September 5, 2008

Sea Levels May Rise 2m Or Less By 2100

A US team of researchers concluded that global sea levels are very unlikely to reach above 2m during this century.

In fact, the speed of glaciers resulting in a rise over 2m is "physically untenable", researchers note in Friday's edition of the journal Science.

Scientists say that news is still nothing to applaud.

"Even a sea level rise of 20cm (8in) in a century will have quite dramatic implications," said Shad O'Neel from the US Geological Survey (USGS).

"This work is in no way meant to undermine the seriousness of climate change, and sea level rise is something we're going to have to deal with."

Last year, the Intergovernmental Panel on Climate Change (IPCC) concluded that sea level rise would probably fit in the range between 28 and 43cm over the century, although 59cm was a possibility.

But the IPCC specifically excluded the mechanism able to produce the biggest amounts of water quickly - acceleration in the flow of ice from the Greenland and Antarctic ice sheets, the world's two major ice masses that would between them raise sea levels by about 70m if they completely melted.

"We find that a total sea level rise of about 2 meters by 2100 could occur under physically possible glaciological conditions but only if all variables are quickly accelerated to extremely high limits," researchers wrote in the journal Science.

Most of the ice comes off in glaciers. Scientists know that many of the glaciers have accelerated in recent years - some quite spectacularly. The Jakobshavn glacier in Greenland, for example, doubled its speed in six years to about 12km per year.

"The gist of the study is that very simple, physical considerations show that some of the very large predictions of sea level rise are unlikely, because there is simply no way to move the ice or the water into the ocean that fast," said co-author Tad Pfeffer, a fellow of CU-Boulder's Institute of Arctic and Alpine Research.

He added that all of the outlet glaciers involved would need to move more than three times faster than the fastest outlet glaciers ever observed for Greenland alone to raise sea levels by two meters this century.

"We don't really know a speed limit for glaciers," said Dr O'Neel, "but we can look at what we have today and ask 'what would happen if they all behaved like Jakobshavn?'

"It's been going fast for several years now and hasn't gone another marked increase in speed. Helheim had a brief period at 14km per year, Columbia at nine or 10; so that kind of figure, in the region of 10km/year, seems to be about as fast as it gets."

Researchers also used varying glacier velocities to calculate sea-rise contribution estimates from the Antarctic Peninsula, Pine Island and Thwaites glaciers.

David Vaughan from the British Antarctic Survey believes the US team has got its figures about right.

"The point is that whatever happens in this century can only start from present conditions and present rates of sea level rise, and that constrains the rise that can occur this century," Vaughan said.

"However, if you're looking further ahead than 2100 - and many governments are, including the Netherlands and the UK which are thinking about infrastructure that would last more than 100 years - then that second century still looks quite scary."


Image Caption: Petermann Glacier, Greenland (NASA/Jesse Allen and Robert Simmon)


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