The thickest sea ice in the Arctic has started to break up, opening waters north of Greenland that are normally frozen, experts have warned.
One meteorologist described the phenomenon - recorded for the first time this year - as "scary", and scientists said it could prove catastrophic for polar bears and seals.
The sea off the north coast of Greenland had long been known as "the last ice area" because it was expected to be the last place to remain frozen, given it had the oldest and thickest ice.
But now scientists are warning that the ice has broken up twice this year, due to warm winds and heatwaves in the northern hemisphere.
As a result, the ice has been pushed further away from the coast than at any time since satellite records began in the Seventies.
The changes could force scientists to revise their theories about which part of the Arctic will withstand global warming the longest.
Professor Peter Wadhams, who heads the Polar Ocean Physics Group at Cambridge University, said the trend would also have "serious" consequences for polar bears and seals.
"The north coast of Greenland, with its very steep cliffs, is a denning area for polar bears," Wadhams said.
"They dig holes in the snow and come out in the spring and go hunting. But, if the pack ice has moved offshore they come out of hibernation and are left without an area to hunt.
"They can't swim very far. If this becomes a permanent feature with ice away from the coast, polar bears won't have any ice to hunt on. You would lose the polar bear habitat."
Ruth Mottram of the Danish Meteorological Institute said: "Almost all of the ice to the north of Greenland is quite shattered and broken up and therefore more mobile.
"Open water off the north coast of Greenland is unusual. This area has often been called 'the last ice area' as it has been suggested that the last perennial sea ice in the Arctic will occur here. The events of the last week suggest that, actually, the last ice area may be further west."
Walt Meier, a senior research scientist at the US National Snow and Ice Data Centre, said: "The ice there has nowhere else to go so it piles up. On average, it's over 4m thick and can be piled up into ridges 20m thick or more. This thick, compacted ice is generally not easily moved around.
"However, that was not the case this past winter [in February and March] and now the ice is being pushed away from the coast by the winds."
Thomas Lavergne, a scientist at the Norwegian Meteorological Institute, described satellite pictures of blue water penetrating white ice as "scary".
Even if the water closes over in a few days, the harm will be done, he said.
"The thick old sea ice will have been pushed away from the coast, to an area where it will melt more easily."