Fundamental limits and trade-offs of stratospheric aerosol geoengineering

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Reducing net emissions of CO2 and other greenhouse gas (GHG) is essential to any response to climate change, but may not occur fast enough to avoid significant climate impacts. Model projections of stratospheric aerosol geoengineering suggest that it could reduce some climate impacts, and thus might potentially become an additional element of a comprehensive climate change strategy. However, current knowledge is insufficient to support informed decisions. A critical question in evaluating geoengineering is what are the fundamental limits or trade-offs in how well geoengineering can manage the climate response from increased GHG? That is, what can geoengineering do, and what can it not do? Building on recent research, this project will address this essential question. The fundamental motivation for this research is to understand a potential option to reduce future climate impacts. Better information is needed both to support future decisions around deployment, and support the development of governance capacity that will be needed to make these decisions. This research will enable a more complete view of the impacts of deploying geoengineering than has previously been possible, by generating simulations that capture a more comprehensive set of deployment options rather than just one or two; and furthermore will assess the extent to which different objectives can or cannot be simultaneously met.This project will generate a set of climate model simulations that each make different choices for which climate goals to prioritize relative to others, and use this to identify potential tradeoffs (sets of objectives that are mutually exclusive) and boundaries (which objectives are achievable and which are not). Throughout this process, the research team will engage policy and governance experts, regarding the potential range of climate goals that might motivate different actors, and on the governance implications of identified trade-offs. The full range of possible strategies has never been explored, in part because optimization over the space of available degrees of freedom – primarily latitudes and seasons of aerosol injection – is complicated by uncertainty and nonlinear interactions (from both microphysics and aerosol-heating-induced changes in stratospheric circulation), and compounded by combinatorial computational complexity. To address these challenges, the research team will combine three innovations. First, the key enabler to this research is an initial assessment on the “size” of the design space; how many usefully-independent degrees of freedom are there? This reduces the combinatorial problem. Second, the computational burden can be reduced by separating the simulations needed to understand the spatial- and seasonal- distribution of stratospheric aerosol optical depth (AOD), which can be short but require a complete stratosphere model, from those needed to assess the climate response to a specified aerosol distribution, which require multi-decadal simulations but not an accurate stratosphere. And third, nonlinearities and uncertainty can be managed through feedback that adjusts injection rates; this enables comparing simulations based on specified objectives rather than specified injection rates. The research team will design a suite of simulations that individually meet different objectives and collectively span the space of possible outcomes. From this, the key tool in evaluating and visualizing trade-offs is through Pareto-optimal surfaces: how do strategies and their responses change as a function of the optimization criteria. Although the simulations will be focused on understanding physical science tradeoffs, social and governance dimensions play a critical role in understanding which objectives may be most important to achieve or which strategies are simply politically infeasible, thus limiting the space in ways not revealed by climate modeling. Therefore, the research team will interface with governance experts throughout to ensure research informs policy. Simulations will also be made available to the wider international community.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.