As the world warms, the tropics are spreading. It's happening because the great spinning loops of air that girdle the planet either side of the equator, known as the Hadley cells (see above), are getting larger. They are expected to keep growing over the next century or so (see main story). But what if global warming continues? 'They can't just keep growing forever," says William Hay of the University of Colorado, Boulder, who studies past climates. According to a study published last year, there could be a tipping point beyond which the cells shrink rapidly and dramatically. This would cause yet more climate chaos, with the deserts suddenly moving much closer to the equator as lush tropical regions dried up.
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