The climate effects of nuclear war

Nuclear weapons pose the most acute existential threat to humankind and the biosphere. Both qualitatively and quantitatively, they are uniquely destructive. Their incendiary effects are likely to exact the greatest toll: the largest cause of acute casualties in nuclear war would be from fires; the greatest cause of long-term casualties would be agricultural collapse and global famine from decade-long worldwide cooling, darkening and drying under a blanket of millions of tons of sooty smoke from burning cities. Only about 2% of current nuclear weapons, exploded on cities, would abruptly cause ice age temperatures, putting over 2 billion people at risk of starvation in just the following 2 years. A nuclear war involving a substantial fraction of the global arsenal would decimate the vast majority of the human population, risk human extinction and that of many other species, and catastrophically disrupt the Earth systems on which the biosphere, including humans depend. The evidence on nuclear winter and famine is the most consequential scientific evidence of the nuclear age. This article reviews the history and status of research on the climate and food production impacts of nuclear war, draws lessons and describes new UN and WHO initiatives on nuclear war effects.