Monday, April 14, 2008

Let's Not Have a Nuclear War, OK?

Total ozone deviations following 5-Tg soot injection in the upper troposphere.
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.

Massive global ozone loss predicted following regional nuclear conflict in PNAS.
We use a chemistry-climate model and new estimates of smoke produced by fires in contemporary cities to calculate the impact on stratospheric ozone of a regional nuclear war between developing nuclear states involving 100 Hiroshima-size bombs exploded in cities in the northern subtropics. We find column ozone losses in excess of 20% globally, 25–45% at midlatitudes, and 50–70% at northern high latitudes persisting for 5 years, with substantial losses continuing for 5 additional years. Column ozone amounts remain near or <220 Dobson units at all latitudes even after three years, constituting an extratropical "ozone hole." The resulting increases in UV radiation could impact the biota significantly, including serious consequences for human health. The primary cause for the dramatic and persistent ozone depletion is heating of the stratosphere by smoke, which strongly absorbs solar radiation. The smoke-laden air rises to the upper stratosphere, where removal mechanisms are slow, so that much of the stratosphere is ultimately heated by the localized smoke injections. Higher stratospheric temperatures accelerate catalytic reaction cycles, particularly those of odd-nitrogen, which destroy ozone. In addition, the strong convection created by rising smoke plumes alters the stratospheric circulation, redistributing ozone and the sources of ozone-depleting gases, including N2O and chlorofluorocarbons. The ozone losses predicted here are significantly greater than previous "nuclear winter/UV spring" calculations, which did not adequately represent stratospheric plume rise. Our results point to previously unrecognized mechanisms for stratospheric ozone depletion.

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