T. M. L. Wigley & S. C. B. Raper
IN 1990, the results of an international assessment of the greenhouse problem were published under the auspices of the Intergovernmental Panel on Climate Change, referred to below as IPCC901. Scenarios were given for future emissions of the ma jor greenhouse gases, and these were interpreted in terms of future changes in greenhouse gas concentrations, global-mean temperature and global-mean sea level. Since then, there have been important scientific and methodological developments. To account for these, and as background material for the forthcoming United Nations Conference on Environment and Development, IPCC has produced an update of the 1990 assessment, IPCC922. This includes a new set of emissions scenarios3,4 which are more comprehensive and, methodologically, more soundly based than the earlier scenarios.
The IPCC update, however, only briefly analyses the implications of these new scenarios for future greenhouse gas concentrations, radiative forcing and global-mean temperature changes5; sea-level implications are not considered. Here we aim to assess the implications in detail. In so doing, we investigate not only the direct consequences of the changes in the emissions scenarios, but also how the results are affected by uncertainties in carbon cycle modelling, the negative feedback on halocarb on radiative forcing through stratospheric ozone depletion6,7, and the possible negative forcing due to anthropogenic SO2 emissions and sulphate aerosol production7-10.
The six new emissions scenarios, IPCC scenarios (IS) 92a-f, differ fundamentally from the four earlier ones (from IPCC90) in a number of ways. First, they account for new policies already implemented or proposed for controlling CO2 emissions a nd halocarbon production, and allow for recent political changes. Second, and more important, they were not developed 'top down' to achieve a prescribed set of radiative forcing targets, but were based on a wide range of assumptions regarding the socioec onomic factors that influence the development of emissions in the absence of new multilateral or unilateral efforts to reduce emissions3,4. The new scenarios thus represent a range of possible futures all corresponding to the continuation of ' Business as Usual'. They therefore replace the single such scenario given by IPCC90. The new scenarios differ from each other because they make different assumptions regarding population growth, economic growth, technological developments, resource limitations, fuel mixes, agricultural development, the implementation or not of already-proposed policies, and so on.
The most important background assumptions for the scenarios and their emissions values for the year 2100 are given in Table 1. Radiative forcing implications and comparisons with the IPCC90 scenarios are discussed later.
The overall range of emissions projections (Table 1) may seem large given that all scenarios are based on an 'existing policies' assumption, and that IPCC90 gave only a single such scenario. Nevertheless, the range realistically reffects the extreme unce rtainties in predicting future population growth and the development of economic, agricultural and industrial activities. For CO2 at least, as noted in ref. 4, the results are in accord with uncertainties expressed in earlier studies11,12. Our main concern here, however, is not the emissions scenarios, but their implications for climate and sea level. They are taken here as the starting point for further analysis.
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