Academic Publication

Peter Irvine, Elizabeth Burns, Ken Caldeira, Frank Keutsch, Dustin Tingley, and David Keith. 2021. “Expert judgments on solar geoengineering research priorities and challenges.” EarthArXiv. Publisher's VersionAbstract
Solar geoengineering describes a set of proposals to deliberately alter the earth’s radiative balance to reduce climate risks. We elicit judgements on natural science research priorities for solar geoengineering through a survey and in-person discussion with 72 subject matter experts, including two thirds of all scientists with ≥10 publications on the topic. Experts prioritized Earth system response (33%) and impacts on society and ecosystems (27%) over the human and social dimensions (17%) and developing or improving solar geoengineering methods (15%), with most allocating no effort to weather control or counter-geoengineering. While almost all funding to date has focused on geophysical modeling and social sciences, our experts recommended substantial funding for observations (26%), perturbative field experiments (16%), laboratory research (11%) and engineering for deployment (11%). Of the specific proposals, stratospheric aerosols received the highest average priority (34%) then marine cloud brightening (17%) and cirrus cloud thinning (10%). The views of experts with ≥10 publications were generally consistent with experts with <10 publications, though when asked to choose the radiative forcing for their ideal climate scenario only 40% included solar geoengineering compared to 70% of experts with <10 publications. This suggests that those who have done more solar geoengineering research are less supportive of its use in climate policy. We summarize specific research recommendations and challenges that our experts identified, the most salient of which were fundamental uncertainties around key climate processes, novel challenges related to solar geoengineering as a design problem, and the challenges of public and policymaker engagement.
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.
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.
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.
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.
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.
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.
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.
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.


Joshua B. Horton. 2018. “Parametric Insurance as an Alternative to Liability for Compensating Climate Harms.” Carbon & Climate Law Review, 12, 4, Pp. 285-296. Publisher's VersionAbstract

Interstate compensation for climate change based on legal liability faces serious obstacles. Structural incongruities related to causation, time, scope, and scale impede application of tort law to climate change, while political opposition from developed countries prevents intergovernmental consideration of liability as a means of compensating for climate damages. Insurance, however, in particular parametric insurance triggered by objective environmental indices, is emerging as a promising alternative to liability. This is manifest in the UNFCCC and the Paris Agreement, which ruled out recourse to legal liability, and in the formation and expansion of regional sovereign climate risk insurance schemes in the Caribbean, Africa, and the Pacific. Theory and early practice suggest that parametric insurance exhibits five key advantages compared to legal liability in the climate change context: (1) it does not require that causation be demonstrated; (2) it has evolved to provide catastrophic coverage; (3) it is oriented toward the future rather than the past; (4) it is contractual, rather than adversarial, in nature; and (5) it provides a high degree of predictability. Compensation based on parametric insurance represents a novel climate policy option with significant potential to advance climate politics.

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.