You know when climate change is biting hard when instead of a vast expanse of snow the North Pole is a vast expanse of water. This year, for the first time, Arctic scientists are preparing for that possibility.

"The set-up for this summer is disturbing," says Mark Serreze, of the US National Snow and Ice Data Center (NSIDC). A number of factors have this year led to most of the Arctic ice being thin and vulnerable as it enters its summer melting season.

In September 2007, Arctic sea ice reached a record low, opening up the fabled North-West passage that runs from Greenland to Alaska.

The ice expanded again over the winter and in March 2008 covered a greater area than it had in March 2007. Although this was billed as good news in many media sources, the trend since 1978 is on the decline.

Young and thin

Arctic ice at its maximum in March, but that maximum is declining by 44,000 km² per year on average, the NSIDC has calculated (see graph on the right). That corresponds to an area roughly twice the size of New Jersey.

What is more, the extent of the ice is only half the picture. Satellite images show that most of the Arctic ice at the moment is thin, young ice that has only been around since last autumn (see picture, right).

                                                                                                     Click here to enlarge graph

Thin ice is far more vulnerable than thick ice that has piled up over several years.                       

Net loss                                                                                                                     

"There is this thin first-year ice even at the North Pole at the moment," says Serreze. "This raises the spectre – the possibility that you could become ice free at the North Pole this year."

Despite its news value in the media, the North Pole being ice free is not in itself significant. To scientists, Serreze points out, "this is just another point on the globe". What is worrying, though, is the fact that multi-year ice – the stuff that doesn't melt in the summer – is not piling up as fast as Arctic ice generally is melting.

On average each year about half of the first year ice, formed between September and March, melts during the following summer. In 2007, nearly all of it disappeared.                                                       Click here to enlarge image

Moreover, an atmospheric phenomenon known as the Arctic oscillation kicked into its strong, "positive", phase this winter. This is known to generate winds which push multi-year ice out of the Arctic along the east coast of Greenland.

Ice still possible

Together, these are the factors that have led to most of the Arctic ice now being so young and thin.

"Even if you lost only half of the first-year ice this year – which would be average – you are still in for a very low ice extent this summer," says Serreze.

Some factors could still save the day, though. In summer 2007, warm winds favoured melting. "If we have an atmospheric pattern like we had last year, we are going to lose a whole bunch of ice this summer, but if we have a cooler, more cyclonic pattern, that might preserve some of that ice," says Serreze.

Catherine Brahic

NewScientist.com news service

Published Friday 25th April 2008