A new report from NASA and the National Snow and Ice Data Center (NSIDC) reveals that the Arctic melt season grows longer by several days each decade. In addition, an earlier start to the melt season allows the Arctic Ocean to absorb enough incoming solar radiation in several locations to melt a maximum of four feet of the Arctic ice cap’s thickness.
Over the last 40 years, Arctic sea ice has declined sharply, and as the ice cap continues to shrink and thin, scientists think an ice-free Arctic Ocean during the summer may well be reached this century. The seven lowest September sea ice extents in the satellite record have all occurred in the past seven years.
“The Arctic is warming and this is causing the melt season to last longer,” said Julienne Stroeve, a senior scientist at NSIDC in Boulder, Colorado and lead author of the new study, which was recently accepted for publication in the journal Geophysical Research Letters. “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 progression of the onset of sea ice melt and freeze-up dates from 1979 to the present, Stroeve’s research team used passive microwave data from NASA’s Nimbus-7 Scanning Multichannel Microwave Radiometer, the Special Sensor Microwave/Imager and the Special Sensor Microwave Imager and Sounder carried onboard Defense Meteorological Satellite Program spacecraft.
As ice and snow start to melt, the presence of water causes spikes in the microwave radiation that the snow grains produce, which these sensors can detect. Once the melt season is in full swing, the microwave emissivity of the ice and snow stabilizes, and it does not change again until the onset of the freezing season causes an additional set of spikes. Scientists can calculate the changes in the ice’s microwave emissivity using a formula developed by Thorsten Markus, coauthor of the paper and chief of the Cryospheric Sciences Laboratory at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
According to a March 3 report from NSIDC, Arctic sea ice extent in February 2014 averaged 5.58 million square miles. This is the fourth lowest February ice extent in the satellite data record, and is 350,000 square miles below the 1981 to 2010 average. The lowest February in the satellite record occurred in 2005. Despite significant regional differences in the start and end of the melt season, the Arctic melt season has lengthened on average by five days per decade from 1979 to 2013.