Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone



Observational analyses have shown the width of the tropical belt increasing in recent decades as the world has warmed1. This expansion is important because it is associated with shifts in large-scale atmospheric circulation2,3,4 and major climate zones5,6. Although recent studies have attributed tropical expansion in the Southern Hemisphere to ozone depletion7,8,9,10, the drivers of Northern Hemisphere expansion are not well known and the expansion has not so far been reproduced by climate models11. Here we use a climate model with detailed aerosol physics to show that increases in heterogeneous warming agents—including black carbon aerosols and tropospheric ozone—are noticeably better than greenhouse gases at driving expansion, and can account for the observed summertime maximum in tropical expansion. Mechanistically, atmospheric heating from black carbon and tropospheric ozone has occurred at the mid-latitudes, generating a poleward shift of the tropospheric jet12, thereby relocating the main division between tropical and temperate air masses. Although we still underestimate tropical expansion, the true aerosol forcing is poorly known and could also be underestimated. Thus, although the insensitivity of models needs further investigation, black carbon and tropospheric ozone, both of which are strongly influenced by human activities, are the most likely causes of observed Northern Hemisphere tropical expansion.

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Figure 1: 1970–2009 annual mean tropospheric trends.
Figure 2: Observed and modelled 1979–1999 Northern Hemisphere tropical expansion based on five metrics.
Figure 3: Northern Hemisphere seasonal tropical expansion based on the combined ALL metric.
Figure 4: Northern Hemisphere 1970–2009 annual mean tropical expansion for each metric versus the expansion index for CAM simulations.


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This study was funded by R.J.A.’s University of California at Riverside initial complement. We acknowledge the individual modelling groups, the Program for Climate Model Diagnosis and Intercomparison and the Working Group on Coupled Modeling of the World Climate Research Programme (WCRP) for their part in making available the WCRP CMIP3 multimodel data set. Support of this data set is provided by the Office of Science, US Department of Energy.

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R.J.A. conceived the project, designed the study, carried out all data analysis and wrote the manuscript. S.C.S. advised on methods and interpretation, and assisted in the writing of the manuscript. J.N. provided homogenized cloud data and assisted in experimental design. C.S.Z. assisted with CAM experiments, including the SNICAR model.

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Correspondence to Robert J. Allen.

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Allen, R., Sherwood, S., Norris, J. et al. Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone. Nature 485, 350–354 (2012).

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