The Fate of Greenland: Lessons from Abrupt Climate Change

Rapid global temperature shifts are nothing new, as Steve Yearley learns from a cold country's core

June 2, 2011

Credit: Gary Comer
Ice spy: Greenland's frozen past reveals that 'big, widespread, abrupt climate changes have happened repeatedly'

Pub-quiz enthusiasts may well know what I did not: that the late Gary Comer, the seriously wealthy founder of the Lands' End clothing company, was also an excellent photographer and discerning funder of climate-change research, especially in Greenland. This very handsome book presents almost 80 photographs of Greenland's glaciers and mountains - with many of the best shots taken by Comer himself - along with detailed, well-written accounts of the scientific insights associated with the field trips that Comer supported. It is both a memorial to Comer and a long, thoughtful essay on the character and causes of sudden climate change.

For the past two and a half million years or so, the Earth has been in an ice age - a period in which there are permanent ice sheets and glaciers. In this ice age, the Earth has mostly been colder than it is now. But the climate during ice ages is not uniformly cold; ice sheets grow and shrink in cycles over tens of thousands of years, and we are currently in a warm episode known as an interglacial.

Greenland is interesting to students of climate change because it has hung on to the ice sheet that covered it during the last glacial episode, which ran from about 100,000 years ago to roughly 10,000 years ago. Whereas Europe, Asia and North America have all lost the vast sheets of ice that used to sprawl south from the Arctic and from mountain regions, Greenland still gets enough snow to replenish the deep ice, up to 3km thick, that covers 90 per cent of the island. Snow builds up annually on Greenland's interior, and at the bottom of this huge pile of ice the compressed snow is very old: it fell well before the start of the last big glaciation.

For this reason, the ice on Greenland provides a fossil record of the snow that has been deposited over the millennia. Because summers are warmer than winters even in a glacial period, the ice plainly records discrete annual accumulations of snow. Taking an ice core from the Greenland ice sheet thus allows one to count back individual years deep into the last glacial period when Britain, northern Europe and chunks of the US were buried under ice, and to analyse the dust and microscopic air bubbles trapped in the ice to assess the state of the atmosphere at the time the snow fell. Greenland is the go-to place for the history of climate in the northern hemisphere.

But it is also the source of a big, wet climate worry. Supposing Greenland followed the example of Canada and let its ice sheet melt or slide off into the ocean: this would result in a worldwide sea-level rise of about 7m. That would be a permanent worldwide inundation to the same level as the surge that overtook New Orleans in 2005.

The emphasis in this book is on what scientists supported by Comer - notably the three last-named authors - have learned about the history and regulation of climate from their studies around Greenland. One key focus is on the way that the climate shifts from the cold, glacial periods within an ice age (which are, so to speak, the default state) to the relatively warm interglacial episodes. One might assume that the transition is gradual and incremental, almost imperceptible. But what the ice cores and other geological data indicate is that the transition is bumpy. Cold periods are interspersed with little bursts of comparative warmth, and warm periods have their own short freezes, such as the so-called Little Ice Age that seems to have driven the Norse settlers out of Greenland in the 14th century.

Even bigger swings from generally cold to generally warm periods do not progress smoothly. The ending of the last glacial period saw warming followed by a sudden cold (known as the Younger Dryas) in which the glaciers advanced down the valleys again, only for the warming to recommence. These sudden swings matter, both because they do not fit well with the idea that warming and cooling are driven exclusively by gradual changes in the Earth's proximity and orientation to the Sun and because they convey the underlying threat that rapid climate change could happen to us - that a big chunk of Greenland's ice could suddenly melt.

The lessons of the Greenland ice cores are very clear, say the authors: "big, widespread, abrupt climate changes have happened repeatedly", even before humans started pumping out lots of greenhouse gases.

The extraordinary thing revealed by research on Greenland is the rapidity with which these changes have happened in the relatively recent past: "A couple of dozen times over the last 100,000 years, a sudden warming occurred over Greenland, often 10 deg C or even more, in roughly a decade or less."

The book charts how the Comer-funded researchers' work helped to make sense of this phenomenon. In eastern Greenland they found a site where they could take good ice cores (from which one can estimate the average annual temperature) and study a series of deposits - moraines - dumped as the glaciers first retreated, then advanced and withdrew again around the time of the Younger Dryas (data from moraines allow one to infer summer temperature trends). What emerges is that the sudden climate switches were predominantly marked by alterations in winter temperatures; from winter lows of around -20 deg C the climate suddenly became Siberia-like (losing another 20 deg C), before abruptly switching back.

Further analyses allowed the team to put forward a mechanism for this. Wallace Broecker is famed for his work on large-scale ocean currents, specifically the Great Ocean Conveyor that brings warm water near the surface from the Indian and Pacific oceans into the Atlantic and returns cold water along the ocean depths. This conveyor brings a stupendous amount of heat to the North Atlantic area, equivalent to about a third of the warmth that the Sun supplies; the current turns over close to Greenland.

The idea is that a rapid and enormous release of cold freshwater, as part of the Canadian ice sheet disintegrated and formed icebergs, would have disrupted this conveyor and provided a pool of water around Greenland that was susceptible to freezing in winter. Winter sea ice would have also reflected more sunlight, thus further driving down the winter temperatures. Such a series of events could bring sudden and dramatic cooling that could persist for decades or more, before being suddenly overturned as the conveyor reasserted itself.

Although the message is that genuinely large and abrupt climate changes have occurred in recent geological time and for reasons that we can begin to comprehend, there is a minor comfort in the story that Philip Conkling and his colleagues tell, since it seems unlikely that human-induced environmental changes would turn off the conveyor in the manner of the Younger Dryas. We might just encourage a rapid melting of ice, but in our warmer world it is very unlikely that winter sea ice would gain a stronghold, so no "mini-glaciation" seems likely to await us.

This book teaches us a lot about how to do science and how to investigate difficult problems about the causes of environmental change. It shows that abrupt climate changes really have happened and puts forward likely mechanisms. It tells us a great deal about the wonders of Greenland and how important the country is as a window on to the recent glaciations. And it makes the whole idea of sudden, grave climate shocks arising from greenhouse emissions seem terribly plausible, even if it does not predict any particular shocks. It is not a story about lands' end, but it is a very fine tribute to Comer.

The Fate of Greenland: Lessons from Abrupt Climate Change

By Philip Conkling, Richard Alley, Wallace Broecker and George Denton

MIT Press, 224pp, £22.95

ISBN 9780262015646

Published 5 June 2011

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