economically feasible. Thus, one strategy for dealing with these dispersed emissions sources involves finding the means to extract CO2 directly from the ambient air. This direct-capture strategy is appealing for numerous reasons: it can be colocated with suitable geological storage sites; it eliminates the need to ship captured CO2 from its source to a disposal site; it could be deployed as soon as it is developed (i.e., one would not have to wait for the phase-out of existing energy infrastructure to begin implementation); and it could likely be carried out at the national level, without the need for new international agreements or governance institutions.

One class of strategies for direct air capture that has emerged thus far involves physical or chemical absorption from airflow passing over some recyclable sorbent such as sodium hydroxide. A few research groups are developing and evaluating prototypes of such systems (Keith et al., 2006; Lackner et al., 1999). Major challenges remain in making such systems viable in terms of cost and energy requirements and improving over-all capture energy efficiency. And of course, the challenges of long-term storage of the captured CO2 are the same as those discussed earlier for CCS from industrial sources. If the technology were to someday become technically and economically feasible, however, the amount that could be captured would face no physical limit (other than global storage capacity) and, thus, could fundamentally alter the picture for efforts to reduce atmospheric GHG concentrations.

Reducing Emissions of Non-CO2Greenhouse Gases

Roughly 15 percent of U.S. GHG emissions (based on CO2 equivalents) come from non-CO2 gases, including methane (CH4), nitrous oxide (N2O), and fluorinated industrial gases such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) (EPA, 2009). Pursuing non-CO2 GHG emissions-reduction opportunities can be an attractive option because these gases are, per molecule, generally much stronger climate forcing agents than CO2; studies have shown that including non-CO2 emissions-reduction options allows involvement of a far wider and more diverse set of economic sectors and opportunities, leading to a substantial reduction in the overall economic cost of limiting GHGs (e.g., Clarke et al., 2009; de la Chesnaye et al., 2007).

There are technically feasible strategies for reducing some non-CO2 GHG emissions at negative or modest incremental costs. Many of these strategies are discussed briefly below, and more detailed discussion can be found in the literature (EPA, 2006, and see Table 3.1). Note that some strategies can yield multiple environmental benefits; for example, later in this chapter we discuss the example of controlling chemical species that affect both climate change and air quality (e.g., black carbon, tropospheric ozone,

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