Scientists have found the first unequivocal evidence that the Arctic region is warming faster than the rest of the world at least a decade before it was predicted to happen.
Climate-change researchers have found that air temperatures in the region are higher than would be normally expected during the autumn because the increased melting of the summer Arctic sea ice is accumulating heat in the ocean.
The phenomenon, known as Arctic amplification, was not expected to be seen for at least 10 or 15 years and the findings will further raise concerns that the Arctic has already passed the climatic tipping-point towards ice-free summers, beyond which it may not recover.
The Arctic is considered one of the most sensitive regions in terms of climate change and its transition to another climatic state will have a direct impact on other parts of the Northern Hemisphere, as well as more indirect effects around the world.
Although researchers have documented a catastrophic loss of sea ice during the summer months over the past 20 years, they have not until now detected the definitive temperature signal that they could link with greenhouse-gas emissions.
But a study by scientists from the United States National Snow and Ice Data Centre in Colorado shows that Arctic amplification has been going on for five years, and will continue to intensify Arctic warming for the foreseeable future.
The study has found that amplification is already showing up as a marked increase in surface air temperatures within the Arctic region during the autumn period, when the sea ice begins to reform after the summer melting period.
Julienne Stroeve, who led the study with her colleague Mark Serreze, said autumn air temperatures this year and in recent years had been anomalously high.
The Arctic Ocean warmed more than usual because heat from the sun was absorbed more easily by the dark areas of open water compared with the highly reflective surface of a frozen sea.
Between September and last month, researchers found "strong surface temperature anomalies over the areas where the sea ice was lost", Dr Stroeve said.
"The warming climate is leading to more open water in the Arctic Ocean. As these open water areas develop through spring and summer, they absorb most of the sun's energy, leading to ocean warming."
October temperature readings were significantly higher than normal in the entire Arctic region - between 3C and 5C above average - but some areas were dramatically higher.
In the Beaufort Sea, north of Alaska, for instance, near-surface air temperatures were more than 7C higher than normal for this time of year.
Computer models have predicted totally ice-free summers in the Arctic by 2070, but many scientists now believe the first ice-free summer could occur within the next 20 years.