Southern
Ocean Current Faces Slowdown Threat
By Michael Byrnes
24 March, 2007
Reuters
HOBART (Reuters)
- The impact of global warming on the vast Southern Ocean around
Antarctica is starting to pose a threat to ocean currents that distribute
heat around the world, Australian scientists say, citing new deep-water
data.
Melting ice-sheets and glaciers in Antarctica are releasing fresh water,
interfering with the formation of dense "bottom water," which
sinks 4-5 kilometers to the ocean floor and helps drive the world's
ocean circulation system.
A slowdown in the system
known as "overturning circulation" would affect the way the
ocean, which absorbs 85 percent of atmospheric heat, carries heat around
the globe.
"If the water gets fresh
enough ... then it won't matter how much ice we form, we won't be able
to make this water cold and salty enough to sink," said Steve Rintoul,
a senior scientist at the Australian government-funded CSIRO Marine
Science.
"Changes would be felt
... around the globe," said Rintoul, who recently led a multinational
team of scientists on an expedition to sample deep-basin water south
of Western Australia to the Antarctic.
Water dense enough to sink
to the ocean floor is formed in polar regions by surface water freezing,
which concentrates salt in very cold water beneath the ice. The dense
water then sinks.
Only a few places around
Antarctica and in the northern Atlantic create water dense enough to
sink to the ocean floor, making Antarctic "bottom water" crucial
to global ocean currents.
But the freshening of Antarctic
deep water was a sign that the "overturning circulation" system
in the world's oceans might be slowing down, Rintoul said, and similar
trends are occurring in the North Atlantic.
For the so-called Atlantic
Conveyor, the surface warm water current meets the Greenland ice sheet
then cools and sinks, heading south again and driving the conveyor belt
process.
But researchers fear increased
melting of the Greenland ice sheet risks disrupting the conveyor. If
it stops, temperatures in northern Europe would plunge.
Rintoul, who has led teams
tracking water density around the Antarctic through decades of readings,
said his findings add to concerns about a "strangling" of
the Southern Ocean by greenhouse gases and global warming.
Australian scientists warned
last month that waters surrounding Antarctica were also becoming more
acidic as they absorbed more carbon dioxide produced by nations burning
fossil fuels.
Acidification of the ocean
is affecting the ability of plankton -- microscopic marine plants, animals
and bacteria -- to absorb carbon dioxide, reducing the ocean's ability
to sink greenhouse gases to the bottom of the sea.
Rintoul said that global
warming was also changing wind patterns in the Antarctic region, drawing
them south away from the Australian mainland and causing declining rainfall
in western and possibly eastern coastal areas.
This was contributing to
drought in Australia, one of the world's top agricultural producers,
he said.
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