Impacts of anthropogenic aerosols on air quality, climate, and extremes

Anthropogenic aerosols have significant impacts on global air quality and climate. However, there are still major uncertainties in our understanding of their characteristics and effects, particularly the mechanisms through which they influence the Earth’s climate and the wider environment. Using simulations from the Community Earth System Model (CESM1) for the recent past and future, this thesis seeks to enhance our understanding of the multifaceted impacts of anthropogenic aerosols. Time-slice model simulations for 1970 and 2010 indicate that once the presentday climate has fully responded to 1970-2010 changes in all forcings including greenhouse gases (GHGs), anthropogenic aerosols and ozone, both the global mean temperature and precipitation responses will be roughly double the transient ones. The temperature response per unit effective radiative forcing (ERF) for short-lived climate forcers (SLCFs) varies considerably across many factors, suggesting that the ERF should be used carefully to interpret the climate impacts of SLCFs. Changes in the probability distribution of global-mean daily precipitation are dominantly driven by GHG changes, but by aerosols when averaged regionally over Asia and Europe. Next, the impacts of two major policy-relevant emission drivers of 1970-2010 aerosol changes, energy use growth and technology advances, are investigated. Energy use growth dominates the total aerosol changes and associated climate impacts, from and within Asia in particular. However, technology advances outweigh the impacts of energy use growth over Europe and North America. The temperature response per unit aerosol ERF varies significantly across many factors, including location and magnitude of aerosol-related emission changes, questioning again the utility and robustness of ERF, and related metrics, in interpreting climate change. The 1970-2010 changes in air pollution are driven predominately by anthropogenic emissions while climate change (i.e., changes in air pollutants attributable to changes in meteorologies driven by GHGs, solar radiation, etc.) also contributes. The overall changes in air pollution lead to an extra 1.7 million deaths per year due to PM2.5 and 87,000 yr-1 related to O3, and losses of 166 million tons yr-1 of staple crop production with value 53 billion USD2010 yr-1. The effects attributable to anthropogenic emissions reflect a “tug-of-war” between energy use growth and emission control measures, emphasizing the key role of policymaking in influencing global environmental wellbeing. Aerosol-related emissions are expected to decline in the future; this may generate large impacts on climate extremes on top of modulating mean climate. This thesis thus makes use of transient model simulations under the Representative Concentration Pathway 8.5, and seeks to understand how future aerosol reductions will influence climate extremes, focusing particularly on heatwaves worldwide and precipitation extremes over Asia. Results show that there will be more severe heatwaves globally due primarily to mean warming, with minor contributions from future temperature variability changes. These changes are mainly associated with GHG increases, while aerosol reductions contribute significantly over the Northern Hemisphere (particularly Europe and China). Further, per unit of global surface warming, aerosol reductions, compared to GHG increases, induce a disproportionally stronger response in heatwave metrics via aerosol-cloud interactions. The Asian monsoon region will get progressively warmer and wetter as GHGs increase, while precipitation extremes will be significantly aggravated due to aerosol reductions. Such aggravations are driven by local-scale aerosol-cloud interactions over northern East Asia but by aerosol changes induced large-scale circulation anomalies over southern East and South Asia. This thesis provides a comprehensive assessment of the impacts of aerosols on air quality, climate and extremes. These impacts, despite large uncertainties related to the representation of aerosols in climate models, are largely under the direct control of policy interventions, offering policymakers significant influence on future global habitability. The importance of aerosols in changing climate up to the present-day is very relevant for projections of future climate, and climate extremes and related risk management in particular. The above findings shed light on, and provide motivation for, further studies aimed at reducing uncertainties in aerosol effects, and constraining aerosol processes in climate models for reliable future climate projections.