NASA satellite data reveals how this year's minimum sea ice extent, reached on Sept. 9 as depicted here, declined to a level far smaller than the 30-year average (in yellow) and opened up Northwest Passage shipping lanes (in red). (Credit: NASA Goddard's Scientific Visualization Studio)
NASA satellites have confirmed that last month Arctic sea ice was close to breaking the record low for summer ice cover. The waxing and waning of the ice around the North Pole is a key measure of climate change say scientists. Each year the ice expands through the winter before melting away as the sun warms the northern hemisphere, reaching its minimum level in September.
The record low for summer ice was hit in 2007 and scientists from the National Snow and Ice Data Centre (NSIDC), which is supported NASA, say that extreme was hit thanks to freak weather conditions.
NSIDC scientist Walt Meier said: "Atmospheric and oceanic conditions were not as conducive to ice loss this year, but the melt still neared 2007 levels. This probably reflects loss of multiyear ice in the Beaufort and Chukchi seas as well as other factors that are making the ice more vulnerable."
According to NASA's Joey Comiso, there is now a 30 year record of decline to draw on. He said: "The sea ice is not only declining, the pace of the decline is becoming more drastic," Comiso said. "The older, thicker ice is declining faster than the rest, making for a more vulnerable perennial ice cover."
Two measures of ice cover are taken by a microwave radiometer on the Aqua satellite. Microwave radiometers are very sensitive gauges of energy transmitted from the Earth which scientists can use to judge the amount of water, ice or water vapour underneath the spacecraft's flight path. Aqua measures the total area of sea covered by the ice, and by that measure the 2007 record low was broken for 10 days early last month. The sea ice extent measure is broader, including areas of ocean where ice covers 15% of the surface area.
The lowest extent this year was recorded on September 9 at 1.67 million square miles. The average for the month was 1.78 million square miles. Both these measures put 2011 as the second lowest on record and this year's ice extent was 938,000 square miles below the average from the period 1979 to 2000.
NASA scientists say the trend is well established over a long period, with a 12% drop in Arctic sea ice in each of the last three decades. "The oldest and thickest ice in the Arctic continues to decline, especially in the Beaufort Sea and the Canada Basin," said NSIDC scientist Julienne Stroeve.
As the rate of sea ice decline speeds up it is starting to exceed the predictions of climate computer models which had previously suggested that by 2100 the Arctic will be ice free in summer.
This video shows Arctic sea ice from March 7, 2011, to Sept. 9, 2011, ending with a comparison of the 30-year average minimum extent (in yellow) and the Northwest Passage (shown in red). (Credit: NASA's Goddard Space Flight Center)