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BOX 15.1

Geoengineering: Solar Radiation Management and GHG Removal

The term geoengineering refers to deliberate, large-scale manipulations of the Earth’s environment designed to offset some of the harmful consequences of GHG-induced climate change (see AGU, 2009; AMS, 2009; NRC, 1992b; The Royal Society, 2009). Geoengineering encompasses two very different classes of approaches: carbon dioxide removal (CDR) and solar radiation management (SRM). Figure 15.1 depicts the most commonly discussed options in both these categories.

CDR approaches (also referred to as post-emission GHG management or carbon sequestration methods) involve removal and long-term sequestration of atmospheric CO2 (or other GHGs) in forests, agricultural systems, or through direct air capture with geological storage. These techniques and their implications are discussed in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c) and are also mentioned in several previous chapters. There is no consensus regarding the extent to which the term geoengineering should be applied to various widely accepted practices that remove CO2 from the atmosphere (e.g., reforestation).

SRM approaches, the focus of this chapter, are those designed to increase the reflectivity of the Earth’s atmosphere or surface in an attempt to offset some of the effects of GHG-induced climate change.


In November of 1965, the Environmental Pollution Panel of the President’s Science Advisory Council (PSAC) for the first time informed a president of the United States about the threats posed by increasing atmospheric CO2 concentrations. Their report stated:

The climatic changes that may be produced by the increased CO2 content could be deleterious from the point of view of human beings. The possibilities of bringing about countervailing climatic changes therefore need to be thoroughly explored. A change in the radiation balance in the opposite direction to that which might result from the increase of atmospheric CO2 could be produced by raising the albedo, or reflectivity, of the earth (PSAC, 1965).

The topic of SRM was also taken up in the National Research Council’s 1992 report Policy Implications of Greenhouse Warming (NRC, 1992b). That report noted:

[W]e are at present involved in a large project of inadvertent “geoengineering” by altering atmospheric chemistry [i.e., by increasing GHG concentrations], and it does not seem inappropriate to inquire if there are countermeasures that might be implemented to address adverse impacts.… Our current project of “geoengineering” involves great uncertainty and risk. Engineering coun-

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