Aerosol Indirect Effects and 21st Century Northern Hemisphere Tropical Widening

Abstract:

Observations show the tropical belt has widened over the past few decades, a phenomenon associated with poleward migration of subtropical dry zones and large-scale atmospheric circulation. In addition to greenhouse gases, studies have also associated tropical widening with stratospheric ozone loss and heterogeneous warming agents. Here, we investigate how tropical belt width will change through the 21st century in response to several climate forcing agents. Models that include aerosol indirect effects yield significantly larger Northern Hemisphere (NH) tropical widening than models that lack aerosol indirect effects, including a reversal of the projected NH tropical contraction under RCP4.5 to significant tropical widening. Additional simulations show that future reductions in anthropogenic aerosols drive NH tropical widening as large as, or larger than, greenhouse gases. Although aerosol indirect effects remain uncertain, we conclude that they are important drivers of 21st century climate change and that future efforts to reduce air pollution may have significant impacts on the width of the tropical belt.