Melting polar ice shelves linked to warmer seas

By STEVE CONNOR

An Antarctic ice shelf the size of Scotland is rapidly disintegrating due to warmer seas, according to the second study in two days to show a dramatic thinning of polar ice.

Scientists believe that the Larsen ice shelf on the Antarctic Peninsula may disappear altogether within 70 years, and even earlier if warming trends continue.

Although the ice shelf itself will not raise sea levels - it is already floating on the ocean - scientists say its loss may trigger a catastrophic release of ice on the peninsula's mainland, causing global sea levels to rise up to 1m.

Researchers led by Andrew Shepherd, a glaciologist from Cambridge University, found that the Larsen ice shelf has thinned by as much as 18m in the past decade and that this can now be explained only by a warmer ocean.

The study, published last week in the journal Science, came a day after another study revealed that the sea ice in the Arctic is also melting rapidly because of rising temperatures, threatening the only natural habitat of the polar bear.

Both studies used radar measurements taken by the European Space Agency's ERS-1 and ERS-2 satellites to monitor the loss of ice over huge swathes of sea at opposite ends of the Earth over 10 years.

"We've discovered that the Larsen ice shelf is thinning due to warmer oceans around it and that's the reason why it's disintegrating," Dr Shepherd says.

Radar measurements - accurate to within 20cm - of the ice shelf's average height above the sea reveal a clear pattern of thinning since measurements began in 1992, he says.

The amount of melting freshwater running off the ice shelf into the surrounding sea is equivalent to the combined flow of eight Rivers Thames.

This could disturb the local sea currents that are part of a much wider global ocean circulation, he says.

On average, the ice shelves of the Antarctic Peninsula have retreated about 300 square kilometres a year since 1980.

But a massive collapse of the Larsen shelf has occurred twice - in 1992 and again in 2001, when huge icebergs floated out to sea.

Dr Shepherd says it is not yet possible to say with certainty whether global warming is directly responsible for the melting, although it is now indisputable that the sea around the Antarctic Peninsula is getting warmer.

But other parts of the Antarctic continent are getting colder.

He says the Larsen ice shelf is about 300m thick.

When two previous sections of the shelf thinned to about 200m in thickness they quickly disintegrated.

Present estimates suggest that the Larsen shelf will begin to disintegrate rapidly by about 2070, although this is likely to happen sooner if warming trends continue unabated, Dr Shepherd says.

"There is going to be no direct damage because these icebergs probably won't affect shipping, and sea levels won't rise from the melting of the ice shelf, but there is a danger that it will affect ocean currents.

"Some people have suggested, for instance, that too much cold freshwater released in this way could cut off the Antarctic ocean currents that replenish the Gulf Stream."

Another unknown is how the disappearance of the ice shelf may affect the local ice sheets - large bodies of ice that are trapped on land by the ice shelf acting as a barrier. Much bigger ice shelves in Antarctica are also being monitored by satellite radar.

The Ronne and the Ross shelves are about 10 times the size of the Larsen shelf and their disintegration would be far more serious.


Ice warnings

Antarctic: A disintegrating ice shelf could disappear within 70 years.

Arctic: Melting sea ice threatens the natural habitat of polar bears.

The reason: Possibly global warming - but other parts of the Antarctic are getting colder.

- INDEPENDENT