Posted by: JeffMasters, 3:42 PM GMT on July 08, 2011
The summer melt season is in full swing in the Arctic, and sea ice there is in record retreat. Arctic sea ice is currently at its lowest extent on record for early July, according to estimates from the National Snow and Ice Data Center and University of Bremen. Moreover, Arctic sea ice volume is at its lowest on record, according to the University of Washington Polar Science Center, and during June 2011, was reduced by nearly half (47%) compared to its maximum at the beginning of the satellite era, in 1979. The latest surface analysis from Environment Canada shows a 1039 mb high pressure system centered north of Alaska, which is bringing clear skies and plenty of ice-melting sunshine to the Arctic. The combined action of the clockwise flow of air around the high and counter-clockwise flow of air around a low pressure system near the western coast of Siberia is driving warm, southerly winds into the Arctic that is pushing ice away from the coast of Siberia, encouraging further melting. This pressure pattern, known as the Arctic Dipole, was dominant over the Arctic during June, leading to June having the 2nd lowest extent on record, and the record low extent observed at the beginning of July. The Arctic Dipole began emerging in the late 1990s, and was unknown before then; thus climate change is suspected as its primary cause. The Arctic Dipole has become increasingly common in the last six years, and has contributed significantly to the record retreat of Arctic sea ice.
Figure 1. Arctic sea ice extent as of July 7, 2011, as estimated by the National Snow and Ice Data Center. Ice extent during the first week of July was slightly less than the previous record low set in 2007 (dashed green line.)
The previous all-time record year for sea ice loss: 2007
The all-time summer Arctic sea ice melt occurred in 2007, when a "perfect storm" of weather conditions came together to cause a stunning amount of ice loss. Unusually strong high pressure over the Arctic led to clear skies and plenty of sunshine. Arctic winds, which usually blow in a circular fashion around the Pole, instead blew from the south over Central Siberia, due to the Arctic Dipole pattern, injecting large amounts of warm air into the Arctic. Sea ice loss, which had been 20% in the summer of 2006 compared to the summer of 1979, doubled to 39% in 2007, according to the National Snow and Ice Data Center. In one year, as much ice was lost as in the previous 28 years. Compared to the 1950s, over half of the Arctic sea ice had disappeared.
Figure 2. Arctic sea ice extent since 1900, as estimated from satellite and ship reports compiled by Walsh and Chapman (2001). Image credit: University of Illinois cryosphere group.
Summertime Arctic sea ice loss since 2007 has not been as severe, due to cooler and cloudier conditions. However, ice loss in 2008 - 2010 was worse than any year prior to 2007, and the amount of old, thick, multi-year ice has suffered steep declines. How often, then, might we expect to see a "perfect storm" of weather conditions capable of triggering record sea ice loss like in 2007? Well, at the December 2008 meeting of the American Geophysical Union, the world's largest scientific conference on climate change, J.E. Kay of the National Center for Atmospheric Research showed that Arctic surface pressure in the summer of 2007 was the fourth highest since 1948. Cloud cover at Barrow, Alaska was the sixth lowest. This suggests that once every 10 - 20 years a "perfect storm" of weather conditions highly favorable for ice loss invades the Arctic. The last two times such conditions existed was 1977 and 1987.
The latest 1-week forecast from the Canadian GEM model shows the Arctic Dipole pattern continuing, but with high pressure gradually weakening over the Arctic. This should decrease the southerly winds blowing warm air into the Arctic, and help slow down the current record retreat to just below record levels. However, the latest 2-week forecast from the GFS model shows high pressure will build back in over the Arctic during the last half of July, which would tend to increase the flow of warm air into the region again. Overall, it appears that the weather conditions during July 2011 will end up not being as favorable for ice loss as July 2007 was, but the ice is more vulnerable to melting than in 2007 due to the significant loss of old, thick, multi-year ice since 2007. It is too early to tell what may occur during August, but the forecast for July leads me to believe that we will come very close to breaking the 2007 record for all-time ice loss in September, but fall just short. Of the seventeen outlooks issued in early June by various scientific groups, only four called for 2011 to exceed 2007 for summer Arctic sea ice loss.
Figure 3. Distribution of individual Pan-Arctic Outlook values (June Report) for September 2011 sea ice extent. Image credit: ARCUS.