Australian scientists yesterday revealed new evidence of global warming, suggesting that sea ice around Antarctica had shrunk 20% in the past 50 years.

The research published in the journal Science traced the pattern of sea ice in the Southern Ocean as far back as 1840.

"Between 1841 and 1950 there was very little change but there is a marked decline in sea ice distribution since 1950 of around 20%," said the lead author, Mark Curran.

ON ANOTHER GLOBAL WARMING FRONT... A large portion of Lake Superior is seen covered in ice in this March 12, 2003 satellite image. In theory, global warming should be a good thing for the Great Lakes, right? Wrong. Global warming means more snow, not less, for the snowbound region along the eastern border between Canada and the United States, researchers said on November 4, 2003. Their study of snowfall records in the Great Lakes region and elsewhere suggests there has been a significant increase in snowfall in the Great Lakes region since the 1930s but not anywhere else. The team, at Colgate University in Hamilton, New York, said that global warming does not mean sunnier weather everywhere. Other researchers have predicted that, as the climate gets warmer overall, it could mean colder temperatures in some parts of the world and more severe weather in general as weather patterns change. Photo by Reuters

The change is important because sea ice - the area around the poles where seawater is frozen into layers no more than a few meters thick - is regarded as a crucial indicator of climate change.

Unlike the ice caps over Greenland and Antarctica and the icebergs they spawn, the size and thickness of the sea ice have little direct effect on worldwide temperatures and sea levels.

But scientists believe it is responsible for the movement of global ocean currents which ensure land masses such as Britain (via its warming Gulf Stream) are kept at certain temperatures.

In winter, pack ice covers an area of more than 7 million square miles around Antarctica, shrinking to 1.5 million square miles during the southern hemisphere summer. At its greatest extent, the ice field is bigger than Russia.

Several studies in recent years have shown that sea ice around Antarctica has increased since the 1970s, but Dr Curran said that the short timescale of those studies failed to take longer-term trends into account. "Until now, records have relied to a large degree on satellite observations since the 1970s. Thirty years is a very short time over which to draw any conclusions," he said.

A Nasa satellite study released last week showed that the growth of pack ice since the end of the 1970s was preceded by a period of shrinkage between 1973 and 1978. The break-up of the Arctic pack ice in recent years has also highlighted concerns about the implications of sea ice decline.

The evidence is taken from ice cores bored out of the Law Dome, a mound of compressed snow from the Antarctic mainland more than half a mile deep, which carries a climate record stretching back 90,000 years.

Compacted snowfall forms annual layers in the ice similar to tree rings. The scientists worked out the extent of past sea ice by measuring the amount of methane sulphonic acid (MSA), produced by marine algae, in the ice cores. MSA-producing algae is linked to sea ice, so the quantity of the chemical in any year's snowfall gives a clue to the extent of that year's pack ice.

"We have only just scratched the surface of this so far," said Dr Curran. "It's only when we start going back into the earlier layers that we'll start to find out whether the changes since 1950 are the norm or an exception."

The new findings are startlingly similar to contentious research published in 1997 based on whaling records. Before the banning of commercial whaling in 1987, ships often moored close to the Antarctic pack ice while taking their catch, allowing researchers to retrace the extent of the pack ice by examining the positions of whaling ships.

Scientists see Antarctica as a barometer of climate trends, with many of the indicators pointing towards global warming. Average temperatures on the Antarctic peninsula have risen 3C in the past 50 years, and the Antarctic ice shelves have retreated dramatically.

B15, which at more than 4,000 square miles was as big as Jamaica and the world's largest iceberg when it calved from the Ross ice shelf in 2000, split apart during storms last month, giving rise to two smaller bergs.

© Guardian Newspapers Limited 2003

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