The impacts of greenhouse warming have been described by Working Group 2 of the 1995 IPCC assessment. They include the impacts of changing vegetation zones on agriculture and silviculture, changes in the occurrence of crop pests and disease-carrying insects, as well as estimates of the effects of increased frequencies of extreme events such as heat waves, dry spells and floods. The present study attempts to quantify and valuate each of these impacts, integrated over the 21st century, under the assumption of continued injection of greenhouse gases into the atmosphere, leading to a doubling of atmospheric content around the middle of the 21st century. The result is a greenhouse warming externality constituting a sizable fraction of the global gross national product, provided that impacts are valued in the way normally done in externality studies for the industrialized world. As the greenhouse gas emissions are chiefly from industrialized countries, while the largest impacts are in those regions of the world least likely to reach stages of high development during the 21st century, the results pose a difficult geo-political problem The implications for estimating externalities of particular energy conversion activities are briefly discussed.