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Recent Sea-Level Contributions of the Antarctic and Greenland Ice Sheets
Andrew Shepherd1 and Duncan Wingham
Science 16 March 2007:
Vol. 315. no. 5818, pp. 1529 - 1532

After a century of polar exploration, the past decade of satellite measurements
has painted an altogether new picture of how Earth's ice sheets are changing. As global temperatures have risen, so have rates of snowfall, ice melting, and glacier flow. Although the balance between these opposing processes has varied considerably on a regional scale, data show that Antarctica and Greenland are each losing mass overall. Our best estimate of their combined imbalance is about 125 gigatons per year of ice, enough to raise sea level by 0.35 millimeters per year. This is only a modest contribution to the present rate of sea-level rise of 3.0 millimeters per year. However, much of the loss from Antarctica and Greenland is the result of the flow of ice to the ocean from ice streams and glaciers, which has accelerated over the past decade. In both continents, there are suspected triggers for the accelerated ice discharge—surface and ocean warming, respectively—and, over the course of the 21st century, these processes could rapidly counteract the snowfall gains predicted
by present coupled climate models.

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The researchers found that most of the warming is happening high above ground. At midsummer, the data shows that the air that has warmed the most is 2 kilometres above land.

This, says Graversen, rules out the theory that Arctic warming is being accelerated by melting ice. Although the researchers remain unsure what is accelerating Arctic warming, they suggest it might be related to how fast energy is being transported towards the North Pole by cyclones.

The team calculated the flow of energy into the Arctic Circle using meteorological data, and looked at how this flow has changed since the 1980s.

They found that the amount of energy transported from the tropics into the Arctic has increased and that the increase corresponds to the rise of temperatures in the region.

"We are not saying this is the only explanation," says Graversen, "this could explain maybe 25% of the amplification of warming in the Arctic."

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"This assumption is that if this [heat flow] has increased—which we see in the data that it has—then it has contributed to the warming in the Arctic, not only at the surface, but higher in the atmosphere," Graversen said.

Increased moisture in northward-moving air also plays a role, he said, because when the water vapor condenses into clouds and snow, it releases energy, warming the air.

Nobody knows how much of this change is the result of human emissions of planet-warming gases such as carbon dioxide, but it's likely that they play a role.

"Many models suggest an increase in energy transport when more greenhouse gases are introduced into them," he said.

"Changes in the circulation in the atmosphere might have had a much larger effect than previously thought, but these changes may also have been induced by greenhouse gases."