Atmospheric consequences of nuclear exchange

Nuclear winter has been hypothesized to dramatically alter the Earth’s climate. Michael Mills et al. have studied ozone depletion as a result of regional nuclear war, developing a computational model that links climate to atmospheric chemistry, and conducting 10-year simulations. The fires that would result from a total exchange of 100 Hiroshima-equivalent nuclear devices (yield 15 kilotons) would generate 5 million tonnes of soot that firestorms and solar heating would loft into the stratosphere. At altitudes up to 60 km, the soot would absorb solar radiation and heat surrounding gases, thus increasing the rate constants for several reactions that break down ozone. The authors found that two chemical reactions in particular would dominate alterations in ozone after a nuclear war: (i) the Chapman cycle, in which an oxygen free radical and ozone combine to form two diatomic molecules; and (ii) a coupled pair of reactions catalyzed by NO and NO2 that similarly produces diatomic oxygen. In the year after a nuclear war, the Chapman cycle would account for the greatest ozone loss, but NOx catalysis would then become dominant and persist for several more years. Mills et al.’s model predicts that the net result in both hemispheres would be an ozone hole extending from 20° north or south latitude to the poles. — K.M.