new physics (dark matter, galactic cosmic ray background variation, very long period modulation of the sun’s output as it moves through galactic dust clouds, space aliens, God’s Will, pink unicorns, or just plain nonlinear dynamics with a life of their own).
I’m not even sure we CAN find sufficient evidence to accept or reject any of these hypotheses.
When I was young, I absolutely loved dinosaurs. I read every book I could get my hands on on paleontology by the time I was in the third grade, and not just children’s books, either. Ditto astronomy. I am struck by just how much of what was taught as “settled science” back in the 1960’s turned out to be complete bullshit, and how many of the alternative explanations that were taught as settled science in between turned out not to be as solidly supported as people might have liked to believe at the time since. The closing of Panama is just one in a long line of things that might — or might not — have happened. It isn’t easy to make completely defensible inferences from what the Earth leaves us after thousands, millions, and tens or hundreds of millions of years, because while some parts of the surface are comparatively stable, other parts are constantly churning and quite new, with things being cast up or pressed down. The Earth is not really “solid”; it just flows on geological time. It is so very easy to see some evidence here or there and overextend it to a complete theory of all sorts of stuff, only to find that the evidence one was basing the whole story on was incomplete or incorrect.
rgb
Re. Painter’s Panama: the critical evidence is paleontological: edentates evolved in isolation in SA until a few million years ago when NA animals moved in. There’s really no question about this. –AGF
agfosterjr,
I am aware of the paleontological side of the question. Where was the sea barrier? The isthmus of Nicaragua or the Isthmus of Tehuantepec? Or it could have been the Isthmus of Panama, with the most likely place at its juncture with S. America, the Atrato Rivet system. In any case, there would have been only shallow depths and no deep seaway.
There has been a heap of conjecture concerning the ice age piled on top of Paleontological evidence and it now falls under the weight of this study.
No one seems to have mentioned the possibility of the sun being powered by variable strength Birkland currents coming out of the center. .They would explain changes in sun and changes of earth’s warming into ice ages and back to variable warming.
That’s because (if I understand what you are saying) there isn’t any possibility of this being correct. The sun is “powered” by thermonuclear fusion taking place in its core. If one looks out at the night sky, one can see that this fusion takes place remarkably consistently and powers similar sized objects in similar ways and at similar temperatures. Birkeland currents are simply charged particles flowing along the magnetic field lines coupling the earth’s magnetic field to the Sun’s; back at the sun they are part of its already enormously complex magnetic field. The variations in this field (responsible for things like sunspots and coronal mass ejections) do modulate the surface state of the sun and do have various effects on the Earth (modulation of atmospheric neutron counts etc) but so far nobody has constructed a truly convincing model for how solar activity can modulate Earth temperatures. The biggest problem is that the effects themselves seem far too small to create the large responses attributed to them, followed by the substantial problem that there is no clean linear relationship between things like sunspot activity and global temperature, so one needs at the very least a complex multivariate model that is both more difficult to construct and to justify or verify in order to explain the times the sun is active but it isn’t warming and vice versa, even over only the last hundred-plus years where we have moderately good records.
rgb
Willis, you need to check the astronomical specialist lierature, which proves the MC cycle effects:
Or falsify the literature: paper from. MASSON-DELMOTTE, V. et el: GRIP Deuderium Excess
Reveals Rapid and Orbital-Scale Changes in Greenland Moisture Origin, Science, vol 309 (2005),
no. 5731, pp.118-121…..here: especially figure 1: Obliquity fluctuations.
I absolutely see a correlation. I don’t have the data, and I don’t have the time, but just looking at the overlapping graphs, you can see that, for the last 500k yrs, they trend down and up together. The only variance from the trend is around 220k yrs ago where the temp trend starts decending while the isolation upticks once more. But this also correlates with the largest spike during a temperature decent.
Willis, I can confirm Lubos Motl’s comment that Lindzen has looked closely at Gerard Roe’s analysis and is completely convinced by it. Lubos post http://motls.blogspot.ca/2010/07/in-defense-of-milankovitch-by-gerard.html was mentioned above, but I don’t think that you paid enough attention to it.
One possible difference in approach is whether one regards d18O values as a proxy for “ice volume” or “temperature”. I haven’t parsed Roe’s article, but the diagram shown in Lubos’ article is very convincing.
http://lh4.ggpht.com/_4ruQ7t4zrFA/TDL7RSCEgZI/AAAAAAAAEGE/0HeA3XYGVmM/milankovitch-roe-fig2.JPG?imgmax=400
From my own reflections on the data, early Holocene NH summer insolation was very elevated and its impact on the ice sheet was enormous. The proxy evidence for the Holocene is accumulating rapidly and in my opinion was underreported in AR5, especially given the overemphasis on Mannian reconstructions.
Steve McIntyre January 23, 2015 at 11:45 am
I didn’t comment at all on Lubos’s post because I didn’t see the link … however, I did discuss Science of Doom’s two-post critique of Roe’s hypothesis, which is here and here. I’d be interested in your comment on his take. He says, for example:
I note that Lubos did NOT actually look at the data himself, he just commented on the Roe paper … whereas SoD did look not only at the data that Roe used, but at the latest update of that data.
And looking at SoD’s analysis, I can’t find any fault in it … but then your eye is keener than mine, so I’m much interested in your take on it.
Best regards,
w.
Re albedo: Graeme Stephens, an accomplished climate scientist, has pointed out that SH and NH albedos are very similar despite very different land-sea arrangements.
Which hasn’t stopped one single person from telling me that the explanation for counterphase correlation between the annual insolation peak and global average temperature is due to variations in NH and SH albedo, but hey, what are facts in the face of a good argument?
The annual variation is roughly 91 Watts/m^2, too, which is absolutely enormous compared to the forcing attributed to CO_2.
Personally, I think that this is moderately incontrovertible evidence that simple linearized models for “sun heats earth” are naive in the extreme — they might capture the average in some sort of imaginary greybody limit, but the actual global climate dynamics can overcome stupendous differences in insolation and move the global average temperature the wrong way compared to the actual incident energy flow.
People comment on how much the earth is supposed to heat due to the tiny, tiny imbalance associated with increased CO_2 while failing to remark upon or adequately explain an actual annual heat imbalance almost two orders of magnitude greater.
Sigh.
rgb
But the summer is shorter which cancels out the extra insolation.
??? SH albedo is highly variable as far as sea ice goes; stable over the long term. NH albedo is variable over the long term. There is no Pleistocene hemispherical albedo symmetry. –AGF
Milankovitch Cycles are only good for the general climate trends but fail for all the abrupt violent short lived climatic changes.
In addition to the issues raised by Willis, the Milankovitch theory is fraught with a lot of other problems, (too many to fully discuss here). But, here are a few. First, let’s look at the basis for the theory—how good is the correlation with glaciations and interglaciations and how good is the dating? There are three cycles: (1) change in the tilt of the Earth’s spin axis-41,000 year cycle, (2) precession of the equinox-21,000 year cycle, and (3) eccentricity of Earth’s orbit-96,000 year cycle. All affect the distribution of solar energy per unit area reaching the Earth. Since each of the cycles has a different length, the sum of all three will sometimes be additive and at other times will cancel each other out. So at any given time, the sum of the three cycles will be different than at other times. To make things even more complicated, different weights can be assigned to each cycle to maximize the overall effect (e.g., you could say that the 41,000 year cycle is twice as important as one of the others and this will give you a different glacial/interglacial cycle.
Another issue is how good is the dating of the glacial/interglacial cycles? The Antarctic oxygen isotope curve is not well dated beyond the range of 14C dating (~40,000 years). So dating the ice hundreds of thousands of years is at best a very rough estimate. So where did the ages of glaciations come from? The ages of astronomical cycles can be calculated, but how can they be correlated with glaciations/interglaciations? This was done by identifying glacial cycles in deep sea cores using oxygen isotope analyses of microfossils and deriving a glacial/interglacial curve. But there is no good way to date the deep sea cores beyond the range of 14C dating. So what Imbrie and others did was to stretch and compress the astronomic and deep-sea curves until they got the fit they wanted. Then, knowing the chronology of the astronomical cycles, they applied this to the deep-sea cores they had just correlated to get a glacial/interglacial chronology. Then they concluded that the correlation of ages of the glacial/interglacial cycles to the astronomical cycles proved that Milankovitch cycles caused glaciations!
Another serious problem came to light with the discovery of Dansgaard-Oeschger sudden climatic reversals during the last glaciation. These were so abrupt that they could not possibility be caused by Milankovitch changes and since the magnitude of the Younger Dryas changes were from full non-glacial to full glacial temperatures and back to full non-glacial temperatures, it is clear that something other than Milannkovitch cycles can cause Pleistocene glaciations. To further complicate the issue, the Southern Hemisphere should be out of phase with the Northern Hemisphere. Strong evidence exists that glaciations in the Northern and Southern hemispheres were not only synchronous but, at least for the last glaciation, almost exactly simultaneous. Although a number of possible explanations have been put forth to explain this vexing anomaly, so far none has proven satisfactory, and, as Mercer (1984) put it, the globally synchronous climatic change “remains the fly in the ointment of the Milankovitch theory: the last interglacial and the last glaciation both appear to have affected the Northern and Southern hemispheres simultaneously, and with comparable severity.” This is also true of the Dansgaard-Oeschger reversals, which are almost exactly simultaneous in both hemispheres.
These are only a few of the issues related to the Milankovitch theory of glaciations. The bottom line that there is a lot of negative evidence related to Milankovitch cycles as the cause of ice ages and no positive proof.
Time will tell
” no positive proof”
###
Or rather, positively no proof.
All well said, and I agree. There is too much unexplained, the explanations that seem to “work” are often rather forced, and it is absolutely clear that it isn’t a predictive theory because any rule one abstracts abruptly changes or fails without warning. It also doesn’t explain, as I mentioned above, the gradual deepening of Pliestocene glaciation and temperatures, or the transition from roughly periodic/symmetric glacial/interglacial episodes to a highly asymmetric 90 ky of glaciation followed by 10 ky of interglacial that is observed over the last million years or so. At this point it wouldn’t (seem to) take much to erase the 10 ky interglacials altogether and remain in cold phase for a million years or three until SOMETHING we do not understand changes. Or to go the other way.
With luck nothing will happen abruptly in the near future either direction, but from what we can infer glacial melting or refreezing can happen remarkably abruptly on a geological scale, with rather enormous variations over as little as a century. And no, I don’t think we can explain any of this in a credible, evidence supported way, however pretty the epicyclic theories are that supposedly are the root causes.
rgb
Milankovitch Cycles have to evaluated against the back drop I present below.
In addition the initial state of the earth’s climate( land arrangements ,elevation, magnetic field ,ice dynamic, random terrestrial events) , internal variation within the sun itself have to be taken into the account, along with the random, chaotic ,non linearity aspects of the climatic system of the earth and the randomness of outer space (say within 20 light years of earth) which is why x+x does not equal an x outcome when a given item changes in a given way that has an effect on the climatic system of the earth.
The one item cause giving an X climate result is not going to work
Willis, it may not come to you as a surprise but as these data were available ten years ago, those observations have been made by others too, especially the missing 400Ky Milankovitch spike but other than that it’s obvious that the frequency spectrums are rather different on a large scale, although occasionally also surprizingly accurate on a smaller scale, for instance for the last 90ky. It’s obvious that the 100ky cycle is not from Milankovitch, hence one of the very few things that Spencer Weart happened to get right is:
“As one reviewer said in 2002, “The sheer number of explanations for the 100,000-year cycle… seems to have dulled the scientific community into a semipermanent state of wariness about accepting any particular explanation.”
source (but not peer reviewed!): http://www.aip.org/history/climate/cycles.htm
But I think one should be focussing on the oceanic 100ky cycle for the last million years (not before), which is likely closer to the cause of the ice core 100ky cycles.
Willis,
Global temperature will respond to changes in global forcing. The differences in insolation at different points on the globe do not cause the same level of differences in temperature. Global changes caused by the Milankovitch cycles are quite small – the below site suggests that the changes globally are between 340 at minimum and 343 at maximum.
http://www.climatedata.info/Forcing/Forcing/milankovitchcycles.html
David
Willis,
I particularly draw your attention to this graph:
http://www.climatedata.info/Forcing/Forcing/milankovitchcycles_files/BIGw01-milankovitch—cycles.gif.gif
Notice that insolation changes in July at 65 North are large. Insolation changes at 65 North for the entire year are smaller. Insolation changes over the entire year over the entire earth are smaller still.
David
If Gerard Roe’s work was so good, then why hasn’t it been projected forward in time. Perhaps Roe wasn’t sufficiently convinced by his own work. Without a projection then it is essentialy useless.
And I apologise: this is what I was responding to in your post:
“Now, I found that surprising. I hadn’t realized the size of the swings. The cycles are about 21,000 years long and the swings are quite large, up to 100 W/m2 from trough to peak. So IF the temperature is following the forcing as the current hypothesis claims, a swing of 100 W/m2 is certainly large enough to cause a very large swing in temperatures. The current hypothesis is that at equilibrium we should see a swing of ~3°C for each additional 3.7W/m2 of forcing. However, we’re talking annual swings. Transient climate sensitivity is about 70% of equilibrium sensitivity, so I’ll use 50% to give some cushion. So according to the current thinking, a swing of an additional 100 W/m2 which is maintained for a thousand years should result in an increased annual temperature swing of about 40°C (73°F) … and we don’t see anything in the geological records even half that size.”
Thanks, Willis and respondents for a fascinating post & discussion…..however, while a simple answer seems unlikely and a unified consensus (here anyway!) seems even less likely….. is, dare I say it, variable solar output the (long-suffering) elephant in the room?
You can fit the Milankovitch Cycles to the Ice core temp quite attractively, though you would lose a lot of money betting on it for extended periods:
http://www.robles-thome.talktalk.net/Milank1.pdf
A key point not often noted is that the variation in eccentricity actually changes the total annual insolation globally, not just its distribution.
R.
Thanks for showing us what professional climate workers who are paid to do this don’t. That mismatch between EPICA and insolation history is puzzling and they should dig deeper into it.
The Milankovitch insolation driver argument assumes that the sun puts out a constant amount of energy. Perhaps that assumption is wrong. Perhaps the sun itself has cycles to be considered.
Also, I wonder about the overall frequency domain characteristics of the ocean-atmospheric oscillator network. We barely knew what ENSO was 50 years ago and only recently started to comprehend PDO, AMO, etc. I would wager real money that such oscillations are not the only ones. Ones with much longer periods are highly likely. The “Ice Age” – interglacial cadence may simply be a long period oscillation of this network.
This graph at Wiki, a colored adaptation from the original Vostok report, doesn’t leave much room for argument over correlation: http://en.wikipedia.org/wiki/Milankovitch_cycles#mediaviewer/File:Vostok_420ky_4curves_insolation.jpg
Comically that report called it evidence of GHG’s running the show. The multi-thousand year lag between insolation and T is of course explained by the albedo lag–it takes thousands of years for the ice to accumulate or melt. When averaging insolation it’s important to specify the period. The most important figure is the roughly 80W/m^2 insolation difference in June at the edge of the ice–on the ground, at noon, on a clear day. All the ice has to do is melt faster than it snowed. And the T difference between a mile high ice field and a pretty lake 10ky later is what? 50K? The cores measure albedo. –AGF
Hi Willis,
Part of the explanation for a lack of obvious effect due to the 20,000 year cycle in high latitude insolation is that while the insolation does change as you note, the length of the summer period moves in the opposite direction. So while it is true that the southern summer currently has about 7% higher maximum solar intensity, the length of that higher intensity (the length of the southern summer) is considerably less than the northern summer… which has lower solar intensity. (Kepler’s laws of planetary motion.)
I suspect the key WRT ice ages is the total solar energy received during the warm half of the year…. which doesn’t change so much with the 20,000 year precession cycle.. Of course, that doesn’t answer why there are in fact ice age cycles. My guess is that ice ages are paced by crustal depression during glacial accumulation, which very gradually lowers (and so warms) the ice sheet melt front. Once the ice front starts to melt, ever more depressed land is exposed, and the melt tends to run away. When the ice sheets are gone, the slow continuing rebound of that now exposed land leads to a gradually cooler local climate and eventually the slow formation of glaciers/ice sheets again. The increase in albedo from ice/snow accumulation likely contributes to ‘locking in’ the ice age for a long time; the crustal depression has to become pretty extreme before the system becomes “unstable” and reverts pretty quickly to an interglacial.
.
The interesting thing is that the magnitude of the crustal rebound (up to 400-500 meters) when combined with the lapse rate (6C to 10C per Km, depending on local humidity) seems enough to ‘trigger’ season-to-season snow/ice accumulation at high latitudes.
(BTW, I am pretty sure there have been published papers, including some recent ones, which suggest an important contribution to glacial cycles from crustal depression/rebound.)
One piece of the puzzle that I have not seen anywhere and perhaps we do not have is the variation in solar output over these longer time scales. It is not likely to be very large percentage-wise, but is likely to be yet another confounding factor in trying to interpret the causes of climate change.
Most are speaking of what causes the ice ages. Isn’t that backwards? The natural state of the Earth is what we call the ice age. The question is what causes the interglacials. Increased insolation at 65 N will melt the great NH ice sheets but only when NH summer is at perihelion and eccentricity is low. Those events happen to come together about every 95-100K years. Since we know that to be true, and we know that interglacials happen about every 100K years at that time, is this a case of it’s right in front of our noses or a case of correlation is not causation?
Forgot to mention obliquity with also has to be at maximum. So max obliquity, low eccentricity, NH summer solstice at perihelion. All three must come together, not just two. Happens roughly ever 100K years. Lasts until obliquity lowers to 23 degrees, summer solstice moves away from perihelion and eccentricity rises.
TIF – Actually if you review geologic time up to the Pleistocene, you will find that the natural state of the Earth is not “ice age” but mostly “hot house”. It is true that we live in an interglacial period (past 10,000 yrs) and that the climate state for most of the past 2 million years has been mostly glacial/ice age, but the earth is 4.5+ b years old and most of its time, based on the rock record, it appears to have been ice free (or at least global ice free. Alpine glaciation excepted).
That’s true. I was referring to only the last 2-3 million years because that’s the period which has the interglacials we are talking about.
Many geologists theorize that the earth was covered completely by a shallow ocean about 2.5 billion years ago. No continents. Over geologic time periods plate tectonics are thought to have played a decisive roll in climate, resultant geothermal activity, continental uplift, ocean plate colapse, continental migration, all playing rolls in climate along with solar output changes and orbital drift. Lots of variables.
Willis,
I have not read all the comments so I don’t know if this is covered. In the main post you compare Northern Hemisphere insolation with Antarctic temperature. But you should recall that the pattern of Southern Hemisphere insolation is the inverse of that in the Northern Hemisphere. One can expect that Southern Hemisphere insolation will have some local impact on Southern Hemisphere temperature, including the interior of Antarctica. Consequently while the Northern Hemisphere ice-age driver dominates the accumulation and ablation of ice in North America and Europe, it is not the only significant factor for Southern Hemisphere temperature. One should not expect a perfect match between Northern Hemisphere insolation (and Ice volume) and Antaractic temperature. A reasonable match is the best that one should expect.
If you look at the smaller swings in polar temperature recorded over the last 100,000 years, more particularly those swings that last just a few thousands of years, there is in fact an anti-phase relationship between ice core records from Greenland and Antarctica.
Willis,
What Don Easterbrook say makes a fair bit of sense. The riddle of the cause of the ice ages has not been fully solved by anybody. The full answer is still a work in progress.
I think that orbital cycles constrain maximum temperatures in the summer and minimum temperatures in the winter so that if we went back to the holocene climate optimum maximum summer temperatures would be significantly higher than they are now.The maximum summer temperature has not risen significantly in the UK in the last hundred years while the average temperature,which climate scientists focus on,has risen significantly.