Publications

    Wake Smith and Gernot Wagner. 11/23/2018. “Stratospheric aerosol injection tactics and costs in the first 15 years of deployment.” Environmental Research Letters, 13. 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.
    Peter J. Irvine, David W. Keith, and John Moore. 7/27/2018. “Brief communication: Understanding solar geoengineering's potential to limit sea level rise requires attention from cryosphere experts.” The Cryosphere, 12, Pp. 2501-2513. Publisher's VersionAbstract
    Stratospheric aerosol geoengineering, a form of solar geoengineering, is a proposal to add a reflective layer of aerosol to the stratosphere to reduce net radiative forcing and so to reduce the risks of climate change. The efficacy of solar geoengineering at reducing changes to the cryosphere is uncertain; solar geoengineering could reduce temperatures and so slow melt, but its ability to reverse ice sheet collapse once initiated may be limited. Here we review the literature on solar geoengineering and the cryosphere and identify the key uncertainties that research could address. Solar geoengineering may be more effective at reducing surface melt than a reduction in greenhouse forcing that produces the same global-average temperature response. Studies of natural analogues and model simulations support this conclusion. However, changes below the surfaces of the ocean and ice sheets may strongly limit the potential of solar geoengineering to reduce the retreat of marine glaciers. High-quality process model studies may illuminate these issues. Solar geoengineering is a contentious emerging issue in climate policy and it is critical that the potential, limits, and risks of these proposals are made clear for policy makers.
    Joshua B. Horton, Jesse L. Reynolds, Holly Jean Buck, Daniel Callies, Stefan Schäfer, David W. Keith, and Steve Rayner. 6/28/2018. “Solar Geoengineering and Democracy.” Global Environmental Politics, Pp. 5-24. Publisher's VersionAbstract
    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.
    Sebastian D. Eastham, Debra K. Weisenstein, David W. Keith, and Steven R. H. Barrett. 5/25/2018. “Quantifying the impact of sulfate geoengineering on mortality from air quality and UV-B exposure.” Atmospheric Environment. Publisher's VersionAbstract
    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.
    Aseem Mahajan, Dustin Tingley, and Gernot Wagner. 5/2018. “Fast, cheap, and imperfect? U.S. public opinion about solar geoengineering.” Environmental Politics. Publisher's VersionAbstract
    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.
    Douglas G. MacMartin, Katharine L. Ricke, and David W. Keith. 4/2/2018. “Solar geoengineering as part of an overall strategy for meeting the 1.5°C Paris target.” Philosophical Transactions of the Royal Society, 376, 2119.Abstract
    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’.
    Jordan P. Smith, John Dykema, and David Keith. 4/2/2018. “Production of Sulfates Onboard an Aircraft: Implications for the Cost and Feasibility of Stratospheric Solar Geoengineering.” Earth and Space Science. Publisher's VersionAbstract
    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, SOor 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.
    Andy Parker and Peter Irvine. 3/11/2018. “The Risk of Termination Shock From Solar Geoengineering.” Earth's Future, 6, Pp. 456-467. Publisher's VersionAbstract
    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.
    Zhen Dai, Debra Weisenstein, and David Keith. 1/2018. “Tailoring Meridional and Seasonal Radiative Forcing by Sulfate Aerosol Solar Geoengineering.” Geophysical Research Letters, 45. Publisher's VersionAbstract
    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.
    Joshua B. Horton. 2018. “Parametric Insurance as an Alternative to Liability for Compensating Climate Harms.” Carbon & Climate Law Review, 12, 4, Pp. 285-296. Publisher's VersionAbstract

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

    Dustin Tingley and Gernot Wagner. 10/31/2017. “Solar geoengineering and the chemtrails conspiracy on social media.” Palgrave Communications, 3, 12. Publisher's VersionAbstract
    Discourse on social media of solar geoengineering has been rapidly increasing over the past decade, in line with increased attention by the scientific community and low but increasing awareness among the general public. The topic has also found increased attention online. But unlike scientific discourse, a majority of online discussion focuses on the so-called chemtrails conspiracy theory, the widely debunked idea that airplanes are spraying a toxic mix of chemicals through contrails, with supposed goals ranging from weather to mind control. This paper presents the results of a nationally representative 1000-subject poll part of the 36,000-subject 2016 Cooperative Congressional Election Study (CCES), and an analysis of the universe of social media mentions of geoengineering. The former shows ~ 10% of Americans declaring the chemtrails conspiracy as “completely” and a further ~ 20–30% as “somewhat” true, with no apparent difference by party affiliation or strength of partisanship. Conspiratorial views have accounted for ~ 60% of geoengineering discourse on social media over the past decade. Of that, Twitter has accounted for >90%, compared to ~ 75% of total geoengineering mentions. Further affinity analysis reveals a broad online community of conspiracy. Anonymity of social media appears to help its spread, so does the general ease of spreading unverified or outright false information. Online behavior has important real-world reverberations, with implications for climate science communication and policy.
    David W. Keith, Gernot Wagner, and Claire L. Zabel. 9/1/2017. “Solar geoengineering reduces atmospheric carbon burden.” Nature Climate Change, 7, Pp. 617–619. Publisher's VersionAbstract

    Solar geoengineering is no substitute for cutting emissions, but could nevertheless help reduce the atmospheric carbon burden. In the extreme, if solar geoengineering were used to hold radiative forcing constant under RCP8.5, the carbon burden may be reduced by ~100 GTC, equivalent to 12–26% of twenty-first-century emissions at a cost of under US$0.5 per tCO2.

    Masahiro Sugiyama, Shinichiro Asayama, Atsushi Ishii, Takanobu Kosugi, John C. Moore, Jolene Lin, Penehuro F. Lefale, Wil Burns, Masatomo Fujiwara, Arunabha Ghosh, Joshua Horton, Atsushi Kurosawa, Andy Parker, Michael Thompson, Pak-Hang Wong, and Lili Xia. 7/4/2017. “The Asia-Pacific’s role in the emerging solar geoengineering debate.” Climatic Change. Publisher's VersionAbstract
    Increasing interest in climate engineering in recent years has led to calls by the international research community for international research collaboration as well as global public engagement. But making such collaboration a reality is challenging. Here, we report the summary of a 2016 workshop on the significance and challenges of international collaboration on climate engineering research with a focus on the Asia-Pacific region. Because of the region’s interest in benefits and risks of climate engineering, there is a potential synergy between impact research on anthropogenic global warming and that on solar radiation management. Local researchers in the region can help make progress toward better understanding of impacts of solar radiation management. These activities can be guided by an ad hoc Asia-Pacific working group on climate engineering, a voluntary expert network. The working group can foster regional conversations in a sustained manner while contributing to capacity building. An important theme in the regional conversation is to develop effective practices of dialogues in light of local backgrounds such as cultural traditions and past experiences of large-scale technology development. Our recommendation merely portrays one of several possible ways forward, and it is our hope to stimulate the debate in the region.
    Peter J. Irvine, Ben Kravitz, Mark G. Lawrence, Dieter Gerten, Cyril Caminade, Simon N.Gosling, Erica J. Hendy, Belay T. Kassie, W. Daniel Kissling, Helene Muri, Andreas Oschlies, and Steven J. Smith. 1/24/2017. “Towards a comprehensive climate impacts assessment of solar geoengineering.” Earth's Future, 5, Pp. 93–106. Publisher's VersionAbstract

    Despite a growing literature on the climate response to solar geoengineering—proposals to cool the planet by increasing the planetary albedo—there has been little published on the impacts of solar geoengineering on natural and human systems such as agriculture, health, water resources, and ecosystems. An understanding of the impacts of different scenarios of solar geoengineering deployment will be crucial for informing decisions on whether and how to deploy it. Here we review the current stateof knowledge about impacts of a solar-geoengineered climate and identify the major research gaps. We suggest that a thorough assessment of the climate impacts of a range of scenarios of solar geoengineering deployment is needed and can be built upon existing frameworks. However, solar geoengineering poses a novel challenge for climate impacts research as the manner of deployment could be tailored to pursue different objectives making possible a wide range of climate outcomes. We present a number of ideas for approaches to extend the survey of climate impacts beyond standard scenarios of solargeoengineering deployment to address this challenge. Reducing the impacts of climate change is the fundamental motivator for emissions reductions and for considering whether and how to deploy solargeoengineering. This means that the active engagement of the climate impacts research community will be important for improving the overall understanding of the opportunities, challenges, and risks presented by solar geoengineering.

    Jesse L. Reynolds, Andy Parker, and Peter Irvine. 12/13/2016. “Five solar geoengineering tropes that have outstayed their welcome.” Earth's Future, 4, Pp. 562–568. Publisher's VersionAbstract

    In the last decade, solar geoengineering (solar radiation management, or SRM) has receivedincreasing consideration as a potential means to reduce risks of anthropogenic climate change. Some ideas regarding SRM that have been proposed have receded after being appropriately scrutinized, while others have strengthened through testing and critique. This process has improved the understanding ofSRM’s potential and limitations. However, several claims are frequently made in the academic and popular SRM discourses and, despite evidence to the contrary, pose the risk of hardening into accepted facts. Here, in order to foster a more productive and honest debate, we identify, describe, and refute five of the most problematic claims that are unsupported by existing evidence, unlikely to occur, or greatly exaggerated. These are: (A) once started, SRM cannot be stopped; (B) SRM is a right-wing project; (C) SRM wouldcost only a few billion dollars per year; (D) modeling studies indicate that SRM would disrupt monsoonprecipitation; and (E) there is an international prohibition on outdoors research. SRM is a controversial proposed set of technologies that could prove to be very helpful or very harmful, and it warrants vigorous and informed public debate. By highlighting and debunking some persistent but unsupported claims, this paper hopes to bring rigor to such discussions.

    David Keith, Debra Weisenstein, John Dykema, and Frank Keutsch. 12/12/2016. “Stratospheric Solar Geoengineering without Ozone Loss.” Proceedings of the National Academy of Sciences. Publisher's VersionAbstract

    Injecting sulfate aerosol into the stratosphere, the most frequently analyzed proposal for solar geoengineering, may reduce some climate risks, but it would also entail new risks, including ozone loss and heating of the lower tropical stratosphere, which, in turn, would increase water vapor concentration causing additional ozone loss and surface warming. We propose a method for stratospheric aerosol climate modification that uses a solid aerosol composed of alkaline metal salts that will convert hydrogen halides and nitric and sulfuric acids into stable salts to enable stratospheric geoengineering while reducing or reversing ozone depletion. Rather than minimizing reactive effects by reducing surface area using high refractive index materials, this method tailors the chemical reactivity. Specifically, we calculate that injection of calcite (CaCO3) aerosol particles might reduce net radiative forcing while simultaneously increasing column ozone toward its preanthropogenic baseline. A radiative forcing of −1 W⋅m−2, for example, might be achieved with a simultaneous 3.8% increase in column ozone using 2.1 Tg⋅y−1 of 275-nm radius calcite aerosol. Moreover, the radiative heating of the lower stratosphere would be roughly 10-fold less than if that same radiative forcing had been produced using sulfate aerosol. Although solar geoengineering cannot substitute for emissions cuts, it may supplement them by reducing some of the risks of climate change. Further research on this and similar methods could lead to reductions in risks and improved efficacy of solar geoengineering methods.

    David W. Keith and Peter J. Irvine. 11/30/2016. “Solar geoengineering could substantially reduce climate risks — A research hypothesis for the next decade.” Earth's Future, 4, Pp. 549–559. Publisher's VersionAbstract

    We offer a hypothesis that if solar geoengineering (SG) were deployed to offset half of the increase in global-mean temperature from the date of deployment using a technology and deployment method chosen to approximate a reduction in the solar constant then, over the 21st century, it would (a) substantially reduce the global aggregate risks of climate change, (b) without making any country worse off, and (c) with the aggregate risks from side-effects being small in comparison to the reduction in climate risks. We do not set out to demonstrate this hypothesis; rather we propose it with the goal of stimulating a strategic engagement of the SG research community with policy-relevant questions. We elaborate seven sub-hypotheses on the effects of our scenario for key risks of climate change that could be assessed in future modeling work. As an example, we provide a defence of one of our sub-hypotheses, that our scenario of SG would reduce the risk of drought in dry regions, but also identify issues that may undermine this sub-hypothesis and how future work could resolve this question. SG cannot substitute for emissions mitigation but it may be a useful supplement. It is our hope that scientific and technical research over the next decade focuses more closely on well-articulated variants of the key policy-relevant question: could SG be designed and deployed in such a way that it could substantially and equitably reduce climate risks?

    Elizabeth T. Burns, Jane A. Flegal, David W. Keith, Aseem Mahajan, Dustin Tingley, and Gernot Wagner. 11/1/2016. “What do people think when they think about solar geoengineering? A review of empirical social science literature, and prospects for future research.” Earth's Future. Publisher's VersionAbstract

    Public views and values about solar geoengineering should be incorporated in science-policy decisions, if decision makers want to act in the public interest. In reflecting on the past decade of research, we review around 30 studies investigating public familiarity with, and views about, solar geoengineering. A number of recurring patterns emerge: (1) general unfamiliarity with geoengineering among publics; (2) the importance of artifice versus naturalness; (3) some conditional support for certain kinds of research; and (4) nuanced findings on the “moral hazard” and “reverse moral hazard” hypotheses, with empirical support for each appearing under different circumstances and populations. We argue that in the coming decade, empirical social science research on solar geoengineering will be crucial, and should be integrated with physical scientific research.

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