The first IPCC Assessment (IPCC, 1990) recognised the existence of a host of agents that can cause climate change including greenhouse gases, tropospheric aerosols, land-use change, solar irradiance and stratospheric aerosols from volcanic eruptions, and provided firm quantitative estimates of the well-mixed greenhouse gas forcing since pre-industrial times. Since that Assessment, the number of agents identified as potential climate changing entities has increased, along with knowledge on the space-time aspects of their operation and magnitudes. This has prompted the radiative forcing concept to be extended, and the evaluation to be performed for spatial scales less than global, and for seasonal time-scales.
IPCC (1992) recognised the importance of the forcing due to anthropogenic sulphate aerosols and assessed quantitative estimates for the first time. IPCC (1992) also recognised the forcing due to the observed loss of stratospheric O3 and that due to an increase in tropospheric O3. Subsequent assessments (IPCC, 1994; SAR) have performed better evaluations of the estimates of the forcings due to agents having a space-time dependence such as aerosols and O3, besides strengthening further the confidence in the well-mixed greenhouse gas forcing estimates. More information on changes in solar irradiance have also become available since 1990. The status of knowledge on forcing arising due to changes in land use has remained somewhat shallow.
For the well-mixed greenhouse gases (CO2, CH4, N2O and halocarbons), their long lifetimes and near uniform spatial distributions imply that a few observations coupled with a good knowledge of their radiative properties will suffice to yield a reasonably accurate estimate of the radiative forcing, accompanied by a high degree of confidence (SAR; Shine and Forster, 1999). But, in the case of short-lived species, notably aerosols, observations of the concentrations over wide spatial regions and over long time periods are needed. Such global observations are not yet in place. Thus, estimates are drawn from model simulations of their three-dimensional distributions. This poses an uncertainty in the computation of forcing which is sensitive to the space-time distribution of the atmospheric concentrations and chemical composition of the species.
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