Most people don’t realize that the Earth is still in a long term ice age that started about three million years ago and has had many alternating cold glacial periods interspersed with warmer interglacial periods. We are currently in an interglacial period where global temperatures have been near our modern “normal” for about 12,000 years now. In addition to our current interglacial period, there have been four previous interglacial periods in the last 500,000 years. Each one has been spaced about 100,000 years apart and lasted about 2,000 to 25,000 years with temperatures at or above our current modern “normal”. These observations are based on the EPICA Antarctic ice core climate reconstruction using oxygen isotope ratios as a proxie for global temperature change.
The graph below shows the current and last four interglacial periods plotted together, normalized to the year where the estimated global temperature first reached the level of our modern “normal” climate. The approximate year where each interglacial episode first reached the modern “normal” temperature is shown in the legend. Notice that all four previous interglacials had global temperatures reaching 2 to 4 degrees Centigrade higher than our current modern “normal” without any help from humans, based on this reconstruction.
I find it amazing how abruptly and similarly each glacial period ended in about 5,000 years to start each following interglacial warm period. In contrast, the duration of the interglacials has been much more variable. The most recent previous interglacial period that started about 130,000 years ago lasted about 14,000 years at temperatures at or above our current modern “normal”. The second previous interglacial lasted about as long as our current interglacial while the third previous was by far the shortest, lasting about 2,000 years, but arguably had somewhat of a double peak. However, the latter secondary peak did not quite reach the warmth of our current modern “normal”. The fourth previous interglacial which started about 418,000 years ago was by far the longest at about 25,000 years.
As repeatable as the glacial cycles have been over the last 500,000 years, I see little reason not to expect more of the same in the future. Using this interglacial comparison as a climate persistence forecast, we might expect about a 75% chance that the global average temperature will begin to drop dramatically sometime within the next few thousand years and about a 25% chance of staying warm for another 10,000 years or so … at most. Perhaps we need all the anthropogenic warming we can muster to stall or prevent the next glacial period?
Our understanding of what causes these glacial cycles, which are relatively recent on a geological scale, is still very limited although there are plenty of hypotheses. Our current climate models cannot predict them and therefore to me are somewhat useless. Until we can create climate models that can accurately track past glacial and interglacial periods I will not be too impressed and I certainly don’t believe our infant and untested climate models should be used to shape policy regarding “climate change”.
Update 2016 November
Below is a link to an interesting analysis of the causes of glacial cycles, along with conclusions made by the author, which seem reasonable to me. The author hypothesizes that evidence suggests that the current interglacial period is likely to be only average in length and therefore should be ending soon, most likely sometime within the next two thousand years.
Nature Unbound I: The Glacial Cycle
1) Obliquity is the main factor driving the glacial-interglacial cycle. Precession, eccentricity and 65°N summer insolation play a secondary role. There is no 100 kyr cycle. Milankovitch Theory is incorrect.
2) The current pacing of interglacial periods is the consequence of the Earth being in a very cold state that prevents almost half of obliquity cycles from successfully emerging from glacial conditions. The rate for the past million years has been 72.7 kyr/interglacial, or 1.8 obliquity cycles between interglacials. This can be generally described as one interglacial every two obliquity cycles except when close to the 413 kyr eccentricity peaks, when interglacials take place at every obliquity cycle.
3) Glacial terminations require, in addition to rising obliquity, the existence of very strong feedback factors manifested as very low glacial maximum temperatures. High northern summer insolation at the second half of the rising obliquity period is a positive factor, and if high enough during eccentricity peaks can drive the termination process.
4) CO2 can only produce a minor effect in glacial terminations since the measured change in concentration (roughly a third of a doubling which represents half of the warming effect of a doubling) is too small to account for any important contribution to the large observed temperature changes.
5) Since the precession cycle has bottomed and the obliquity cycle is half way down we should expect the next glacial inception to take place within the next two millennia.