Global Warming Slows the Winds
By Betsy Mason
ScienceNOW Daily News
3 May 2006
Climate models predict that global warming will upset the delicately balanced
atmospheric circulation that controls global climate and weather patterns.
Now scientists are finding evidence that man-made greenhouse gas emissions
may already be tipping the equilibrium, which could have severe
repercussions, including altered weather patterns and a decline in important
fishing grounds.
A key feature of Earth's atmospheric circulation is a steady flow of tropical
air known as the Walker circulation. Warm air rises over the equatorial
western Pacific and cools and sinks in the east. This sets up a flow of air
over the ocean from high pressure in the east to low pressure in the west.
The winds push water from east to west, which causes an upwelling of cold,
nutrient-rich water in the eastern Pacific. Simple climate theories predict
that global warming will weaken this circulation.
But is it actually happening? A team led by climate scientist Gabriel Vecchi
of the U.S. National Oceanic and Atmospheric Administration examined
historical weather records from the equatorial Pacific Ocean dating back to
1861. They found that the difference in pressure between the east and west
Pacific has declined since that time, suggesting a weakening Walker
circulation.
Vecchi's team then compared the pressure changes with the predictions of
three global climate models. The numbers agreed with a model that included
both natural and human forces as well as one that only took human activity
into account. The numbers didn't mesh with the predictions of a model that
only considered natural forces, which suggests that humans are responsible
for a significant part of the reduction in pressure gradient.
Weakened winds across the Pacific could reduce the upwelling of nutrient-rich
water off the coasts of Peru and Ecuador, which could in turn put a damper on
biological activity and cause a decline in fishing stocks, the researchers
argue in the 4 May Nature. The Walker circulation is also largely responsible
for the El Nino-La Nina cycle, and Vecchi's work suggests an increase in El
Nino episodes may be in store.
More El Ninos would "be good for the American west and a number of other
mid-latitude regions that are drought-prone ... but not great for tropical
regions that see drought during El Nino," says climate scientist Mark Cane of
Columbia University's Lamont Doherty Earth Observatory. But Cane warns that
Vecchi's team studied only a fairly short time period, and longer geological
records of sea surface temperature show the opposite effect from warming.
Changes on the scale of decades are so great that it is hard to know if the
apparent trend since 1861 is significant, he says.
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