The term geoengineering refers to deliberate, large-scale manipulations of Earth's environment designed to offset some of the harmful consequences of GHG-induced climate change. Geoengineering encompasses two different classes of approaches: carbon dioxide removal (CDR) and solar radiation management (SRM) (see Figure 2.9). CDR approaches (also referred to as postemission GHG management, atmospheric remediation, or carbon sequestration methods), several of which were discussed in the sections above, involve removal and long-term sequestration of atmospheric CO2 (or other GHGs) in forests, agricultural systems, or through direct air capture and geologic
17 For additional discussion and references, see Chapter 15 in Part II of the report.
storage. Additional details about these techniques and their implications can be found in the companion report Limiting the Magnitude of Future Climate Change (NRC, 2010c).
SRM approaches, the focus of this section, are those designed to increase the reflectivity of Earth's atmosphere or surface in an attempt to offset some of the effects of GHG-induced climate change. SRM approaches seek to either reduce the amount of sunlight reaching Earth's surface or reflect additional sunlight back into space. There is a limited body of research on this topic. While some SRM approaches may be technologically and economically feasible (only considering direct deployment costs), they all involve considerable risk and potential for unintended (albeit currently understudied) side effects. It is unclear at the present time, therefore, whether SRM could actually reduce the overall risk associated with climate change and whether it could realistically be employed as quickly as is technically possible, especially in light of the full range of environmental and sociopolitical complexities involved.
Although few, if any, voices are promoting SRM as a near-term alternative to GHG emissions-reduction strategies, the concept has recently been gaining more serious attention as a possible "backstop" measure, because strategies attempted to date have failed to yield significant emissions reductions, and climate trends may become significantly disruptive or dangerous. Further research is necessary to better understand the physical science of the impacts and feasibility of SRM as well as issues related to governance, ethics, social acceptability, and political feasibility of planetary-scale, intentional manipulation of the climate system.
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