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Scenarios of net greenhouse gas and aerosol precursor emissions for the next 100 years or more are necessary to support study of potential anthropogenic impacts on the climate system. The scenarios both provide inputs to climate models and also assist in assessing the relative importance of relevant trace gases and aerosol precursors in changing atmospheric composition and hence climate. Scenarios can also help to improve the understanding of key relationships among factors that drive future emissions.

Scenarios are not predictions of the future and should not be used as such. This becomes increasingly true as the time horizon increases, because the basis for the underlying assumptions becomes increasingly speculative. Considerable uncertainties surround the evolution of the types and levels of human activities (including economic growth and structure), technological advances, and human responses to possible environmental, economic and institutional constraints. Since completion of the 1990 Scenario A (SA90), events and new information have emerged which relate to that scenario's underlying assumptions. These developments include: the London Amendments to the Montreal Protocol; revision of population forecasts by the World Bank and the United Nations; publicadon of the IPCC Energy and Industry Sub-group scenario of greenhouse gas emissions to AD 2025; political events and economic changes in the former USSR, Eastern Europe and the Middle East; re-estimation of sources and sinks of greenhouse gases (reviewed in this Assessment); revision of preliminary FAO data on tropical deforestation; and new scientific …