NewsAugust 18, 2011

Antarctic Ice Melt: The Big Picture

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Michael D. Lemonick

By Michael D. Lemonick

First complete map of the speed and direction of ice flow in Antarctica, derived from radar data from Japanese, European, and Canadian satellites, and processed by NASA-funded research. The thick black lines delineate major ice divides. Subglacial lakes in Antarctica's interior are also outlined in black. Thick black lines along the coast indicate ice sheet grounding lines. Credit: NASA/JPL-Caltech/UCI

The vast ice sheets that cover Antarctica are sliding inexorably into the sea. This is hardly news: it was happening last year, and in 1900, and thousands — even millions — of years before that. The slide is so slow that it’s balanced, more or less, by new ice built from the snow that falls every year. If that balance were to shift dramatically, sending all the ice into the ocean, sea level would shoot up by a catastrophic 200 feet or so.

That’s not likely to happen any time soon (“soon” being the next few hundred years, at the very least). But the flow of ice has accelerated in recent years, both in Antarctica and in Greenland, and scientists who study moving ice are understandably anxious to figure out how that trend will play out during the coming decades. Back in 2007, the U.N. Intergovernmental Panel on Climate Change's (IPCC) last major report deliberately left out changes in the rate of ice sheet flow from its calculations for future sea-level rise, because scientists simply didn’t have enough information to say anything reliable.

But a new map, just published online by the journal Science, may help change that. It doesn’t provide any firm answers. But it does give scientists a detailed look at how, where, and how fast ice flows across the continent today — and that will be a crucial piece of information for monitoring changes over the coming years.

The map, compiled by Eric Rignot and his colleagues at the University of California, Irvine, assembles radar measurements from five different satellites that have crisscrossed the frozen continent from 1996 to 2009. Each satellite alone gives only part of the picture. But together, they show the entire continent in motion — in some places, slowly, in others much more quickly. There’s an impressive animation from the website of the Jet Propulsion Laboratory, which conducted much of the data analysis.

What’s significant here, says Rignot, is that the classical model of Antarctic ice flow has it all happening by deformation — the weight of the ice pressing down and literally squeezing itself out from the thickest points at the center of the continent (they’re brown on the image below) toward the sea. “But the only way you can explain these long, fast-flowing fingers,” he says (they’re red and purple here) “is that ice is sliding along the bedrock.”

Understanding these details, he says, fills in a major missing piece of the puzzle regarding how ice drains from Antarctica, and future versions of the map will show how that drainage may accelerate in a warming world. That most likely won’t happen due primarily melting, since the Antarctic interior will remain frigid for hundreds of years to come. The more important factor will be the warming of ocean waters, which melt ice shelves and glacial tongues that reach into the sea.

With those natural brakes removed, upstream ice can flow faster — quickening the pace of sea level rise. “Information about what the ocean is doing, both here and in Greenland, is still a missing piece of the puzzle,” says Rignot. “We need to know what impact that’s having on outlet glaciers, and how that will change in the future.”