|The decline in sea ice extent from 1978-2005|
Accelerating melting has shrunk Arctic ice to its smallest extent in at least a century, according to scientists from NASA, the National Snow and Ice Data Center (NSIDC), and the University of Washington.
For the fourth consecutive year, NSIDC and NASA scientists using satellite data have tracked a “stunning” reduction in arctic sea ice at the end of the northern summer. The persistence of near-record low extents leads the group to conclude that Arctic sea ice is likely on an accelerating, long-term decline.
Considering the record low amounts of sea ice this year leading up to the month of September, 2005 will almost certainly surpass 2002 as the lowest amount of ice cover in more than a century. If current rates of decline in sea ice continue, the summertime Arctic could be completely ice-free well before the end of this century.—Julienne Stroeve, NSIDC
That conclusion echoes last year’s findings from the Arctic Council, an eight-nation report by 250 experts.
Arctic sea ice extent, or the area of ocean that is covered by at least 15% ice, typically reaches its minimum in September, at the end of the summer melt season. On September 21, 2005, the five-day running mean sea ice extent dropped to 5.32 million square kilometers (2.05 million square miles), the lowest extent ever observed during the satellite record from 1978.
A recent assessment of trends throughout the past century indicates that the current decline also exceeds past low ice periods in the 1930s and 1940s.
For the period 1979 through 2001, before the recent series of low extents, the rate of September decline was slightly more than 6.5 percent per decade. After the September 2002 minimum, which was the record before this year, the trend steepened to 7.3 percent.
Incorporating the 2005 minimum, with a projection for ice growth in the last few days of September, the estimated decline in end-of-summer Arctic sea ice is now approximately 8 percent per decade. All four years have ice extents approximately 20 percent less than the 1978 through 2000 average. This decline in sea ice amounts to approximately 1.3 million square kilometers (500,000 square miles). This is an area roughly equivalent to twice the size of Texas.
With four consecutive years of low summer ice extent, confidence is strengthening that a long-term decline is underway.
Having four years in a row with such low ice extents has never been seen before in the satellite record. It clearly indicates a downward trend, not just a short-term anomaly—Walt Meier, NSIDC
The winter recovery of sea ice extent in the 2004-2005 season was the smallest in the satellite record. Cooler winter temperatures allow the sea ice to rebound after summer melting. But with the exception of May 2005, every month since December 2004 has set a new record low ice extent for that month.
In addition, arctic temperatures have increased in recent decades. Compared to the past 50 years, average surface air temperatures from January through August, 2005, were 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit) warmer than average across most of the Arctic Ocean.
The trend in sea ice decline, lack of winter recovery, early onset of spring melting, and warmer-than-average temperatures suggest a system that is trapped in a loop of positive feedbacks, in which responses to inputs into the system cause it to shift even further away from normal.
Feedbacks in the system are starting to take hold. Right now, our projections for the future use a steady linear decline, but when feedbacks are involved the decline is not necessarily steady—it could pick up speed.—NSIDC Lead Scientist Ted Scambos