The Weekend Australian
By: Bruce Montgomery
Antarctica's ice sheets, once considered to be almost immune to global warming, are now key indicators of change.
Antarctica accounts for 90 per cent of the world's ice and 70 per cent of its fresh water. The average depth of the ice is more than 2000m.
If all of the Antarctic ice melted, it has the hypothetical potential to raise sea levels by 54m, but with the average temperature minus 37C, there is in no danger of it melting.
The dramatic harbinger of tomorrow's world is the West Antarctic Ice Sheet, which Australian research scientists study remotely since it is not in Australian Antarctic territories (they are in the east) but whose stability is crucial to sea levels across the world.
WAIS is the size of NSW. Its ice mass is bedded on rock 2km below the surface.
Should it release itself from that anchor, it has the capacity to raise global sea levels by 5m to 6m. That is 10 times the Intergovernmental Panel on Climate Change's mid-range projection for sea-level rise by 2050.
What are the chances?
First, the physics should be understood. Existing icebergs are no threat to the sea level. Like ice cubes floating in a glass of water, they have already displaced the volume of liquid that is equal to the weight of the ice and the water level has risen accordingly.
A solid block of ice, anchored to the sea floor, will not exert its full effect on the sea level until it melts or it breaks free from its bedrock and its weight displaces the equivalent volume of the sea.
This is the danger the WAIS presents. If it were to break free, its effect on global sea levels could be relatively instantaneous.
Satellites reveal that the ice sheets are thinning and their slide into the sea is accelerating.
The satellites measure the changes to the ice sheets using lasers and other instruments that indicate the position and depth of the ice.
They are accurate to the centimetre and constantly monitor what is happening.
Ian Allison leads the Ice, Ocean, Atmosphere and Climate Program with the Australian Antarctic Division and the Antarctic Climate and Ecosystems Co-operative Research Centre, both in Hobart.
He has studied the Antarctic for nearly 40 years and taken part in more than 25 research expeditions. He was a lead author of the IPCC'S Fourth Assessment Report chapter on changes to snow and ice.
"The first reports from the satellites in 2003-04 showed a sea level rise attributable to warming of Antarctica of 0.2mm to 0.3mm a year,'' Allison says.
"The latest monitoring by the satellites is showing 0.6mm to 0.7mm a year.''
Part of the cause is thermal expansion of the sea.
The oceans absorb and store heat. Water is most dense at 4C. Above that temperature its density decreases, it occupies more space, the oceans rise.
"That might appear to be only 6cm each century.'' Allison says.
"But if it keeps doubling every five years, it's a very different story.
"The concern about the West Antarctic Ice Sheet is that melting at the edges of the ice sheet tends to accelerate, dragging more ice with it. It can become intense.
"We don't really know how stable the West Antarctic Ice Sheet is at the point where it is bedded.''