The possibility of using geoengineering - the deliberate manipulation of the planetary environment - to counteract anthropogenic climate change is deeply controversial. At least in name, geoengineering has largely been ignored in recent climate assessments (Bruce, Lee et al., 1996; Watson, Zinyowera et al., 1996). Under close examination, however, the distinction between geoengineering and other responses to the CO2-climate problem proves to be fuzzy. Use of the term geoengineering is shifting, as advocates of response strategies that were formerly labeled geoengineering now seek to avoid the term. Section 10.2 elaborates a definition of geoengineering; assessment of the implications of its shifting meaning are deferred to the concluding discussion.

Historical perspective is vital to understanding the role of geoengineering in human choices about climate. The historical background sketched in Section 10.3 shows that proposals to engineer the climate are deeply woven into the history of the CO2-climate problem. The focus is on the postwar rise of weather and climate modification and the interweaving of its decline with rising concern about inadvertent climate modification. The evolving status of geoen-gineering as a response to anthropogenic climate change is examined through a review of US climate assessments and the IPCC assessment reports.

Section 10.4 reviews proposals to geoengineer the climate. Structure for the review is provided by a taxonomy of anthropogenic climate modification that includes geoengineering to counter anthropogenic climate forcing as a special case. Whereas the structure is broad, treatment of detailed proposals focuses on recent work that was not covered by previous reviews of geoengineering (Keith and Dowlatabadi, 1992; Flannery, Kheshgi et al., 1997; Kitzinger and

1 This article originally appeared under the same title in Annual Reviews of Energy and Environment, 25: 245-284 (2000). The two versions are identical excepting minor editorial corrections.

Frankel, 1998; Michaelson, 1998; Allenby, 1999). Recent developments include analysis of very low-mass scattering systems for altering planetary albedo (Teller, Wood et al., 1997), climate model simulation of the effect of albedo geoengineering, improved scientific understanding of the role of iron as a limiting nutrient in oceanic ecosystems (Cooper, Watson et al., 1996; Behrenfeld and Kolber, 1999), and speculation about the use of genetically modified organisms to enhance carbon sinks (Reichle, Houghton et al., 1999).

Frameworks for assessing geoengineering are surveyed in Section 10.5; they include economics, risk and uncertainty, politics and law, and environmental ethics. Finally, the concluding section suggests that the fuzziness of the boundary demarcating geoengineering from conventional mitigation arises from deep uncertainties about the appropriate extent of deliberate human management of global biogeochemical systems. Although most geoengineering methods may reasonably be viewed as marginal to the debate about climate change, the failure of modern assessments to consider the implications of geoengineering has encouraged avoidance of questions about the appropriate extent of deliberate planetary management - questions that warrant serious debate.

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