In response to the buildup of U.S. and Russian nuclear arsenals during the Cold War, a series of major scientific studies conducted in the 1980s issued warnings about the potential for a "nuclear winter" scenario - the possibility that a large-scale nuclear exchange could inject massive amounts of soot and particulates into the upper atmosphere that would block incoming solar radiation and cause major ecosystem and societal disruptions. In the decades since that concept emerged, profound military, political, and technological changes have reshaped the nuclear landscape, while scientific advances have deepened the understanding of, and ability to model, Earth system processes. It is in this context that the U.S. Congress asked for this report to re-examine the potential environmental, social, and economic effects that could unfold over the weeks to decades after a nuclear war.The effects of any given nuclear exchange would depend on key processes and interactions along a causal pathway with six stages: weapon employment scenarios and effects; fire dynamics and emissions; plume rise, fate, and transport; physical Earth system impacts; ecosystem impacts; and socioeconomic impacts. Impacts of radioactive fallout were not included in the assessment. Potential Environmental Effects of Nuclear War identifies major uncertainties and data gaps at each stage of the causal pathway that currently limit researchers' ability to understand and model the effects of a nuclear war. This report recommends that relevant U.S. agencies coordinate the development of and support for a suite of model intercomparison projects to organize and assess models to reduce uncertainties in projections of the climatic and environment effects of nuclear war.