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It’s the most urgent call for geoengineering yet: begin cooling the Arctic by 2013 or face runaway global warming. But the warning – from a voice on the scientific fringe – may be premature, according to experts contacted by Â鶹´«Ã½.

, a former software engineer who has become alarmed at the possibility of reaching a climate “tipping point” argued for Arctic geoengineering as soon as possible in a poster presentation at the meeting in San Francisco last week.

“We’ve got to pull out all the stops to prevent a runaway situation,” Nissen says. He suggests using stratospheric aerosols to cool the surface and subsurface below, or increasing the reflectance of low-level clouds by pumping a fine spray of salt water into them.

Although Nissen’s opinion is not in the scientific mainstream, he has the backing of a leading expert on sea ice, of the University of Cambridge, who recently suggested that the Arctic ocean may be ice-free at the end of each summer . Wadhams says that accelerating climate change in the Arctic has forced him to abandon his scepticism about geoengineering. “One has to consider doing something,” he says.

Gas leak

As the Arctic loses its shield of ice in the summer months, shallow waters over the east Siberian Arctic continental shelf will warm to several degrees above freezing. This is the largest continental shelf on the planet, covering 2.1 million square kilometres, and the sea above it is just 50 metres deep on average. The seabed consists largely of methane-rich permafrost, which began to be submerged about 8000 years ago, as the sea level rose following the last ice age. Without a protective cap of sea ice over the shallow water, the permafrost will warm rapidly and release huge amounts of methane, Nissen fears.

Nissen’s alarm about catastrophic methane release stems in part from the findings of a team led by of the International Arctic Research Center at the University of Alaska Fairbanks. Last year she reported large amounts of methane bubbling from the east Siberian Arctic shelf.

Ice-free when?

However, both the sea-ice projections and the fears about catastrophic methane releases are shrouded in uncertainty. of the Naval Postgraduate School in Monterey, California, has developed a regional model that suggests an ice-free Arctic Ocean by the end of the summer from . But global climate models suggest that this won’t happen until 2030 at the earliest.

One contributor to the Arctic Sea Ice Blog, meanwhile, has fitted exponential curves to data on ice volume and for the complete loss of late-summer sea ice. The problem is that other curves fit the data similarly well, but give much later dates when extrapolated forward. “If you pick one curve over another, I’d like to see a good reason for doing so,” says of the University of Washington in Seattle.

Uncertainty analysis

It’s also unclear how much methane, in total, is bubbling from the east Siberian Arctic shelf – and whether the methane release observed by Shakhova and her colleagues is due to present warming or is the result of the permafrost slowly melting since it was inundated beginning eight millennia ago. “There are still more questions than answers,” says , a member of the team.

What’s more, says of Royal Holloway, University of London, it seems that the largest current releases of methane are coming from the southern hemisphere tropics, rather than the Arctic.

Given the uncertainties, Nissen’s proposal seems unlikely to take off. However, it heightens the need for governments to develop guidelines for future geoengineering that may become necessary. “There is very much an urgent need to addresses governance issues,” says of the University of Oxford, part of a team that has developed a “” for geoengineering research.