for National Geographic News
Over the past 25 years the tropics have expanded by as much as 300 miles (500 kilometers) north and south—evidence of climate change in action, a new study says.
This not only means that rain-drenched regions near the Equator are growing, experts say, but also that global warming may be pushing deserts poleward in places such as the U.S. Southwest, Mexico, Australia, South Africa, South America, and the Mediterranean.
"The rate of increase is pretty big," said study lead author Dian Seidel of the National Oceanographic and Atmospheric Administration's (NOAA) Air Resources Laboratory in Silver Spring, Maryland.
"It's several degrees of latitude over the course of 25 years."
Tracking Air Circulation
If it sounds strange to think of the tropics as expanding, that's because geographers and climate scientists view them differently. To mapmakers, the tropics are simply the regions between 23.5 degrees north and south latitude, where at least once a year the sun is directly overhead.
But Seidel and her team based their definition of the tropical belt on air circulation.
Near the Equator, moist air rises, producing clouds and ideal conditions for rain forests.
That air then moves poleward—north in the Northern Hemisphere, south in the Southern Hemisphere.
Wrung dry of moisture, the air eventually descends back to Earth, producing deserts. Surface breezes then angle back toward the Equator, completing the cycle.
Seidel's team used meteorological and satellite data to find the northern and southern edges of this zone, mapping changes from the late 1970s to the present.
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