Sea Ice at Record Low

Arctic sea ice extent is now at a record low -- as of yesterday, below even last year's number. This is based on IARC-JAXA data (International Arctic Research Center - Japan Aerospace Exploration Agency.

The National Snow and Ice Data Center (NSIDC) has the same result:

Note: This is ice extent, which is not the same as area (though easier to measure). (For that matter, what really matters is volume, but though I hear people are working on ways to measure that, they're not there yet.)

Here's a way of thinking about the difference between extent and area: imagine a checkboard with lots of checkers on it, more than usual. Some places the checkers will touch, but there will be spaces between them. Their extent is the area of the entire checkerboard, but their area is the area of the checkerboard minus the gaps in-between all the checkers.

Thus, extent is always greater than area.

There are, of course, uncertainties associated with each day's measured ice extent, but the IARC-JAXA site doesn't give them (as far as I have been able to tell). So to within the precision of the satellite measurements, today's numbers are essentially the same as last year's. But the ice extent growth rate is very small in the last few weeks:

Ice Extent growth rate, Nov 30 - Dec 21 (thousands of sq-km/day):

2002	55
2003	84
2004	67
2005	64
2006	106
2007	79
2008	53

Why is this happening? Warmth. Here's the NSIDC commentary from Dec 3 (2008):

The period of very rapid ice growth that characterized October and early November has ended. The rise in ice extent over the past three weeks has been much slower, and should continue to slow until the expected seasonal ice extent maximum is reached sometime in March. Air temperatures over the Arctic Ocean stayed well above average during November, partly because of continued heat release from the ocean to the atmosphere and partly because of a pattern of atmospheric circulation transporting warm air into the region.