for National Geographic News
Global warming in the Arctic is mysteriously occurring more quickly 1.2 miles (2 kilometers) above the surface than at ground level, a new study says.
Scientists have long known from ground-level measurements that the Arctic is warming at nearly twice the global average rate.
Experts had presumed that this was an effect of solar heating, in which melting snow reveals darker underlying land and water that better absorb heat—an effect that escalates as snow and ice continue to shrink. (Related: "Warming Oceans Contributed to Record Arctic Melt" [December 14, 2007].)
So the finding that the air is warming even more rapidly higher up is especially surprising, reports a team led by Rune Graversen of Stockholm University in tomorrow's issue of the journal Nature.
Graversen's team drew on decades of weather data, mostly from satellites and trans-Arctic airplane flights, to examine temperature changes at various elevations across the Arctic.
They found that during the summer, the rate at which the upper atmosphere was warming up was two times faster each decade.
"It's a remarkable result," Graversen said. "I think nobody expected that."
The finding also means that solar heating can't be the only cause, because that should produce the greatest warming close to the surface.
"Retreating snow and ice cannot explain the vertical structure of the warming that we show," Graversen said. "So snow and ice retreat is not as important as we previously thought."
The effect also occurred in winter, when there is little sunlight to warm the surface.
Graversen theorizes that the upper-elevation warming is linked to changes in atmospheric circulation, such as heat flow northward each month.
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