Academic Publications

Clark, Britta. “How to Argue about Solar Geoengineering.” Journal of Applied Philosophy 40, no. 3 (2023): 505-520. Publisher's VersionAbstract
Should high-income countries engage in solar geoengineering research and possible deployment? On the assumption that the speed of the energy transition will be insufficient to abate catastrophic climate impacts, research into solar geoengineering begins to look like a technically and socially feasible route to mitigate such impacts. But on the assumption that a rapid and relatively just energy transition is still within the realm of political possibility, research into solar geoengineering looks more like an ideological tool designed to divert time and resources from less risky climate solutions. At the heart of debates over solar geoengineering, then, is disagreement over what political actors can be expected to do in the future. In this article, I argue that both objectors to and proponents of solar geoengineering research often make background assumptions regarding expected future actions that are either (a) inaccurate or (b) inconsistent. I propose an account of expected future actions that avoids these problems and sketch what the debate over solar geoengineering looks like with these assumptions in place.
Harding, Anthony R., Mariia Belaia, and David W. Keith. “The value of information about solar geoengineering and the two-sided cost of bias.” Climate Policy 23, no. 3 (2023): 355-365. Publisher's VersionAbstract
Solar geoengineering (SG) might be able to reduce climate risks if used to supplement emissions cuts and carbon removal. Yet, the wisdom of proceeding with research to reduce its uncertainties is disputed. Here, we use an integrated assessment model to estimate that the value of information that reduces uncertainty about SG efficacy. We find the value of reducing uncertainty by one-third by 2030 is around $4.5 trillion, most of which comes from reduced climate damages rather than reduced mitigation costs. Reducing uncertainty about SG efficacy is similar in value to reducing uncertainty about climate sensitivity. We analyse the cost of over-confidence about SG that causes too little emissions cuts and too much SG. Consistent with concerns about SG’s moral hazard problem, we find an over-confident bias is a serious and costly concern; but, we also find under-confidence that prematurely rules out SG can be roughly as costly. Biased judgments are costly in both directions. A coin has two sides. Our analysis quantitatively demonstrates the risk-risk trade-off around SG and reinforces the value of research that can reduce uncertainty.
Horton, Joshua B., Kerryn Brent, Zhen Dai, Tyler Felgenhauer, Oliver Geden, Jan McDonald, Jeffrey McGee, Felix Schenuit, and Jianhua Xu. “Solar geoengineering research programs on national agendas: a comparative analysis of Germany, China, Australia, and the United States.” Climatic Change 176 (2023). Publisher's VersionAbstract

Solar geoengineering (SG), or the proposed use of technology to reflect sunlight back to space as a means of partially counteracting climate change, requires systematic research funded by public bodies, yet no dedicated national SG research programs (“programs”) currently exist. To explain why and understand how things might change in the future, we add concepts from role theory, a research tradition focused on international relations and foreign policy analysis, to the Multiple Streams Approach, a theoretical framework devel- oped to study agenda setting at the national level, to assess policy processes related to SG research in four countries: Germany, China, Australia, and the United States (US). The results of our analysis indicate that, among these four states, only the US might plausi- bly consider initiating a program under present conditions. Germany, China, and Australia appear likely to seriously consider comparable efforts only in response to a US program, although their reasons for doing so and specific program designs would differ. The source of this variation, we argue, is the different foreign policy paradigms—or “national role conceptions”—prevailing in each state, which mediate between domestic and international politics and help define which policy proposals qualify as viable in different countries. From a policy perspective, this suggests that the global trajectory of SG depends disproportionately on developments in the US.

Rabitz, Florian, Marian Feist, Matthias Honegger, Joshua Horton, Sikina Jinnah, and Jesse Reynolds. “A preliminary framework for understanding the governance of novel environmental technologies: Ambiguity, indeterminateness and drift.” Earth System Governance 12 (2022). Publisher's VersionAbstract

We propose a conceptual framework to explain why some technologies are more difficult to govern than others in global environmental governance. We start from the observation that some technologies pose transboundary environmental risks, some provide capacities for managing such risks, and some do both. For “ambiguous” technologies, potential risks and risk management capacities are uncertain, unknown or even unknowable. Governance systems are indeterminate towards ambiguous technologies, as existing norms, rules, scripts and routines do not imply default solutions under institutional focal points. Indeterminateness can lead to institu- tional drift, with risks accordingly remaining unmitigated and risk management capacities remaining unex- ploited. We use the cases of solar geoengineering, gene drive systems and bioinformatics for illustrating this framework. As technological ambiguity may often be irresolvable, we conclude that it might force us to confront the limits to anticipatory global decision-making on matters of long-term environmental sustainability.

Belaia, Mariia, Juan B. Moreno-Cruz, and David W. Keith. “Optimal Climate Policy in 3D: Mitigation, Carbon Removal, and Solar Geoengineering .” Climate Change Economics 12, no. 3 (2021). Publisher's VersionAbstract
We introduce solar geoengineering (SG) and carbon dioxide removal (CDR) into an integrated assessment model to analyze the trade-offs between mitigation, SG, and CDR. We propose a novel empirical parameterization of SG that disentangles its efficacy, calibrated with climate model results, from its direct impacts. We use a simple parameterization of CDR that decouples it from the scale of baseline emissions. We find that (a) SG optimally delays mitigation and lowers the use of CDR, which is distinct from moral hazard; (b) SG is deployed prior to CDR while CDR drives the phasing out of SG in the far future; (c) SG deployment in the short term is relatively independent of discounting and of the long-term trade-off between SG and CDR over time; (d) small amounts of SG sharply reduce the cost of meeting a 2∘2°C target and the costs of climate change, even with a conservative calibration for the efficacy of SG.
Dove, Zachary, Joshua Horton, and Katharine Ricke. “The middle powers roar: Exploring a minilateral solar geoengineering deployment scenario.” Futures 132 (2021). Publisher's VersionAbstract

The prospect of solar geoengineering, which would entail reflecting a small fraction of incoming sunlight back to space to cool the planet, has been slowly but steadily rising on the climate policy agenda. Early research suggests that solar geoengineering could substantially reduce climate risks, but its development and potential use would be accompanied by an array of ecological and sociopolitical risks and governance challenges. Here we reflect on our participation in a solar geoengineering governance scenario exercise conducted at the 2019 International Summer School on Geoengineering Governance. In the scenario with which we engaged, a group of ‘middle powers’ intend to force the issue of solar geoengineering onto the international agenda after decades of deadlock and in the face of intensifying climate impacts. As participants in this ex- ercise, we confronted a range of problems and issues we judged likely to arise. In this article, we discuss a number of these, including the manner in which political considerations are likely to influence the physical and technical aspects of deployment schemes, as well as ways in which emergency framing may undermine political legitimacy. These and other aspects of possible future deployment of solar geoengineering warrant additional targeted scenario analysis.

Felgenhauer, Tyler, Joshua Horton, and David Keith. “Solar geoengineering research on the U.S. policy agenda: when might its time come?Environmental Politics (2021): 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.
Fan, Yuanchao, Jerry Tjiputra, Helene Muri, Danica Lombardozzi, Chang-Eui Park, Shengjun Wu, and David Keith. “Solar geoengineering can alleviate climate change pressures on crop yields.” Nature Food 2, no. 5 (2021): 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 ~10% 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.
Irvine, Peter, Elizabeth Burns, Ken Caldeira, Frank Keutsch, Dustin Tingley, and David Keith. “Expert judgments on solar geoengineering research priorities and challenges.” EarthArXiv (2021). 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.
Dai, Zhen, Elizabeth Burns, Peter Irvine, Dustin Tingley, Jianhua Xu, and David Keith. “Elicitation of US and Chinese expert judgments show consistent views on solar geoengineering.” Humanities and Social Sciences Communications 8, no. 1 (2021). 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.
Golja, C. M., L. W. Chew, J. A. Dykema, and D. W. Keith. “Aerosol Dynamics in the Near Field of the SCoPEx Stratospheric Balloon Experiment.” Journal of Geophysical Research (2021). 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.
Seeley, Jacob T., Nicholas J. Lutsko, and David W. Keith. “Designing a radiative antidote to CO2.” Geophysical Research Letters (2021). 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.
Harding, Anthony R., Katharine Ricke, Daniel Heyen, Douglas G. MacMartin, and Juan Moreno-Cruz. “Climate econometric models indicate solar geoengineering would reduce inter-country income inequality.” Nature Communications 11 (2020).Abstract
Exploring heterogeneity in the economic impacts of solar geoengineering is a fundamental step towards understanding the risk tradeoff associated with a geoengineering option. To evaluate impacts of solar geoengineering and greenhouse gas-driven climate change on equal terms, we apply macroeconomic impact models that have been widely applied to climate change impacts assessment. Combining historical evidence with climate simulations of mean annual temperature and precipitation, we project socio-economic outcomes under high anthropogenic emissions for stylized climate scenarios in which global temperatures are stabilized or over-cooled by blocking solar radiation. We find impacts of climate changes on global GDP-per-capita by the end of the century are temperature-driven, highly dispersed, and model dependent. Across all model specifications, however, income inequality between countries is lower with solar geoengineering. Consistent reduction in inter-country inequality can inform discussions of the distribution of impacts of solar geoengineering, a topic of concern in geoengineering ethics and governance debates.
Reynolds, Jesse, and Joshua Horton. “An earth system governance perspective on solar geoengineering.” Earth System Governance 3 (2020). Publisher's VersionAbstract
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.
Dai, Zhen, Debra K. Weisenstein, Frank N. Keutsch, and David W. Keith. “Experimental reaction rates constrain estimates of ozone response to calcium carbonate geoengineering.” Communications Earth & Environment 1, no. 63 (2020). 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.
Horton, Joshua B., and Barbara Koromenos. “Steering and Influence in Transnational Climate Governance: Nonstate Engagement in Solar Geoengineering Research.” Global Environmental Politics 20, no. 3 (2020): 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.
Lutsko, Nicholas J., Jacob T. Seeley, and David W. Keith. “Estimating Impacts and Trade‐offs in Solar Geoengineering Scenarios With a Moist Energy Balance Model.” Geophysical Research Letters 47, no. 9 (2020). Publisher's VersionAbstract
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.
Horton, Joshua B., Penehuro Lefale, and David Keith. “Parametric Insurance for Solar Geoengineering: Insights from the Pacific Catastrophe Risk Assessment and Financing Initiative.” Global Policy, no. Special Issue (2020). 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.
Keith, David, and Peter Irvine. “Halving warming with stratospheric aerosol geoengineering moderates policy-relevant climate hazards.” Environmental Research Letters 15, no. 4 (2020). 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.
MacMartin, Douglas, Peter Irvine, Ben Kravitz, and Joshua Horton. “Technical characteristics of a solar geoengineering deployment and implications for governance.” Climate Policy 19, no. 10 (2019): 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.
Dagon, Katherine, and Daniel Schrag. “Quantifying the effects of solar geoengineering on vegetation.” Climatic Change 152, no. 1-2 (2019): 235–251. Publisher's VersionAbstract
Climate change will have significant impacts on vegetation and biodiversity. Solar geoengineering has potential to reduce the climate effects of greenhouse gas emissions through albedo modification, yet more research is needed to better understand how these techniques might impact terrestrial ecosystems. Here, we utilize the fully coupled version of the Community Earth System Model to run transient solar geoengineering simulations designed to stabilize radiative forcing starting mid-century, relative to the Representative Concentration Pathway 6 (RCP6) scenario. Using results from 100-year simulations, we analyze model output through the lens of ecosystem-relevant metrics. We find that solar geoengineering improves the conservation outlook under climate change, but there are still potential impacts on terrestrial vegetation. We show that rates of warming and the climate velocity of temperature are minimized globally under solar geoengineering by the end of the century, while trends persist over land in the Northern Hemisphere. Moisture is an additional constraint on vegetation, and in the tropics the climate velocity of precipitation dominates over that of temperature. Shifts in the amplitude of temperature and precipitation seasonal cycles have implications for vegetation phenology. Different metrics for vegetation productivity also show decreases under solar geoengineering relative to RCP6, but could be related to the model parameterization of nutrient cycling. The coupling of water and carbon cycles is found to be an important mechanism for understanding changes in ecosystems under solar geoengineering.
Keith, David, and Joshua Horton. “Multilateral parametric climate risk insurance: a tool to facilitate agreement about deployment of solar geoengineering?Climate Policy (2019). 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.
Vattioni, Sandro, Debra Weisenstein, David Keith, Aryeh Feinberg, Thomas Peter, and Andrea Stenke. “Exploring accumulation-mode H2SO4 versus SO2 stratospheric sulfate geoengineering in a sectional aerosol–chemistry–climate model.” Atmospheric Chemistry and Physics 19 (2019). Publisher's VersionAbstract
Stratospheric sulfate geoengineering (SSG) could contribute to avoiding some of the adverse impacts of climate change. We used the SOCOL-AER global aerosol–chemistry–climate model to investigate 21 different SSG scenarios, each with 1.83 Mt S yr−1 injected either in the form of accumulation-mode H2SO4 droplets (AM H2SO4), gas-phase SO2 or as combinations of both. For most scenarios, the sulfur was continuously emitted at an altitude of 50 hPa (≈20 km) in the tropics and subtropics. We assumed emissions to be zonally and latitudinally symmetric around the Equator. The spread of emissions ranged from 3.75 S–3.75 N to 30 S–30 N. In the SO2 emission scenarios, continuous production of tiny nucleation-mode particles results in increased coagulation, which together with gaseous H2SO4 condensation, produces coarse-mode particles. These large particles are less effective for backscattering solar radiation and have a shorter stratospheric residence time than AM H2SO4 particles. On average, the stratospheric aerosol burden and corresponding all-sky shortwave radiative forcing for the AM H2SO4 scenarios are about 37 % larger than for the SO2 scenarios. The simulated stratospheric aerosol burdens show a weak dependence on the latitudinal spread of emissions. Emitting at 30 N–30 S instead of 10 N–10 S only decreases stratospheric burdens by about 10 %. This is because a decrease in coagulation and the resulting smaller particle size is roughly balanced by faster removal through stratosphere-to-troposphere transport via tropopause folds. Increasing the injection altitude is also ineffective, although it generates a larger stratospheric burden, because enhanced condensation and/or coagulation leads to larger particles, which are less effective scatterers. In the case of gaseous SO2 emissions, limiting the sulfur injections spatially and temporally in the form of point and pulsed emissions reduces the total global annual nucleation, leading to less coagulation and thus smaller particles with increased stratospheric residence times. Pulse or point emissions of AM H2SO4 have the opposite effect: they decrease the stratospheric aerosol burden by increasing coagulation and only slightly decrease clear-sky radiative forcing. This study shows that direct emission of AM H2SO4 results in higher radiative forcing for the same sulfur equivalent mass injection strength than SO2 emissions, and that the sensitivity to different injection strategies varies for different forms of injected sulfur.
Heyen, Daniel, Joshua Horton, and Juan Moreno-Cruz. “Strategic implications of counter-geoengineering: Clash or cooperation?Journal of Environmental Economics and Management 95 (2019): 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.
Svoboda, Toby, Peter Irvine, Daniel Callies, and Masahiro Sugiyama. “The potential for climate engineering with stratospheric sulfate aerosol injections to reduce climate injustice.” Journal of Global Ethics (2019). Publisher's VersionAbstract
Climate engineering with stratospheric sulfate aerosol injections (SSAI) has the potential to reduce risks of injustice related to anthropogenic emissions of greenhouse gases. Relying on evidence from modeling studies, this paper makes the case that SSAI could have the potential to reduce many of the key physical risks of climate change identified by the Intergovernmental Panel on Climate Change. Such risks carry potential injustice because they are often imposed on low-emitters who do not benefit from climate change. Because SSAI has the potential to reduce those risks, it thereby has the potential to reduce the injustice associated with anthropogenic emissions. While acknowledging important caveats, including uncertainty in modeling studies and the potential for SSAI to carry its own risks of injustice, the paper argues that there is a strong case for continued research into SSAI, especially if attention is paid to how it might be used to reduce emissions-driven injustice.
Irvine, Peter, Kerry Emanuel, Jie He, Larry Horowitz, Gabriel Vecchi, and David Keith. “Halving warming with idealized solar geoengineering moderates key climate hazards.” Nature Climate Change (2019). 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 forecastoriented 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.

 

Horton, Joshua B.Parametric Insurance as an Alternative to Liability for Compensating Climate Harms.” Carbon & Climate Law Review 12, no. 4 (2018): 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.
Smith, Wake, and Gernot Wagner. “Stratospheric aerosol injection tactics and costs in the first 15 years of deployment.” Environmental Research Letters 13 (2018). Publisher's VersionAbstract
We review the capabilities and costs of various lofting methods intended to deliver sulfates into the lower stratosphere. We lay out a future solar geoengineering deployment scenario of halving the increase in anthropogenic radiative forcing beginning 15 years hence, by deploying material to altitudes as high as ~20 km. After surveying an exhaustive list of potential deployment techniques, we settle upon an aircraft-based delivery system. Unlike the one prior comprehensive study on the topic (McClellan et al 2012 Environ. Res. Lett. 7 034019), we conclude that no existing aircraft design—even with extensive modifications—can reasonably fulfill this mission. However, we also conclude that developing a new, purpose-built high-altitude tanker with substantial payload capabilities would neither be technologically difficult nor prohibitively expensive. We calculate early-year costs of ~$1500 ton−1 of material deployed, resulting in average costs of ~$2.25 billion yr−1 over the first 15 years of deployment. We further calculate the number of flights at ~4000 in year one, linearly increasing by ~4000 yr−1. We conclude by arguing that, while cheap, such an aircraft-based program would unlikely be a secret, given the need for thousands of flights annually by airliner-sized aircraft operating from an international array of bases.
Irvine, Peter J., David W. Keith, and John Moore. “Brief communication: Understanding solar geoengineering's potential to limit sea level rise requires attention from cryosphere experts.” The Cryosphere 12 (2018): 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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