The Arctic is being hit by melting ice, hotter air and dying wildlife, according to a US government report on the impact of global warming there.
Sun-reflecting Arctic ice has traditionally cooled the whole planet
A new wind circulation pattern is blowing more warm air towards the North Pole than in the 20th Century, scientists found.
Shrubs are now growing in tundra areas while caribou herds are dwindling in Canada and parts of Alaska.
The report stresses that the fate of the Arctic affects the entire planet.
The US National Oceanic and Atmospheric Administration (Noaa) report found that in 2007 winter and spring temperatures were "all above average throughout the whole Arctic and all at the same time" unlike in previous years.
"This is an unusual feature and it looks like the beginning of a signal from global warming," the Noaa's James Overland told reporters.
Scientists have expected polar regions to feel the first impacts of global warming, and the 2006 US State of the Arctic report provided a benchmark for tracking changes, the Associated Press news agency notes.
Wednesday's report was the first update on it.