with limited and manageable impacts on neighbors. On the other hand, difficulties in undertaking targeted applications, or in limiting unintended or unanticipated side effects, could cause serious problems.

The potential efficacy of adaptation options for large-scale catastrophes is high. Relocating individuals and rebuilding structures out of harm’s way would be very effective, assuming threatened areas are identified. Large increases in protected natural areas to mitigate some of the stresses on natural ecosystems also could be effective, though this would depend on the size and connectivity of the areas and the particular threats being addressed—issues about which we still have limited understanding. Adaptation options to short-circuit cascading events could be effective, but even the best-informed and prepared response capacities could be undercut by surprise—especially since locations of potential cascading catastrophes will likely remain quite uncertain. It is important to stress, though, that even if the efficacy of an adaptation (or another measure) is uncertain or anticipated to be only partial, it still may be worth undertaking.

# _4.4 Anticipated Costs of the Response Options_

The costs of the response options will vary, although all will be high. Achieving dramatic and rapid reductions in global GHG emissions will likely be very costly, in the absence of major breakthroughs in low-carbon energy technology or carbon sequestration. Assumptions matter crucially. As a rough guide to the costs, the IPCC estimates that at stabilization between 445– 535 ppmv CO2-equivalent, GDP in 2050 is no more than 5.5 percent below baseline (this is well above current GDP given the effect of continued economic growth). The authors also note there has been little analysis of achieving this stringent target compared to others (Fisher et al. 2007). The IPCC figures also reflect ideal conditions for policy implementation. With less than fully cost-effective implementation, as must be expected, whether from delayed participation by developing countries or the use of some cost-ineffective mitigation policy mechanisms, the total cost would be much larger. For example, DICE runs suggest that with only 50 percent of countries participating, abatement costs for the same goal would increase by 250 percent relative to full global participation (Nordhaus 2008).

The cost of geoengineering remains highly uncertain. One estimate of particulate seeding puts the cost at $25–$50 billion per year (Crutzen 2006). While this is a large number in absolute terms, it is a very small fraction of anticipated world GDP. However, anticipated negative side effects also are a critical part of the cost analysis. Negative side effects of scattering particulates in the atmosphere could include alteration of the hydrologic cycle and acidification of the ocean due to continued buildup of carbon dioxide. These developments in turn could have severe economic consequences that should be a part of the cost analysis. In addition, the costs of geoengineering R&D and deployment are a matter of speculation. Research and pilot-scale experiments would be needed not only to establish an adequate capacity to control large-scale geoengineering and to gather even rough estimates of cost, but also to identify and then find ways to ameliorate side effects. Given the complexity of the natural systems with which the world would be experimenting, it is possible research could not fully put to rest concerns about side effects.<sup>20</sup> The costs of adaptation measures are sure to vary widely and will be difficult to assess a priori in many cases. Adaptation options involving very large land set-asides in the relatively

> 20 See also McCracken (2009) for a discussion of a range of different possible geoengineering approaches and the uncertainties surrounding them at present.

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