Summary: I know that I’ve implied that the change from the Pleistocene glacial period to the warmer Holocene was quite abrupt, about 10 to 12 thousand years ago. And it was, generally speaking, but it wasn’t a particularly smooth change. Dryas octopetala, photo by Jörg Hempel, used under Creative Commons license. Toward the end of the glacial time, as the continental ice sheets were melting back quite rapidly, various things happened to tweak the climate from one that was warming to one that was cooling again. Three of these cooling episodes are called the Dryas – Younger Dryas, Older Dryas, and Oldest Dryas. The Dryas is an Alpine and tundra-loving shrub of the rose family, the national flower of Iceland, which typifies these cool periods. The peak of glaciation, with glaciers as far south as the Ohio and Missouri Rivers in North America and covering the British Isles in Europe, was about 21,000 or 22,000 years ago. The warming and melting that began by about 20,000 years ago was interrupted by the Oldest Dryas interval, which lasted from about 18,000 to 14,700 years ago. It appears to mirror the overall trends of the ice ages – a gradual fall in temperatures to a low point, followed by a relatively abrupt warm up over a short time span. The temperature estimates for all these events are based largely on measurements of oxygen, nitrogen, and argon ratios, which are proportional to temperature, from gases trapped in ice in Greenland and Antarctica, but they are supported by other lines of evidence too. During each of the Dryas periods, much of Europe was tundra or taiga – Arctic conditions, but that does not mean lifeless. The taiga or boreal forest is one of the largest biomes on earth today, supporting vast forests and wide diversity of large animals, from caribou and yaks to bears and many birds. The treeless tundra is less biodiverse, but still not really barren. After a fairly short warming period, fewer than 1,000 years, the Older Dryas cooling took place for a short time, from about 14,100 to 13,900 years ago, only a couple centuries. Its expression is largely European, so the changes may not have been global in scope. The Younger Dryas is the best-known of these cool periods. It lasted from about 12,800 until about 11,570 years ago. It seems to have ended in a step-wise manner, in increments of 5 or 10 years over as short a period as 50 or so years. The end of the Younger Dryas is dated by various means quite accurately, to between 11,545 and 11,640 years ago, with 11,570 a common estimate. The Younger Dryas, like the earlier events, was felt most strongly in Europe, though there is evidence for it in the Pacific Northwest of the United States. Scandinavia and Finland were under ice sheets – still, or again. Britain was largely tundra or taiga, as was most of what is now the North Sea, which was dry land supporting an extensive flora and fauna. By now, you can probably guess at some of the speculated causes for the Younger Dryas. It’s been suggested that there was some impact at about 12,900 years ago that initiated the cool period, but I think that idea has been largely discredited. There was a decent-sized eruption of a volcano at Laacher See, near Koblenz in Germany, also at about 12,900 years ago. It was comparable to the eruption of Mt. Pinatubo in 1991, and while it may have had some effects, it’s pretty hard to see it as THE single cause of a 1,300-year cooling event. I think the most likely cause is some change in the fundamental heat engines of the Northern Hemisphere. The focus of this line of reasoning is the circulation of warm waters to the north in the Atlantic Ocean – specifically, the Gulf Stream and the more important deep-water exchange that keeps the North Atlantic warmer. This works because of the variable density of sea water at different temperatures, so it sets up a continuous cycle of circulation and exchange. For the Younger Dryas, the idea is that this circulation was shut down because of an influx of
