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Trade winds weaken with global warming

Ian Sample, science correspondent
Thursday May 4, 2006


Trade winds that sweep around half the globe are weakening as global
warming disrupts normal atmospheric circulation, scientists report
today.
The winds, which bring rains to the west and churn up the oceans,
turning surface waters into rich feeding grounds, cover 20,000km to
drive weather conditions around the world.

Scientists fear that as the winds lose their puff, weather patterns
will become less predictable and marine organisms will suffer, as fewer
vital nutrients are forced up to the surface from the ocean depths.


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Gabriel Vecchi, a climate scientist at the National Oceanic and
Atmospheric Administration in New Jersey, used air pressure records,
including measurements from the Royal Navy dating back to the 1860s, to
work out how a major air current, known as the Walker circulation, had
changed with time, particularly since the Industrial Revolution.
The air current drives the Pacific Ocean's trade winds from the western
coast of South America to Indonesia. Along the way, the winds pick up
moisture from the warm waters, which falls as rain as the air rises
over Indonesia.

But the winds have another feature that directly affects the marine
ecosystems of the Pacific. As the wind passes over the ocean surface,
it pushes the water and sets up ocean currents. In the tropical region
of the Walker current, the ocean flow causes cool, nutrient-rich water
from the depths of the Pacific to circle up to the surface, where it
provides vital nourishment for marine life.

Dr Vecchi found that since the mid-1800s, the Walker current had
weakened by 3.5% and was expected to fall by 10% by the year 2100. Any
drop in the strength of winds has a larger effect on ocean currents,
and calculations show that the flow of the ocean has dropped by 7.5%
because of the slacking trade wind. The study appears in the journal
Nature today.

The weakening of the Walker current is one of the most consistent
predictions of climate change models. "One of the most robust
predictions of climate change is that this Walker circulation should
slow down, so we wanted to test that prediction to see how good the
models are. Right now they are the only thing we have to guide us into
the future," said Dr Vecchi.

In follow-on experiments, the researchers teased apart the contribution
to the current's slowing of natural climate variability and that caused
by human activity, such as greenhouse gas emissions from the burning of
fossil fuels. They used different simulations to predict what would
happen to the Walker circulation with and without the hike in
greenhouse gas emissions. The simulations showed that the Walker
current only weakened when recent increases in greenhouse gas emissions
were taken into account. "Nearly all of this is down to increases in
greenhouse gas emissions," said Dr Vecchi.

"We know this air current has an influence on weather in many regions,
so we can expect there to be changes locally. Ecosystems are complex,
but our concern would be that this could also affect biological
productivity over a large part of the Pacific," he added.

http://www.guardian.co.uk/climatecha...767125,00.html


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