many of the approaches, there are tools to carry out better evaluations, but, lacking an adequately funded research effort, only rarely have the most advanced models been used.

**_Reversibility:_** Because there are uncertainties, the natural reaction is to urge more research and to not actually do anything until there is full clarity about what should be expected from geoengineering. While this would be desirable, the world is rapidly changing, so there is some urgency to taking action. Indeed, discounting the offsetting (cooling) effects of sulfate aerosols, the loading of which would quickly be diminished if CO2 emissions were cut sharply, the present anthropogenic contribution to greenhouse gas forcing is, using the 100-year Global Warming Potential (GWP) that is most appropriate for consideration of long-term climate change, roughly equivalent to a CO2 concentration of 450-460 ppmv (parts per million by volume). This is the level that scientific studies (e.g., Meinshausen, 2006) suggest would have roughly a 50% likelihood of leading to warming greater than about 2oC over preindustrial conditions, which is the amount of warming that the CEC has concluded would lead to the “dangerous anthropogenic interference with the climate system” that the nations of the world pledged to avoid in agreeing to the UN Framework Convention on Climate Change. To avoid such significant warming, it would, therefore, make sense to initiate limited geoengineering as soon as possible, even though there are uncertainties. In the face of both necessity and urgency, a prerequisite would seem to be that the approach would be at least largely reversible; that is, in case negative side effects were to arise or there were problems or higher than affordable costs of implementation, it would be desirable if stopping its implementation did not lead to significant or permanent adverse consequences.

**_Research and development needs and prospects:_** Recognizing the potential need for early invocation of geoengineering, especially if the sensitivity of the climate to greenhouse gases is near the high end of current estimates, it is important to have a sense of the research and development needed to gain the understanding that would make geoengineering a viable option. Thus, what needs to be done, at what cost, and how long might it take? The types of questions that arise include: What sorts of efforts would be needed and what can be carried out on a computer and what would best be carried out by actually undertaking the effort at a small and inconsequential scale? What new technologies would be needed to implement the approach cost effectively, and what are the prospects for such developments? To date, the elements of a comprehensive, international research plan have not been assembled.

**_Governance and international responsibility:_** Even if one or more of the suggested approaches is found to be workable and likely to be beneficial, there is the overarching question of how might geoengineering be undertaken. How would the decision be made, or even considered? What body, if any, has the authority to take on overall management of the Earth’s climate? What set of information would they need and how would this be developed? How would a decision be made? What level of confidence in the results would be required? How would nations that might be adversely impacted be compensated, and how would this be judged—relative to the impacts of geoengineering alone, or in combination with the greenhouse-induced changes?

Based on the enumeration of the possible approaches and issues contained in the following sections, the final section of the report attempts an integration and synthesis, including identification of those approaches that, with modest research and development, might, separately 11