This chapter investigates greenhouse gases whose atmospheric burdens 1
and climate impacts generally depend on atmospheric chemistry. These greenhouse
gases include those listed in the Kyoto Protocol – methane (CH4), nitrous
oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur
hexafluoride (SF6) – and those listed under the Montreal Protocol and
its Amendments – the chlorofluorocarbons (CFCs), the hydrochlorofluorocarbons
(HCFCs), and the halons. A major focus of this assessment is the change in tropospheric
ozone (O3). Stratospheric water vapour (H2O) is also treated
here, but tropo-spheric H2O, which is part of the hydrological cycle
and calculated within climate models, is not discussed. This chapter also treats
the reactive gases carbon monoxide (CO), volatile organic compounds (VOC), and
nitrogen oxides (NOx = NO+NO2), termed indirect greenhouse
gases. These pollutants are not significant direct greenhouse gases, but through
atmospheric chemistry they control the abundances1
of direct greenhouse gases. This chapter reviews the factors controlling the current
atmospheric abundances of direct and indirect greenhouse gases; it looks at the
changes since the pre-industrial era and their attribution to anthropogenic activities;
and it calculates atmospheric abundances to the year 2100 based on projected emissions
of greenhouse gases and pollutants. Carbon dioxide (CO2) is treated
in Chapter 3; and aerosols in Chapter
5. The atmospheric abundances of greenhouse gases and aerosols from all chapters
are combined in Chapter 6 to calculate current and future radiative forcing. This
chapter is an update of the IPCC WGI Second Assessment Report (IPCC, 1996) (hereafter
SAR). For a review of the chemical processes controlling the abundance of greenhouse
gases see the SAR (Prather et al., 1995) or Ehhalt (1999). More recent assessments
of changing atmospheric chemistry and composition include the IPCC Special Report
on Aviation and the Global Atmosphere (Penner et al., 1999) and the World Meteorological
Organization / United Nations Environmental Programme (WMO/UNEP) Scientific Assessment
of Ozone Depletion (WMO, 1999).
