Arctic sea ice – a prime indicator of climate change – this year retreated to the third-lowest extent since 1979 when satellite measurements began.

Sea ice cover seems to have reached its minimum on September 10, clocking in at 4.76 million square kilometres. Arctic sea ice now appears to have entered into its annual growth mode lasting throughout the Northern Hemisphere autumn and winter.

At last week’s minimum, sea ice extent was 340,000 sq km below 2009, but 240,000 sq km above 2008 and 630,000 sq km above the record low in 2007, according to the National Snow and Ice Data Center (NSIDC) in Boulder, Colorado.

The 2010 minimum is 1.95 million sq km – about the size of Mexico – below the 1979 to 2000 average minimum and 1.62 million sq km below the 1979 to 2009 average minimum.

This is “well outside the range of natural variability”, NSIDC notes.

“Despite a late start to the melt season, the ice extent declined rapidly thereafter, with record daily average ice loss rates for the Arctic as a whole for May and June. Assuming that we have indeed reached the seasonal minimum extent, 2010 would have the shortest melt season in the satellite record, spanning 163 days between the seasonal maximum and minimum ice extents.”

This year there remained abundant sea ice in the East Siberian Sea which was ice free in 2007. But there was less ice this year then in 2007 in the Beaufort Sea and in the East Greenland Sea, and both the Canadian Northwest Passage and the Northeast Passage along the Russian north shore – partly blocked in 2007 – have been open in the last few weeks.

NSIDC will in October issue a full analysis of this year’s Arctic melt season.

Map: Daily Arctic sea ice extent on September 10, 2010 The orange line shows the 1979 to 2000 median extent for that day. (NSIDC)