The polar ice cap is expected to grow more slowly this winter due to a higher-than-expected sea temperature in Arctic waters, scientists have claimed.
Oceanographers from Wales's Bangor University are measuring water turbulence in the seas off Siberia.
Ice crystals were not forming as they usually did at this time of year, said Dr Tom Rippeth from the university.
He warned this was likely to create a "positive feedback," accelerating the shrinking of the Arctic ice cap.
"At this time of year the water should be starting to turn into a "soup" with ice crystals floating round in it," said Dr Rippeth, from Bangor University's School of Ocean Sciences.
"The fact that the water's so warm means it's now going to take longer for the ice to start to grow.
"So it won't grow as big as in previous years. Because ice reflects the sun's rays and sea water absorbs them, this creates a positive feedback accelerating the warming."
The scientists have found the ocean is unseasonably warm
Two of the university's oceanographers, Dr Phil Wiles, and Ben Powell, originally from Cefn Mawr near Wrexham, are part of a multinational team from the UK, USA, Canada and Russia.
They are on board the Victor Buynitsky surveying the Lapev Sea and East Siberian Sea areas of the Arctic Ocean.
They are studying the turbulence of the waters and how this might affect the transfer of heat and freshwater through the water column.
So far, the expedition has found the water temperature is 3C. Sea water's freezing point is -2C.
Dr Rippeth said data from the US-based National Snow and Ice Data Centre (NSIDC) had shown that on 16 September this year the polar ice cap had shrunk to just 4.2 million square km.
He said: "It's very worrying. It appears that the Arctic ice is shrinking at a faster rate than was previously thought.
'Spiral of decline'
"This is nearly a third smaller than it was last year, with the difference in ice cover between last year and this being roughly equivalent to seven times the area of the UK.
"When compared to the typical September ice coverage for the 1960s and 70s the change is even more stark, as the ice cap has shrunk to about half the size it was then.
"The magnitude of the decline in Arctic ice cover has taken everyone by surprise.
"It's far too big to be a result of natural variability and tends to suggest that we are actually locked into a spiral of decline."
The UK team includes scientists from London, Cambridge and Southampton.
The work is funded by the UK's Natural Environment Research Council and is part of the International Polar Year, a two-year scientific programme which started in March.