Arctic sea ice has reached its minimum extent for the year, setting a record for the lowest summer cover since satellite data collection began.
The 2012 extent has fallen to 3.41 million sq km (1.32 million sq mi) - 50% lower than the 1979-2000 average.
Arctic sea ice has long been regarded as a sensitive indicator of changes in the climate.
Scientists who have been analysing the startling melt think it is part of a fundamental change.
"We are now in uncharted territory," said Mark Serreze, director of the National Snow and Ice Center (NSIDC) in Colorado, US.
"While we've long known that as the planet warms up, changes would be seen first and be most pronounced in the Arctic, few of us were prepared for how rapidly the changes would actually occur."