The principal goal of this project was to incorporate the science and policy of short-lived pollutants (with an emphasis on aerosols) into analysis of the coupled human-climate system ? a system that has largely been characterized and studied only in terms of long-lived greenhouse gases like CO2. Since aerosols and their precursors are emitted by the same processes that create greenhouse gases, an understanding of how they behave in the climate, how and where they impact human systems, and whether they potentially alter the human response to climate change is critical for both adaptation and mitigation.

Our interdisciplinary team combined methods from earth systems science and economics/policy analysis to probe the physical and human dynamics in this system, as well as the couplings between the natural and human systems (i.e., the influence of each on the other). Our work has led to a much deeper understanding of how both scientists and policymakers should jointly consider short-lived pollutants and long-lived greenhouse gases, as aerosols alter the physical distribution of climate impacts as well as the cost-benefit analysis of what humans consider to be climate damages and therefore use to motivate mitigation. This work is directly influencing policy through increased attention to aerosol and pollution damages from various sources (and the injustices posed by heterogeneity in those damages), including regional and federal decarbonization plans. 

This project has significantly advanced our scientific understanding of aerosol-climate interactions, the impacts of aerosols on human health and agriculture, and how these impacts modify the social costs of human emissions. The project findings and outcomes related to these three areas are detailed below:

Aerosol-Climate Interactions: The core climate modeling underlying this project was a set of experiments in which we simulated modern CO2 levels but an otherwise fairly pristine (preindustrial) aerosol background as a control condition. We then ran eight experiments in which we emitted a modern aerosol & aerosol precursor mix from one region only. By simulating the same set of emissions, but released from different locations, we were able to show that aerosol-climate interactions are strongly determined by the location of emission. This is due to both differential transport and removal, as well as differential coupling with general circulation (resulting in different radiative forcing efficiencies and precipitation patterns, for example). The same aerosol causes a different set of climate impacts depending on where it is emitted. (See Figure 1).

Aerosol Impacts on Human Systems: Although there is a strong public health literature linking pollution, especially respirable particulate matter (PM2.5), to premature mortality and other health impacts, this project propelled new estimates of aerosol health damages in regions with few monitors (sub-Saharan Africa; Figure 2), health impacts due to sources like wildfires and dust, and impacts due to closure of coal-fired power plants in the United States (Figure 3). In most of these studies, our collaboration was able to leverage novel data over large geographies and research designs that enabled stronger causal inference than prior work. Perhaps the most novel component of this project was a vastly improved understanding of the impacts of aerosols and other criteria pollutants on agricultural yields. Most of the foundational empirical work showing the impacts of aerosols ? both via direct radiative changes and climate-mediated effects ? and other pollutants on crop yields has been produced as part of this project (Figures 4,5).

Social Costs and Mitigation Dynamics: Finally, this project helped to show how considerations of aerosols and other short-lived pollutants can alter perceptions, benefit-cost calculations, and decisions around climate change mitigation, particularly when geographic heterogeneity of impacts is considered. First and most important, we have produced the methods and infrastructure to estimate a locally-specific social cost of emissions that takes into account both CO2 and co-emitted pollutants, and estimates the geographically-resolved damages from a set of emissions released from different regions of the planet (Figure 6). At the international level, we showed that depending on whether or not they chose to consider co-emitted aerosols in meeting their NDC agreements to the Paris Accord, countries could dramatically alter their local co-benefits. At national and more local levels like the state of California, we found that not all types of decarbonization would be expected to improve equitable air quality. As a result, our team has helped frame the importance of joint consideration of short-lived pollutants and long-lived greenhouse gases across spatial scales.

Finally, beyond the intellectual contributions and broad impacts noted above, this project helped train a strong team of interdisciplinary young scientists, including the PIs. Students and postdocs supported by this grant have been hired by leading institutions are building their own interdisciplinary groups, and the intellectual and computational infrastructure generated by this project will serve as a foundation for follow-on and related inquiries for years to come.

Last Modified: 01/04/2023
Modified by: Jennifer A Burney