The Arctic sea ice this year has shrunk to sixth-lowest extent on record, bringing the days of an entirely ice-free Arctic during the summer closer to reality, scientists claim.

However, Arctic sea ice melted less this summer and continued to cover a greater expanse than last year’s record minimum, researchers said.

After an unusually cold summer in the northernmost latitudes, Arctic sea ice appears to have reached its annual minimum summer extent for 2013 on September 13, the NASA-supported National Snow and Ice Data Center (NSIDC) at the University of Colorado in Boulder has reported.

Analysis of satellite data by NSIDC and NASA showed that the sea ice extent shrunk to 5.10 million square kilometres.

This year’s sea ice extent is substantially higher than last year’s record low minimum.

On September 16, 2012, Arctic sea ice reached its smallest extent ever recorded by satellites at 3.41 million square kilometres. That is about half the size of the average minimum extent from 1981 to 2010.

This summer’s minimum is still the sixth lowest extent of the satellite record and is 1.12 million square kilometres lower than the 1981-2010 average.

The ice cap covering the Arctic Ocean shrinks and expands with the passing of the seasons, melting in the summer and refreezing during the long, frigid Arctic winter. This year, cooler weather in the spring and summer led to a late start of the melt season and overall less melt.

This year, Arctic temperatures were 1 to 2.5 degrees Celsius lower than average, according to NASA’s Modern Era Retrospective analysis for Research and Applications, a merging of observations and a modelled forecast.

Satellite imagery, submarine sonar measurements, and data collected from NASA’s Operation Ice-Bridge, an airborne survey of polar ice, indicate that the Arctic sea ice thickness is as much as 50 per cent thinner than it was in previous decades, going from an average thickness of 3.8 meters in 1980 to 1.9 meters in recent years.

The thinning is due to the loss of older, thicker ice, which is being replaced by thinner seasonal ice.

“Thinner ice melts completely at a faster rate than thicker ice does, so if the average thickness of Arctic sea ice goes down, it’s more likely that the extent of the summer ice will go down as well,” said Joey Comiso, senior scientist at Goddard.

“At the rate we’re observing this decline, it’s very likely that the Arctic’s summer sea ice will completely disappear within this century,” said Comiso.

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