Within the tropics, for example, there is less stratospheric ozone—the compound that absorbs harmful UV radiation from the sun—than in the zones to the north and south.
Jet streams—fast-moving air currents traveling through Earth's atmosphere—are also influenced by the air circulation boundary between the tropics and other zones.
Also, satellites can "see" the temperature difference between cold, high-altitude cloud tops near the Equator and warmer surface lands in the cloudless dry zones to the north and south.
Seidel's team tracked changes in five measures of the tropical-subtropical air circulation boundary.
All five showed that the tropical zone was steadily broadening, at a rate of 2.5 to 4.8 degrees latitude every 25 years.
The study appears in the advance online edition of the new journal Nature Geoscience.
Faster Than Anticipated
Global climate-change models predict that the tropics will expand as Earth warms up, Seidel said, but her team's observed expansion was much faster than predicted.
"The models indicate only fractions of a degree," she said.
Seidel and colleagues aren't sure why the tropics' spread has been so much more rapid.
But she thinks it might have something to do with the stratosphere, the second layer of Earth's atmosphere, which isn't well represented in most climate models.
Because current models don't explain what's going on, it's impossible to tell whether the tropics expansion is part of a cycle that might reverse in the future or an indicator that global warming is having stronger-than-anticipated effects.
If the trend continues, however, the impacts would extend well beyond the tropics.
That's because the shift in the Northern and Southern Hemisphere jet streams could alter the tracks of storms, such as those that hit the U.S. West Coast each winter.
"If some of that storminess moves farther north, there is potential for changes in water supply [and] snowpack—things that determine climate in mid-latitudes, where a lot of agriculture is dependent on reliable rainfall," Seidel said.
(Related news: "Humans Changing Rainfall Patterns, Study Says" [July 23, 2007].)
Steven Running, a professor of ecology at the University of Montana who was not involved in the study, said there could be ecological changes.
"Biogeographers define tropical ecosystems [as existing] in climate zones where it never freezes—a very fundamental biophysical threshold," Running said by email.
That keeps cold-intolerant species—such as mosquitoes—from migrating into today's temperate latitudes, he said. (Read about dengue fever's spread and its link to climate change.)
"If tropical climates move, the tropical-temperate geographic transition of ecosystems will be disrupted—with very unpredictable consequences."
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