What did temperatures do as the last glacial minimum ended 120K years ago? [A Few Things Ill Considered]
How would temperature data have been seen during the last 10,000 years prior to the peak of each of the previous Milankovich cycles? What caused the temperature to reverse course in those cycles and why would we not expect it to occur again this time?
First, here are the quick answers to those three questions, then some discussion. 1. It is not currently possible to resolve the temperature record that long ago to anything close to what we have today. 2. The cause of the temperature reversal is not well understood but probably due to a feedback of falling temperatures, growing ice sheets and falling CO2 levels initially triggered by Earth’s orbital variations (Milankovich cycles). 3. We do not expect this to occur again at this time because the orbital cycle is different and not due to exert a cooling effect for several 10′s of thousands of years and because CO2 levels are simply through the roof compared to anything in the past two or more million years.
So to expand a bit about the temperature records during past inter-glacials: it is true that temperature proxies that far in the past can not currently resolve to time scales finer than centuries. The current warming period has barely cleared one century. From a strictly data-centric, context-free, statistical viewpoint, a warming like ours, were it to reverse as quickly as it has come, could come and go in between the dots on our graphs of ice core analyses. However, many possibilities can be eliminated as implausible or very improbable through our understanding of the factors that control annualized and globalized average surface temperature. These data are, after all, emphatically NOT just abstract, context-free bits on a hard drive, they represent real physical properties with known behaviors and constraints.
One of the most compelling contextual factors comes from our understanding of the way temperature correlates with CO2 levels.
(image credit: http://en.wikipedia.org/wiki/File:Co2-temperature-plot.svg)
The magnitude and rapidity of the CO2 spike today is stunning and there is no natural mechanism, known or even speculative, that can reduce that level of atmospheric CO2 on anything less than millennial timescales. Such a pulse could not be invisible in the ice core records. So were we to postulate a warming as rapid as today’s it would imply a very drastic decoupling of the CO2 – temperature record and therefore some other mechanism of climate forcing. Now there are of course other climate forcings in nature, but the only truly dramatic ones cause cooling, things like violent volcanic eruptions and asteroid impacts. Non-greenhouse gas causes of warming are generally very slow: melting ice sheets, continental drift, orbital changes. It is theoretically possible that the sun brightened rather dramatically and then dimmed equally as dramatically in between our data points, but as well as seeming highly unlikely, that would not be very relevant to understanding what is going on today. We know what the sun is doing now, even if we don’t know what it was doing on short timescales 120K years ago.
The global climate may well be changing constantly on many timescales but there simply are no plausible mechanisms that fit the data or the current understanding of the earth-atmosphere system that could have produced an as yet undetectable spike in the temperature record analogous to today’s.
As for our understanding of what actually did happen at the end of previous inter-glacial periods, as I said above that understanding is not perfect. Modeling can explain it roughly as follows: small changes in orbital variations caused the growth of ice sheets in the northern hemisphere. This increased the albedo (reflectivity) of the earth, a further cooling influence. Dying forests became permafrost and carbon drawn from the atmosphere became sequestered under the growing ice. Cooling oceans also began to draw down CO2 levels in the atmosphere. Falling levels of this greenhouse gas contributed to even more cooling in a self-limiting feedback loop eventually seeing the average global temperature drop around 5 degrees Celsius. The precise timing of all these factors is somewhat murky. It is not a simple story, to be sure, and as such is an opportunity for denialists and contrarians to cherry pick and obfuscate their way to many standard climate “skeptic” talking points. You can refer to this article or perhaps this one as well. No one said life was simple!
Today, a process like this would not start for many thousands of years as the Milankovich orbital cycles are not yet aligned in a way to reduce insolation. Further, if we continue unabated on our current carbon trajectory we may well see the actual end of all permanent ice on the planet, a truly astonishing achievement. The CO2 we are pumping out today will be here for centuries, even thousands of years, which may be long enough to see the near complete melting of Greenland’s and Western Antarctica’s ice sheets. I doubt anyone can say with confidence what would happen to the East Antarctica ice sheet. If it did all melt, who knows when planet Earth would again see permanent land ice?
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