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.
These temperature patterns were a surprise. Climate models looking at the effects of global warming have captured the poleward migration of the jet streams. But models also suggested that "the tropics would almost behave like a slab," warming rapidly but fairly evenly between 35 degrees north and south, notes John Wallace, a University of Washington atmospheric scientist and another member of the team. Instead, the outer tropics are warming faster than the deep tropics.
"Are the models missing something?" he asks. "Or are we seeing something unrepresentative? Maybe the same thing won't happen over the next 25 years and the deep tropics will catch up."
Several team members are now focusing their efforts on uncovering the factors driving the apparent trend.
Several lines of evidence suggest that these patterns may be triggered by a long-term increase in sea-surface temperatures in the western Pacific and Indian Oceans, says Ngar-Cheung Lau, with the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory at Princeton University.
Dr. Lau notes that the long-term shift uncovered by the new research mirrors a shorter-term change in circulation patterns that El Niños leave behind as they wane. One implication: If the ocean temperatures continue to rise and the tropics keep expanding, the results could weaken El Niño and strengthen its sister, La Niña, he says. That could be of particular interest for the US and countries that border the Caribbean; La Niñas tend to encourage the formation of Atlantic hurricanes.
Meanwhile, two independent studies also published Friday suggest that as greenhouse-gas emissions continue to increase, warming will probably reach as much as 2 degrees Centigrade higher than the current range of 1.5 to 4.5 degrees that United Nations reports suggest. The evidence comes from ice-core records of temperature and CO2 changes during the rise and fall of continental glaciers, notes Margaret Torn, with the US Department of Energy's Lawrence Berkeley Laboratory in Berkeley, Calif.
In one of those studies, she and colleague John Harte used ice-core data from Antarctica along with climate models to estimate the amount of warming one can expect from additional carbon dioxide released from plants, soils, and the ocean.