Wednesday, September 15, 2010

Arctic sea ice reaches annual minimum extent

NSIDC Images of the Cryosphere

NSIDC, 15th September 2010
Arctic sea ice appears to have reached its annual minimum extent on 10 September. The minimum ice extent was the third-lowest in the satellite record, after 2007 and 2008, and continues the trend of decreasing summer sea ice.

Overview of conditions

On September 10, 2010 sea ice extent dropped to 4.76 million square kilometers (1.84 million square miles). This appears to have been the lowest extent of the year; sea ice has now begun its annual cycle of growth.

The 2010 minimum ice extent is the third-lowest recorded since 1979. The 2010 minimum extent is 240,000 square kilometers (93,000 square miles) above 2008 and 630,000 square kilometers (240,000 square miles) above the record low in 2007. This is 340,000 square kilometers (130,000 square miles) below 2009. The 2010 minimum is 1.95 million square kilometers (753,000 square miles) below the 1979 to 2000 average minimum and 1.62 million square kilometers (625,000 square miles) below the thirty-one-year 1979 to 2009 average minimum.

Conditions in context

This is only the third time in the satellite record that ice extent has fallen below 5 million square kilometers (1.93 million square miles), and all those occurrences have been within the past four years. The minimum for 2009 was 5.10 million square kilometers (1.97 million square miles), fourth lowest in the satellite record.

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 andJune. 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.

A word of caution on calling the minimum
Because of the variability of sea ice at this time of year, the National Snow and Ice Data Center determines the minimum using a five-day running mean value. We have now seen four days of gains in extent. It is still possible that ice extent could fall slightly, because of either further melting or a contraction in the area of the pack due to the motion of the ice. For example, in 2005, the time series began to level out in early September, prompting speculation that we had reached the minimum. However, the sea ice contracted later in the season, again reducing sea ice extent and causing a further drop in the absolute minimum. When all the data for September are in, we will confirm the minimum ice extent for the season.

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