Sea ice naturally increases from about mid-September to March and then recedes again during the warmer summer months. However, measurements from satellites and nuclear-powered submarines all show a significant long-term average decline in volume and surface area extent over the past four decades.
Although there was a significant return of the sea ice in October compared to the previous October, it was still less than half the estimated volume of the same time period in the early 1980s, said Professor Andrew Shepherd from University College London.
"The 9000 cubic kilometres we measured in October is still very much smaller than the 20,000 cubic kilometres we estimate for the same time in the early 1980s. So today's minimum still ranks among the lowest for the past 30 years," Shepherd said. "We do see year-to-year variations in the sea ice due to changes in weather patterns."
The European Space Agency's Cryosat satellite found 90 per cent of the rebound was the result of sea ice that had survived the summer melt season, the so-called "multiyear" ice. Just 10 per cent was first-year ice.
It found that the multiyear ice, which is generally thicker than single-year ice, is now on average about 20 per cent or around 30cm thicker than the previous year.
An Arctic "report card" for 2013, presented to the American Geophysical Union last week, found the long-term trends in the polar region continued to show it is warming rapidly and as a result has experienced dramatic changes, such as decreased snow cover, melting permafrost and diminishing ice on land and sea.
- Independent