Antarctic boulders may point to sea level rise

Boulders as big as soccer balls show that a thinning of West Antarctic glaciers has become 20 times faster in recent decades and may hold clues to future sea level rise, scientists said yesterday.

Rocks trapped in glacier ice start to react like clockwork when exposed to the air because of a bombardment of cosmic rays. Scientists studied boulders by three glaciers to find how long they have been out of the ice and so judge the pace of thinning.

"Boulders the size of footballs could help scientists predict the west Antarctic ice sheet's contribution to sea level rise," according to scientists at British and German research institutes in a report in the journal Geology.

"Initial results show that Pine Island Glacier has 'thinned' by around four centimetres per year over the past 5,000 years, while Smith and Pope Glaciers thinned by just over two centimetres per year during the past 14,500 years," they said.

"These rates are more than 20 times slower than recent changes: satellite, airborne and ground based observations made since the 1990s show that Pine Island Glacier has thinned by around 1.6 meters per year in recent years," it said.

No one even saw Antarctica before sailors spotted the coast in 1820 so there are scant historical records and little understanding of how ice sheets might react to rising temperatures linked to global warming.

If that happened, world sea levels would rise by 1.5 metres. If all of Antarctica melted over thousands of years it would raise sea levels by 57 metres, drowning many of the world's biggest cities and many low-lying islands.

"We've seen a much quicker rate of thinning over the last few decades and we're wondering if that's going to continue or if it will slow down," the British Antarctic Survey's Joanne Johnson said of the West Antarctic glaciers.

"It's possible that there may have been some very fast periods of thinning in the past," Ms Johnson, who was lead author of a study, said.

"We don't have the data to know," she said, adding that scientists were worried that "the acceleration seems to be increasing." Scientists at Britain's Durham University and Germany's Alfred Weener Institute also took part.

Mike Bentley from the University of Durham said "when rocks are left high and dry by thinning glaciers they are exposed to high energy cosmic rays which bombard the rocks."

"This creates atoms of particular elements that we can extract and measure in the laboratory - the longer they have been exposed the greater the build-up of these elements," he said in a statement.