In a recent contribution to this journal Ellis and Schramm [Ellis, J. & Schramm, D. N. (1995) Proc. Natl. Acad. Sci. USA 92, 235-238] claim that supernova explosions can cause massive biological extinctions as a result of strongly enhanced stratospheric NO_x (NO + NO_2) production by accompanying galactic cosmic rays. They suggested that these NO_x productions which would last over several centuries and occur once every few hundred million years would result in ozone depletions of about 95/100, leading to vastly increased levels of biologically damaging solar ultraviolet radiation.
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