Since 1988, a remarkable international scientific collaboration has progressed under the auspices of the Intergovernmental Panel on Climate Change (IPCC), sponsored jointly by the World Meteorological Organization and the U.N. Environmental Program. These volumes, which are also available on the Internet (http://www.IPCC.ch), provide the backup papers, summaries for policymakers, and technical synopses of their "Third Assessment" on climate change (the first two were in 1990 and 1995). Three IPCC working groups tackled separate issues: first, current scientific knowledge regarding global climate change; second, the likely social impact and vulnerabilities; and third, the prospects for mitigating climate change. Readers interested in the scientific basis for climate change will profit from the first volume, which lays out the current consensus views of climatologists as well as the remaining areas of ignorance or uncertainty. The other two are more tentative and general, since they build on the scientific consensus and all its uncertainties -- and add their own uncertainties and disagreements as well. Popular reports suggest that the IPCC envisions higher global temperatures than it did in 1995, citing more scientific knowledge. But that conclusion is misleading. This time, the IPCC sensibly considered a range of scenarios for greenhouse-gas emissions, recognizing that the human ability to foresee global social and economic developments over the next century is limited. The higher temperature increases that it reports result from this range of possibilities, not from new scientific information applied to any particular scenario.