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The point of Category 2 is that the relative attractiveness of the various options is not adequately captured in the simple index of cost per ton of CO2 equivalent. The relative appeal of Category 2 options is greatly affected by other social, environmental, and economic externalities.

Category 3 Options

Category 3 options (Table 29.4), mainly geoengineering options, are those that appear to be feasible with the limited information now available, which may—with additional investigation, research, and development—provide the ability to change atmospheric concentrations, or radiative forcing, and the ultimate impact of greenhouse warming on a substantial scale. By and large, they deal with the symptoms rather than the causes. Some of these actions could be initiated after a deleterious climate change was clearly identifiable, if research and development had been completed earlier. Near-term research of the Category 3 options as a "backstop" measure is likely to be beneficial and relatively inexpensive. In the end, as discussed in Chapter 28, some of these options could be inexpensive, safe, and reversible.

Many of the Category 3 options appear relatively inexpensive from an implementation standpoint but have large unknowns as to their environmental or carbon cycle side effects should they be implemented. For example, increasing phytoplankton growth through addition of iron to the oceans may be a feasible mitigation option, but the impact of tinkering with the oceanic balances of iron, carbon, oxygen, and other nutrients is unknown at this time. These options should be investigated further and should be well understood before implementation is considered seriously.

Comparing the Different Mitigation Options

Tables 29.5, 29.6, and 29.7 summarize the information in Tables 29.2, 29.3, and 29.4 by adding up the maximum potential emission reduction available from each sector and placing the net cost of the various options in categories. Categorization of cost numbers helps to illustrate that a great deal of uncertainty is, of course, associated with many of these numbers. Because this is a "first-order" analysis, the Mitigation Panel has used information from many sources, most of which were not intended to be used for comparative cost analysis. Improvement of the cost estimates will undoubtedly modify the priority ordering of options; at this time, therefore, categories are an appropriate way to compare alternatives. In addition, cost categorization allows comparison of costs of different options relative to a wide-ranging policy instrument such as a carbon tax and its impact on fuel prices.

Figure 29.1 illustrates how mitigation options might be ranked both on