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NASA study: Melting Arctic sea ice concern

NASA study: Melting Arctic sea ice concern

Apr 2, 2014

According to a new study by National Snow and Ice Data Centre (NSIDC) and NASA researchers, the length of the melt season for Arctic sea ice is growing by several days each decade, and an earlier start to this period is allowing the Arctic Ocean to absorb enough additional solar radiation in some places to melt as much as four feet of the Arctic ice cap’s thickness.

In an article on NASA’s website, NASA’s Earth Science News Team member Maria-José Viñas writes that Arctic sea ice has been in sharp decline during the last 40 years and sea ice cover is shrinking and thinning, making scientists think an ice-free Arctic Ocean during the summer might be reached this century. The seven lowest September sea ice extents in the satellite record have all occurred in the past seven years.

Julienne Stroeve, a senior scientist at NSIDC, Boulder and lead author of the new study, which has been accepted for publication in Geophysical Research Letters, says: “The Arctic is warming and this is causing the melt season to last longer. The lengthening of the melt season is allowing for more of the sun’s energy to get stored in the ocean and increase ice melt during the summer, overall weakening the sea ice cover.”

To study the evolution of sea ice melt onset and freeze-up dates from 1979 to the present day, Stroeve’s team used passive microwave data from NASA’s Nimbus-7 Scanning Multichannel Microwave Radiometer, and the Special Sensor Microwave/Imager and the Special Sensor Microwave Imager and Sounder carried on board Defence Meteorological Satellite Program spacecraft.

When ice and snow begin to melt, the presence of water causes spikes in the microwave radiation that the snow grains emit, which these sensors can detect. Once the melt season is in full force, the microwave emissivity of the ice and snow stabilises, and it doesn’t change again until the onset of the freezing season causes another set of spikes. Scientists can measure the changes in the ice’s microwave emissivity using a formula developed by Thorsten Markus, co-author of the paper and chief of the Cryospheric Sciences Laboratory at NASA’s Goddard Space Flight Centre.

Results show that although the melt season is lengthening at both ends, with an earlier melt onset in the spring and a later freeze-up in the fall, the predominant phenomenon extending the melting is the later start of the freeze season.

Some areas, such as the Beaufort and Chukchi Seas, are freezing up between six and 11 days later per decade. But while melt onset variations are smaller, the timing of the beginning of the melt season has a larger impact on the amount of solar radiation absorbed by the ocean, because its timing coincides with when the sun is higher and brighter in the Arctic sky.

Despite large regional variations in the beginning and end of the melt season, the Arctic melt season has lengthened on average by five days per decade from 1979 to 2013. Still, weather makes the timing of the autumn freeze-up vary a lot from year to year.

“There is a trend for later freeze-up, but we can’t tell whether a particular year is going to have an earlier or later freeze-up,” Stroeve says. “There remains a lot of variability from year to year as to the exact timing of when the ice will reform, making it difficult for industry to plan when to stop operations in the Arctic.”

To read the full story, click here.

* Pictured here is an image mosaic of sea ice in the Canadian Basin, taken by Operation IceBridge’s Digital Mapping System on Mar. 28, 2014.

Image Credit: Digital Mapping System/NASA Ames

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