The global atmosphere is a chemically complex and dynamic system, interacting both internally, mostly within the troposphere and stratosphere, and with the oceans, land, and living organisms. Its composition is changing today, and has also changed markedly over the last 400,000 years. Current understanding of this complex system resulting from recent observations, theory, and laboratory and modeling studies is reviewed. Also, results are presented from the Integrated Global System Model (IGSM). This is a coupled model of economic development, atmospheric chemistry, climate dynamics and ecosystem processes that explores possible future changes in atmospheric composition and climate. The results of an uncertainty analysis involving hundreds of runs of the IGSM imply that, without specific mitigation policies, the global average surface temperature may rise between 1.0 and 4.9 °C from 1990 to 2100 (95% confidence limits). Polar temperatures, absent policy, are projected to rise from about 1 to 12 °C (95% limits) with obvious great risk for high latitude ecosystems and ice sheets at the high end of this range. Analysis of the Kyoto Protocol, and a more stringent climate mitigation policy, shows the difficulties in accounting simply for the effects of other greenhouse gases relative to carbon dioxide. Also, the greatest effect of these policies is to lower the probability of extreme changes as opposed to lowering the medians. (© 2004 International Union of Geodesy and Geophysics and the American Geophysical Union)