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Humanmade Pollutants May Be Driving Earth's Tropical Belt Expansion May 17, 2012 Black carbon aerosols and tropospheric ozone, both humanmadepollutants emitted predominantly in the Northern Hemisphere's low-to mid-latitudes, are most likely pushing the boundary of thetropics further poleward in that hemisphere, new research by a teamof scientists shows. While stratospheric ozone depletion has already been shown to bethe primary driver of the expansion of the tropics in the SouthernHemisphere, the researchers are the first to report that blackcarbon and tropospheric ozone are the most likely primary driversof the tropical expansion observed in the Northern Hemisphere. Led by climatologist Robert J. Allen, an assistant professor ofEarth sciences at the University of California, Riverside, theresearch team notes that an unabated tropical belt expansion wouldimpact large-scale atmospheric circulation, especially in thesubtropics and mid-latitudes. "If the tropics are moving poleward, then the subtropics willbecome even drier," Allen said. "If a poleward displacement of themid-latitude storm tracks also occurs, this will shift mid-latitudeprecipitation poleward, impacting regional agriculture, economy andsociety." Study results appear in the May 17 issue of Nature. Observations show that the tropics have widened by 0.7 degreeslatitude per decade, with warming from greenhouse gases alsocontributing to the expansion in both hemispheres. To study thisexpansion, the researchers first compared observational data withsimulated data from climate models for 1979-1999. The simulateddata were generated by a collection of 20 climate models called theCoupled Model Intercomparison Project version 3 or "CMIP3." The researchers found that CMIP3 underestimates the observed 0.35degrees latitude per decade expansion of the Northern Hemispheretropics by about a third. But when they included either blackcarbon or tropospheric ozone or both in CMIP3, the simulationsmimicked observations better, suggesting that the pollutants wereplaying a role in the Northern Hemisphere tropical expansion. Next, to ensure that their results were not influenced by intrinsicdifferences between CMIP3's 20 models, the researchers expanded thetime period studied to 1970-2009, comparing available observed datawith simulated data from NCAR's Community Atmosphere Model (CMIP3data did not extend to 1970-2009). They then repeated the exercisewith the GFDL Atmospheric Model. Using these models allowed theresearchers to directly isolate the effects of black carbon andtropospheric ozone on the location of the tropical boundaries. As before, they found that the models underestimate the observedNorthern Hemisphere expansion of the tropics by about a third. Whenblack carbon and tropospheric ozone were incorporated in thesemodels, however, the simulations showed better agreement withobservations, underscoring the pollutants' role in widening thetropical belt in the Northern Hemisphere. "Both black carbon and tropospheric ozone warm the tropics byabsorbing solar radiation," Allen explained. "Because they areshort-lived pollutants, with lifetimes of one-two weeks, theirconcentrations remain highest near the sources: the NorthernHemisphere low- to mid-latitudes. It's the heating of themid-latitudes that pushes the boundaries of the tropics poleward." Allen further explained that with an expansion of the tropics, windpatterns also move poleward, dragging other aspects of atmosphericcirculation with them, such as precipitation. "For example, the southern portions of the United States may getdrier if the storm systems move further north than they were 30years ago," he said. "Indeed, some climate models have been showinga steady drying of the subtropics, accompanied by an increase inprecipitation in higher mid-latitudes. The expansion of thetropical belt that we attribute to black carbon and troposphericozone in our work is consistent with the poleward displacement ofprecipitation seen in these models." Black carbon aerosols are tiny particles of carbon produced frombiomass burning and incomplete combustion of fossil fuels. Most ofthe world's black carbon production occurs in the NorthernHemisphere, with Southeast Asia being a major producer. The same istrue of tropospheric ozone, a secondary pollutant that results whenvolatile organic compounds react with sunlight. "Greenhouse gases do contribute to the tropical expansion in theNorthern Hemisphere," Allen said. "But our work shows that blackcarbon and tropospheric ozone are the main drivers here. We need toimplement more stringent policies to curtail their emissions, whichwould not only help mitigate global warming and improve humanhealth, but could also lessen the regional impacts of changes inlarge-scale atmospheric circulation in the Northern Hemisphere." Thomas Reichler, an associate professor of atmospheric sciences atthe University of Utah, noted that the new work by the Allen-ledteam represents a major advance in climate dynamics research. "For a long time it has been unclear to the research community whyclimate models were unable to replicate the observed changes in theatmospheric wind structure," said Reichler, who was not involved inthe study. "This work demonstrates now in very convincing ways thatchanges in the amount and distribution of tiny absorbing particlesin the atmosphere are responsible for the observed changes. Sinceprevious model simulations did not account properly for the effectsof these particles on the atmosphere, this work provides asurprisingly simple but effective answer to the original question." Allen, who conceived the research project and designed the study,was joined in the research by Steven C. Sherwood at the Universityof New South Wales, Australia; Joel Norris at the ScrippsInstitution of Oceanography, San Diego; and Charles S. Zender at UCIrvine. Next, the research team will study the implications of the tropicalexpansion from a predominantly hydrological perspective. "The question to ask is how far must the tropics expand before westart to implement policies to reduce the emissions of greenhousegases, tropospheric ozone and black carbon that are driving thetropical expansion?" said Allen, who joined UCR in 2011. We are high quality suppliers, our products such as China Heating Furnace Parts , Tank Agitator Manufacturer for oversee buyer. 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