May 3, 2006

Global warming weakens vast Pacific climate system

By Deborah Zabarenko

WASHINGTON (Reuters) - Climate scientists identified a
likely new victim of global warming on Wednesday: the vast
looping system of air currents that fuels Pacific trade winds
and climate from South America to Indonesia.

This could mean more El Nino-like weather patterns in the
United States, more rain in the western Pacific and less
nourishment for marine life along the Equator and off the South
American coast.

Known as the Walker Circulation, this system of currents
functions as a huge belt stretching across the tropical
Pacific, with dry air moving eastward at high altitude from
Asia to South America and moist air flowing westward along the
ocean's surface, pushing the prevailing trade winds.

When the moist air gets to Asia, it triggers massive rains
in Indonesia. Then it dries out, rises and starts the cycle
again, heading east.

This important system has weakened by 3.5 percent over the
last 140 years, and the culprit is probably human-induced
global warming, scientists reported in the current edition of
the journal Nature.

"This is the impact of humans through burning coal, burning
benzene, gasoline, everything," said Gabriel Vecchi of the U.S.
National Oceanic and Atmospheric Administration and an author
of the study. "It's principally the greenhouse gases from
fossil-fuel burning."

The observed slowdown has been more pronounced in the last
50 years, Vecchi said in a telephone interview, noting this
fits with what theorists and computer models predict should
happen as a result of human-induced global warming.

It is not consistent with any natural fluctuation in the
system, Vecchi said.

Even this relatively small weakening in the Walker
Circulation means a much larger slowing of wind-forced ocean
currents, the scientists found. This could spur "El Nino-like"
effects, Vecchi said, and these in turn could have an impact as
far as the United States, South America and Australia.

While these potential effects are being studied, Vecchi
said it could mean more rain in the southern United States,
droughts elsewhere in North America, and more rain in Pacific
islands like Kiribati.

The slowdown in ocean currents is also expected to cut down
on bottom-to-top ocean circulation that brings nutrients up to
the surface where marine life can feed on them, which could
have an impact on fishing in the equatorial Pacific.

The weakening of the Walker Circulation is projected to
continue, and could weaken another 10 percent by 2100, the
scientists reported. This could mean ocean flow could decrease
by close to 20 percent.