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Formal human 'fingerprints' on polar climate

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The length of temperature records varied between the north and south. The team worked with Arctic records spanning 1900 to 1999. Antarctic records were understandably shorter, covering 1950 to '99. And the warming in Antarctica was far less pronounced continent-wide, although stark regional differences are apparent.

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Still, "this is a clear detection of human influence on climate in the Arctic and Antarctica," says Peter Stott, a modeler with the Hadley Center for Climate Prediction and Research, an arm of Britain's Met Office. Dr. Stott is a member of the team conducting the study.

One big reason for the difference in response to warming north and south, researchers point out, is what lies under the large expanses of ice at each pole.

In the Arctic, it's sea water. When the highly reflective ice cap melts back during the northern summer, it exposes the water to sunlight. The darker water stores the heat and releases it in ways that retard ice formation in the fall. In addition, warmer water can enter the Arctic Ocean Basin from the south. Thinner ice during the winter leads to a faster, earlier melt in spring. The open water captures more heat, and the cycle repeats in what researchers term a positive feedback loop. And changes in seasonal wind patterns  can steer the loose ice toward Greenland, driving it out into the North Atlantic. To see the cumulative effect, one need only look at the record lows in summer sea ice over the past several years. While 2008 saw the second-largest loss of summer sea ice since satellite measurements began in 1979 (2007 smashed previous records), preliminary data suggest that the overall volume of summer ice hit a record low this year.

At the other end of the world, by contrast, the vast expanse of ice sits thousands of feet thick atop a continent.  In the interior, there is little change in the ability of the region's surface to reflect, rather than absorb, sunlight.  Here, the concerns focus more on coastal regions. While East Antarctica shows little or no warming, the Antarctic Peninsula and portions of the coast along the edge of the West Antarctic Ice Sheet have warmed. And relatively warm water working its way into the Southern Ocean from toastier latitudes is a lead suspect behind a melt-back and break-up of ice shelves, which can act as brakes on continental glaciers heading seaward.  Indeed, one of the potential gotchyas in this study is that many of the measurements are taken from relatively warmer areas, so the results may be somewhat biased toward the warm end of things. That may not be all bad.  Monaghan suggests that it may be at least as  important to track climate change where ice is on the threshold of melting as it is to focus on building continent-wide averages. "They seem to be doing a reasonable job where it matters," he says.

Indeed, this is an area of increased scientific focus, Monaghan notes. The Nature Geophysics study shows that efforts to improve simulations of polar changes are working, he says. This is a nod from a scientist who in May published a study indicating that models up to that point tended to overstate Antarctic warming.  The next step, he says, is to build into a new generation of climate models that include processes that govern the ebb and flow of ice sheets. This would give researchers and policymakers a better handle on the speed and size of potential ice sheet losses and sea-level rise as the climate warms.

Note: Eoin O'Carroll is on vacation. He'll resume posting on Tuesday, Nov. 4.

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