The major policy implication of nuclear winter was that a full-scale nuclear attack would produce climatic effects which would so disrupt the food supply that it would be suicide for the attacking country and would also impact non-combatant countries. The subsequent end of the arms race and reduction of superpower tensions can be traced back to the world being forced to confront both the direct and indirect consequences of the use of nuclear weapons by the public policy debate in response to nuclear winter theory. While significant reductions of American and Russian nuclear arsenals followed, each country still retains enough weapons to produce a nuclear winter. Several other countries now possess enough nuclear weapons to not only severely damage themselves and others directly by a regional nuclear war, but also to damage the rest of the world through significant global climate changes. We use a modern climate model and new estimates of smoke generated by fires in contemporary cities to calculate the response of the climate system to a regional nuclear war between emerging third world nuclear powers using 100 Hiroshima-size bombs on cities in the subtropics. We find significant cooling and reductions of precipitation lasting years, which would impact the global food supply. The global average surface air temperature would cool rapidly to a value lower than experienced on Earth in more than 1000 years, and would stay there for more than 10 years. The climate changes are large because the fuel loadings in modern cities are quite high and the subtropical solar insolation heats the resulting smoke cloud and lofts it into the high stratosphere, where removal mechanisms are slow. The results presented here need to be tested with other climate models, and the detailed consequences on agriculture, water supply, global trade, communications, travel, air pollution, and many more potential human impacts need further study. Each of these potential hazards deserves careful analysis by governments advised by a broad section of the scientific community.

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