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Investigation of Climate Forcing Agents on Tropical Belt Width Through the 21st Century


Recent studies suggest that the tropics have expanded from ~2-5° over the past few decades and that this widening may continue into the future in association with global climate change. This expansion has potentially important implications for subtropical societies that include profound changes in the hydrological cycle and large-scale atmospheric circulation. Consistent with theory, models forced by greenhouse gases (GHG's) produce this expansion, however, some measurements have showed that observed tropical expansion is significantly larger than simulations of the 20th and 21st century from the Coupled Model Intercomparison Project version 3 (CMIP3). Recent climate simulations have shown that direct heating of the troposphere and surface, such as that caused by absorbing aerosols can drive expansion.

In this study, we quantify future changes in the tropical belt width using CAM3 and the 4 Representative Concentration Pathway's (RCP's) through the end of the 21st century. The RCP scenarios all assume stringent emissions controls on aerosols and their precursors, and hence include progressive decreases through the 21st century. Changes in tropical belt width are analyzed using 5 metrics. We find that the global, annual expansion rates vary from .09 +/- .05º and .14 +/- .03º decade-1 for GHG and SO2 forcings respectively under RCP 4.5. Furthermore, Northern hemisphere (NH) expansion rates vary from .05 +/- .04º and .12 +/- .02º decade-1 for GHG and SO2 forcings respectively under RCP 4.5. As widening increases with RCP, SO2 still accounts for as much widening as GHG's. Given the large reductions in future sulfate emissions with all RCP's reaching preindustrial values by 2100, we find that sulfate aerosols may be just as dominant a driver of tropical expansion in the northern hemisphere as GHG's through the end of the 21st century.

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