ABSTRACT
This chapter quantifies the relative importance of different anthropogenic gases in modifying past and future climate, together with the associated uncertainties. Past radiative forcing changes or future changes under the 1992 IPCC emissions scenarios are used as a basis for comparison. For the past, CO2 is by far the most important contributor to forcing changes, followed by SO2-derived sulphate aerosols and CH4. The largest uncertainties are those associated with aerosol forcing. For the future, the largest forcing contributions come from CO2, followed by aerosols, CH4 and then either N2O or the halocarbons (depending on scenario). Future forcing uncertainties arising from the following factors are quantified: emissions uncertainties; possible changes in lifetimes (for CH4 and hydrogenated halocarbons); stratospheric ozone feedback (for halocarbons); and global-warming-related emissions increases (for CH4). The most important absolute uncertainties are, in order of magnitude, those associated with CO2 changes, aerosols and methane. Research priorities should be determined by the absolute magnitudes of uncertainty (which requires the use of a common yardstick such as radiative forcing) and by the potential for reductions in uncertainty. On this basis, carbon cycle and aerosol research (including the issue of determining the spatial patterns of aerosol forcing) are judged of highest priority, followed by the atmospheric chemistry of methane and climate-related methane emissions changes. Other important issues, but of lower priority, are the current N20 budget, possible climate-related changes in N20 emissions, stratospheric water vapour changes arising from methane emissions and 194the indirect radiative fordng from halocarbons arising through stratospheric ozone depletion.
