A graph showing the history of the earth's climate over the past 450,000 years, based on three different records. Note how steep the slopes of the lines are at the start and end of ice ages. We do not understand how such rapid, dramatic changes take place.
Saturday, March 20, 2010
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It seems to my untrained eye that, for the period graphed, the trend is long ice ages punctuated at 75-130,000 year intervals by short periods of intense warming and then rapid cooling. Have I got this right, and if so what are the hypotheses about why this cycle occurs?
I note also that, if the last few these putative cycles are any guide, we are on a rising temperature curve that may well go higher before it rapidly plummets. In this case, we would be in the midst of a global warming, but one that is part of a long planetary cycle, and not our own doing. (Of course the pattern also shows we could be at the top of a warming cycle, and about to go over the hump and down.)
You read the graph correctly. And it is indeed true that the current warm period has never gotten as warm as the last one did at its peak. On the other hand, the current warm period has already lasted longer than the last one did.
One hypothesis is that the cycle is driven by variations in solar radiation, but there isn't much evidence for that.
Another is a sort of feedback loop involving plants and CO2 from volcanoes -- at the height of an ice age, there is less plant life to absorb CO2, so it builds up until the ice melts, then more plants grow and they soak up CO2 until the greenhouse effect is reduced so much that another ice age begins.
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