Thursday, August 30, 2012

Lowest ever Arctic sea ice cover

In the northern summer of 2007 the extent of Arctic sea ice reached a low of 4.17 million square kilometers, the lowest since satellite measurements began in 1979. A few days ago the extent of Arctic sea ice was just 4.10 million square kilometers, just breaking the 2007 record. But, the ice is likely to keep melting well into September. 

The minimum sea ice coverage of 2007, recorded on September 18 (left) next to the new record set on August 26 this year (right). Previous years suggest that ice will continue to melt until mid-September (image NSDIC).
Of concern is that this year was cooler than 2007. However, the extent of sea ice coverage measures just two of the three important dimensions. Like its area, the thickness of sea ice has also been falling. This means that the loss of ice area is accelerated because there is a smaller volume that needs to melt. 

Arctic sea ice volume estimated from ice extent and thickness data (image Wikipedia).
The ice loss exposes a greater expanse of open water, which feeds back via two mechanisms to further accelerate ice loss. The sea ice reflects sunlight back into space and as more dark water is exposed more energy is absorbed, which raises temperature. Larger areas of open ocean also allow storms to generate larger waves that break up the ice into smaller pieces, which melt faster.

Of note is that the six years with the lowest Antarctic ice covers are the last six years. Listed from lowest to highest ice cover, they are 2012, 2007, 2011, 2008, 2010 and 2009. To round out the top ten years with the least ice cover add 2005, 2006, 2002 and 2004 to the list. That's right, the top ten years of lowest sea ice cover have occurred in the last eleven years. And 2003 sits at number twelve after 1995.

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