The drastic melting of Arctic sea ice has finally ended for the year, scientists announced on Wednesday, but not before demolishing the previous record - and setting off new warnings about the rapid pace of change in the region.
The apparent low point for 2012 was reached on Sunday, according to the National Snow and Ice Data Centre, which said that sea ice that day covered about 3.41 million square kilometres, or 24 per cent, of the surface of the Arctic Ocean. The previous low, set in 2007, was 29 per cent.
When satellite tracking began in the late 1970s, sea ice at its lowest point in the summer typically covered about half the Arctic Ocean, but it has been declining in fits and starts over the decades.
"The Arctic is the Earth's air-conditioner," said Walt Meier, a research scientist at the snow and ice centre, an agency sponsored by the US government. "We're losing that. It's not just that polar bears might go extinct, or that native communities might have to adapt, which we're already seeing - there are larger climate effects."
His agency waited a few days before announcing the low to be sure sea ice had started to refreeze, as it usually does at this time of year, when winter closes in rapidly in the high Arctic. A shell of ice will cover much of the Arctic Ocean in coming months, but it is likely to be thin and prone to melting when summer returns.
Scientists consider the rapid warming of the region to be a consequence of the human release of greenhouse gases, and they see the melting as an early warning of big changes to come in the rest of the world.
Some of them also think the collapse of Arctic sea ice has already started to alter atmospheric patterns in the Northern Hemisphere, contributing to greater extremes of weather in the United States and other countries, but that case is not considered proven.
The sea ice is declining much faster than had been predicted in the last big UN report on the state of the climate, published in 2007. The most sophisticated computer analyses for that report suggested that the ice would not disappear before the middle of this century, if then.
Now, some scientists think the Arctic Ocean could be largely free of summer ice as soon as 2020. But governments have not responded to the change with any greater urgency about limiting greenhouse emissions. To the contrary, their main response has been to plan for exploitation of newly accessible minerals in the Arctic, including drilling for more oil.
Scientists said on Wednesday that the Arctic has become a prime example of the built-in conservatism of their climate forecasts. As dire as their warnings about the long-term consequences of heat-trapping emissions have been, many of them fear they may still be underestimating the speed and severity of the impending changes.
In a panel discussion in New York sponsored by Greenpeace, James E. Hansen, a prominent NASA climate scientist, said the Arctic melting should serve as a warning to the public of the risks that society is running by failing to limit emissions.
"The scientific community realises that we have a planetary emergency," Hansen said. "It's hard for the public to recognise this because they stick their head out the window and don't see that much going on."
A prime concern is the potential for a large rise in the level of the world's oceans. The decline of Arctic sea ice does not contribute directly to that problem, since the ice is already floating and therefore displacing its weight in water.
But the disappearance of summer ice cover replaces a white, reflective surface with a much darker ocean surface, allowing the region to trap more of the sun's heat, which in turn melts more ice. The extra heat in the ocean appears to be contributing to an accelerating melt of the nearby Greenland ice sheet, which does contribute to the rise in sea level.
At one point this summer, surface melt was occurring across 97 per cent of the Greenland ice sheet, a development not seen before in the era of satellite measurements, although geological research suggests that it has happened in the past.
The sea is now rising at a rate of about a 30 centimetres per century, but scientists like Hansen expect this rate to increase as the planet warms, putting coastal settlements at risk.
A scientist at the snow and ice centre, Julienne C. Stroeve, hitched a ride on a Greenpeace ship in recent weeks to inspect the Arctic Ocean for herself. Interviewed this week after putting into port at the island of Spitsbergen, she said one of her goals had been to debark on ice floes and measure them, but that it had been difficult to find any large enough to support her weight.
Ice floes were numerous in spots, she said, but "when we got further into the ice pack, there were just large expanses of open water."