Earth's Future

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.

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.

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.

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?

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.

The literature on the costs of climate change often draws a link between climatic ‘tipping points’ and large economic shocks, frequently called ‘catastrophes’. The phrase ‘tipping points’ in this context can be misleading. In popular and social scientific discourse, ‘tipping points’ involve abrupt state changes. For some climatic ‘tipping points,’ the commitment to a state change may occur abruptly, but the change itself may be rate-limited and take centuries or longer to realize. Additionally, the connection between climatic ‘tipping points’ and economic losses is tenuous, though emerging empirical and process-model-based tools provide pathways for investigating it. We propose terminology to clarify the distinction between ‘tipping points’ in the popular sense, the critical thresholds exhibited by climatic and social ‘tipping elements,’ and ‘economic shocks’. The last may be associated with tipping elements, gradual climate change, or non-climatic triggers. We illustrate our proposed distinctions by surveying the literature on climatic tipping elements, climatically sensitive social tipping elements, and climate-economic shocks, and we propose a research agenda to advance the integrated assessment of all three.