Earth's Tropics Arid Belt Expands
Pastor Dale Morgan
Dec 2, 1:11 PM EST
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Earth's Tropics Arid Belt Expands*
By SETH BORENSTEIN
AP Science Writer
WASHINGTON (AP) -- Earth's tropical belt seems to have expanded a couple
hundred miles over the past quarter century, which could mean more arid
weather for some already dry subtropical regions, new climate research
shows.
Geographically, the tropical region is a wide swath around Earth's
middle stretching from the Tropic of Cancer, just south of Miami, to the
Tropic of Capricorn, which cuts Australia almost in half. It's about
one-quarter of the globe and generally thought of as hot, steamy and
damp, but it also has areas of brutal desert.
To meteorologists, however, the tropics region is defined by long-term
climate and what's happening in the atmosphere. Recent studies show
changes that indicate an expansion of the tropical atmosphere.
The newest study, published Sunday in the new scientific journal Nature
Geoscience, shows that by using the weather definition, the tropics are
expanding toward Earth's poles more than predicted. And that means more
dry weather is moving to the edges of the tropics in places like the
U.S. Southwest.
Independent teams using four different meteorological measurements found
that the tropical atmospheric belt has grown by anywhere between 2 and
4.8 degrees latitude since 1979. That translates to a total north and
south expansion of 140 to 330 miles.
One key determination of the tropical belt is called the Hadley
circulation, which is essentially prevailing rivers of wind that move
vertically as well as horizontally, carrying lots of moisture to rainy
areas while drying out arid regions on the edges of the tropics. That
wind is circulating over a larger area than a couple decades ago.
But that's not the only type of change meteorologists have found that
shows an expansion of the tropics. They've seen more tropical conditions
by measuring the amount of ozone in the atmosphere, measuring the depth
of the lower atmosphere, and the level of dryness in the atmosphere at
the edges of the tropics.
Climate scientists have long predicted a growing tropical belt toward
the end of the 21st century because of man-made global warming. But what
has happened in the past quarter century is larger and more puzzling
than initially predicted, said Dian Seidel, a research meteorologist
with the National Oceanic and Atmospheric Administration lab in Silver
Spring, Md. She is the author of the newest study.
"They are big changes," she said. "It's a little puzzling."
She said this expansion may only be temporary, but there's no way of
knowing yet.
Seidel said she has not determined the cause of this tropical belt
widening. While a leading suspect is global warming, other suspects
include depletion in the ozone layer and changes in El Nino, the
periodic weather phenomenon in the Pacific Ocean.
Other climate scientists are split on the meaning of the research
because it shows such a dramatic change - beyond climate model
predictions. Some scientists, such as Richard Seager at Columbia
University's Lamont-Doherty Earth Observatory, say changes in El Nino
since the 1970s probably are a big factor and could make it hard to
conclude there's a dramatic expansion of the tropical belt.
But climate scientists Andrew Weaver of the University of Victoria and
Richard Somerville of the Scripps Institution of Oceanography said
Seidel's work makes sense and that computer models have consistently
been underestimating the ill effects of global warming.
"Every time you look at what the world is doing it's always far more
dramatic than what climate models predict," Weaver said.
Both Weaver and Seidel said the big concern is that dry areas on the
edge of the tropics - such as the U.S. Southwest, parts of the
Mediterranean and southern Australia - could get drier because of this.
"You're not expanding the tropical jungles, what you're expanding is the
area of desertification," Weaver said.
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On the Net:
Nature Geoscience: http://www.nature.com/ngeo