Their widening - by an average 140 miles - could shift storm tracks, dry out southern Europe, and grow some deserts.
The tropics - the globe's most torrid climate belt - have widened during the past 27 years, expanding toward the poles by an average of about 140 miles, according to new research.
If the trend continues through the end of the century, it would drive rain-bearing storms toward higher latitudes, deprive heavily populated southern Europe of much-needed winter rain and snow, and expand the world's subtropical deserts, atmospheric scientists say.
"It's a big deal," notes Thomas Reichler, a University of Utah atmospheric scientist and a member of the research team, which reported its results in Friday's issue of the journal Science.
Some aspects of the results are consistent with global-warming projections, team members note. If the cause does prove to be global warming, these results would represent the first direct satellite evidence of its impact on worldwide atmospheric circulation, says team leader Qiang Fu, a researcher at the University of Washington.
But some of their results also are strikingly at odds with the models, leaving the door ajar for other suspects.
Although tantalizing hints of the expansion of the tropics came from balloon-borne sensors, it took satellite temperature information to nail the full scope of the change. From 1979 to 2005, the highest temperature increases in the lowest layer of the atmosphere, the troposphere, occurred in vast swaths centered on 30 degrees latitude. Meanwhile the steepest cooling in the next layer of atmosphere, the stratosphere, occurred in these same regions. The net effect, researchers say, has been to nudge the average paths of swift rivers of air known as the subtropical jet streams farther north and south. These paths mark the meteorological border between the tropics and temperate regions, and the landscape beneath these boundaries tend to be hot and dry.