News•September 13, 2012

New Study Shows How Fast Ice Sheets Can Change

How high and how fast sea level will rise is a crucial question for the coming century, and it all depends on how fast the giant ice sheets atop Greenland and Antarctica melt back and slide into the ocean as the planet warms. To try and figure out how rapidly ice sheets respond to climate change, scientists look to the ancient past — and a new paper released Thursday in Science says that ice can respond a lot faster than anyone had thought.

During a very brief episode of cooling that happened about 8,200 years ago, glaciers spread across Baffin Island, in the Canadian Arctic, at an almost astonishing pace, said the authors. “Based on the prehistoric record,” said lead author Nicolás Young, at the Lamont-Doherty Earth Observatory near New York City, “ice sheets can respond really quickly to short-term and modest temperature change.”

Credit: NASA

That’s an important and worrisome conclusion. Scientists have already measured a sea level rise of 8 inches over the past century, and their best projections suggest another 3 feet or so by 2100, as ice sheets melt and slide into the ocean. That seems to be happening faster all the time. This summer alone, ice experts noted unusually widespread melting in Greenland, along with the disintegration of part of a major glacier. No one knows for sure whether the melting might accelerate, leading to even faster-rising seas.

While this new study addresses the growth of ice sheets — the last thing anyone is concerned about in a warming world — the fact that ice can spread so quickly in such a short time suggests it could retreat quickly as well.

In this case, “short time” is 150 years — the length of the cooling interlude known to paleoclimatologists as the 8.2-ka Event. At that point, global temperatures were on the rise, and the glaciers that had dominated the Earth during the last Ice Age were in full retreat. Meltwater from the giant Laurentide Ice Sheet had formed a gigantic freshwater lake known as Lake Agassiz just south of Hudson Bay, which was bigger than all of the Great Lakes combined.

As the planet kept warming, a natural dam made of ice burst, and trillions of gallons of water emptied from Lake Agassiz into Hudson Bay, and from there into the North Atlantic, where it disrupted a major ocean current that brings warm water northward to Europe and North America.

With this meridional overturning circulation shut down, the region cooled by about 4-7° F, and stayed that way for about 150 years.

That’s just a blink of the eye in geologic time, but it’s about the same amount of temperature change that climate scientists project for the coming century. That being the case, Young said, “one of the main questions we need to answer is whether ice sheets can respond on timescales that matter to humans.”

To try and find out, Young, who was at the University at Buffalo at the time, made the trek with his co-authors to Ayr Valley, on Baffin Island, in Arctic Canada, to look for rocks pushed ahead of expanding glaciers, then left behind when the glaciers retreated.

What they found was that glaciers grew dramatically, virtually overnight. Not only that: they grew bigger in just 150 years than they’d grown during a much longer episode of cooling known as the Younger Dryas, when temperature dropped even more and stayed low for 1,200 years.

The difference, the scientists believe, is that while average temperatures were lower in the Younger Dryas, summers may actually have been cooler during the shorter 8.2-ka Event. That would have kept glaciers from melting much during the summers, giving them a head start on expansion during the following winters.

Expanding glaciers is not something anyone is worried about at the moment, of course — it’s the retreat, along with the resulting sea level rise, that has everyone worried. But the more scientists can understand about how glaciers and ice sheets respond to climate change in both directions, the better they can project what’s likely to happen in the near future.

“Classically, the retreat of glaciers and ice sheets happens more quickly than the advance,” Young said. “Now we’re showing that the advance can happen as quickly as well.”

That being the case, Young said it’s a good bet that what happens to the ice on Greenland, especially, where the ice sheet is looking increasingly precarious, “is going to be tightly coupled to temperature change over the next century.”

Nobody thinks all of Greenland’s ice will melt away by 2100 — a very good thing, since that could raise sea level by a catastrophic 20 feet or more. But given what scientists are learning about how ice sheets respond to temperature, there’s still plenty to worry about.