Chapter 5 addresses the potential use of climate engineering technologies as a way of counteracting anthropogenic global warming. These technologies encompass methods for the permanent removal of atmospheric carbon dioxide, for instance via its direct capture from ambient air and subsequent geological storage. They also include more controversial technologies that would modify the planetary albedo in order to reflect a small fraction of incoming sunlight back into space in order to induce an artificial cooling effect. These two sets of technologies play out against the background of the increasingly dim prospects of achieving international temperature targets in line with the 2015 Paris Agreement. The chapter traces institutional responses to both negative emissions technologies and solar geoengineering across a multitude of international forums, including the Convention on Biological Diversity, the London Convention on the Dumping of Waste at Sea, and the Paris Agreement itself.

Greenhouse gas emissions abatement, negative emissions technologies, and adaptation are not, and most likely will not, be enough to prevent dangerous climate change and its deleterious impacts on humans, other species, and ecosystems. Some scientists and others are increasingly considering and researching solar geoengineering, which would reflect or block some of the sun's incoming solar radiation, as a potential complementary response. This introductory chapter offers an initial explanation of climate change and solar geoengineering, including its leading proposed techniques of stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning. Solar geoengineering should be taken seriously, as its governance is both important and challenging. Among the major challenges is that solar geoengineering presents a high-stakes risk-risk tradeoff under conditions of great uncertainty. Another is that although earlier governance can be more effective, little is then known of such an emerging technology’s salient characteristics. The chapter outlines the topics covered by the remainder of the book and makes the author’s prior assumptions explicit.

Solar geoengineering is being considered and researched as a potential response to anthropogenic climate change. After exploring the causes and risks of climate change and other responses to it, this chapter describes solar geoengineering’s history and proposed methods, including stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning. The current evidence regarding their potential capabilities, costs, and technical feasibility is presented. Evidence from models and natural analogs indicates that a moderate deployment of solar geoengineering would globally reduce climate change. It also appears to be technically feasible, rapid in its effects, inexpensive in its direct deployment costs, and reversible. Among solar geoengineering’s physical risks are imperfect compensation of climatic changes and consequent residual climatic anomalies, delayed recovery of stratospheric ozone, and irresolvable uncertainty. Social challenges include decision-making regarding deployment, problematic uni- or minilateral implementation, strained international relations, displacement of emissions abatement, biased future decision-making, and disagreement regarding ethics.