photo: NASA ICE/CC BY
New data from the National Snow and Ice Data Center shows that the average Arctic sea ice extent in July set a new monthly record low--even though the rate of ice loss slowed "substantially" in the last two weeks of the month.
Average sea ice extent for July 2011 was 3.06 million square miles, 81,000 square miles lower than the previous record low, set in July 2007.
Sea ice declined at an average pace of 90,200 square kilometers (34,800 square miles) per day, which is slightly faster than the average for 1979 to 2000 of 84,400 square kilometers (32,600 square miles) per day. Ice loss slowed towards the end of July as a high-pressure cell centered over the northern Beaufort Sea broke down and a series of low-pressure systems moved over the central Arctic Ocean. This change brought cooler conditions and likely pushed the ice apart into a thinner but more extensive ice cover.
Continuing a trend as least as important as the diminishing extent of sea ice, NSIDC reports that the oldest and thickest sea ice continues to decline:
Until recently, the central Arctic Ocean and Canadian Archipelago served as refuges for some of the oldest, thickest ice. However, the new data show that ice age is now declining in these areas. A map of ice age for the third week of July, combined with sea ice concentration for July 31, 2011 shows that in the eastern Beaufort Sea, the ice has essentially melted back to the edge of the multi-year ice cover (ice older than one year). Multi-year ice is more resistant to melting completely in summer, so it is not yet clear how much more ice will melt. Another tongue of old ice extends from near the pole towards the New Siberian Islands.
These maps show sea ice concentration (left) and ice age (right) over the Arctic Ocean. In the Beaufort Sea off the coast of Alaska, ice has melted back to the edge of a tongue of older, thicker ice. In the ice age image, red shows ice 5 years old and older, green shows 4-year-old ice, light blue shows 3-year old ice, dark blue is second-year ice, and purple shows first-year ice. Image and caption: NSIDC
Completing the picture: NSIDC also reports that the edge of sea ice has retreated from the shores of Siberia and Eurasia, meaning that the the Northern Sea Route shipping lane may fully open up. Reduced ice cover means it is already possible this year to travel the route, however.
Above Canada and Alaska though, the Northwest Passage "remains choked with ice", NSIDC says--however ice loss in the Passage is "well ahead of average" and is nearly matching the rapid loss seen in 2010.