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Warming Temperatures, Slowing Glaciers?

Meltwater on the surface of a Greenland glacier. Credit: NASA

Warming temperatures mean melting ice, which means a rise in sea level. (Sometimes, that is. If the ice is already floating in the ocean, it doesn't affect sea level; it does, however, cause other problems). 

But in places covered with massive ice sheets, like Antarctica and Greenland, melting is only part of the story. There's also the fact that ice flows slowly out of the interior out to the sea, where it breaks off into icebergs. When these plop into the ocean, they do raise sea level, like ice cubes dropped into a glass of water. This has been going on for ages — but in a warming world, that process might speed up. In its most recent major report, in 2007, the U.N. Intergovernmental Panel on Climate Change (IPCC) talked about this possibility.

It could happen in two ways. First, water melting from the surface could percolate down through cracks to a glacier's underside, where it would act as a lubricant, letting the glacier slide more easily along bedrock. Second, glaciers often reach for a distance out into the sea, grinding against the sea floor before breaking off into icebergs. If the ocean warms, the grinding ice could melt back and even float free of the sea floor, removing another major source of friction. Since that report, satellite observations have proven that glaciers in both places actually are flowing faster.

But according to a new study in Nature, lubrication by meltwater may not be the reason behind this trend. By looking back at records of both air temperature and ice flow, a team of scientists in England, Scotland and Belgium has found that in warmer years, when more melting was happening on the glaciers' surfaces, the glaciers did speed up at first, but then slowed way down later on in the summer — to the point, says co-author Andrew Shepherd, of the University of Leeds, that "in warm summers, the overall flow is actually slower than in cooler summers." 

It turns out, he says, that meltwater does indeed lubricate the bedrock and make glaciers flow faster. But as the summer goes on and meltwater increases, individual water pockets under the ice merge to form relatively narrow, faster-flowing streams. There's more water, but it's less spread out — and therefore lubricates less of the glacier's underside. "This is something we already know happens with Alpine glaciers," says Shepherd. Since changes in glacial flow could be caused by two different mechanisms, Shepherd and his colleagues deliberately chose glaciers that don't reach the sea, so they could avoid being confused by changes caused by ocean warming. 

None of this means that Greenland is going to shed less ice as the climate warms. Glaciers that don't reach the sea in the first place aren't dropping icebergs into the ocean anyway. Those that do reach the sea (they're called tidewater glaciers) are still subject to speedup mechanism # 2.

But the worst-case scenario, in which tidewater glaciers speed up in two different ways at once, may not end up happening. As glaciologists try and project sea level rise over the coming century and beyond, this new study — especially if it's findings are borne out by future research — could make their calculations more realistic.

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