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

The primary drivers of the Northern Hemisphere expansion of the tropical climate zone over the past several decades are shown to be the recent increases in black carbon aerosols and tropospheric ozone rather than in greenhouse gases, which contribute to a lesser extent. Factors underlying northerly spread of the tropics The tropical climate zone has been expanding for several decades. Stratospheric ozone depletion has been identified as a factor in the expansion to the south, but the mechanisms responsible for expansion in the Northern Hemisphere have remained unclear. Now Allen et al . report that increases in black carbon and tropospheric ozone as a result of atmospheric heating and a shift in the tropospheric jet are the most likely cause of the expansion to the north. Greenhouse gases are a secondary influence. In terms of climate-mitigation policy, this work suggests that emission controls on black carbon and ozone precursors would not only help to mitigate global warming, but could also lessen the impacts of changes in large-scale atmospheric circulation in the Northern Hemisphere. Observational analyses have shown the width of the tropical belt increasing in recent decades as the world has warmed 1 . This expansion is important because it is associated with shifts in large-scale atmospheric circulation 2 , 3 , 4 and major climate zones 5 , 6 . Although recent studies have attributed tropical expansion in the Southern Hemisphere to ozone depletion 7 , 8 , 9 , 10 , the drivers of Northern Hemisphere expansion are not well known and the expansion has not so far been reproduced by climate models 11 . 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 jet 12 , 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 li