surface would need to be constructed and put into orbit each year—or approximately an additional 10 square miles per day each and every day—for as long as CO₂ emissions continue increasing at rates comparable to today’s (Govindasamy and Caldeira, 2000). Due to the magnitude of spaced-based deployment required for such an undertaking, and the enormous cost of putting objects into orbit, these options appear impractical for addressing threats posed by climate change this century.

One of the most widely discussed options for SRM involves the injection of sulfate aerosols into the stratosphere, although other types of particles could potentially serve the same function. As discussed in Chapter 6, particles can reflect solar radiation back to space, offsetting some of the warming associated with GHGs. The amount of sulfur that would need to be supplied to the stratosphere to offset the radiative forcing associated with GHG emissions could be delivered through a variety of means, including aircraft and artillery shells, with relatively small direct costs (Crutzen, 2006; NRC, 1992b; Robock et al., 2009; The Royal Society, 2009). Since sulfate particles are also injected into the stratosphere by volcanic eruptions, cooling following recent eruptions serves at least as a general “proof of concept” for this approach. For example, in the year following the eruption of Mount Pinatubo in June 1991, global temperatures cooled by approximately 0.9°F (0.5°C; Trenberth and Dai, 2007). Process understanding could be developed through small-scale tests, but an understanding of global climate effects would require either reliance on models or tests that would be of global scale and at least one-tenth the size of a full deployment. Full deployment would require a long-term, uninterrupted commitment to continued injection at the scale of tens of kilograms of material per second injected quasi-continuously. A sudden cessation after a sustained deployment could result in rapid temperature increases over a period of a few years, causing potentially severe impacts on ecological and social systems (Matthews and Caldeira, 2007).

A range of options have been proposed to “whiten” clouds, or make them more reflective, by increasing the number of water droplets in the clouds. The most widely discussed proposal involves whitening low clouds over remote parts of the ocean by