Climate, air quality, and human health impacts of tropical aerosol emissions

The effects of aerosols on local and remote climates are myriad and complex. They are incompletely understood, particularly those due to aerosols from tropical regions. The health impacts are poorly constrained, with the future effects uncertain. Prior studies neglected the impacts of African and tropical aerosol perturbations, and the impacts of single continents following different transient emissions scenarios have not been explored. The new generation of climate models simulate the processes which mediate aerosol-driven changes in atmospheric composition and the climate in unprecedented detail. The Shared Socioeconomic Pathway emissions scenarios provide coherent narratives for future anthropogenic emissions and demographics, with much broader projected ranges in aerosol emissions than prior scenarios. To address some of these gaps, this PhD uses the recent UKESM1 model to investigate the climate and health impacts of idealised perturbations to tropical and African aerosol emissions, and scenarios of 21st century African aerosol and CO2 emissions. Tropical carbonaceous aerosol is found to cause overall warming in UKESM1 due to Black Carbon absorption, with surface cooling locally. This cooling persists when including additional CO2 emissions from Africa. The aerosols drive complex responses in precipitation, including significant Inter-Tropical Convergence Zone shifts and rainfall decreases due to atmospheric energy budget changes. Differing African emissions scenarios affect temperature and precipitation extremes, with variations from the mean responses. PM2.5 and O3 decrease globally into the future, remaining more constant in Africa. The human health impacts of air pollution therefore decrease when using present-day populations, but increase when using future projections. O3's impact is currently lower than PM2.5's, but this switches under strong mitigation. This project contributes to the understanding of the effect of tropical aerosols on the climate, both from idealised experiments and realistic scenarios, and shows the substantial impact of high air pollutant emissions in Africa in the 21st century.