"We have now shown that a war involving relatively minor nuclear powers could irreparably disrupt human society globally, and perhaps life on Earth," Mike Mills of the University of Colorado told environmentalresearchweb. "Such knowledge lends urgent relevancy to taking care of the world’s conflicts in regions far from our own and reducing the proliferation of nuclear weapons."
Mills and colleagues from the University of Colorado, University of California and US National Center for Atmospheric Research used a chemistry-climate model and new estimates of the smoke produced by fires in contemporary cities to calculate the impact of 100 Hiroshima-size bombs exploded in cities in the northern subtropics.
They calculated that for the first five years following such a conflict, the ozone loss would average about 20% globally, 25 to 45% over mid-latitudes such as North America and Europe, and 50 to 70% further north. They say these losses are significantly greater than previous "nuclear winter/UV spring" calcuations, which did not adequately represent stratospheric plume rise.
"By the definition of the Antarctic ozone hole, we calculate a near-global ozone hole lasting for several years, with substantial depletion continuing for 10 years following the war," said Mills. "Compare this with a few per cent ozone depletion, which caused the banning of chlorofluorocarbons, and you can see that the consequences would be truly catastrophic."
The increase in ultraviolet radiation reaching Earth as a result of ozone depletion could cause human health problems such as cataracts and skin cancer, damage plants, animals and ecosystems, increase smog formation and damage outdoor materials.
"UV radiation has been shown to be particularly damaging to inhabitants of aquatic ecosystems, including amphibians, shrimp, fish and phytoplankton," said Mills. "Most organisms could do little to avoid exposure, so one of the big unanswered questions is how the biota would respond to these big UV increases triggered by a nuclear exchange. The biological consequences of such deep cuts in ozone should be studied further."
According to the researchers, their results point to previously unrecognized mechanisms for stratospheric ozone depletion. The main cause of the ozone depletion would be heating of the stratosphere by smoke, which absorbs solar radiation strongly.
"Higher stratospheric temperatures would accelerate catalytic reaction cycles, particularly reactions of nitrogen oxide gases known collectively as NOx that destroy ozone," explained Mills. "Our study confirms calculations from an independent global climate model in showing that the smoke from firestorms in megacities would be heated by sunlight, causing it to rise as high as 80 km, into the upper stratosphere."
Removal mechanisms for the smoke in the upper stratosphere are slow, and the convection created by rising smoke plumes would alter stratospheric circulation, redistributing ozone and sources of ozone-depleting gases.
In 2006, writing in the journal Climatic Change, Paul Crutzen proposed soot as an "interesting alternative" to sulphur particles for cooling the Earth’s surface and creating minor nuclear winter conditions.
"He describes soot as 'much cheaper and less messy' than sulphur, and suggests an added benefit would be inhibiting formation of the Antarctic ozone hole by heating the stratosphere," said Mills. "The global-scale ozone reductions that our study predicts for relatively small injections of soot into the upper troposphere and lower stratosphere indicate an unexpected sensitivity associated with such perturbations, and suggest that geoengineering schemes based on absorbing aerosols – such as soot – might pose an unprecedented hazard to the biosphere worldwide."
Mills is now modelling the effects of geoengineering schemes that use sulphate aerosols on global ozone. He plans to present some of his results at the European Geophysical Union in Vienna next week.
The researchers reported their work in PNAS.