A few days back I linked to a report that pushed back recorded climate history to some 650,000 years ago. In a nutshell, today’s greenhouses are completely off the charts as far as history goes.
I’ve posted one figure from the papers* that I think captures everything that I was trying to say in a fairly easy-to-digest format. It’s a big file, so click through the extended text to view it.
The time scale runs horizontally from the left to right, from zero thousand years ago (today) on the left to 650 thousand years ago on the right. The colored traces represent different measurements that the scientists made continuously down the length of the ice core. The purple line represents atmospheric temperature, which you estimate using oxygen isotopes. Working your way up, the next three lines represent different greenhouse gases, with nitrous oxide in navy, carbon dioxide in red and methane in light blue. If you start at 650,000 years ago and work left you can see a fairly regular rise and fall in each gas that correlates more or less with a rise and fall in the estimated atmospheric temperature. That’s the world coming in and out of ice ages. Now look at what happens when each greenhouse gas reaches the far left, corresponding to today. Cripes.
You can see another important point on this graph. Human civilization started anywhere between ten and fourteen thousand years ago, by which I mean centrally-governed populations with borders and urban centers; loose tribes with fire in caves go back much farther than that. If you look at the last few millimeters on the left side of the temperature record you can see that for ten thousand years we’ve had a surprisingly warm and stable climate. As far as civilization is concerned I don’t think that’s an accident.
Based in part on some ideas that Jared Diamond brought up in his Pulitzer Prize-winning book Guns, Germs and Steel, I think that we owe a good bit of our global success to that climate stability. If that’s the case then we won’t do civilization any favors if we manage to bring our current island of stability to an end. Looking at those crazy spikes at the far end of the greenhouse gas traces it’s hard to imagine how we won’t.
(*) Science vol. 310: pages 1313 and 1317