Arctic's personal greenhouse turns up the heat

by Catherine Brahic, New Scientist, February 18, 2009

It might be one of the coldest regions on the planet but the Arctic is warming twice as fast as the rest of the globe -- and now we know the reasons why. Two new studies show that the greenhouse effect is stronger above the North Pole, and that the waters of the Arctic Ocean are acting like a radiator to heat the region's atmosphere.

The warming of the Arctic warming has been explained before as being due to a positive feedback loop: as the ice cap melts and disappears, more of the dark ocean is exposed: the Arctic's reflectivity, or albedo, decreases. This means less energy is reflected back out into space and the region warms still further.

But that infamous arctic albedo feedback is only a small part of the problem, Rune Graversen of the Royal Netherlands Meteorological Institute and Minguiai Wang of the University of Michigan have now shown.

Driving factors

The pair ran two nearly identical computer models of Arctic conditions. One model reflected the idea that the albedo of the Arctic reduces as temperatures rise and ice melts, but in the other the albedo didn't change.

Graversen and Wang found that even in the model where albedo was "locked," Arctic temperatures continued to warm at well above the rate for the rest of the globe. Preventing the albedo feedback only decreased warming by 15%, indicating that it is not a driving factor behind the Arctic hothouse.

"The results are consistent with earlier work," says Andrew Barrett of the US National Snow and Ice Data Center in Boulder, Colorado, suggesting that decreasing reflectivity was not the dominant factor driving Arctic warming.

Graversen and Wang then looked at what other factors drive warming. Outside of the tropics their models show that the greenhouse effect gradually gets stronger at latitudes higher than 30°N. And the researchers say this regional Arctic greenhouse effect is getting stronger as the ice cap melts.

Exposed seas

This is because less ice means more exposed sea, and a larger surface from which water can evaporate. Since water vapour is a strong greenhouse gas, the evaporation effectively creates an Arctic energy trap.

Barrett and colleagues have also recently pin-pointed another factor contributing to Arctic warming: the ocean is acting like a radiator and pushing energy into the lower atmosphere (The Cryosphere, Vol. 3, p. 11).

By combining computer models and meteorological observations the team found that over the last five years air temperatures have been warming near the Earth's surface more than they have been at higher altitudes. The phenomenon is strongest in autumn and over areas of open water that would have in the past been iced over. A darker pole absorbs more solar energy, water stores that energy and later releases it to the atmosphere.

All this means the shrinking ice cap is playing a triple role in warming the Arctic. The ice is reflecting less energy, the open water is storing more energy, and is also supplying greenhouse gas to the atmosphere in the form of water vapour. Those three factors combine to produce a strong regional greenhouse over the Arctic.

"The surface albedo feedback certainly still has a role but it is one of a number of factors," says Barrett.

Journal reference: Climate Dynamics (DOI: 10.1007/s00382-009-0535-6)

Link to article: http://www.newscientist.com/article/dn16623-arctics-personal-greenhouse-turns-up-the-heat.html