This report is a Supplement to the 1990 Report of the Scientific Assessment Working Group of the Intergovernmental Panel on Climate Change (IPCC). The IPCC was set up jointly by the World Meteorological Organization and the United Nations Environment Programme in 1988 to provide an authoritative international consensus of scientific opinion on global warming. This Supplement reviews the key conclusions of the 1990 Report in the light of new evidence, focussing on five main areas: * sources and sinks of greenhouse gases and the precursors of tropospheric ozone * radiative forcing of the atmosphere, with emphasis on the concept of Global Warming Potential * updated scenarios for emission of greenhouse gases up to the end of the 21st century (as the basis for climate modelling studies) * new developments in climate modelling and model validation * recent observational data on climate variability and change. Several hundred international scientists participated in the preparation and review of this assessment, thus continuing the approach which gained such widespread acceptance and authority for the 1990 IPCC report. It represents the continuing effort of the international scientific community to communicate to policymakers, at both national and international levels, the very latest scientific knowledge and understanding of the complex issues surrounding climate and climate change. The original report, Climate Change The IPCC Scientific Assessment, is still available from Cambridge University Press.