The Impact of Perturbations to Tropical Aerosols and their Precursors on Local and Remote Climates.

Abstract:

Due to their short lifetime, aerosols have large impacts near their emission region, but they also have effects on the climate in remote locations. This work uses the UK Earth System Model 1 (UKESM1) to investigate the atmospheric composition, climate, and health effects of tropical aerosols and precursors. We performed four idealised perturbation experiments in which a) tropical SO2 emissions were multiplied by a factor of 10; b) tropical biomass burning carbonaceous aerosol emissions were multiplied by 10; c) African biomass burning carbonaceous aerosol emissions were multiplied by 10; and d) tropical biomass burning carbonaceous aerosol emissions were entirely removed. Impacts on radiation fluxes, temperature, circulation and precipitation are investigated, both over the emission regions, where microphysical effects dominate, and remotely, where dynamical influences become more relevant. Increasing tropical SO2 emissions causes a global cooling, and the asymmetric forcing (stronger negative forcing in the Northern Hemisphere) drives a southward shift of the intertropical convergence zone (ITCZ). The experiment with the large increase in tropical biomass burning organic carbon (OC) and black carbon (BC) features a net warming globally, and a local cooling in locations where the aerosol load increases the most. This is because both OC and BC reduce radiation at the surface locally, causing cooling, but BC causes a warming of the atmospheric column which wins out on the global scale. Despite this warming, global precipitation decreases due to the atmospheric stabilising effect of BC. Using the Shared Socioeconomic Pathway scenarios (SSPs), transient future experiments have also been performed, testing the effect of Africa following a relatively more polluting route (SSP3-RCP7.0) to the rest of the world (SSP1-RCP1.9), relative to a global SSP1-RCP1.9 control. Impacts on both climate and health are investigated, both locally and remotely.