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).
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