April 9, 2011

Rising Seas Forecast For Some Cities By 2100

New forecasts on rising sea levels suggest that New York will be a big loser, while some regions, including those closer to polar regions, will win big, reports BBC News..

A 2007 report from the Intergovernmental Panel on Climate Change forecast sea levels to rise by as much as 1 foot by 2100. But that forecast was a global average.

A Dutch team has now made an attempt to model all the factors leading to regional variations. And whatever the global figure turns out to be, there will be regional differences.

Factors used to paint a picture of how sea levels vary across the world include ocean currents and temperature and salinity differences of seawater. Currently, sea levels vary by up to three feet. These factors do not include short-term changes due to tides and winds.

Scientists say that as regions, such as the Arctic Ocean, become less saline as ice sheets shed their contents due to global warming, regional patterns of peaks and lows will change.

"Everybody will still have the impact, and in many places they will get the average rise," said Roderik van der Wal from the University of Utrecht, who presented regional projections with a team of researchers at the European Geosciences Union (EGU) meeting in Vienna.

"But places like New York are going to have a larger contribution than the average - 20 percent more in this case - and Reykjavik will be better off," said van der Wal.

The team, which had made specific projections for thirteen regions, said New York would see the biggest increase of rising sea levels from the global average, but Vancouver, Tasmania and The Maldives are also forecast to see higher sea levels as well.

One oddity surrounding such projections is that areas closer to melting ice sheets will see smaller sea levels rises than areas further away. The reason for this is because ice sheets such as those on Greenland and Antarctica attract water through gravitational influences.

These forces pull water towards the coast, effectively causing rises of centimeters. When the ice melts, it raises the average sea level simply by entering the sea, but the gravitational pull is now less effective, so locally sea levels may recede.

"So if the Greenland sheet melts more, that's better for New York; but if Antarctica melts, that's worse for New York - and it's equally true for northwestern Europe," van der Wal told BBC News reporter Richard Black.

The effects would be especially pronounced for Reykjavik, Greenland, which is forecast to receive less than half the global average sea level rise.

The next IPCC assessment, due out in 2013, will feature projections from van der Wal and other scientists on sea level rises. But other scientists have likely completed more regional models that can be offered up as well.

"We're right at the beginning of making regional projections, and at this point there is still a lot of uncertainty," said Stefan Rahmstorf, a sea level specialist from the Potsdam Institute for Climate Impact Research in Germany.

"But it is clear that some parts of the world will feel sea level rise much more quickly than other parts; and an additional factor is land movements," Rahmstorf told BBC.

"In some places such as a lot of the Scandinavian coastline, the land is rising so fast that they will not have any problem with sea level rise in the near future, whereas in other places the land is subsiding - that includes some of the world's big delta cities," he noted.

Rahmstorf published research in 2007 -- ahead of the last IPCC report -- showing that sea levels had been rising faster than climate models had forecast. Since then, he and other researchers have concluded that somewhere between 1.5 feet and 6.5 feet is likely by the end of the century.

After further analysis, Rahmstorf put the range closer to 2.25 to 6 feet -- the range reflecting uncertainties in how ice sheets may melt, and in how society may or may not respond to the findings of climate scientists by controlling greenhouse gas emissions.


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