Antarctic ice fringe 'melting faster'

Ice meets sea: Warmer water means faster melting

By Alex Kirby BBC News Online environment correspondent

US scientists say the floating fringes of the Antarctic ice sheet are melting faster than previous studies had suggested.

They say the rate of melting is linked to the temperature of the surrounding seawater.

They estimate that each 0.1 Celsius rise in sea temperature can increase the rate of melting by one metre annually.

The scientists say their findings could have implications for the stability of the West Antarctic Ice Sheet (WAIS).

The research, by Eric Rignot, at the Jet Propulsion Laboratory in Pasadena, and Stanley Jacobs, at Lamont-Doherty Earth Observatory, Columbia University, is reported in Science.

When ice from the interior of the continent reaches the grounding line - the point where glacier, sea and seabed meet - and begins to float, its underside melts into the seawater.

Rignot and Jacobs used satellite radar interferometry, which can distinguish between floating and grounded ice, to estimate how quickly the shelves are melting.

Marked increase

They found that the rate varies for different shelves, from less than 4 metres thickness per year to more than 40 metres in the case of the Pine Island glacier (PIG), the largest in West Antarctica.

Antarctica's climate is hard to read

Using similar techniques, a UK team estimated in 2001 that the PIG's mass was shrinking by about four Gt (four billion tonnes) annually, and found that the grounding line had retreated five kilometres inland between 1992 and 1996.

The US team examined 23 glaciers. They say: "Most of the melt rates we calculate near glacier grounding lines exceed the area-average rates for the largest ice shelves by one to two orders of magnitude."

They say the rates "are strongly correlated with ocean thermal forcing... Our results demonstrate that bottom melting near an ice shelf grounding line is strongly leveraged by the temperature of seawater that comes into contact with the ice in that region."

An increase in seawater temperature, they say, which increases the bottom melting rate, will steepen the ice thickness gradient near the grounding line.

"That will increase the driving stress and flow velocity, and it will reduce the ice shelf resistance to ice discharge, potentially causing the glacier to accelerate and its grounding line to retreat.

Verification needed

"If enhanced melting caused a grounding line to move landward into a deeper basin, a positive feedback will occur as the ocean progressively reaches deeper ice."

Dr Keith Nicholls, of the British Antarctic Survey, told BBC News Online: "The approach is interesting, and the results they've found are a good first go.

Scientific efforts are intensifying

"I do have a slight problem with their technique, though - it's not easy using satellite observations to get melting rates.

"I'm a bit unhappy with the figures they've come up with, which are higher than ours, and the technique does need validating.

"But even if their figures are too high, this is the first time the correlation between warming and melting has been established, and that is interesting.

Confusing picture

"Once we confirm the satellite base measurements we'll be really comfortable."

While the East Antarctic Ice Sheet is considered relatively safe, there have been fears that climate change could cause the WAIS to disintegrate, raising global sea levels by as much as five metres.

But research published last January found that parts of the WAIS may be getting thicker, not thinner.

Most climate scientists agree that West Antarctica, and probably much of the rest of the continent as well, defies simple analysis.

The Intergovernmental Panel on Climate Change (IPCC) says: "There are considerable uncertainties about the mass balance of the Antarctic ice sheets and the future behaviour of the WAIS."

Images courtesy of the British Antarctic Survey