The PSAC (1965) and NRC (1992b) reports suggested that proposals to increase the reflectivity of the Earth (and to remove GHGs from the atmosphere) be thoroughly examined. This sentiment was echoed by many participants at the geoengineering workshop held in June 2009 as part of the suite of activities for the America’s Climate Choices study (Appendix F), as long as such research does not undermine other critical climate research efforts (see the discussion of ethical issues below), including research on adapting to the impacts of climate change and on conventional strategies for limiting the magnitude of future climate change (i.e., reducing fossil fuel consumption, deforestation, and other activities that contribute to climate forcing). Critically, these evaluations should explore the intended effects of geoengineering approaches and their potential unintended side effects, as well as the ethical, institutional, social, and political aspects of intentional manipulation of the climate system.

A number of different SRM methods have been proposed. This subsection briefly outlines some of the approaches that have been discussed in the literature (Keith, 2000; Rasch et al., 2008) and briefly summarizes their potential to reduce total radiative forcing. Other sources, including a recent report by the Royal Society (2009), provide a more comprehensive description. The relative advantages and disadvantages, potential for unintended consequences, and governance and ethical issues associated with these approaches are discussed in the next subsection. It should be noted that, unlike many other areas of research discussed in this report, these issues have undergone relatively little scientific scrutiny, with most of the relevant research done by a few small groups of scientists working with limited resources. Thus, many of the conclusions presented here must be regarded as preliminary and subject to revision.

A variety of options have been proposed for placing vast satellites in space, typically at the L1 point1 between Earth and the Sun (Early, 1989). However, to compensate for the increase in GHGs, nearly 4,000 square miles (10,000 square kilometers) of reflective