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84.26 On Human Survival: Nuclear Winter

The Presidents of Science for Peace and of Physicians for Social Responsibility (Canada) sent a letter dated March 28, 1984, to Members of Parliament, drawing their attention urgently to work published recently by Dr. Paul Ehrlich, Dr. Carl Sagan and their colleagues.

It is estimated that as many as one billion people would die as a direct result of a 5000 megaton exchange in a major nuclear war. A team of scientists has recently shown that the indirect effects would be even more catastrophic. They have calculated how much dust and smoke is produced in nuclear wars of various sizes, how much sunlight is absorbed and how much the temperature falls. In addition to the known results showing widespread radioactive fallout followed by increased ultraviolet radiation, they have found that soot from burning cities obscures sunlight in a manner not previously predicted. Within a week of the war an unbroken pall of darkness covers the Northern Hemisphere. This lasts for several weeks and disrupts plant growth and hence food chains. The effects spread to the Southern Hemisphere and the harsh Nuclear Winter lasts for months, having a major impact on climate for several years.

Most survivors of the war, both human and animal, would die of starvation, cold and thirst in this Nuclear Winter. In the Third World even more people would die of the indirect effects than died of the direct effects in the Northern Hemisphere. The radioactive fallout appears to be worse and more widespread than previously estimated and would cause extreme susceptibility to epidemic infectious disease. Uncontrolled fires in forests and especially the cities release large quantities of toxic gases, which would inhibit the recovery of vegetation damaged by blast and fire. Ozone depletion leads about 3 months after the war to increased exposure to ultraviolet light, which damages biological systems in many ways — for instance, depressing immunity to infectious disease and causing blindness in humans and animals. Tropical forests are especially at risk, and most animal and vegetable species in the tropics might become extinct.

Even a relatively small nuclear exchange of 100 megatons targeted on cities, where there is a great deal of combustible material, could produce a two-month period of subfreezing land temperatures reaching down to -23°C. The cold and dark would be almost as bad as after a much larger war targeted on military facilities.

In summary: after a 5000 megaton nuclear exchange, or perhaps even after a much smaller nuclear war, survivors would face extreme cold, lack of food, fuel and water, heavy burdens of radiation and pollutants, disease and severe psychological stress. It seems likely that a nuclear war, even of relatively limited dimensions, would extinguish human civilization in the Northern Hemisphere and possibly human life on this planet.


1 R.P. Turco et al.: “Nuclear Winter: Global Consequences of Multiple Nuclear Explosions”, Science 222, 1283-92, December 23, 1983. ^

2 Paul R. Ehrlich et al.: “Long-Term Biological Consequences of Nuclear War”, Science 222, 1293-1300, December 23, 1983. ^

3 Carl Sagan: “Nuclear War and Climatic Catastrophe: Some Policy Implications”, Foreign Affairs 62, 257-292, Winter, 1983/84. ^

The letter to Members of Parliament ends:

“Our organizations believe that these predictions of global catastrophe resulting from nuclear war are well grounded in scientific fact. The threat to Canadians and all humanity is clear and urgent. We ask you with respect to ponder deeply the attached summary of these new scientific findings and to reflect upon their consequences for Canadian international policy.”

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