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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Some scientists suggest that it might be possible to reflect a portion of incoming sunlight back into space to reduce climate change and its impacts. Others argue that such solar radiation management (SRM) geoengineering is inherently incompatible with democracy. In this article, we reject this incompatibility argument. First, we counterargue that technologies such as SRM lack innate political characteristics and predetermined social effects, and that democracy need not be deliberative to serve as a standard for governance. We then rebut each of the argument’s core claims, countering that (1) democratic institutions are sufficiently resilient to manage SRM, (2) opting out of governance decisions is not a fundamental democratic right, (3) SRM may not require an undue degree of technocracy, and (4) its implementation may not concentrate power and promote authoritarianism. Although we reject the incompatibility argument, we do not argue that SRM is necessarily, or even likely to be, democratic in practice.
If solar geoengineering were to be deployed so as to mask a high level of global warming, and then stopped suddenly, there would be a rapid and damaging rise in temperatures. This effect is often referred to as termination shock, and it is an influential concept. Based on studies of its potential impacts, commentators often cite termination shock as one of the greatest risks of solar geoengineering. However, there has been little consideration of the likelihood of termination shock, so that conclusions about its risk are premature. This paper explores the physical characteristics of termination shock, then uses simple scenario analysis to plot out the pathways by which different driver events (such as terrorist attacks, natural disasters, or political action) could lead to termination. It then considers where timely policies could intervene to avert termination shock. We conclude that some relatively simple policies could protect a solar geoengineering system against most of the plausible drivers. If backup deployment hardware were maintained and if solar geoengineering were implemented by agreement among just a few powerful countries, then the system should be resilient against all but the most extreme catastrophes. If this analysis is correct, then termination shock should be much less likely, and therefore much less of a risk, than has previously been assumed. Much more sophisticated scenario analysis—going beyond simulations purely of worst‐case scenarios—will be needed to allow for more insightful policy conclusions.
Sulfate geoengineering is a proposed method to partially counteract the global radiative forcing from accumulated greenhouse gases, potentially mitigating some impacts of climate change. While likely to be effective in slowing increases in average temperatures and extreme precipitation, there are known side-effects and potential unintended consequences which have not been quantified. One such consequence is the direct human health impact. Given the significant uncertainties, we take a sensitivity approach to explore the mechanisms and range of potential impacts. Using a chemistry-transport model, we quantify the steady-state response of three public health risks to 1 °C global mean surface cooling. We separate impacts into those which are “radiative forcing-driven”, associated with climate change “reversal” through modification of global radiative forcing, and those “direct impacts” associated uniquely with using sulfate geoengineering to achieve this. We find that the direct (non-radiative forcing driven) impact is a decrease in global mortality of ∼13,000 annually. Here the benefits of reduced ozone exposure exceed increases in mortality due to UV and particulate matter, as each unit of injected sulfur incurs 1/25th the particulate matter exposure of a unit of sulfur emitted from surface sources. This reduction is exceeded by radiative forcing-driven health impacts resulting from using sulfate geoengineering to offset 1 °C of surface temperature rise. Increased particulate matter formation at these lower temperatures results in ∼39,000 mortalities which would have been avoided at higher temperatures. As such we estimate that sulfate geoengineering in 2040 would cause ∼26,000 (95% interval: −30,000 to +79,000) early deaths annually relative to the same year without geoengineering, largely due to the loss of health benefits associated with CO2-induced warming. These results account only for impacts due to changes in air quality and UV-B flux. They do not account for non-mortality impacts or changes in atmospheric dynamics, and must be considered in the wider context of other climate change impacts such as heatwave frequency and sea level rise.
Solar geoengineering, which seeks to cool the planet by reflecting a small fraction of sunlight back into space, has drawn the attention of scientists and policymakers as climate change remains unabated. Unlike mitigation, solar geoengineering could quickly and cheaply lower global temperatures. It is also imperfect. Its environmental impacts remain unpredictable, and its low cost and immediate effects may result in “moral hazard,” potentially crowding out costly mitigation efforts. There is little understanding about how the public will respond to such tradeoffs. To address this, a 1,000-subject nationally representative poll focused on solar geoengineering was conducted as part of the Cooperative Congressional Election Study (CCES) of the US electorate in October-November 2016. The importance that individuals place on solar geoengineering’s speed and cost predicts their support for it, but there is little to no relationship between their concerns about its shortcomings and support for its research and use. Acquiescence bias appears to be an important factor for attitudes around solar geoengineering and moral hazard.
Counter-geoengineering is the idea that a country might seek or threaten to counteract the cooling effect of solar geoengineering through technical means. Although this concept has been mentioned with increasing frequency in commentary on geoengineering, it has received little scholarly attention. We offer a preliminary analysis. We begin by distinguishing two kinds of counter-geoengineering: countervailing with a warming agent, and neutralising with a physical disruption. Based on this distinction, we review prior suggestions and describe novel methods by which either method might be accomplished, within the constraints imposed by deep technical uncertainties and substantial technical challenges. We then reflect on the strategic requirements and motivations for developing counter geoengineering and use a simple game-theoretic framework to demonstrate how counter-geoengineering might interact with the free-driver dynamic of solar geoengineering to shape climate geopolitics. We find that any state that could credibly threaten counter-geoengineering would effectively have a veto over the use of solar geoengineering, which could reduce the prospects of unilateral deployment. Alternatively, the development of geoengineering and countergeoengineering capabilities could lead to dangerous brinkmanship. We conclude that the development of counter-geoengineering would face considerable practical obstacles and would signal continuing political failure to manage climate risks on a cooperative basis.
Injection of sulfate aerosols into the stratosphere, a form of solar geoengineering, has been proposed as a means to reduce some climatic changes by decreasing net anthropogenic radiative forcing. The cost and technical feasibility of forming aerosols with the appropriate size distribution are uncertain. We examine the possibility of producing the relevant sulfur species, SO2 or SO3, by in situ conversion fromelemental sulfur onboard an aircraft. We provide afirst-order engineering analysis of an open cycle chemicalplant for in situ sulfur to sulfate conversion using a Brayton cycle combustor and a catalytic converter. We find that such a plant could have sufficiently low mass that the overall requirement for mass transport to the lower stratosphere may be reduced by roughly a factor of 2. All else equal, this suggests that—for a given radiative forcing—the cost of delivering sulfate aerosols may be nearly halved. Beyond reducing cost, the use of elemental sulfur reduces operational health and safety risks and should therefore reduce environmental side effects associated with delivery. Reduction in cost is not necessarily beneficial as it reduces practical barriers to deployment, increasing the urgency of questions concerningthe efficacy, risks, and governance of solar geoengineering.
Solar geoengineering refers to deliberately reducing net radiative forcing by reflecting some sunlight back to space, in order to reduce anthropogenic climate changes; a possible such approach would be adding aerosols to the stratosphere. If future mitigation proves insufficient to limit the rise in global mean temperature to less than 1.5°C above preindustrial, it is plausible that some additional and limited deployment of solar geoengineering could reduce climate damages. That is, these approaches could eventually be considered as part of an overall strategy to manage the risks of climate change, combining emissions reduction, net-negative emissions technologies and solar geoengineering to meet climate goals. We first provide a physical science review of current research, research trends and some of the key gaps in knowledge that would need to be addressed to support informed decisions. Next, since few climate model simulations have considered these limited-deployment scenarios, we synthesize prior results to assess the projected response if solar geoengineering were used to limit global mean temperature to 1.5°C above preindustrial in an overshoot scenario that would otherwise peak near 3°C. While there are some important differences, the resulting climate is closer in many respects to a climate where the 1.5°C target is achieved through mitigation alone than either is to the 3◦C climate with no geoengineering. This holds for both regional temperature and precipitation changes; indeed, there are no regions where a majority of models project that this moderate level of geoengineering would produce a statistically significant shift in precipitation further away from preindustrial levels. This article is part of the theme issue ‘The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels’.
We study the possibility of designing solar radiation management schemes to achieve a desired meridional radiative forcing (RF) profile using a two-dimensional chemistry-transport-aerosol model. Varying SO2 or H2SO4 injection latitude, altitude, and season, we compute RF response functions for a broad range of possible injection schemes, finding that linear combinations of these injection cases can roughly achieve RF profiles that have been proposed to accomplish various climate objectives. Globally averaged RF normalized by the sulfur injection rate (the radiative efficacy) is largest for injections at high altitudes, near the equator, and using emission of H2SO4 vapor into an aircraft wake to produce accumulation-mode particles. There is a trade-off between radiative efficacy and control as temporal and spatial control is best achieved with injections at lower altitudes and higher latitudes. These results may inform studies using more realistic models that couple aerosol microphysics, chemistry, and stratospheric dynamics.