Freshening of deep Antarctic waters worries experts
http://www.enn.com/top_stories/article/34921
Published April 18, 2008 09:09 AM

SINGAPORE (Reuters) - Scientists studying the icy depths of the sea
around Antarctica have detected changes in salinity that could have
profound effects on the world's climate and ocean currents.

The scientists returned to the southern Australian city of Hobart on
Thursday after a one-month voyage studying the Southern Ocean to see
how it is changing and what those changes might mean for global
climate patterns.

Voyage leader Steve Rintoul said his team found that salty, dense
water that sinks near the edge of Antarctica to the bottom of the
ocean about 5 km (3 miles) down was becoming fresher and more buoyant.

So-called Antarctic bottom water helps power the great ocean conveyor
belt, a system of currents spanning the Southern, Pacific, Indian and
Atlantic Oceans that shifts heat around the globe.

"The main reason we're paying attention to this is because it is one
of the switches in the climate system and we need to know if we are
about to flip that switch or not," said Rintoul of Australia's
government-backed research arm the CSIRO.

"If that freshening trend continues for long enough, eventually the
water near Antarctica would be too light, too buoyant to sink and
that limb of the global-scale circulation would shut down," he said
on Friday.

Cold, salty water also sinks to the depths in the far north Atlantic
Ocean near Greenland and, together with the vast amount of water that
sinks off Antarctica, this drives the ocean conveyor belt.

This system brings warm water into the far north Atlantic, making
Europe warmer than it would otherwise be, and also drives the large
flow of upper ocean water from the tropical Pacific to the Indian
Ocean through the Indonesia Archipelago.

If these currents were to slow or stop, the world's climate would
eventually be thrown into chaos.

"We don't see any evidence yet that the amount of bottom water that's
sinking has declined. But by becoming fresher and less dense it's
moving in the direction of an ultimate shutdown."

Rintoul said results of the bottom water samples in the Ross Sea
directly south of New Zealand and off Antarctica's Adelie Land
further to the west, were a crucial finding.

"We didn't know that before we left but it's now clear that both of
those regions are becoming fresher for some reason."

GLOBAL WARMING TO BLAME?

During the voyage, scientists from Australia, Britain, France and the
United States measured salinity, carbon dioxide and iron
concentrations as well as currents between Antarctica and Australia.

Rintoul said his team are studying if faster melting of ice sheets or
sea ice is the source of the fresher water but he said it was too
early to tell if global warming was to blame.

Over the coming months, his team will study oxygen isotopes collected
from water samples.

"Oxygen isotopes act as a tracer of ice melt and that information
should help pin down exactly what the cause of the freshening is in
the deep ocean," said Rintoul, of the Antarctic Climate and
Ecosystems Cooperative Research Centre.

"The leading hypothesis at the moment for why it's freshening is that
the floating ice around Antarctica is melting more rapidly than in
the past."

He pointed to studies showing winds around Antarctica changing
because of global warming and the ozone hole.

"The most likely scenario is that those changes in winds have changed
the circulation of the ocean, in particular caused more upwelling of
relatively warm water from below and that could have caused the
increased melting of ice around Antarctica," he said.

"The next challenge over the coming months and year will be to see
just how well we can this pin down."

(Editing by Jeremy Laurence)
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