Monday, August 13, 2007

A Positive Feedback in the Arctic

The Artic sea ice extent is shockingly small this summer and may become ice free sooner than expected (condition not observed for at least a million years):

Sea ice cover (src: NCEP). The image itself displays the ice concentration in intervals. Special colors are pale purple ('weather'), darker purple (no data), gray (too much land near the cell for reliable ice concentrations), and black (land). Red indicates low concentrations (16 to 28 percent), while blues indicate high ice concentrations (over 85%).

It's not surprising knowing that sea surface temperatures in the Artic have been 3 to 5 deg. higher than normal:

Sea Surface Temperature Anomaly (src: UNISYS).

The last three points in the chart below seem to indicate a significant departure from the the linear model used until now. This departure may suggest that a non linear model is maybe more appropriate indicating a possible acceleration of the melting sign of a self-enhancing or positive feedback process ( greater loss of sea ice and albedo (the degree of reflecting ability), brings about more warming, leading to greater loss of arctic ice).

Trends in ice extent anomalies show how the expanse covered by ice is changing from year to year for a given month. Anomalies are given in percentage difference from the mean extent for that month. The mean is calculated using the period 1979-2000 (src: NSIDC).

It seems to confirm the most pessimistic forecasts:

(a) Northern Hemisphere sea ice extent in September from one integration of the Community Climate System Model version 3 (CCSM3) with observations from the satellite era shown in red. The light blue line is a 5-yr running mean. The three lower panels show the September ice concentration (ice floes are separated by open water) in three select decades. (src: RealClimate and "Future abrupt reductions in the summer Arctic sea ice" (Holland et al.)).