which together with some post-emission GHG management strategies is sometimes referred to as geoengineering (see Box 5.1). NRC, Advancing the Science and Limiting the Magnitude review the technologies and practices that are available for pursuing these various opportunities. Here we provide a very brief overview, starting with the more general issue of setting goals for limiting the magnitude of climate change.
Geoengineering, applied to climate change, refers to deliberate, large-scale manipulations of the Earth’s environment intended to offset some of the harmful consequences of GHG emissions, and it encompasses two very different types of strategies: solar radiation management and post-emission GHG management.a Many proposed geoengineering approaches are ambitious concepts with global environmental consequences; as such, they have attracted a great deal of attention. In general, however, current scientific knowledge of the efficacy and overall risk reduction potential of most geoengineering approaches is limited.
Solar radiation management (SRM) involves increasing the reflection of incoming solar radiation back into space. Some SRM approaches can, in theory at least, produce substantial cooling quickly and thus could potentially be used in the case of “climate emergencies” involving unexpectedly rapid warming or severe impacts. A much-discussed example is the proposal to continuously inject large quantities of small reflective particles (aerosols) into the stratosphere.This would mimic some effects of sustained large volcanic eruptions, which have been observed to cool the earth’s surface measurably for months. Another SRM strategy sometimes proposed is to increase the reflectivity of the Earth’s surface through widespread use of “white roofs.”
The potential benefits of many SRM strategies are offset by potential risks. In the case of aerosol injection strategies, for example, significant regional or global effects on precipitation patterns could occur,b potentially placing food and water supplies at risk. SRM alone would also do nothing to slow ocean acidification, since CO2 concentrations in the atmosphere and ocean would continue to rise. Thus, it is unclear if any of the proposed large-scale SRM strategies could actually reduce the overall risk associated with human-induced climate change.c Large-scale proposals for post-emission GHG management generally involve either removing CO2 from the atmosphere by direct air capture technologies or managing ecosystems on land or in the ocean to increase their natural uptake and storage of carbon. Strategies for enhancing carbon sequestration in soils and forests (which are often viewed as standard strategies for limiting climate change rather than geoengineering) are relatively well understood and offer important opportunities for reducing net GHG emissions in some parts of the world. However, changes in the sequestration rates of these systems are often difficult to quantify, and the potential effectiveness of these strategies may decline over time due to saturation effects.d A more controversial post-emission GHG management strategy which has actually been tested at small scales is fertilizing the ocean by adding iron to iron-poor waters (or adding other limiting nutrients or minerals) to increase removal of CO2 from the atmosphere by phytoplankton. Concerns have been raised both about the efficacy of this approach and about possible risks that it might pose to marine