Little Cold Spell Grows Massive Ice Sheet

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How fast can glaciers and ice sheets expand and shrink in response to rapidly changing climatic conditions? It's a question that scientists have been pondering with particular interest of late, with Greenland's Peterman Glacier calving large amounts of ice two years in succession, and much of the island's surface ice melting earlier this summer. 

Because abrupt climate changes have occurred, across various spatial and temporal scales, at several previous points in the planet's history, scientists can look for prehistorical clues, to see what happened then and thus infer what might happen in a warming 21st century. A team of geologists has done just that, although it has looked for evidence not during previous warm spells, but by looking at two major cooling events in Earth's past.

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One, called the Younger Dryas period, began about 13,000 years ago and lasted more than a millennium. A second, less prosaically dubbed the 8.2 kiloyear event (because it occurred approximately 8,200 years ago), was less intense and was far shorter in duration – no more than 150 or so years. In this week's journal Science, Nicolas Young of Columbia University and colleagues write that, by dating moraines – piles of rocks and debris that glaciers deposit while expanding – on Canada's Baffin Island, they found that glaciers had been significantly more expansive during those cold periods.

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No surprise there, of course. What was interesting and seemingly counter-intuitive, however, was that those glaciers appeared to cover a larger area during the more recent, shorter-lived, and less intense cold spell than during the Younger Dryas period.

"It's not at all amazing that a small local glacier would grow in response to an event like this, but it is incredible that a large ice sheet would do the same," Young, who conducted the study while at the University at Buffalo, said in a press release.

Young and colleagues suggest that the reason may be because, although the Younger Dryas period was colder year round, there was much more seasonal variation during the 8.2 kiloyear event, with greater summer cooling perhaps feeding the glaciers' advance. As for why there should have been such seasonal variation, the authors are uncertain, although they propose that the apparently catastrophic trigger for the more recent event – a rapid influx of meltwater into the Labrador Sea as a result of warming leading to the final collapse of the massive Laurentide Ice Sheet – may have played a role. 

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While much analysis remains, the authors say that their study adds to existing evidence that ice sheets reacted rapidly in the past to cooling or warming, and raises concerns that they could do so again as the Earth heats up.

"One of the questions scientists have been asking is how long it takes for these huge chunks of ice to respond to a global climate phenomenon," said study co-author Jason Briner, a University at Buffalo associate professor of geology. "People don't know whether glaciers can respond quickly enough to matter to our grandchildren, and we're trying to answer this from a geological perspective, by looking at Earth's history."

"What we're seeing," he added, "is that these ice sheets are surprisingly sensitive to even short periods of temperature change."

IMAGE: Icicles melting in the Arctic midnight sun, Baffin Island, Canada. (Louise Murray/Corbis)

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