Human-made Pollutants May Be Expanding Tropical Zone, Study Says

U.S. scientists say emissions in the Northern Hemisphere of black carbon aerosols and ozone, both of which absorb solar radiation, are likely causing the hemisphere’s tropical regions to expand poleward. After comparing observations of tropic expansion — which suggest that the tropics have widened 0.7 degrees per decade since 1970, largely because of global warming — with climate models, researchers at University of California, Riverside, found that the climate models tended to underestimate that shift by about a third. But when they included either black carbon or tropospheric ozone — or both — into the models, the simulations mimicked observations better, suggesting that the emissions are playing a role in tropical expansion because of their radiation-absorbing effect. “If the tropics are moving poleward, then the subtropics will become even drier,” said Robert J. Allen, a professor Earth sciences and lead author of the study, published in the journal Nature. “If a poleward displacement of the mid-latitude storm tracks also occurs, this will shift mid-latitude precipitation poleward, impacting regional agriculture, economy, and society.”