Publications

2023
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.
clark_2023.pdf
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.
harding_belaia_and_keith_2023.pdf
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.

horton_et_al._2023.pdf
2022
Horton, Joshua. Carbon Removal Scoping Study, 2022. carbon_removal_scoping_study.pdf
Horton, Joshua. Solar Geoengineering Scoping Study, 2022. solar_geoengineering_scoping_study_v2.pdf
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.

rabitz_et_al._2022.pdf
2021
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.
belaia_moreno-cruz_and_keith_2021.pdf
Horton, Joshua B.Solar Geoengineering at a Standstill?Global Policy Opinion, 2021. Publisher's VersionAbstract

Joshua B. Horton argues that developing countries have the most to gain and to lose from resolving the impasse stalling global governance for geoengineering.

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.

dove_horton_and_ricke_2021.pdf
Aldy, Joseph E, Tyler Felgenhauer, William A Pizer, Massimo Tavoni, Mariia Belaia, Mark E Borsuk, Arunabha Ghosh, et al.Social science research to inform solar geoengineering: What are the benefits and drawbacks, and for whom?Science 374, no. 6569 (2021): 815-818. Publisher's Version aldy_et_al._2021.pdf
Aldy, Joseph E, Tyler Felgenhauer, William A Pizer, Massimo Tavoni, Mariia Belaia, Mark E Borsuk, Arunabha Ghosh, et al.Social science research to inform solar geoengineering.” Science 374, no. 6569 (2021): 815-818. Publisher's Version
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.
felgenhauer_et_al_2021.pdf
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.
fan_et_al_2021_nature_food.pdf
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.
irvine-etal-solar-geo-research-priorities-eartharxiv.pdf
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.
s41599-020-00694-6_1.pdf
Horton, Joshua, and David Keith. “Can Solar Geoengineering Be Used as a Weapon?Council on Foreign Relations, 2021. Publisher's Version
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.
Aerosol dynamics SCoPEx Golja et al.pdf
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.
Designing a radiative antidote to CO2
2020
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.
harding_et_al._2020.pdf
Keith, David. “The world needs to explore solar geoengineering as a tool to fight climate change.” Boston Globe, 2020. Publisher's Version

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