OT: Scientists forecast metre rise in sea levels this century

Scientists forecast metre rise in sea levels this century
Ian Sample, science correspondent
(The Guardian)

Half of Greenland and vast areas of Antarctica are destined to melt if
global warming continues at the same pace until the end of the century,
scientists warned yesterday. Their research shows that the loss of so
much ice will trigger dramatic rises in sea levels, ultimately swamping
low-lying regions of Essex, Lincolnshire and Norfolk and threatening the
flood defences of cities such as London, Liverpool and Bristol. The last
time so much ice was lost from the poles - in a period between ice ages
129,000 years ago - global sea levels rose by four to six metres.

Experts believe many coastal regions would suffer long before sea levels
rose significantly, because even a minor rise will make storm surges
more devastating and increase the risk of flooding. A rise of one metre
would in effect close the port of London as the Thames barrier would
need to be raised for 300 days a year to protect the city, according to
one scientist.

The warning comes from climate scientists who combined historical
records of Arctic and Antarctic ice melting with advanced computer
models capable of predicting future environmental conditions. They found
that if nothing is done to put the brakes on climate change, Greenland,
the west Antarctic ice *** and other expanses of polar ice will be
warmed beyond a "tipping point" after which their melting is inevitable.

Jonathan Overpeck, director of the Institute for the Study of Planet
Earth at the University of Arizona, and Bette Otto-Bliesner, at the US
National Centre for Atmospheric Research in Colorado, looked back in
history to find a time when sea levels were higher than they are today.
They identified a period when glacial melting increased sea levels by
several metres.

They used a computer model of the environment to work out how warm the
world must have been to trigger the mass melting and concluded that the
Earth was between 3C and 5C warmer than it is today. Using the same
model, the scientists spun the clock forward to see when climate change
is likely to make the world as warm again. "We showed that that level of
warming will come later in this century unless we act on carbon
emissions," said Professor Overpeck. "An Arctic warming of 3C to 5C is
enough to cause four to six metres of sea level rise."

If temperatures do rise as the scientists predict, the ice at the poles
will not be lost immediately. Enough ice is likely to melt within the
next 100 years to raise sea levels by a metre, but ultimately the fresh
water pouring into the North Atlantic would slow down the Gulf stream,
which bathes Britain in warm water from the tropics, by a quarter.
"These ice sheets have melted before and sea levels rose. The warmth
needed isn't that much above present conditions," said Dr Otto-Bliesner.

The major concern is that unless climate change slows down
significantly, the eventual loss of polar ice and subsequent six-metre
rise in sea levels will be unavoidable. "There has been an increasing
number of observations from the ice sheets suggesting they are
responding faster to climate change than anticipated. Now along come our
results showing these kinds of changes occurred in the past and lead to
large ice *** retreat and sea level rise.

There's a threshold beyond which we'll be committed to this melting and
sea level rise irreversibly in the future and that will come later this
century," said Prof Overpeck. A one-metre rise in sea level would see
the Maldives disappear, make most of Bangladesh uninhabitable and put
cities such as New Orleans "out of business", according to Prof
Overpeck. The research is published in two papers in the US journal
Science today.

The melting 129,000 years ago was driven by natural processes. This time
greenhouse gas emissions have been warming the planet since the
industrial age. "We know what natural causes occurred 129,000 years ago
and we know what's causing climate change now. What's going on now is
driven by human activity and if we want these changes to occur we just
carry on as we are. If we don't want them, we have to stop
concentrations of carbon dioxide in the atmosphere from going much above
560 parts per million, double the pre-industrial level," said Prof
Overpeck.

Carbon dioxide levels in the atmosphere stand at around 380 parts per
million, but many scientists believe they will rise to 550ppm by the
middle of the century. "If we were to experience a rise ofone metre [in
sea levels], we would have to improve sea defences around the country
and that would be extremely costly. We wouldn't be able to use the port
of London because the Thames barrier would have to be closed for much of
the year," said Professor David Vaughan, a climate scientist at the
British Antarctic Survey in Cambridge.
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