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rocky and ginnicus wrote:
Global warming weakens Pacific trade winds
Long-term effect could disrupt marine food chain
This illustration show how the loop of circulating wind over the
Pacific Ocean known as the Walker circulation works.
View related photos
Gabriel Vecchi / UCAR

Updated: 1:45 p.m. ET May 3, 2006
NEW YORK – The trade winds in the Pacific Ocean are weakening as a
result of global warming, according to a new study that indicates
changes to the region’s biology are possible.

Using a combination of real-world observations and computer modeling,
researchers conclude that a vast loop of circulating wind over the
Pacific Ocean, known as the Walker circulation, has weakened by about
3.5 percent since the mid-1800s. The trade winds are the portion of
the Walker circulation that blow across the ocean surface.

The researchers predict another 10 percent decrease by the end of the
21st century.

The effect, attributed at least in part to human-induced climate
change, could disrupt food chains and reduce the biological
productivity of the Pacific Ocean, scientists said.

The study was led by Gabriel Vecchi of the University Corporation for
Atmospheric Research and is detailed in the May 4 issue of the
journal Nature.

The researchers used records of sea-level atmospheric pressure
readings from as far back as the mid-1800s to reconstruct the wind
intensity of the Walker circulation over the past 150 years. A
computer climate model replicated the effect seen in the historical
record.

Some of the computer simulations included the effects of human
greenhouse gas emissions; others included only natural factors known
to affect climate such as volcanic eruptions and solar variations.

“We were able to ask ‘What if humans hadn’t done anything? Or what if
volcanoes erupted? Or if the sun hadn’t varied?'” Vecchi said. “Our
only way to account for the observed changes is through the impact of
human activity, and principally from greenhouse gases from fossil
fuel burning.”

Earth’s average temperature has risen by about 1 degree Fahrenheit
over the past century and many scientists believe greenhouse gases
and carbon dioxide emissions from human activities are to blame.

“This is evidence supporting global warming and also evidence of our
ability to make reasonable predictions of at least the large scale
changes that we should expect from global warming,” Vecchi told
LiveScience.

By extrapolating their data and combining it with results from other
models, the researchers predict the Walker circulation could slow by
an additional 10 percent by 2100.

Driving force

The trade winds blow from the east at an angle towards the equator
and have been used by sailors for centuries seeking to sail west.
Christopher Columbus relied on the Atlantic’s trade winds to carry
him to North America. The winds get their name from their
reliability: To say that a “wind blows trade” is to say that it blows
on track.

The overall Walker circulation is powered by warm, rising air in the
west Pacific Ocean and sinking cool air in the eastern Pacific.

This looping conveyer belt of winds has far-reaching effects on
climate around the globe. It steers ocean currents and nourishes
marine life across the equatorial Pacific and off the coast of South
America by driving the upwelling of nutrient-rich cold water from
ocean depths to the surface.

The Walker circulation is also primarily responsible for transporting
water vapor that evaporates from the ocean surface west, towards
Indonesia; there, the moisture rises up into the atmosphere,
condenses, and falls back to Earth as rain.

Several theories on the effects of global warming predict a weakening
of the Walker circulation. Scientists think it works like this:

To remain energetically balanced, the rate at which the atmosphere
absorbs water vapor must be balanced by the rate of rainfall. But as
temperatures rise and more water evaporates from the ocean, water
vapor in the lower atmosphere increases rapidly. Because of various
physical processes, however, the rate of rainfall does not increase
as fast.

Since the atmosphere is absorbing moisture faster than it can dump
it, and because wind is the major transporter of moisture into the
atmosphere, air circulation must slow down if the energy balance is
to be maintained.

A drop in winds could reduce the strength of both surface and
subsurface ocean currents and dampen cold water upwelling at the
equator.

“This could have important effects on ocean ecosystems,” Vecchi
said. “The ocean currents driven by the trade winds supply vital
nutrients to near-surface ocean ecosystems across the equatorial
Pacific, which is a major fishing region.”

Copyright 2006 The Associated Press. All rights reserved.