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US: December 3, 2007

Story by Deborah Zabarenko, Environment Correspondent

WASHINGTON - Earth's tropical belt is expanding much faster than expected, and that could bring more storms to the temperate zone and drier weather to parts of the world that are already dry, climate scientists reported on Sunday.

"Remarkably, the tropics appear to have already expanded – during only the last few decades of the 20th century – by at least the same margin as models predict for this century," the scientists said in the current edition of Nature Geoscience.
Scientists forecast the tropic belt would spread by about 2 degrees of latitude north and south of the Equator by the end of the 21st century. That amount of tropical expansion has already occurred, and was confirmed by five independent ways of measuring it, the report found.

For mapmakers and astronomers, there is no question about where the tropic zone ends: it is at 23.5 degrees north and south of the Equator at the Tropics of Capricorn and Cancer, boundaries determined by Earth's tilt on its axis. These tropical borders are the furthest point from the Equator where the sun shines directly overhead at the summer solstice.

But climate scientists define the tropic band by what happens on the land, in the water and in the air, and that is what is changing, the study said.


Tropical temperatures are warm, and it rains a lot, with little seasonal or day-to-day change. The subtropics, by contrast, are generally dry. If the warm, wet tropical climate is spreading poleward, the dry subtropic climate may head for the poles too.

Those dry subtropical bands could include some of the most heavily populated places on Earth, the scientists said: the Mediterranean, the US Southwest, northern Mexico, southern Australia, southern Africa and parts of South America.