With the concentrations of atmospheric greenhouse gases (GHGs) rising to levels unprecedented in the current glacial epoch, due in large part to human activities, the earth's climate system appears to be rapidly shifting into a warmer regime [1] . Studies by Raupach et al. [2] and Canadell et al. [3] have concluded that global GHG emissions are growing at a rate in excess of 3% per year (Princiotta, this book). The human and ecosystems costs associated with the consequent climate change are expected to be great.

Many in the international science and policy communities fear that the fundamental changes in human behavior, and in the global economy, that will be required to significantly reduce GHG emissions in the very near term are unattainable. This fear has stimulated discussion in the scientific and policy communities, and in the popular press, of a radical alternative strategy for arresting climate change -intentional, direct manipulation of the earth's energy balance. The term coined to describe this strategy is "geoengineering" [4].

The notion that weather and climate could be modified through technological means was proposed early in the twentieth century, and evolved to include schemes ranging from cloud seeding to stimulate rainfall to strategies for warming the climate in the northern latitudes [5]. The concept of geoengineering greatly enlarges upon these more local- and regionally-focused ideas to consider the earth's climate system, as a whole. Directly and intentionally modifying the global climate system to achieve specific ends was initially dismissed as science fiction. More recently, the sensational aspects of geoengineering have made the topic an attractive subject for the popular press and television [6-11].

With the consensus conclusion by the Intergovernmental Panel on Climate Change (IPCC) that climate change is underway, publications concerning geoengineering in prominent scientific journals have proliferated, and the level of discourse on the subject has rapidly grown more sophisticated and more energetic. The range of opinion concerning geoengineering as a legitimate policy option is broad. Geoengineering appears to be a plausible means of "buying time" while humanity undergoes the profound changes in its use of energy and choices in transportation that are needed to eliminate GHG emissions. Many fear, however, that the adoption of geoengineering as a climate management strategy will lessen the sense of urgency needed to drive the global-scale changes critical for reducing future climate change.

While the concepts underlying geoengineering strategies are simple, given our current level of knowledge, the downstream consequences of their implementation are not at all clear. A critical, missing piece in the discussion of geoengineering as a strategy for managing climate is a systematic evaluation of the downstream costs versus benefits, from a global perspective, based on rigorous scientific study. Should the geoengineering approach become necessary for minimizing the risk of a major climate catastrophe, such an evaluation will be essential for the design of a successful climate management program.

This chapter introduces the basic concepts underlying the geoengineering strategy for mitigating climate warming, provides an overview of the more widely discussed project proposals, and highlights the critical uncertainties regarding the implementation of each proposal. Also included is a discussion of potential co-benefits along with the undesirable consequences of the implementation of these proposals that should be factored into the risk assessment of a global climate management program. The chapter concludes with a discussion of the need for an integrated risk assessment/risk management framework that incorporates the available quantitative information on the risks associated with the intentional manipulation of the climate system - should direct manipulation of the climate system become a policy choice. Recommendations for research targeted at minimizing some of the larger uncertainties are included in this discussion.

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