2021
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. 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 >10 publications on SG. Both expert groups supported greatly increased research, recommending SG research funding of ~5% on average (10th–90th percentile range was 1–10%) of climate science budgets compared to actual budgets of <0.3% in 2018. Climate experts chose far less SG deployment in future climate policies than did SG experts.
C. M. Golja, L. W. Chew, J. A. Dykema, and D. W. Keith. 2021. “Aerosol Dynamics in the Near Field of the SCoPEx Stratospheric Balloon Experiment.” Journal of Geophysical Research. Publisher's VersionAbstract
Stratospheric aerosol injection (SAI) might alleviate some climate risks associated with accumulating greenhouse gases. Reduction of specific process uncertainties relevant to the distribution of aerosol in a turbulent stratospheric wake is necessary to support informed decisions about aircraft deployment of this technology. To predict aerosol size distributions we apply microphysical parameterizations of nucleation, condensation and coagulation to simulate an aerosol plume generated via injection of calcite powder or sulphate into a stratospheric wake with velocity and turbulence simulated by a three‐dimensional (3D) fluid dynamic calculation. We apply the model to predict the aerosol distribution that would be generated by a propeller wake in the Stratospheric Controlled Perturbation Experiment (SCoPEx). We find that injecting 0.1 g s‐1 calcite aerosol produces a nearly monodisperse plume and that at the same injection rate, condensable sulphate aerosol forms particles with average radii of 0.1 µm at 3 km downstream. We test the sensitivity of plume aerosol composition, size, and optical depth to the mass injection rate and injection location. Aerosol size distribution depends more strongly on injection rate than injection configuration. Comparing plume properties with specifications of a typical photometer, we find that plumes could be detected optically as the payload flies under the plume. These findings test the relevance of in situ sampling of aerosol properties by the SCoPEx outdoor experiment to enable quantitative tests of microphysics in a stratospheric plume. Our findings provide a basis for developing predictive models of SAI using aerosols formed in stratospheric aircraft wakes.
2020
Jacob T. Seeley, Nicholas J. Lutsko, and David W. Keith. 12/6/2020. “Designing a radiative antidote to CO2.” Geophysical Research Letters. Publisher's VersionAbstract
Solar Radiation Modification (SRM) reduces the CO2‐induced change to the mean global hydrological cycle disproportionately more than it reduces the CO2‐induced increase in mean surface temperature. Thus if SRM were used to offset all CO2‐induced mean warming, global‐mean precipitation would be less than in an unperturbed climate. Here we show that the mismatch between the mean hydrological effects of CO2 and SRM may partly be alleviated by spectrally tuning the SRM intervention (reducing insolation at some wavelengths more than others). By concentrating solar dimming at near‐infrared wavelengths, where H2O has strong absorption bands, the direct effect of CO2 on the tropospheric energy budget can be offset, which minimizes perturbations to the mean hydrological cycle. Idealized cloud‐resolving simulations of radiative‐convective equilibrium confirm that spectrally‐tuned SRM can simultaneously maintain mean surface temperature and precipitation at their unperturbed values even as large quantities of CO2 are added to the atmosphere.
David W. Keith and John M. Deutch. 12/2020. “Climate Policy Enters Four Dimensions.” Securing our Economic Future, Aspen Institute Press.
Zhen Dai, Debra K. Weisenstein, Frank N. Keutsch, and David W. Keith. 12/2020. “Experimental reaction rates constrain estimates of ozone response to calcium carbonate geoengineering.” Communications Earth & Environment, 1, 63. Publisher's VersionAbstract
Stratospheric solar geoengineering (SG) would impact ozone by heterogeneous chemistry. Evaluating these risks and methods to reduce them will require both laboratory and modeling work. Prior model-only work showed that CaCO3 particles would reduce, or even reverse ozone depletion. We reduce uncertainties in ozone response to CaCO3 via experimental determination of uptake coefficients and model evaluation. Specifically, we measure uptake coefficients of HCl and HNO3 on CaCO3 as well as HNO3 and ClONO2 on CaCl2 at stratospheric temperatures using a flow tube setup and a flask experiment that determines cumulative long-term uptake of HCl on CaCO3. We find that particle ageing causes significant decreases in uptake coefficients on CaCO3. We model ozone response incorporating the experimental uptake coefficients in the AER-2D model. With our new empirical reaction model, the global mean ozone column is reduced by up to 3%, whereas the previous work predicted up to 27% increase for the same SG scenario. This result is robust under our experimental uncertainty and many other assumptions. We outline systematic uncertainties that remain and provide three examples of experiments that might further reduce uncertainties of CaCO3 SG. Finally, we highlight the importance of the link between experiments and models in studies of SG.
Joshua B. Horton, Penehuro Lefale, and David Keith. 10/10/2020. “Parametric Insurance for Solar Geoengineering: Insights from the Pacific Catastrophe Risk Assessment and Financing Initiative.” Global Policy, Special Issue. Publisher's VersionAbstract
Solar geoengineering (SG) entails using technology to modify the Earth's radiative balance to offset some of the climate changes caused by long‐lived greenhouse gases. Parametric insurance, which delivers payouts when specific physical indices (such as wind speed) cross predefined thresholds, was recently proposed by two of us as a compensation mechanism for SG with the potential to ease disagreements about the technology and to facilitate cooperative deployment; we refer to this proposal as reduced‐rate climate risk insurance for solar geoengineering, or ‘RCG’. Here we probe the plausibility of RCG by exploring the Pacific Catastrophe Risk Assessment and Financing Initiative (PCRAFI), a sovereign risk pool providing parametric insurance coverage against tropical cyclones and earthquakes/tsunamis to Pacific island countries since 2013. Tracing the history of PCRAFI and considering regional views on insurance as compensation necessitates reconfiguring RCG in a way that shifts the focus away from bargaining between developed and developing countries toward bargaining among developed countries. This revised version of RCG is challenged by an assumption of broad developed country support for sovereign climate insurance in the developing world, but it also better reflects the underlying incentive structure and distribution of power.
Joshua B. Horton and Barbara Koremenos. 8/31/2020. “Steering and Influence in Transnational Climate Governance: Nonstate Engagement in Solar Geoengineering Research.” Global Environmental Politics, 20, 3, Pp. 93-111. Publisher's VersionAbstract
Theorists of transnational climate governance (TCG) seek to account for the increasing involvement of nonstate and substate actors in global climate policy. While transnational actors have been present in the emerging field of solar geoengineering—a novel technology intended to reflect a fraction of sunlight back to space to reduce climate impacts—many of their most significant activities, including knowledge dissemination, scientific capacity building, and conventional lobbying, are not captured by the TCG framework. Insofar as TCG is identified with transnational governance and transnational governance is important to reducing climate risks, an incomplete TCG framework is problematic for effective policy making. We attribute this shortcoming on the part of TCG to its exclusive focus on steering and corollary exclusion of influence as a critical component of governance. Exercising influence, for example, through inside and outside lobbying, is an important part of transnational governance—it complements direct governing with indirect efforts to inform, persuade, pressure, or otherwise influence both governor and governed. Based on an empirical analysis of solar geoengineering research governance and a theoretical consideration of alternative literatures, including research on interest groups and nonstate advocacy, we call for a broader theory of transnational governance that integrates steering and influence in a way that accounts for the full array of nonstate and substate engagements beyond the state.
Dharik Sanchan Mallapragada, Emre Gençer, Patrick Insinger, David Keith, and Francis Martin O’Sullivan. 8/19/2020. “Can Industrial-Scale Solar Hydrogen Supplied from Commodity Technologies Be Cost Competitive by 2030?” Cell Reports Physical Science, 100174. Publisher's VersionAbstract
Expanding decarbonization efforts beyond the power sector are contingent on cost-effective production of energy carriers, like H2, with near-zero life-cycle carbon emissions. Here, we assess the levelized cost of continuous H2 supply (95% availability) at industrial-scale quantities (100 tonnes/day) in 2030 from integrating commodity technologies for solar photovoltaics, electrolysis, and energy storage. Our approach relies on modeling the least-cost plant design and operation that optimize component sizes while adhering to hourly solar availability, production requirements, and component inter-temporal operating constraints. We apply the model to study H2 production costs spanning the continental United States and, through extensive sensitivity analysis, explore system configurations that can achieve $2.5/kg levelized costs or less for a range of plausible 2030 technology projections at high-irradiance locations. Notably, we identify potential sites and system configurations where PV-electrolytic H2 could substitute natural gas-derived H2 at avoided CO2 costs (%$120/ton), similar to the cost of deploying carbon capture and sequestration
Nicholas J. Lutsko, Jacob T Seeley, and David W. Keith. 5/2020. “Estimating Impacts and Trade‐offs in Solar Geoengineering Scenarios With a Moist Energy Balance Model.” Geophysical Research Letters, 47, 9.Abstract
There are large uncertainties in the potential impacts of solar radiation modification (SRM) and in how these impacts depend on the way SRM is deployed. One open question concerns trade‐offs between latitudinal profiles of insolation reduction and climate response. Here, a moist energy balance model is used to evaluate several SRM proposals, providing fundamental insight into how the insolation reduction profile affects the climate response. The optimal SRM profile is found to depend on the intensity of the intervention, as the most effective profile for moderate SRM focuses the reduction at high latitudes, whereas the most effective profile for strong SRM is tropically amplified. The effectiveness of SRM is also shown to depend on when it is applied, an important factor to consider when designing SRM proposals. Using an energy balance model allows us to provide physical explanations for these results while also suggesting future avenues of research with comprehensive climate models.
David Keith and Peter Irvine. 3/20/2020. “Halving warming with stratospheric aerosol geoengineering moderates policy-relevant climate hazards.” Environmental Research Letters, 15, 4. Publisher's VersionAbstract
Stratospheric aerosol geoengineering is a proposal to artificially thicken the layer of reflective aerosols in the stratosphere and it is hoped that this may offer a means of reducing average climate changes. However, previous work has shown that it could not perfectly offset the effects of climate change and there is a concern that it may worsen climate impacts in some regions. One approach to evaluating this concern is to test whether the absolute magnitude of climate change at each location is significantly increased (exacerbated) or decreased (moderated)relative to the period just preceding deployment. In prior work it was found that halving warming with an idealized solar constant reduction would substantially reduce climate change overall, exacerbating change in a small fraction of places. Here, we test if this result holds for a more realistic representation of stratospheric aerosol geoengineering using the data from the geoengineering large ensemble (GLENS). Using a linearized scaling of GLENS we find that halving warming with stratospheric aerosols moderates important climate hazards in almost all regions. Only 1.3% of land area sees exacerbation of change in water availability, and regions that are exacerbated see wetting not drying contradicting the common assumption that solar geoengineering leads to drying in general. These results suggest that halving warming with stratospheric aerosol geoengineering could potentially reduce key climate hazards substantially while avoiding some problems associated with fully offsetting warming.
Jesse L. Reynolds and Joshua B. Horton. 2/2020. “An earth system governance perspective on solar geoengineering.” Earth System Governance, 3.Abstract
Solar geoengineering appears capable of reducing climate change and the associated risks. In part because it would be global in effect, the governance of solar geoengineering is a central concern. The Earth System Governance (ESG) Project includes many researchers who, to varying degrees, utilize a common vocabulary and research framework. Despite the clear mutual relevance of solar geoengineering and ESG, few ESG researchers have considered the topic in substantial depth. To stimulate its sustained uptake as a subject within the ESG research program, we identify significant contributions thus far by ESG scholars on the subject of solar geoengineering governance and survey the wider solar geoengineering governance literature from the perspective of the new ESG research framework. Based on this analysis, we also suggest specific potential lines of inquiry that we believe are ripe for research by ESG scholars: nonstate actors’ roles, polycentricity, public engagement and participation, and the Anthropocene.
2019
Douglas MacMartin, Peter Irvine, Ben Kravitz, and Joshua Horton. 9/23/2019. “Technical characteristics of a solar geoengineering deployment and implications for governance.” Climate Policy, 19, 10, Pp. 1325-1339. Publisher's VersionAbstract
Consideration of solar geoengineering as a potential response to climate change will demand complex decisions. These include not only the choice of whether to deploy solar engineering, but decisions regarding how to deploy, and ongoing decisionmaking throughout deployment. Research on the governance of solar geoengineering to date has primarily engaged only with the question of whether to deploy. We examine the science of solar geoengineering in order to clarify the technical dimensions of decisions about deployment – both strategic and operational – and how these might influence governance considerations, while consciously refraining from making specific recommendations. The focus here is on a hypothetical deployment rather than governance of the research itself. We first consider the complexity surrounding the design of a deployment scheme, in particular the complicated and difficult decision of what its objective(s) would be, given that different choices for how to deploy will lead to different climate outcomes. Next, we discuss the on-going decisions across multiple timescales, from the sub-annual to the multi-decadal. For example, feedback approaches might effectively manage some uncertainties, but would require frequent adjustments to the solar geoengineering deployment in response to observations. Other decisions would be tied to the inherently slow process of detection and attribution of climate effects in the presence of natural variability. Both of these present challenges to decision-making. These considerations point toward particular governance requirements, including an important role for technical experts – with all the challenges that entails.
David Keith and Joshua Horton. 4/23/2019. “Multilateral parametric climate risk insurance: a tool to facilitate agreement about deployment of solar geoengineering?” Climate Policy. Publisher's VersionAbstract
States will disagree about deployment of solar geoengineering, technologies that would reflect a small portion of incoming sunlight to reduce risks of climate change, and most disagreements will be grounded in conflicting interests. States that object to deployment will have many options to oppose it, so states favouring deployment will have a powerful incentive to meet their objections. Objections rooted in opposition to the anticipated unequal consequences of deployment may be met through compensation, yet climate policy is inhospitable to compensation via liability. We propose that multilateral parametric climate risk insurance might be a useful tool to facilitate agreement on solar geoengineering deployment. With parametric insurance, predetermined payouts are triggered when climate indices deviate from set ranges. We suggest that states favouring deployment could underwrite reduced-rate parametric climate insurance. This mechanism would be particularly suited to resolving disagreements based on divergent judgments about the outcomes of proposed implementation. This would be especially relevant in cases where disagreements are rooted in varying levels of trust in climate model predictions of solar geoengineering effectiveness and risks. Negotiations over the pricing and terms of a parametric risk pool would make divergent judgments explicit and quantitative. Reduced-rate insurance would provide a way for states that favour implementation to demonstrate their confidence in solar geoengineering by underwriting risk transfer and ensuring compensation without the need for attribution. This would offer a powerful incentive for states opposing implementation to moderate their opposition.
Sandro Vattioni, Debra Weisenstein, David Keith, Aryeh Feinberg, Thomas Peter, and Andrea Stenke. 4/11/2019. “Exploring accumulation-mode H2SO4 versus SO2 stratospheric sulfate geoengineering in a sectional aerosol–chemistry–climate model.” Atmospheric Chemistry and Physics, 19. Publisher's Version
Daniel Heyen, Joshua Horton, and Juan Moreno-Cruz. 3/20/2019. “Strategic implications of counter-geoengineering: Clash or cooperation?” Journal of Environmental Economics and Management, 95, Pp. 153-177. Publisher's VersionAbstract
Solar geoengineering has received increasing attention as an option to temporarily stabilize global temperatures. A key concern is that heterogeneous preferences over the optimal amount of cooling combined with low deployment costs may allow the country with the strongest incentive for cooling, the so-called free-driver, to impose a substantial externality on the rest of the world. We analyze whether the threat of counter-geoengineering technologies capable of negating the climatic effects of solar geoengineering can overcome the free-driver problemand tilt the game in favour of international cooperation. Our game-theoreticalmodel of countries with asymmetric preferences allows for a rigorous analysis of the strategic interaction surrounding solar geoengineering and counter-geoengineering.We find that countergeoengineering prevents the free-driver outcome, but not always with benign effects. The presence of counter-geoengineering leads to either a climate clash where countries engage in a non-cooperative escalation of opposing climate interventions (negative welfare effect), a moratorium treaty where countries commit to abstain from either type of climate intervention (indeterminate welfare effect), or cooperative deployment of solar geoengineering (positivewelfare effect).We show that the outcome depends crucially on the degree of asymmetry in temperature preferences between countries.
Peter Irvine, Kerry Emanuel, Jie He, Larry Horowitz, Gabriel Vecchi, and David Keith. 3/11/2019. “Halving warming with idealized solar geoengineering moderates key climate hazards.” Nature Climate Change. Publisher's VersionAbstract

Solar geoengineering (SG) has the potential to restore average surface temperatures by increasing planetary albedo, but this could reduce precipitation. Thus, although SG might reduce globally aggregated risks, it may increase climate risks for some regions. Here, using the high-resolution forecast-oriented low ocean resolution (HiFLOR) model—which resolves tropical cyclones and has an improved representation of present-day precipitation extremes—alongside 12 models from the Geoengineering Model Intercomparison Project (GeoMIP), we analyse the fraction of locations that see their local climate change exacerbated or moderated by SG. Rather than restoring temperatures, we assume that SG is applied to halve the warming produced by doubling CO2 (half-SG). In HiFLOR, half-SG offsets most of the CO2-induced increase of simulated tropical cyclone intensity. Moreover, none of temperature, water availability, extreme temperature or extreme precipitation are exacerbated under half-SG when averaged over any Intergovernmental Panel on Climate Change (IPCC) Special Report on Extremes (SREX) region. Indeed, for both extreme precipitation and water availability, less than 0.4% of the ice-free land surface sees exacerbation. Thus, while concerns about the inequality of solar geoengineering impacts are appropriate, the quantitative extent of inequality may be overstated.

Toby Svoboda, Peter Irvine, Daniel Callies, and Masahiro Sugiyama. 2/7/2019. “The potential for climate engineering with stratospheric sulfate aerosol injections to reduce climate injustice.” Journal of Global Ethics. Publisher's Version
2018
Lee Miller and David Keith. 10/4/2018. “Climatic Impacts of Wind Power.” Joule, 2. Publisher's VersionAbstract

We find that generating today’s US electricity demand (0.5 TWe) with wind power would warm Continental US surface temperatures by 0.24C. Warming arises, in part, from turbines redistributing heat by mixing the boundary layer. Modeled diurnal and seasonal temperature differences are roughly consistent with recent observations of warming at wind farms, reflecting a coherent mechanistic understanding for how wind turbines alter climate. The warming effect is: small compared with projections of 21st century warming, approximately equivalent to the reduced warming achieved by decarbonizing global electricity generation, and large compared with the reduced warming achieved by decarbonizing US electricity with wind. For the same generation rate, the climatic impacts from solar photovoltaic systems are about ten times smaller than wind systems. Wind’s overall environmental impacts are surely less than fossil energy. Yet, as the energy system is decarbonized, decisions between wind and solar should be informed by estimates of their climate impacts.

Lee Miller and David Keith. 10/4/2018. “Observation-based solar and wind power capacity factors and powerdensities.” Environmental Research Letters, 13. Publisher's VersionAbstract

Power density is the rate of energy generation per unit of land surface area occupied by an energy system. The power density of low-carbon energy sources will play an important role in mediating the environmental consequences of energy system decarbonization as the world transitions away from high power-density fossil fuels. All else equal, lower power densities mean larger land and environmental footprints. The power density of solar and wind power remain surprisingly uncertain: estimates of realizable generation rates per unit area for wind and solar power span 0.3–47Wem−2 and 10–120Wem−2 respectively. We refine this range using US data from 1990–2016. We estimate wind power density from primary data, and solar power density from primary plant-level data and prior datasets on capacity density. The mean power density of 411 onshore wind power plants in 2016 was 0.50Wem−2. Wind plants with the largest areas have the lowest power densities. Wind power capacity factors are increasing, but that increase is associated with a decrease in capacity densities, so power densities are stable or declining. If wind power expands away from the best locations and the areas of wind power plants keep increasing, it seems likely that wind’s power density will decrease as total wind generation increases. The mean 2016 power density of 1150 solar power plants was 5.4Wem−2. Solar capacity factors and (likely) power densities are increasing with time driven, in part, by improved panel efficiencies. Wind power has a 10-fold lower power density than solar, but wind power installations directly occupy much less of the land within their boundaries. The environmental and social consequences of these divergent land occupancy patterns need further study.

Peter J. Irvine, David W. Keith, and John Moore. 7/27/2018. “Brief communication: Understanding solar geoengineering's potential to limit sea level rise requires attention from cryosphere experts.” The Cryosphere, 12, Pp. 2501-2513. Publisher's VersionAbstract
Stratospheric aerosol geoengineering, a form of solar geoengineering, is a proposal to add a reflective layer of aerosol to the stratosphere to reduce net radiative forcing and so to reduce the risks of climate change. The efficacy of solar geoengineering at reducing changes to the cryosphere is uncertain; solar geoengineering could reduce temperatures and so slow melt, but its ability to reverse ice sheet collapse once initiated may be limited. Here we review the literature on solar geoengineering and the cryosphere and identify the key uncertainties that research could address. Solar geoengineering may be more effective at reducing surface melt than a reduction in greenhouse forcing that produces the same global-average temperature response. Studies of natural analogues and model simulations support this conclusion. However, changes below the surfaces of the ocean and ice sheets may strongly limit the potential of solar geoengineering to reduce the retreat of marine glaciers. High-quality process model studies may illuminate these issues. Solar geoengineering is a contentious emerging issue in climate policy and it is critical that the potential, limits, and risks of these proposals are made clear for policy makers.