Thoughts on Long Term Climate
The graph shows global temperatures over the past quarter of a million years; the horizontal scale is in thousands of years, the vertical in degrees C. The source is Jouel et. al. 1996. Two things strike me about it:
1. We are in an unusually warm period, and have been for about the past ten thousand years. It has only gotten this warm once before over the period shown.
2. Judging at least by the previous peak about a hundred and twenty-five thousand years ago,and the smaller peaks between, this warm period has already lasted for an unusually long time.
From point 1 one might conclude that current worries about global warming are misplaced—on the long term evidence, we ought to be worrying about cooling. One might also include that arguments about anthropogenic warning are misplaced, since the current warm period started long before modern technological society with its large consumption of fossil fuel.
Point 2 suggests a somewhat different conclusion. The beginning of the long warm period we are still in roughly coincides with the invention of agriculture and the consequent large expansion in human population as we went from hunter/gathering to farming. One can imagine a variety of mechanisms by which that change might have affected climate, some described in a recent blog post at FuturePundit, one of my favorite blogs.