New research in the United States says temperatures in the Arctic region are now the highest they have been for 2000 years.
The researchers from the Northern Arizona University say it is one of the clearest signs that human activity is warming the planet.
Evidence was obtained from ice cores, tree rings and lake sediments.
Writing in the journal Science, they say this confirms that the Arctic is very sensitive both to changes in solar heating and to greenhouse warming.
Twenty three sample sites provided a decade-by-decade picture of temperatures across the region.
The BBC reports the last decade - 1998-2008 - stands out as the warmest in the entire series.