December 29. The Ends of the Ice Age

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 fresh water to the North Atlantic. This isn’t really unreasonable. The huge continental ice sheets of North America were melting, and the water had to go somewhere, but it’s a little more challenging to explain the abruptness of the changes. But we have a likely smoking gun. I talked about some of the glacial meltwater lakes in North America the other day – but I left out one of the largest – Glacial Lake Agassiz. Named for the eminent glacial geologist Louis Agassiz, this lake fronted the retreating ice sheet in what are now Manitoba, Saskatchewan, North Dakota, and Minnesota, with a possible connection to a similar glacier-margin lake that covered much of northern Ontario. The surface area was much larger than today’s Great Lakes.

Lake Agassiz map by Warren Upham, USGS Monograph 25, 1895 (public domain). The extent of the lake was actually larger than shown here.

While the continental ice sheet was still present to the north and east, Lake Agassiz drained to the south, through valleys now occupied by the Minnesota and upper Mississippi Rivers. When the ice melted enough, there could have been an emptying – either catastrophic or not, into Hudson Bay and thence into the North Atlantic. This is the influx of fresh water that is the most likely culprit in the shutdown of the North Atlantic circulation, and the cause of the Younger Dryas.

There’s one more cool period to mention – the Little Ice Age. It wasn’t really an ice age, but it was a distinctly cooler time, approximately 500 years, from 1350 A.D. until about 1850 A.D. There were several pulses of cold during this interval, well documented historically, including between about 1460 and 1550, from 1650 to 1715, and from 1770 until 1820 – and now I’m not using years ago, but the actual dates, A.D.

The likely causes are the usual suspects. One interesting one is changes in the sun’s output. Two of the coldest times coincide with periods when the sun had virtually no sunspot activity. The best known of those is the Maunder Minimum, from 1645 to 1715. Volcanic activity is of course another possibility. The eruption of Tambora, in Indonesia in 1815 famously caused the “Year Without a Summer” in 1816, and there were other major eruptions in the early 1800s, which could have impacted that cold period.

But it’s also possible that that slowdown of the North Atlantic circulation was a factor, if not THE factor. The Little Ice Age follows the Medieval Warm Period, which lasted from 950 to 1250 A.D. It was a time when the Vikings colonized Iceland and Greenland and northern Newfoundland; the latter two colonies were abandoned after the Little Ice Age began. The warm period could have caused more melting, but a more likely possibility is that it affected atmospheric circulation patterns, resulting in a persistent jet stream that might have kept Europe, especially, cooler, and might have had even global implications. For much more on this idea, and the Little Ice Age in general, I strongly recommend a book, The Little Ice Age: How Climate Made History, 1300-1850, by Brian Fagan (Basic Books, 2001)  

And for a good look at how we understand recent climate changes, I recommend The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future, by Richard Alley (Princeton University Press, 2002). 

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Before I close today I want to thank my friends, colleagues, and listeners for their suggestions, but in particular I thank geologists Patricia Dickerson in Texas, Stephen Henderson in Georgia, and Colleen Elliott right here in Butte, Montana, for their support and suggestions for topics in this series. Thanks!

—Richard I. Gibson

Younger Dryas causes 

More Younger Dryas causes 

Dryas octopetala, photo by Jörg Hempel, used under Creative Commons license.

Lake Agassiz map by Warren Upham, USGS Monograph 25, 1895 (public domain). The extent of the lake was actually larger than shown here.