This group is a fast-growing team of researchers working at the intersection of climate science and technology with a focus on the science and public policy of solar geoengineering under the leadership of David Keith, Professor of Applied Physics at Harvard’s School of Engineering and Applied Sciences and Professor of Public Policy at the Harvard Kennedy School.

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Recent Publications

An interactive stratospheric aerosol model intercomparison of solar geoengineering by stratospheric injection of SO2 or accumulation-mode sulfuric acid aerosols

Debra Weisentein, Daniele Visioni, Henning Franke, Ulrike Niemeier, Sandro Vattioni, Garbiel Chiodo, Thomas Peter, and David Keith. 3/4/2022. “An interactive stratospheric aerosol model intercomparison of solar geoengineering by stratospheric injection of SO2 or accumulation-mode sulfuric acid aerosols.” Atmospheric Chemistry and Physics, 22, 5, Pp. 2955-2973. Publisher's VersionAbstract
Studies of stratospheric solar geoengineering have tended to focus on modification of the sulfuric acid aerosol layer, and almost all climate model experiments that mechanistically increase the sulfuric acid aerosol burden assume injection of SO2. A key finding from these model studies is that the radiative forcing would increase sublinearly with increasing SO2 injection because most of the added sulfur increases the mass of existing particles, resulting in shorter aerosol residence times and aerosols that are above the optimal size for scattering. Injection of SO3 or H2SO4 from an aircraft in stratospheric flight is expected to produce particles predominantly in the accumulation-mode size range following microphysical processing within an expanding plume, and such injection may result in a smaller average stratospheric particle size, allowing a given injection of sulfur to produce more radiative forcing. We report the first multi-model intercomparison to evaluate this approach, which we label AM-H2SO4 injection. A coordinated multi-model experiment designed to represent this SO3- or H2SO4-driven geoengineering scenario was carried out with three interactive stratospheric aerosol microphysics models: the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM2) with the Whole Atmosphere Community Climate Model (WACCM) atmospheric configuration, the Max-Planck Institute’s middle atmosphere version of ECHAM5 with the HAM microphysical module (MAECHAM5-HAM) and ETH’s SOlar Climate Ozone Links with AER microphysics (SOCOL-AER) coordinated as a test-bed experiment within the Geoengineering Model Intercomparison Project (GeoMIP). The intercomparison explores how the injection of new accumulation-mode particles changes the large-scale particle size distribution and thus the overall radiative and dynamical response to stratospheric sulfur injection. Each model used the same injection scenarios testing AM-H2SO4 and SO2 injections at 5 and 25 Tg(S) yr−1 to test linearity and climate response sensitivity. All three models find that AM-H2SO4 injection increases the radiative efficacy, defined as the radiative forcing per unit of sulfur injected, relative to SO2 injection. Increased radiative efficacy means that when compared to the use of SO2 to produce the same radiative forcing, AM-H2SO4 emissions would reduce side effects of sulfuric acid aerosol geoengineering that are proportional to mass burden. The model studies were carried out with two different idealized geographical distributions of injection mass representing deployment scenarios with different objectives, one designed to force mainly the midlatitudes by injecting into two grid points at 30◦ N and 30◦ S, and the other designed to maximize aerosol residence time by injecting uniformly in the region between 30◦ S and 30◦ N. Analysis of aerosol size distributions in the perturbed stratosphere of the models shows that particle sizes evolve differently in response to concentrated versus dispersed injections depending on the form of the injected sulfur (SO2 gas or AM-H2SO4 particulate) and suggests that prior model results for concentrated injection of SO2 may be strongly dependent on model resolution. Differences among models arise from differences in aerosol formulation and differences in model dynamics, factors whose interplay cannot be easily untangled by this intercomparison.
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Developing a Plume-in-Grid Model for Plume Evolution in the Stratosphere

Hongwei Sun, Sebastian Eastham, and David Keith. 3/21/2022. “Developing a Plume-in-Grid Model for Plume Evolution in the Stratosphere.” Journal of Advances in Modeling Earth Systems, 14, 4. Publisher's VersionAbstract
Stratospheric emissions from aircraft or rockets are important sources of chemical perturbations. Small-radius high-aspect-ratio plumes from stratospheric emissions are smaller than global Eulerian models' grid cells. To help global Eulerian models resolve subgrid plumes in the stratosphere, a Lagrangian plume model, comprising a Lagrangian trajectory model and an adaptive-grid plume model with a sequence of plume cross-section representations (from a highly resolved 2-D grid to a simplified 1-D grid based on a tradeoff between the accuracy and computational cost), is created and embedded into a global Eulerian (i.e., GEOS-Chem) model to establish a multiscale Plume-in-Grid (PiG) model. We compare this PiG model to the GEOS-Chem model based on a 1-month simulation of continuous inert tracer emissions by aircraft in the stratosphere. In the PiG results, the final injected tracer is more concentrated and approximately 1/3 of the tracer is at concentrations 2–4 orders of magnitude larger compared to the GEOS-Chem results. The entropy of injected tracer in the PiG results is 6% lower than the GEOS-Chem results, indicating less tracer mixing. The total product mass from a hypothetical second-order process (applied to the injected tracer) in the PiG results is 2 orders of magnitude larger than the GEOS-Chem results. Increasing the GEOS-Chem model's horizontal resolution 4-fold is insufficient to resolve this product difference, while requiring over seven times the computational resources of the PiG model. This paper describes the PiG model framework and parameterization of plume physical processes. Chemical and aerosol processes will be introduced in the future.
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Elicitation of US and Chinese expert judgments show consistent views on solar geoengineering

Zhen Dai, Elizabeth T. Burns, Peter J. Irvine, Dustin H. Tingley, Jianhua Xu, and David W. Keith. 2021. “Elicitation of US and Chinese expert judgments show consistent views on solar geoengineering.” Humanities and Social Sciences Communications, 8, 1, Pp. 1–9. Publisher's VersionAbstract
Expert judgments on solar geoengineering (SG) inform policy decisions and influence public opinions. We performed face-to-face interviews using formal expert elicitation methods with 13 US and 13 Chinese climate experts randomly selected from IPCC authors or supplemented by snowball sampling. We compare their judgments on climate change, SG research, governance, and deployment. In contrast to existing literature that often stress factors that might differentiate China from western democracies on SG, we found few significant differences between quantitative judgments of US and Chinese experts. US and Chinese experts differed on topics, such as desired climate scenario and the preferred venue for international regulation of SG, providing some insight into divergent judgments that might shape future negotiations about SG policy. We also gathered closed-form survey results from 19 experts with \textgreater10 publications on SG. Both expert groups supported greatly increased research, recommending SG research funding of \textasciitilde5% on average (10th–90th percentile range was 1–10%) of climate science budgets compared to actual budgets of \textless0.3% in 2018. Climate experts chose far less SG deployment in future climate policies than did SG experts.
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