WASHINGTON, 25 May 2006 - The tropical regions of the world have been expanding since 1979, according to a study by U.S. climatologists who are unsure whether the phenomenon is caused by global warming or natural climate change.
After analysing satellite temperature data collected from 1979 to 2005, researchers estimated that the tropics expanded by two degrees of latitude, or 225 kilometres, during that period.
The phenomenon could explain the increase in droughts and decrease in precipitation observed in recent years in the subtropical regions of southwestern United States and Europe's Mediterranean basin, the scientists said in a paper published in the journal Science's May 26 issue.
"It's a big deal. The tropics may be expanding and getting larger," said study co-author Thomas Reichler, a University of Utah meteorology professor.
The tropical zone, geographically speaking, lies between the Tropic of Cancer at 23.5 degrees north latitude and the Tropic of Capricorn at 23.5 degrees south latitude. Meteorologists, however, generally consider that the tropics extend 30 degrees latitude north and south of the Equator.
Should the tropics "move another two or three degrees poleward in this century," said University of Washington atmospheric sciences professor and study co-author John Wallace, "very dry areas such as the Sahara Desert could nudge farther toward the pole, perhaps by a few hundred miles."
While global averages show warming of the lower atmosphere, or troposphere, the new study shows specifically that Earth's mid-latitudes grew about 0.83 degrees Celsius warmer over the past 26 years, researchers said.
They said they excluded the record mid-latitude temperatures in 1998 caused by the strong current warming phenomenon in the Pacific Ocean, dubbed El Nino, of 1997.
Earth has two polar jet streams at polar latitudes, one in each hemisphere, and two subtropical jet streams closer to the Equator, also one in each hemisphere.
The data show that the jet streams in both hemispheres have moved toward the poles.
"The jet streams mark the edge of the tropics, so if they are moving poleward that means the tropics are getting wider," Wallace said.
"We analyzed 26-year-long satellite measurements of atmospheric temperatures and found a distinct and very robust pattern of warming, which suggests that each subtropical jet stream has moved poleward by about one degree latitude," or 110 kilometres, Reichler said.
Weather balloons have provided independent confirmation of the satellite observation, he added.
"The possible expansion of the tropics may be a totally new aspect of climate change," Reichler said. "We don't know for sure what triggered it."
Researchers said that besides global warming, another possible cause for tropical expansion is the depletion of the stratospheric ozone layer due to pollutants such as refrigerant gases.
