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The study, published in Nature, offers a new explanation for the rise in the Arctic's surface temperature, which over the past century has been nearly 2?C or twice the global average.

Until now, the big suspect in "Arctic amplification" has been reflectivity of sunlight.

When the sun's rays hit snow or ice, most of that solar energy bounces back into space - but as those melting surfaces give way to dark-blue sea, the heat is absorbed instead.

This self-reinforcing process, called a feedback, is an established factor in accelerating warming in snow and ice.

But Stockholm University scientists, led by Rune Graversen, believe a possibly bigger cause for Arctic warming could be changes in heat transport in the middle of the troposphere, an atmospheric band that extends 10km above Earth's surface.