Steve Connor: Vicious cycle will affect climate of northern hemisphere

Since the first satellites began monitoring the frozen ocean of the Arctic 27 years ago they have given us a bird's eye view of the relentless loss of the northern hemisphere's floating sea ice.

There is an annual cycle of sea ice growth and retreat each polar winter and summer, but the monthly averages collected since 1978 show an unambiguous, long-term decline. Satellite data shows the sea ice in September - the month when it melts to its minimum extent each year - has declined by about 8 per cent each decade. Since the late 1970s, the Arctic has lost about 20 per cent of its September sea ice.

The decline was constant, but in recent years it appears to have accelerated, dramatically so this summer, suggesting a sort of feedback mechanism may have kicked in, adding to the extra melting caused by man-made greenhouse gases and global warming. The most obvious explanation is that as the summer ice has retreated, it has exposed more open ocean to the warming influence of direct sunlight.

This causes the ocean to warm further, triggering the loss of still more summer ice, the exposure of more open water and the absorption of more sunlight and heat, a vicious cycle or "positive" feedback that would inevitably cause an irreversible loss of sea ice if left unchecked.

The latest satellite data for August suggests such an irreversible process has begun. Scientists studying the Arctic now fear that a threshold or "tipping point" may have been reached beyond which the Arctic may not recover, at least not on a human timescale. "It appears that the Arctic is the warmest it has ever been since at least 400 years ago," said Mark Serreze of the US National Snow and Ice Data Centre at Colorado University. "In global climate change, the loss of Arctic sea ice cover is the one thing that really stands out. The decline is really rather remarkable."

Native people who live in the Arctic have given many anecdotal accounts of the changes to their frozen world. The ice on which they hunt is thinner and less predictable. The sea ice protecting their villages from the wind-lashed oceans is no longer the weather barrier it once was, and unexpected storms appear as if from nowhere. The Inuit people of North Baffin in the Canadian Arctic even have a word for the unusual and unwelcome weather - uggianaqtuq.

Biologists have also detected serious changes to the flora and fauna of the region. Plants and insects from the south are moving further north and the polar bear is finding it difficult to build up enough fat reserves for winter because there is so little floating ice to hunt from during summer. But it is the wider climate implications for the loss of sea ice that is the greatest cause of concern. "If you were to remove the Arctic ice cover it would radically change the heat sink which drives the circulation of the atmosphere," Dr Serreze said.

In other words, the frozen Arctic helps to shape and moderate the climate of the entire northern hemisphere. If sea ice is lost - and computer models suggest summer ice will disappear altogether by 2070 - then it would lead to a rapid warming of the land glaciers on Greenland.

Sea ice itself floats on water so its melting would not raise sea levels substantially. But if all the land ice on Greenland were to melt it would raise sea levels around the world by six metres, enough to inundate the major population centres of nearly every country with a coastline.

The Snow and Ice Data Centre has found a clear link between the rate of loss of sea ice and the rate at which the land ice of Greenland is melting. The fear is that the melting of this land ice could also suddenly accelerate.

But could anything hold back the accelerating loss of ice? Unlikely, leading American scientists said last month in the geology journal EOS. They found that the Arctic is moving out of the "envelope" of natural fluctuations that is part and parcel of the geological periods between ice ages, so-called interglacials.

"The future Arctic is likely to have dramatically less permanent ice than exists at present," their study said. "At the present rate of change, a summer ice-free Arctic Ocean within a century is a real possibility. The change appears to be driven largely by feedback-enhanced global climate warming, and there seem to be few if any processes for feedbacks capable of altering the trajectory towards this 'super interglacial' state."