Tuesday, September 01, 2009

Greenland's Glaciers

In a few posts over the last year or so, I have linked to news stories about Greenland's ice sheet (and the Arctic generally) melting much faster than previously expected in response to climate change. Most of these stories have reported new photographs or data without much detail about why or how the melting has outpaced predictions. This article in The Guardian interviews several scientists involved in Arctic research who are trying to answer those questions. One anecdote stood out to me immediately:

Before their first expedition, Hamilton and his colleague Leigh Stearns, from the University of Kansas, used satellite data to plan exactly where they would land on a glacier.

"When we arrived there was no glacier to be seen. It was way up the fjord," he says. "We thought we'd made some stupid goof with the co-ordinates, but we were where we were supposed to be." It was the glacier that was in the wrong place. A vast expanse had melted away.

When Hamilton and Stearns processed their first measurements of the glacier's speed, they thought they had made another mistake. They found it was marching forwards at a greater pace than a glacier had ever been observed to flow before. "We were blown away because we realised that the glaciers had accelerated not just by a little bit but by a lot," he says. The three glaciers they studied had abruptly increased the speed by which they were transmitting ice from the ice sheet into the ocean.

One reason that Greenland's ice sheet is melting faster than ice sheets in Antarctica is that Greenland's is somewhat farther from the pole. But that is not the only reason. One contributing factor is that the ocean around it is warming as well.
She has been surprised by early results measuring sea water close to the melting glaciers: one probe recovered from last year recorded a relatively balmy 2C at 60 metres in the fjord in the middle of winter. Straneo said: "This warm and salty water is of subtropical origin – it's carried by the Gulf Stream. In recent years a lot more of this warm water has been found around the coastal region of Greenland. We think this is one of the mechanisms that has caused these glaciers to accelerate and shed more ice."

Straneo's research is looking at what scientists call the "dynamic effects" of the Greenland ice sheet. It is not simply that the ice sheet is melting steadily as global temperatures rise. Rather, the melting triggers dynamic new effects, which in turn accelerate the melt.

"It's quite likely that these dynamic effects are more important in generating a near-term rapid rise in sea level than the traditional melt," says Hamilton. Another example of these dynamic effects is when the ice sheet melts to expose dirty layers of old snow laced with black carbon from forest fires and even cosmic dust. These dark particles absorb more heat and so further speed up the melt.
The faster that Greenland's glaciers melt, the faster and higher we can expect sea levels to rise. Higher sea levels, of course, pose a major threat to any coastal city. Additional feedback effects are hard to predict until they start to happen. In any case the original article is worth a read. It gives a detailed look at what is happening with Arctic climate research.