In this chapter we leap over the divider in our geoengineering toolbox to examine the implements on the other side of our portfolio by considering several prospective solar radiation management techniques. These begin with surface modifications that might in some way increase the albedo of the earth and cause it to absorb less solar energy, including lightening roofs and other built structures, bioengineering crops to be slightly lighter, and manipulating ice to reflect more sunlight. Glaciers could be reinforced to slow their slides into the sea. Turning our gaze upward, marine cloud brightening is a technique that could increase the reflectivity of clouds, while cirrus cloud thinning could reduce the heat-trapping effect of these high wispy clouds. Yet higher up, it may someday be possible to deploy assets in space the might deflect some of the incoming sunlight. However, the surface modification techniques don’t appear to be properly scaled to the problem, and the cloud and space-based ideas remain too speculative to rely upon.

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.