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Ice floes float in Baffin Bay above the Arctic circle from the Canadian Coast Guard icebreaker Louis S. St-Laurent on July 10, 2008. Sea ice has been shrinking at an average rate of about 5 per cent each decade.Jonathan Hayward/The Canadian Press

A new record low for the extent of Arctic sea ice continues a decades-long trend that is quickly upending the northern ecosystem and possibly affecting southern weather.

"There might be some sea-ice growth yet, but I think it's pretty much done," said Julienne Stroeve of the U.S.-based National Snow and Ice Data Center, which marked Thursday as the day of maximum ice spread for this winter.

Sea ice has been shrinking at an average rate of about 5 per cent each decade.

The centre said the maximum extent of this year's ice is more than a million square kilometres below the 30-year average, a difference of about 7 per cent. It's also about 1 per cent below the previous winter record low set in 2011.

At the same time, the record for minimum ice coverage, which happens after the summer melt season, was set in 2012.

As well, the ice has been reaching its annual maximum earlier and earlier. The melt season has also been starting earlier.

Dr. Stroeve said although this year's figure is a record low, most of that deficit is accounted for by open water in the Barents Sea north of Russia. Summer sea-ice levels at the end of the melt season are low all over.

"Typically, all of our low winter sea-ice years have been driven by low conditions in the Barents Sea."

The centre doesn't measure how thick the ice is, a crucial factor in how quickly it melts.

But Dr. Stroeve said recent research drawing on both satellite data and measurements from submarines suggests that ice thickness has declined in the Central Arctic to about 1.25 metres from 3.6 metres in the 1970s.

The changes are already distorting fragile Arctic food webs, altering the timing of crucial algae blooms that kick-start the entire ecosystem, from birds to bowheads.

Communities along northern coastlines – which, in the Arctic, is almost all of them – have less time in which they can safely travel on or hunt and fish from the ice.

Some scientists link vanishing sea ice with abnormal weather in the south. They say a shrinking temperature differential between the Arctic and the middle of the continent is slowing the jet stream, causing it to drift into lower latitudes.

Dr. Stroeve said the jury is still out on that theory.

"There's a lot of variability, so it's hard to attribute changes in weather patterns with what's going on in the Arctic.

"That doesn't mean there can't be a link. The Arctic warming faster than the rest of the planet is going to change weather patterns."