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 Niño, 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 Niño 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.
Copyright 2007 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.
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