Publications

    Sebastian Eastham, Sarah Doherty, David Keith, Jadwiga H. Richter, and Lili Xia. 2021. “Improving Models for Solar Climate Intervention Research.” Eos. Publisher's VersionAbstract

    Solar climate intervention, also known as solar radiation modification, is an approach intended to mitigate the impacts of climate change by reducing the amount of solar energy that the Earth system traps. It sits alongside three other plausible responses to climate risk: emission cuts and decarbonization, atmospheric carbon dioxide (CO2) removal, and adaptation to a changing climate.

    Unlike the other approaches, solar climate intervention (SCI), which comprises various techniques, aims to modify Earth’s radiation budget—the amounts and balance of solar energy that Earth absorbs and reflects—directly. Implementing SCI means either decreasing inbound solar (shortwave) radiation by reflecting it back into space before it is absorbed or increasing the amount of outbound terrestrial (longwave) radiation.

    Potential methods of SCI include stratospheric aerosol injection (SAI), marine cloud brighteningcirrus cloud thinningsurface albedo modification, and space-based methods involving, for example, mirrors (Figure 1). At present, the potential efficacy and risks of implementing these approaches to reduce climate change are highly uncertain and likely depend on how they are implemented.

    The Geoengineering Modeling Research Consortium (GMRC) was founded to coordinate SCI modeling research and to identify and resolve relevant issues with physical models, especially where existing climate research is unlikely to do so. Here we synthesize 2 years of GMRC meetings and research, and we offer specific recommendations for future model development.

    Yuanchao Fan, Jerry Tjiputra, Helene Muri, Danica Lombardozzi, Chang-Eui Park, Shengjun Wu, and David Keith. 2021. “Solar geoengineering can alleviate climate change pressures on crop yields.” Nature Food, 2, 5, Pp. 373–381. Publisher's VersionAbstract
    Solar geoengineering (SG) and CO2 emissions reduction can each alleviate anthropogenic climate change, but their impacts on food security are not yet fully understood. Using an advanced crop model within an Earth system model, we analysed the yield responses of six major crops to three SG technologies (SGs) and emissions reduction when they provide roughly the same reduction in radiative forcing and assume the same land use. We found sharply distinct yield responses to changes in radiation, moisture and CO2, but comparable significant cooling benefits for crop yields by all four methods. Overall, global yields increase \textasciitilde10% under the three SGs and decrease 5% under emissions reduction, the latter primarily due to reduced CO2 fertilization, relative to business as usual by the late twenty-first century. Relative humidity dominates the hydrological effect on yields of rainfed crops, with little contribution from precipitation. The net insolation effect is negligible across all SGs, contrary to previous findings.
    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, 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.
    Tyler Felgenhauer, Joshua Horton, and David Keith. 2021. “Solar geoengineering research on the U.S. policy agenda: when might its time come?” Environmental Politics, Pp. 1–21. Publisher's VersionAbstract
    Solar geoengineering (SG) may be a helpful tool to reduce harms from climate change, yet further research into its potential benefits and risks must occur prior to any implementation. So far, however, organized research on SG has been absent from the U.S. national policy agenda. We apply the Multiple Streams Approach analytical framework to explain why a U.S. federal SG research program has failed to materialize up to now, and to consider how one might emerge in the future. We argue that establishing a federal program will require the formation of an advocacy coalition within the political arena that is prepared to support such a policy objective. A coalition favoring federal research on SG does not presently exist, yet the potential nucleus of a future political grouping is evident in the handful of ‘pragmatist’ environmental organizations that have expressed conditional support for expanded research.
    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.
    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.
    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.
    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.
    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.

     

    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.

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