The Tropics Are Expanding

Atmospheric temperature measurements by U.S. weather satellites indicate Earth's hot, tropical zone has expanded farther from the equator since 1979. Researchers from the University of Utah and University of Washington say the apparent north-south widening of the tropics amounts to 2 degrees of latitude or 225 kilometers. But they do not know yet if the tropical expansion was triggered by natural climate variation or by human-caused phenomena such as depletion of the atmosphere's ozone layer or global warming due to the greenhouse effect.

"It's a big deal. The tropics may be expanding and getting larger," says study co-author Thomas Reichler, an assistant professor of meteorology at the University of Utah. "If this is true, it also would mean that subtropical deserts are expanding into heavily populated midlatitude regions." He added that droughts and unusually dry conditions in recent years in the subtropical American Southwest and Mediterranean Europe may be related to expansion of the tropics.

Reichler said the study makes no conclusion about the cause of the tropical expansion, but is purely observational, based on 1979-2005 measurements by nine weather satellites. While previous measurements dealt with global averages, the new study shows specifically that Earth's midlatitudes got about 17 degrees Celsius warmer during the past 26 years, suggesting there has been a change in the average position of the subtropical jet streams. These rivers of air, one in the Northern Hemisphere and one in the Southern Hemisphere, move west to east and mark the meteorological transition from tropical to subtropical climates.

"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 pole ward by about 1 degree latitude," Reichler says. "This poleward movement took place over both hemispheres, indicating that the tropics have been widening. Independent [weather balloon] observations of the atmosphere confirm these findings."

He adds: "The possible expansion of the tropics may be a totally new aspect of climate change. We don't know for sure what triggered it. My research is investigating whether it is related to global warming or not. One can certainly think of various mechanisms of how global warming-related changes in the atmosphere could induce the changes we see. But it's very speculative at this point. That's what our research is going to look at."

The researchers considered the possibility that the 26-year warming trend might be an illusion caused by data from the strong El Nino of 1997, which caused record midlatitude temperatures in 1998. But the midlatitude warming trend remained even when data from the 1997 El Nino was excluded.

If global warming isn't responsible for tropical expansion, another possible cause is the depletion of the stratospheric ozone layer due to pollutants such as refrigerant gases. Ozone loss cools the stratosphere while the troposphere warms the same pattern from global warming due to greenhouse gases.

Picture: The map shows areas of particularly strong warming of the lower atmosphere in yellow, orange and reddish colors. Note the enhanced warming of midlatitude regions north and south of the equator, indicating the expansion of the tropics. The map also shows pronounced warming at Arctic latitudes. Credit: Qiang Fu, University of Washington