An University of California - Riverside led team has identified black carbon and tropospheric ozone as the most likely drivers of large-scale atmospheric circulation change in the Northern Hemisphere tropics zone. While stratospheric ozone depletion has already been shown to be the primary driver of the expansion of the tropics in the Southern Hemisphere, the researchers are the first to report that black carbon and tropospheric ozone are the most likely primary drivers of the tropical expansion observed in the Northern Hemisphere. Led by climatologist Robert J. Allen, an assistant professor of Earth sciences at the University of California, Riverside, the research team notes that an unabated tropical belt expansion would impact large-scale atmospheric circulation, especially in the subtropics and mid-latitudes. \"If the tropics are moving poleward, then the subtropics will become even drier,\" Allen said.  \"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.\" Study results appear in the May 17 issue of Nature. Atmospheric circulation is the large-scale movement of air, and the means (together with the smaller ocean circulation) by which thermal energy is distributed on the surface of the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic climatological structure generally remains fairly constant and stable.  However, it can shift over long periods of time.   Observations show that the tropics have widened by 0.7 degrees latitude per decade, with warming from greenhouse gases also contributing to the expansion in both hemispheres. To study this expansion, the researchers first compared observational data with simulated data from climate models for 1979-1999 using a computer model called CMIP3. The researchers found that CMIP3 underestimates the observed 0.35 degrees latitude per decade expansion of the Northern Hemisphere tropics by about a third.  But when they included either black carbon or tropospheric ozone or both in CMIP3, the simulations mimicked observations better, suggesting that the pollutants were playing a role in the Northern Hemisphere tropical expansion. They then repeated the exercise with the GFDL Atmospheric Model.  Using these models allowed the researchers to directly isolate the effects of black carbon and tropospheric ozone on the location of the tropical boundaries. As before, they found that the models underestimate the observed Northern Hemisphere expansion of the tropics by about a third.  When black carbon and tropospheric ozone were incorporated in these models, however, the simulations showed better agreement with observations, underscoring the pollutants’ role in widening the tropical belt in the Northern Hemisphere. \"Both black carbon and tropospheric ozone warm the tropics by absorbing solar radiation,\" Allen explained. \"Because they are short-lived pollutants, with lifetimes of one-two weeks, their concentrations remain highest near the sources: the Northern Hemisphere low- to mid-latitudes.  It’s the heating of the mid-latitudes that pushes the boundaries of the tropics poleward.\" Black carbon aerosols are tiny particles of carbon produced from biomass burning and incomplete combustion of fossil fuels.  Most of the world’s black carbon production occurs in the Northern Hemisphere, with Southeast Asia being a major producer.  The same is true of tropospheric ozone, a secondary pollutant that results when volatile organic compounds react with sunlight.. Next, the research team will study the implications of the tropical expansion from a predominantly hydrological perspective. \"The question to ask is how far must the tropics expand before we start to implement policies to reduce the emissions of greenhouse gases, tropospheric ozone and black carbon that are driving the tropical expansion?\" said Allen, who joined UCR in 2011.