Summers in the northern hemisphere are now warmer than at any period in six centuries, according to climate research published on Wednesday in the science journal Nature.
Harvard University researchers analysing evidence from Arctic tree rings, ice cores, lake sediments and thermometer records said recent warm temperature extremes in high northern latitudes "are unprecedented in the past 600 years" both for magnitude and frequency.
"The summers of 2005, 2007, 2010 and 2011 were warmer than those of all prior years back to 1400," they reported.
"The summer of 2010 was the warmest in the previous 600 years in western Russia and probably the warmest in western Greenland and the Canadian Arctic as well," they said.
"These and other recent extremes greatly exceed those expected from a stationary climate."