The Last 10,000 Years
AN ALMOST STABLE CLIMATE
The calm after the storm: this is the image we have from the climatic records of polar regions. In Antarctica the contrast between the hills and the valleys that followed each other during the last glacial period, then the deglaciation, and the stability of those same records since the beginning of the Holocene, a bit more than 10,000 years ago, is clear. And above all, there is nothing in common between the highs and the lows, rapid warming and slower cooling, which punctuated the temperature variations in Greenland between 100,000 years ago and the end of the Younger Dryas, 11,500 years ago, and the “flatness” of the same records since then. Climatically speaking, they are two different worlds.
Not entirely, however. Around 10,000 years ago we were already in an interglacial period—temperatures were even a bit warmer than they are today, at least in Antarctica, but our planet was not yet completely free of the jolts characteristic of colder periods. The reason for this is the slow pace at which the enormous glacial ice sheet that covered the northern part of North American disappeared. A good third of it still existed 10,000 years ago. Mild temperatures favored the melting of the ice, which sometimes led to the formation of “glacial” lakes imprisoned by ice barriers. This was the case 8,200 years ago in northeastern Canada.1 But the barrier that closed Lakes Agassiz and Ojibway gave way, freeing enormous quantities of freshwater, on the order of 200,000 km3, which poured into the North Atlantic in less than a hundred years. The consequences of a stop in the thermohaline circulation that followed were very widely felt in the Northern Hemisphere, in the Atlantic Ocean, in Western Europe, in Africa, and as far as the regions affected by monsoons. This is also seen in the decrease in the concentration of methane analyzed in the ice of Greenland in which the chronicle of that event is