For over 45 years, proposals for deliberate, large-scale manipulation of Earth's environment—or geoengineering (see Box 15.1 and Figure 15.1 )—have been put forward as ways to potentially offset some of the consequences of climate change. For example, whitening clouds, injecting particles into the stratosphere, or putting sunshades in space could increase Earth's reflectivity, thereby reducing incoming solar radiation and offsetting some of the warming associated with increasing GHG concentrations. Although few if any voices are promoting geoengineering as a near-term option to limit the magnitude of climate change, the concept has recently been gaining more serious attention as a possible backstop measure to be used if traditional strategies to limit emissions fail to yield significant emissions reductions or if climate trends become disruptive enough to warrant extreme and risky measures.
Questions decision makers are asking, or will be asking, about solar radiation management and other geoengineering approaches include the following:
• Can the negative impacts associated with increasing atmospheric greenhouse gas (GHG) concentrations be reduced or offset by intentionally intervening in the climate system? If so, how?
• What undesirable, unintended consequences might result from such interventions? How could these consequences be anticipated or detected?
• Who should decide, whether, when, and how to intentionally intervene in the climate system?
• What institutional mechanisms would be needed to initiate, carry out, monitor, and respond to the impacts—foreseen and unforeseen—of such an effort?
• Which types of interventions might be most socially acceptable and what frameworks for evaluation, governance, and compensation should be used?
In this chapter, we briefly review what is known about proposed solar radiation management (SRM) approaches and related governance and ethical issues and conclude with a discussion of the research needed to better understand SRM. Carbon dioxide removal approaches are addressed in Chapters 9, 10, and 14 and in the companion report Limiting the Magnitude of Climate Change (NRC, 2010c). Note that SRM research is in its infancy and that most conclusions should be regarded as preliminary.
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.
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