**_Economic Cost and Phasing:_** Each of the approaches will require some investment. In that very little research has been done, information on costs is generally quite uncertain. There has been a sense, however, that the costs might well be less than the costs of switching to alternative energy technologies to reduce greenhouse gas emissions (NAS, 1992; Barrett, 2008)--how long this will be the case as the costs of renewables drop is not clear. As a general rule, however, early studies indicate that geoengineering approaches having the least significant adverse side effects would require the most investment, and vice-versa.

**_Coupling to other problems and issues:_** Each of the proposed geoengineering approaches is coupled to other issues, some technological, some economic, and some cultural. A number of the approaches would basically trade one kind of pollution for another. For example, as long as coal is being used for some degree of energy generation, continuing to emit SO2 into the troposphere (or choosing to augment such emissions) to increase the tropospheric aerosol loading would be relatively inexpensive and straightforward, although it would likely sustain or increase acid deposition. For both tropospheric and stratospheric injection of sulfate aerosols, increasing planetary reflectivity would also have the effect of sharply reducing the ratio of direct to diffuse solar radiation, likely leading to consequences for ecosystems and perhaps for wildlife as well as diminishing the capacity of solar technologies depending on the direct solar beam (e.g., mirrorbased technologies).

**_Duration and impact on future generations:_** Like the enhanced greenhouse effect that it would be addressing, geoengineering would need to extend over a prolonged period, leading to effects and obligations that would be imposed on future generations, generally without their consent. There are differences, however, among the approaches in the degree of action required over time, with some approaches requiring continued effort over time and some only needing to be implemented over a finite period. It is an open question whether different standards should apply to imposing actions over decades as opposed to imposing obligations and costs that would persist for centuries or even indefinitely. This issue is complicated by its relation to how it affects society’s efforts to reduce emissions. In particular, to the extent that the reduced impacts that are brought about by geoengineering ease the pressure and success of efforts to reduce emissions, geoengineering may rightly be seen as passing not just one problem (i.e., the obligation to continue geoengineering) to future generations, but two (i.e., an amplified extent of climate change and an even stronger obligation to continue geoengineering). This is akin to basically borrowing money to double down on a bet. With much more at risk, any failure in the system would lead to the risk being even worse than when geoengineering was begun.

**_Uncertainties:_** Although suggestions that geoengineering should actively be considered as a policy option are increasing (e.g., Wigley, 2006; Ragaini, 2008), there has been very limited research on the major aspects, much less the subtleties, of most of these approaches. For those approaches on which investigations have started, the tools used have tended to be the very climate models that some of those advocating geoengineering have said introduce uncertainties in projections of the impacts of the greenhouse gases themselves. In addition, most of the studies evaluating geoengineering options have been done for very idealized cases (e.g., smoothly spread aerosols in the stratosphere) with no examination of what the consequences of treating the approaches realistically might mean to, for example, weather patterns and extremes, the natural variability and oscillations of the atmosphere-ocean system, or ecosystems and other species. For

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