By Quirin Schiermeier

Arctic sea ice has declined slightly less dramatically this year than in the past couple of years. But the seasonal minimum, reached this week, is still the third-lowest on record since satellite radar measurements began in 1979, reinforcing a marked 30-year downward trend in summertime ice extent.

Scientists with the University of Colorado's National Snow and Ice Data Center (NSIDC) in Boulder announced yesterday that the 5.1 million square kilometres sea-ice extent observed on 12 September is the lowest point of the year. With Arctic autumn days getting cooler, and sunlight rapidly fading after the equinox next week, sea ice will now enter its six-month growth cycle again.

"We've turned the corner," says Mark Serreze, a sea-ice expert with the NSIDC.

Dwindling summertime sea-ice extent is a prime indicator of climate change at high northern latitudes, which in the past decades have warmed faster than most other parts of the planet. As ice reflects back some of the sunlight that reaches the surface, reduced ice cover further accelerates the warming trend.

But this year's minimum is almost one million square kilometres -- around twice the size of Spain -- above that for 2007, when the minimum sea-ice extent dropped to its lowest ever, at 4.1 million square kilometres. In 2008, the second-lowest year on record, the minimum sea-ice extent clocked in at 4.52 million square kilometres.

Downward spiral

The modest recovery this year, says Serreze, was probably thanks to a shift in late-summer weather patterns. The typical combination of a high-pressure system over the Beaufort Sea and low pressure over eastern Siberia, which carries a lot of warm air across the Siberian Sea and pushes the ice edge towards the pole, broke down in August, preserving the remaining ice pack.

But there is no indication that 2009 marks the beginning of a trend reversal, scientists stress. This year's minimum was in fact low enough to reinforce the long-term negative trend, says Bill Chapman, an atmospheric scientist at the University of Illinois at Urbana-Champaign. The 2009 minimum is 1.28 million square kilometres below the 1979-2008 average summer minimum, and 1.61 million square kilometres below the 1979-2000 average minimum.

"There's no reason to think that the downward trajectory will abate," Chapman says. "This was a pretty normal season. Rather quiet weather actually, with not much blowing around of ice. And yet we have seen this whole lot of melting, and Arctic ship routes opening up for several weeks."

Since 2004, when satellite-based observations of ice thickness first became available, laser-altimetry measurements have indicated a substantial drop in the overall volume of multi-year sea ice. If the melt season starts out with thin ice in spring it takes less energy to melt large patches of ice during summer than if the ice pack had been solidly thick at the onset.

"Favourable atmospheric patterns just add to the melting," says Serreze. "Twenty years ago they would not nearly have had the same effect. The much thicker ice then could have easily taken some punishment from wind and sun."

Ice retreat

Climate models disagree as to when the Arctic Ocean might be ice-free in summer, but some scientists think it might happen as early as 2030. Apart from possibly harmful effects on Arctic species that have adapted to the annual cycle of ice growth and melt, open-ocean conditions during summer months will have considerable implications for disputes over military operations, sea routes and mineral and oil exploration.

"We're entering a new epoch of sea-ice melt in the Arctic Ocean due to climate change," says Peter Wadhams, an oceanographer at the University of Cambridge, UK, who is conducting research in the Fram Strait off Greenland aboard the Greenpeace ship Arctic Sunrise. "In five years' time most of the sea ice could be gone in summer with just an 'Alamo of ice' remaining north of Ellesmere Island."

Puzzlingly, the trend in Antarctica is in stark contrast to what is going on in the Arctic. Having reached its largest seasonal extension, Antarctic sea-ice extent is some 400,000 square kilometres higher than the 1979-2000 average winter maximum. Scientists think that ozone depletion and increased evaporation and snowfall in the Southern Ocean might explain the abundance of ice around Antarctica.