Aerosol Impacts on Climate Projections

Increasing anthropogenic aerosol emissions have profoundly impacted the climate over the industrial era. However, the radiative forcing of aerosols has persisted as one of the most uncertain aspects of present-day and industrial era climate modelling. This uncertainty in aerosol radiative forcing limits our ability to constrain estimates of climate sensitivity and regional climate changes. Over the coming decades reductions in anthropogenic aerosol emissions are expected in line with climate change and air quality mitigation policies. It is therefore vital to understand how the uncertainty in aerosol radiative forcing effects climate projections, and continue working towards reducing uncertainty in aerosol radiative forcing. The research in this thesis quantifies the range of changes in aerosol radiative forcing up to the mid-21st century for anthropogenic aerosol emission reduction scenarios. The effect of uncertainty in aerosol radiative forcing on future climate responses such as global mean temperature evolution and tropical precipitation shifts is explored. The usefulness of model performance in simulating observed surface solar radiation trends as a constraint on aerosol forcing is evaluated. The results demonstrate anthropogenic aerosol emission reductions scenarios cause a positive global mean radiative forcing up to mid-21st century, relative to 2000. There is a large uncertainty in the near-term projections of aerosol radiative forcing due to both model parametric uncertainty and scenario uncertainty. Sea spray emissions and updraft velocity are the dominant causes of variance in near-term global mean aerosol radiative forcing for a middle of the road emission scenario in the modelling framework we explore. The uncertainty in near-term aerosol radiative forcing in the middle of the road emissions scenario alone can cause a 5-year window in projecting the exceedance year of a global temperature rise of 1.5 °C, which the Paris Agreement aims to limit temperature rise to. A correlation between the uncertainty in aerosol radiative forcing and climate sensitivity would increase the projected exceedance window by many years. Observed shifts in tropical precipitation have been attributed to anthropogenic aerosol radiative forcing. Previous studies suggested the strength of the hemispheric contrast in aerosol radiative forcing can modulate the magnitude of tropical precipitation shifts. However, the results in this thesis show accounting for parametric model uncertainty and a greater influence from internal variability can obscure such a relationship between anthropogenic aerosol forcing and tropical precipitation shifts over the 20th century. Yet, there is a clear influence of major volcanic eruptions on shifts. However, in the future, under a high greenhouse gas emission scenario, there is relationship between the magnitude of present-day aerosol forcing and tropical precipitation shifts up to mid-21st century. The results in this thesis suggest that projections of both global mean temperature change and tropical precipitation shifts will be improved by reducing aerosol radiative forcing uncertainty. For tropical precipitation, any predictive gains may be offset by temporary shifts caused by potential future major volcanic eruptions. Trends in surface solar radiation are one line of evidence that has been suggested as a constraint on aerosol radiative forcing because the observations capture trends that coincide with the long-term evolution of aerosol emissions. The research in this thesis shows that caution is needed when using surface solar radiation as a model constraint because model performance, parameter influence and the relationship with aerosol radiative forcing varies between time periods when anthropogenic aerosol emissions increased compared to periods when emissions decreased, seasons, the degree of ocean coupling in the model, and the model ensemble size. The results in this thesis provide insight into the important impacts that the uncertainty in aerosol radiative forcing has on future climate projections, and highlight areas of research needed to better understand and reduce the uncertainty of aerosol-driven responses in climate projections.