buffering process that occurs as a result of the weathering of rocks. Such an enhancement of natural weathering processes—equivalent to giving a bromide to the ocean—would likely pose a very large logistical challenge as the mass of material required for buffering is typically several times as large as the mass of CO2 to be sequestered.

# **<u>6.0 Creating a Decision Process for Geoengineering</u>**

Research on the scientific aspects of geoengineering, including the cost of actually carrying it out, is one aspect of determining whether such approaches merit consideration as a policy option that could be considered along with mitigation, adaptation, and suffering through the impacts of climate change. A traditional approach for evaluating the merits of a course of action is comparison of costs and benefits of such efforts. Comparing the direct costs of the geoengineering itself relative to the costs of reducing emissions by an equivalent amount is the most straightforward (e.g., see NAS, 1992), but this leaves out the potential benefits of the avoided impacts of climate change, which seem likely to be important.

As made clear in analyses by Stern (2007) and Nordhaus (2007), evaluating the benefits of limiting the pace of future climate change can lead to quite different results as a result of differing assumptions about the appropriate manner in which to weigh the present importance of future impacts. In addition, reductions in impacts are necessarily estimated based on the results of extended term simulations of very complex climatic, ecological and socioeconomic models, which leads to outcomes that are at least as uncertain as the projections of climatic impacts without mitigation or geoengineering. For example, in evaluating approaches for solar radiation management, in addition to all the climate-related impacts, an analysis would have to weigh the direct effects of the further rise in the CO2 concentration, including the negative consequences of allowing ongoing acidification of the ocean and the positive effects of enhanced water use efficiency and productivity of some crops. That many analyses (e.g., IPCC, 2007b) comparing fossil versus renewable sources of energy are so limited in their analysis of the consequences of the changes in CO2 and climate makes clear that considerable further research and analysis will be needed, including considering the results in both cost-benefit and risk-based frameworks.

One important complication of any such analysis involves the time dimension, because, assuming that one or more viable and cost-effective approaches to geoengineering can be found (e.g., injection of aerosols into the stratosphere), the commitment to the approach must match the lifetime of the CO2 increment that is supposedly being offset. To first order, this means that continued emission of CO2 would need to be offset by roughly a 200-year commitment to the stratospheric injection because, were the geoengineering effort to be halted for any reason, the radiative forcing of the remaining CO2 would no longer be offset and the atmospheric temperatures would likely jump up toward a higher equilibrium temperature relatively rapidly (Brovkin et al., 2008; Robock et al., 2008). In that each of the approaches requires some maintenance or renewal (e.g., additional launching of mirrors, lofting of particles or reflecting balloons, or augmenting the number of mist-dispersing ships) the commitment to the offset will, without the approval of future generations, impose responsibility and costs on them.

Coming to an understanding of what creating a multi-generational commitment might mean is likely to be very difficult. While there have been national, and even international commitments 26