Scientists have detected signs that the planet's tropics may have expanded much farther north in the past 3 decades than climate models had predicted for the next century. If the findings are confirmed and the trend continues, it could place major strains on subtropical ecosystems, hasten the spread of tropical diseases, and generally make life less pleasant for populations living with the zones of change.
The tilt of Earth's axis creates the tropical zones, which form a 47°-latitude belt around the planet's midsection. Also helping form these warm-weather regions are the distribution of water, land, winds, and currents. These processes take warm, moist air from the equator and send it toward the poles, where it raises temperatures in regions such as most of Central and South America, central and southern Africa, Southeast Asia, and a good bit of Australia. Climate models predict that global warming could be causing the tropics to expand. So far, they have suggested a creep of 2° of latitude north and south, but only over the next century.
To find out what has happened so far, a team led by climate scientist Dian Seidel of the National Oceanic and Atmospheric Administration in Silver Spring, Maryland, examined the stratosphere for signs of change in the tropics. She and colleagues surveyed five sets of data collected by satellites and weather balloons from 1979 to 2000. The data showed that tropical climate patterns, such as increased ozone concentrations and temperatures, in the stratosphere had expanded by up to 4.5° of latitude--depending on the observations--in the Northern Hemisphere during that short period.
Seidel says the reason for the difference between the model predictions and the observed data could be that the models tend to concentrate on the lower atmosphere and Earth's surface, not on the interactions with the stratosphere. Although the changes there are indeed occurring, "we don't know yet what that means for the surface," says Seidel, whose team reported the findings online 2 December in Nature Geoscience.
Atmospheric scientist John Wallace of the University of Washington, Seattle, says the survey "makes a compelling case that the tropical belt has widened substantially over the past 30 years," and if it continues at the same rate, "it will have major societal implications."