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Greenland not melting as fast as feared
Scientist: As warm or warmer in '30s, '40s – many of glaciers smaller then than now

It was big news when the rate of melting suddenly doubled in 2004 as ice sheets began moving more quickly into the sea. That inspired predictions of the imminent demise of Greenland’s ice — and a catastrophic rise in sea level. But a paper published online this afternoon by Science reports that two of the largest glaciers have suddenly slowed, bringing the rate of melting last year down to near the previous rate. At one glacier, Kangerdlugssuaq, “average thinning over the glacier during the summer of 2006 declined to near zero, with some apparent thickening in areas on the main trunk.”

I asked the lead author of the paper, Ian Howat of the University of Washington, for some perspective. Here’s his take:

    Over the past few years there has been a major revolution in the way scientists think about ice sheet response to climate change. Previously, it was assumed that the big ice sheets react very slowly to climate, on the order of centuries to millenia. This is because surface melting and precipitation was thought to be the dominant way in which ice sheets gain and lose mass under changes in climate. However, over the past five years we have observed that the flow speed of the ice sheets, and therefore the rate at which the ice flows to ocean can change dramatically over very short time scales.

By short, he means months or less.

I also asked Dr. Howat about the argument that, since Greenland went through decades of relatively warm weather in the first half of the 20th century without catastrophic consequences, it’s unlikely that the glaciers are suddenly going to plunge into the ocean because of the current warming. His response:

  Greenland was about as warm or warmer in the 1930’s and 40’s, and many of the glaciers were smaller than they are now. This was a period of rapid glacier shrinkage world-wide, followed by at least partial re-expansion during a colder period from the 1950’s to the 1980’s. Of course, we don’t know very much about how the glacier dynamics changed then because we didn’t have satellites to observe it. However, it does suggest that large variations in ice sheet dynamics can occur from natural climate variability. The problem arises in the possibility that, due to anthropogenic warming, warm phases will become longer and more severe, so that each time the glaciers go through a period of retreat like this, they won’t fully grow back and they will retreat farther the next time.

That sounds like a reasonable concern. But for now, with the glaciers moving in fits and starts, it’s wise not to make any sweeping predictions based on a few measurements. Although the Intergovernmental Panel on Climate Change was criticized for not incorporating the recent scary data from Greenland into its long-range projections, these new results seem to vindicate its caution. As Dr. Howat and his co-authors warn: “Special care must be taken in how these and other mass-loss estimates are evaluated, particularly when extrapolating into the future because short-term spikes could yield erroneous long-term trends.”