release of the gas. The IPCC calculates GWPs for the well-mixed gases for three discrete time horizons of 20, 100, and 500 years. The Kyoto Protocol recommends that parties to the treaty use the 100-year value for comparing emissions reductions of different gases toward meeting targeted greenhouse emissions reductions for the first commitment period of 2008-2012 (Kyoto Protocol, 1997).

Application of the GWP concept has mainly been restricted to the long-lived greenhouse gases. In principle, it could be applied to short-lived forcing agents such as ozone and aerosols or, more specifically, to the emissions of their precursors (e.g., Schwartz, 1993), but there are a number of complicating factors including (1) the often poorly defined relationship between the precursor and the radiative forcing agent; (2) the inhomogeneity of the forcing; and (3) the much shorter time horizons (decades or less) relevant to the radiative forcing from these short-lived agents. In addition, the current concept is not useful for evaluating how the rate of technical transformation, which depends on economic and policy drivers, affects the trade-off between two greenhouse gases. At present, integrated assessment models are used to consider the combined scientific and economic factors that contribute to the global warming impacts of different forcings (e.g., Manne and Richels, 2001).

Many criticisms of the oversimplicity of the GWP approach have been published (Lashof, 2000; O’Neill, 2000; Smith and Wigley, 2000a,b). More complex equivalence calculations, such as the “forcing equivalence index” of Wigley (1998), have been developed to address its shortcomings. The essence of the forcing equivalence index is that a time series of emissions of a greenhouse gas produces a time series of radiative forcings. By inverting this temporal profile of radiative forcing in terms of the atmospheric properties of another greenhouse gas, the “equivalent” emissions of the alternative gas are estimated. This calculation, like the GWP, does not fully treat the complexities of the long-term behavior of the two gases (O’Neill, 2000).

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