The current retreat of ice shelves in the Antarctic due to global warming is nothing new — but this time the problem is manmade and therefore potentially more serious, according to research released Wednesday.
Writing in the latest issue of the journal "Geology," British scientists said a survey had shown that ice shelves had retreated thousands of years ago as a result of rising air and ocean temperatures.
"What this tells us is that ice shelves don't just break up because they get too big — as the global warning skeptics argue," said Dominic Hodgson, a scientist with the British Antarctic Survey and one of the leading investigators.
He said previous periods of warming — about 9,500 years ago and some 2,000 years to 4,000 years ago — were caused by natural causes, including the ending of ice ages, rather than man's emissions and the ice shelves had been able to reform.
"This time, the problem is man-made and if we don't take steps, the damage will be worse," he said. "There is no room for complacency."
The study, by scientists from the universities of Durham, Edinburgh and from the British Antarctic Survey, or BAS, said the George VI Ice Shelf on the Antarctic Peninsula is the first to show that a currently 'healthy' ice shelf experienced an extensive retreat about 9,500 years ago, more than anything seen in recent years.
BAS scientists Carol Pudsey and Jeff Evans reported that the Prince Gustav Ice Shelf, which collapsed in 1995, had also collapsed several thousand years ago.
This, along with the recent collapse of other ice shelves like Larsen B, has been put down to an overall increase in temperatures caused by damage to the ozone layer that protects the earth from the sun.
The study is particularly relevant for other surveys on the West Antarctic Ice Sheet where scientists have found that a relatively warm current, Circumpolar Deep Water, is causing high melt rates on the underside of an ice shelf in Pine Island Bay.
The gradual removal of this ice shelf may be causing the glaciers inland to flow faster, which could lead to enhanced drainage of part of the West Antarctic Ice Sheet, and a rise in sea level.
The BAS says that over the past 900,000 years, there has been an average of 280 parts per million of carbon dioxide in the atmosphere, one of the main causes of global warming. In the period since the Industrial Revolution, and particularly in recent years, that has risen to 350 parts per million.
The scientists analyzed sediments from the bottom of a freshwater lake close to the edge of the present George VI Ice Shelf. The results revealed that about 9,500 years ago the shelf retreated, allowing the sea to flood into the lake. The ice shelf didn't reform until 1,500 years later, and has been present ever since.
Ice shelves are formed when glaciers flow into the sea and freeze, then coalesce with other frozen flows.