by Catherine Brahic

NewScientist, 03 January 2008

Energy flowing from the equator up towards the North Pole may partly explain the rapid warming of the Arctic, say researchers.

It is well documented that the Arctic is warming about twice as fast as the rest of the globe, but the reason for this remains a mystery.

The leading hypothesis is that ice disappearing as a result of climate change is largely to blame. Warmer temperatures melt the Arctic ice and exposes water, which absorbs more sunlight than ice. This causes temperatures to rise further, melting more ice, and so on.

But a team led by Rune Graversen at the University of Stockholm in Sweden now challenges this theory.

The researchers analysed temperature measurements taken during the 1980s and 1990s by satellite instruments. But instead of just looking at which regions have warmed the most, they also examined the height in the atmosphere where the warming took place.

Lofty changes

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."

Antarctic question

The team’s findings fit well with suggestions that more and more cyclones, which carry warm air, have been moving into the Arctic Circle.

Clouds could also explain the movement of energy into the Arctic. Some studies have suggested that there are more and more clouds over the Arctic, says Graversen. These might be soaking up energy from the sun and warming up the atmosphere.

One question Graversen's findings raise is whether energy from the tropics is also being directed towards the South Pole.

The surface of the Antarctic is not warming nearly as fast as the rest of the globe, but in 2006, John Turner of the British Antarctic Survey published findings showing that the air 5 kilometres above Antarctica has warmed more than anywhere else on Earth over the last 30 years.

Journal reference: Nature (vol 451, p 53)

Link to article: http://environment.newscientist.com/channel/earth/climate-change/dn13134-melting-ice-may-not-explain-warming-arctic.html