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."
This year's minimum caps a summer of low ice extents in the Arctic. On 26 August, sea ice extent fell to 4.10 million sq km (1.58 million sq mi), breaking the previous record low set on 18 September 2007 of 4.17 million sq km (1.61 million sq mi).
On 4 September, it fell below four million sq km (1.54 million sq mi), another first in the 33-year satellite record.
"The strong late season decline is indicative of how thin the ice cover is," said NSIDC scientist Walt Meier.
"Ice has to be quite thin to continue melting away as the sun goes down and Fall approaches."
The sea ice extent is defined as the total area covered by at least 15% of ice, and varies from year to year because of changeable weather conditions.
However, ice extent has shown a dramatic overall decline over the past 30 years.
A 2011 study published in Nature journal, used proxies such as ice cores and lake sediments to reconstruct sea ice extent in the Arctic over the last 1,450 years. The results suggested the duration and magnitude of the current decline in sea ice could be unprecedented over that period.
Paul.Rincon-INTERNET@bbc.co.uk
Source: http://www.bbc.co.uk/news/science-environment-19652329#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa
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