Hot questions of shelf life

REPORTS FROM THE BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE

Over the past decade, we have heard more and more about blocks of ice the size of countries breaking adrift from the Antarctic continent. Very often, this is misleadingly attributed to global warming.

Antarctic scientists are trying to fathom what is really going on. David Vaughan at the British Antarctic Survey told the BA: "These massive ice-shelf break-ups are due to local warming events, not global warming, and have no substantial impact on sea level."

Myths are difficult to shake, but the fact is that ice shelves are already floating. Thus they cannot cause a rise of sea level when they melt, as Archimedes would be quick to point out.

But the shelves are not totally afloat, they are at points "grounded" or in contact with the sea floor. These points may be what gives the ice shelves their stability, says Dr Vaughan. "They act like foundations of the arch in a bridge, and the expanses of ice between the grounding points give the ice shelf a stability we can understand using the same engineering principles that apply to an arch built of bricks." Unlike the bridges, of course, ice shelves span hundreds of kilometres, but the factors that make an ice shelf stable or unstable are basically the same as those familiar to bridge builders and structural surveyors.

Water slowly erodes the bases of the arches of a bridge spanning a river. It is the same for the foundations or walls of ice in an ice shelf. Dr Vaughan demonstrates the process using children's building blocks. "The arch forms naturally and is stable. Quite a few bricks can be removed from under the arch and it still stays up, but eventually too many bricks will be removed and the arch will collapse."

So it is with the giant expanses supporting the ice shelves. The rise in atmospheric temperature is gradually eroding material from them and eventually they will become unstable and break into icebergs.

Only 3 per cent of the total Antarctic ice shelves have been lost over the past 50 years, and the impact has been negligible. "However, we should expect to be surprised since climate will not do simple things," Dr Vaughan says. The local warming of 2.5°C registered in the Antarctic peninsula region since 1940 has claimed five ice shelves, and if it continues more will follow. However, scientists at BAS are keen to stress that this is due to a strong regional warming, not particularly related to global increases in temperature.

Today's global circulation models do not consistently reproduce a similar warming of the peninsular region, and BAS scientists are looking to connections with local climate and processes such as El Nino to explain this puzzling rise.