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Last Updated: Wednesday, 6 June 2007, 16:06 GMT 17:06 UK
Antarctic glaciers 'flow faster'
The Eureka Glacier lit by the midnight Sun (Pete Bucktrout/Bas)
The satellites tracked the movements of over 300 glaciers
Satellite data confirms glaciers on the Antarctic Peninsula are flowing faster.

British Antarctic Survey (BAS) scientists used Europe's ERS-1 and -2 spacecraft to track the flow rate of over 300 "ice rivers".

They found a 12% increase in the speed of the glaciers over the period from 1993 to 2003.

The team, which reports its work in the Journal of Geophysical Research, says the study will inform estimates of future sea level rise.

The BAS scientists calculate this group of glaciers alone is currently contributing about 0.047mm a year to global ocean height.

"The Antarctic Peninsula has experienced some of the fastest warming on Earth, nearly 3C over the last half-century," explained lead author Dr Hamish Pritchard.

"Eighty-seven percent of its glaciers have been retreating during this period and now we see these glaciers are also speeding up."

Building knowledge

The new study builds on earlier work by BAS published in 2005.

This used more than 2,000 aerial photographs dating from 1940, and over 100 satellite images from the 1960s onwards, to assess the change in position of glacier fronts over time.

The study found the vast majority of the ice bodies to be in retreat.

EUROPE'S ERS SPACE MISSION
Artist's impression of ERS-2 (Esa)
The European Remote-Sensing (ERS) satellites were launched in 1991 and 1995
Instruments flown include specialised radars, infrared imagers, and an ozone monitor
Spacecraft put in 800km-high polar orbits; covers entire globe in just three days
Wealth of data on ice cover, surface winds, vegetation, trace gases, Earth movement etc
Only ERS-2 still works; but both platforms' archives of data feed long-term scientific study
"The work we reported on a few years ago was only half the story," co-author Professor David Vaughan told BBC News recently.

"Most of what was retreating was already floating so there was no sea-level estimation from that. We've now done the study of the same batch of glaciers to ask if they are accelerating - are they taking more snow into the ocean? And the answer is yes."

The latest work underlines the importance of the different ways in which ice can respond to climate change in addition to simply melting in warmer air.

The new data would seem to indicate that the cause of the flow-rate increase is the result of melting of the lower glaciers, which flow directly into the sea. As they thin, the buoyancy of the ice can lift the glaciers off their rock beds, allowing them to slide faster.

"This turns into a positive loop," explained Dr Pritchard. "They thin a bit, and they flow faster because of that; and then because they are flowing faster, they discharge more ice; and that just makes them thin more and flow faster again - so they get into a cycle."

Dynamic contribution

In its recent projections of future sea level rise, the International Panel on Climate Change (IPCC) forecasted an increase this century in the range of 18-59cm (7-23in).

The numbers excluded the additional contributions that may come from the type of dynamical ice flow processes seen in the Antarctic Peninsula's glaciers.

It was felt these processes were still too poorly understood; but knowledge is growing fast, say scientists.

Aerial shot of glacial terrain (Neil Stevenson/Bas)
If more landed ice gets into the oceans, sea levels will rise
The BAS work has echoes in Greenland where surface melt waters are thought to be draining to the underside of landed ice, lubricating its movement towards the ocean.

And elsewhere on the Antarctic Peninsula, too, it has been shown that the removal of floating ice shelves will speed up the flow into the ocean of the previously bounded ice streams behind.

When all these positive accelerations are taken into account, they are almost certain to make the current IPCC estimates look conservative.

"Observations like ours will feed into the modelling and better predictions will come out. But what the study shows is that the dynamic effects are happening and on quite a large scale. They are a factor already and are only likely to be more of a factor in the future," Dr Pritchard said.

Recent satellite altimetry studies have shown global sea levels to be rising by about 3mm a year. Some 50% of this is probably the result of the expansion of waters as they react to a warmer climate.




SEE ALSO
Antarctic glaciers show retreat
21 Apr 05 |  Science/Nature
Arctic dips as global waters rise
15 Jun 06 |  Science/Nature

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