The loss of sea ice in the Arctic is the main reason why temperatures are rising much faster there than elsewhere, a new study has found.
Dr James Screen and Dr Ian Simmonds from the University of Melbourne report their findings today in the journal Nature.
"The warming in the Arctic over the last couple of decades has been proceeding much faster than the global average," says Screen, a post-doctoral fellow studying polar climate.
"It's roughly speaking, twice as fast."
He says the cause of this Arctic temperature 'amplification' has been the subject of debate among scientists.
Some experts have argued that changes in cloud cover, which can trap heat as well as block the Sun, explains the anomaly.
Others say changes in atmospheric and oceanic circulation, which can bring more heat to the poles, are behind it.
But, says Screen, he and Simmonds have confirmed a third theory, that the decline in sea ice is the main factor driving the temperature amplification.
Central to Screen and Simmonds' conclusions is their analysis of data that shows warming in the Arctic is concentrated near the surface of the Earth.
Since this is close to where the changes in ice are happening, it supports the idea that sea ice decline is a key driver, says Screen.
He says if the warming was due to changes in circulation, one would expect it to extend to a greater depth through the atmosphere.
Loss of ice
Unlike Antarctica which is dominated by thick ice sheets covering land, the Arctic is dominated by a relatively thin layer of ice floating on water.
Sea ice reflects energy from the Sun and helps keep the Earth cool.
But Screen and Simmonds found Arctic warming between 1989 and 2008 correlated strongly with about a 16% loss of sea ice over the same period.
"The amount of the ocean that's being covered by ice is decreasing so you're getting more open water," says Screen.
"Coupled with that, there's been a thinning of the ice."
Screen says the decrease in ice means less reflection of heat (the 'albedo effect'), and more evaporation from oceans, causing an accumulation of water vapour in the atmosphere, which itself is a greenhouse gas.
Screen says understanding this climate feedback is important in terms of developing more accurate climate change models.
He says current Intergovernmental Panel on Climate Change models do include this feedback process, but have underestimated the observed loss of Arctic sea ice to date.
Screen and Simmonds restricted their study to recent decades when the most marked decrease in ice had occurred.
Screen says the shortage of meteorological stations in the Arctic means he and Simmonds relied heavily on data from satellites, which have only been around for the past 20 to 30 years.
"We're analysing a new data set which is probably the highest quality data set that we have for the Arctic," says Screen.
The new findings conflict with an earlier study, also in Nature, that found warming higher up in the atmosphere, weighing against the role of sea ice.
But Screen says the authors of that study used an earlier data set and did not interpret it correctly.
Still, he acknowledges debate will continue about what exactly is the main driver of temperature amplification in the Arctic and stresses he is not ruling out the contribution of other factors.
"It's where you place the emphasis," he says.