This document comprises both the Summary for Policymakers and the Technical Summary of the Working Group I (WGI) report. It represents, in conjunction with the 11 chapters of the underlying WGI report from which this material was drawn, the most comprehensive assessment of the science of climate change since WGI of the IPCC produced its first report Climate Change: The IPCC Scientific Assessment in 1990. It enlarges and updates information contained in that assessment and also in the interim reports produced by WGI in 1992 and 1994. The first IPCC Assessment Report of 1990 concluded that continued accumulation of anthropogenic greenhouse gases in the atmosphere would lead to climate change whose rate and magnitude were likely to have important impacts on natural and human systems. The IPCC Supplementary Report of 1992, timed to coincide with the final negotiations of the United Nations Framework Convention on Climate Change in Rio de Janeiro (June 1992), added new quantitative information on the climatic effects of aerosols but confirmed the essential conclusions of the 1990 assessment concerning our understanding of climate and the factors affecting it. The 1994 WGI report Radiative Forcing of Climate Change examined in depth the mechanisms that govern the relative importance of human and natural factors in giving rise to radiative forcing, the "driver" of climate change. The 1994 report incorporated further advances in the quantification of the climatic effects of aerosols, but it also found no reasons to alter in any fundamental way those conclusions of the 1990 report which it addressed.