Addendum to Emissions of greenhouse gases from the use of transportation fuels and electricity. Effect of 1992 revision of global warming potential (GWP) by the Intergovernmental Panel on Climate Change (IPCC).

Abstract

This addendum contains 2 important messages. (1) This document supersedes all previous versions of this work. Please do not use any older versions any more. (2) The atmospheric-science community now believes that it cannot estimate confidently the ''Global Warming Potentials'' (GWPs) of the indirect effects of greenhouse gases. A GWP is a number that converts a mass-unit emission of a greenhouse gas other than CO{sub 2} into the mass amount of CO{sub 2} that has an equivalent warming effect over a given period of time. This report refers to GWPs as ''CO{sub 2}-equivalency factors.'' For example, a forthcoming report by the Intergovernmental Panel on Climate Change disavows many of the GWPs estimated in an earlier IPCC report, and states that GWPs for the indirect effects of the non-CO{sub 2} greenhouse gases cannot be estimated accurately yet. However, this does not mean that in principle there are no GWPs for the non-CO{sub 2} greenhouse gases; rather, it means that some of the GWPs are uncertain, and that the earlier IPCC estimates of the GWPs may or may not turn out to be right (albeit, in at lease one case, discussed in this paper, the earlier estimates almost certainly will be wrong). In this report the author used the IPCC's 1990 estimates of the GWPs for 20-, 100-, and 500-year time horizons, and expressed the bottom-line results for each of these three time horizons. However, the recent uncertainty about the GWPs affects how you should interpret the results. Because the IPCC has disclaimed some of its GWPs, the GWPs as a group no longer are the best estimates of the warming effects over 20, 100, and 500 years. Instead, they are just a collection of possible values for the GWPs--in short, scenarios. Therefore, you should interpret the ''20-, 100-, and 500-year time horizons'' as three general GWP scenarios--say, scenarios, A, B, and C.--and not as time-period scenarios. For example, you should not think that the results shown here under the ''100-year time horizon'' actually embody the scientific community's best estimates of the relative warming potentials of the various greenhouse gases over a 100-year period. Instead, you should understand the results to be the outcome of making a particular set of assumptions about what the GWPs might be. The ''time horizons'' no longer necessarily represent time horizons, but rather general scenarios for, or assumptions about, the GWPs.