Nuclear Darkness
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Reduced Canadian wheat-growing potential after small drops in average temperature

The global changes in average surface temperatures caused by the detonation of 100 Hiroshima-size weapons in the cities of India and Pakistan (50 weapons in each nation) would act to decrease the length of growing seasons in the Northern Hemisphere. In some places like Canada (see map below) the decreases in average temperature would have drastic effects upon the ability to grow the annual wheat crop. It is predicted that wheat exports would cease for three or more years; the global decreases in grain production would be severe and would likely lead to a global nuclear famine affecting hundreds of millions of already hungry people (see the article by Dr. Ira Helfand, "An Assessment of the Extent of Projected Global Famine Resulting from Limited, Regional Nuclear War")

Reduced Canadian wheat-growing potential after small drops in average temperature
Reproduced/modified by permission of American Geophysical Union.

Map showing the present principal area of Canadian wheat production, and the reduction that would result from small decreases in average surface temperature.

Source

Scientific Committee on Problems of the Environment (SCOPE), Figure 4.8a on p. 302 of SCOPE Report 28, Environmental Consequences of Nuclear War: Vol. I, Physical and Atmospheric Effects by A. Ptticok, T.P. Ackerman, P.J. Crutzen, M.C. MacCracken, C.S. Shapiro & R.P. Turco (eds.), 1986, 2nd ed. 1989, Wiley & Sons, New York; Vol. II, Ecological and Agricultural Effects by M.A. Harwell and T.C. Hutchinson (eds.), 1985, 2nd ed. 1989, Wiley & Sons, New York; PDF files available at www.icsu-scope.org.
reduced temps after war with 5 million tons of smoke
Reproduced/modified by permission of American Geophysical Union.
Temperature changes in summer (°C) following a small nuclear war (India-Pakistan Conflict)

Surface air temperature changes following the injection of 5 million tons of smoke into the stratosphere after the detonation of a total of 100 Hiroshima-size nuclear weapons in the largest cities of India and Pakistan [for the 5 Tg standard case]. Temperatures are averaged for June, July, and August of the first year following the smoke injection. Effects are largest over land, but there is substantial cooling over tropical oceans, too. The warming over Antarctica is for a small area, is part of normal winter interannual variability, and is not significant.

Source