"Tropical expansion is a relatively newly discovered climate change signal, with hemispheric implications, as opposed to being isolated to smaller-scale local or regional implications," Robert Allen of the University of California Riverside, US, told environmentalresearchweb. "It has important societal implications particularly from a hydrological perspective."

According to Allen, expansion of the subtropical dry zones near +/– 30 degrees, where large-scale descending air favours dry conditions, is likely to make many places near the poleward edge of the zones drier. Tropical expansion may also cause the mid-latitude storm tracks to move towards the poles, decreasing rainfall in locations on the equatorward flank of the storm track.

Allen and colleagues from the University of New South Wales, Australia, the University of California, San Diego and University of California, Irvine compared climate models from the Coupled Model Intercomparison Project version 3 (CMIP3) with observations from 1979–1999. The models significantly underestimated the expansion of the Northern Hemisphere tropics but including black carbon and tropospheric ozone gave estimates closer to the observations.

"We show that components of air pollution – those that absorb solar radiation, heating the atmosphere – are pushing the edge of the tropical belt farther poleward in the Northern Hemisphere," said Allen. "Prior work showed greenhouse gases are the main driver of tropical expansion. They do contribute, but not as much as black carbon and tropospheric ozone in the Northern Hemisphere."

Black carbon is produced by biomass burning and incomplete combustion of fossil fuels while tropospheric ozone is produced when volatile organic compounds react in sunlight. "Cutting down on aerosol emissions would not only enhance air quality and human health, but lessen large-scale changes in atmospheric circulation," said Allen.

Black carbon and tropospheric ozone could both explain the peak in tropical expansion seen in the summer, since the compounds cause warming mainly by absorbing solar radiation, which is more abundant at this time, say the researchers.

"Because black carbon and tropospheric ozone are short-lived pollutants, with lifetimes of one-two weeks, their concentrations remain highest near the sources: the Northern Hemisphere low- to mid-latitudes," explained Allen. "It's the heating of the mid-latitudes that pushes the boundaries of the tropics poleward."

Even including black carbon and tropospheric ozone, the models still underestimate the tropical expansion – the team believes this may be due to factors such as inaccurate observations of black-carbon levels, non-linear responses or models being insufficiently sensitive.

In the Southern Hemisphere, stratospheric ozone depletion is fingered as the main culprit for tropical expansion, with greenhouse gases also contributing. "Because there are less aerosol emissions in the Southern Hemisphere, black carbon and tropospheric ozone do not significantly enhance Southern Hemisphere tropical expansion," said Allen.

Now the researchers, who reported their work in Nature, plan to focus on the hydrological impacts of a widening tropical belt.