This is a sobering bit of conjecture. A hypothetical exhange of 100 15-kiliton nuclear weapons (Hiroshima-sized) could pretty much wipe out the ozone layer.
From the NewScientist link:
Apart from the human devastation, a small-scale nuclear war between India and Pakistan would destroy much of the ozone layer, leaving the DNA of humans and other organisms at risk of damage from the Sun’s rays, say researchers.
Michael Mills of the University of Colorado at Boulder, US, and colleagues used computer models to study how 100 Hiroshima-sized bombs would affect the atmosphere.
They say that their scenario – in which each country launches 50 devices of 15 kilotons – is realistic, given the countries’ nuclear arsenals.
“The figure of 100 Hiroshima-sized bombs compares pretty accurately to the approximately 110 warheads that both states reportedly possess between them,” agrees Wyn Bowen, professor of non-proliferation and international security in the War Studies Group at King’s College, UK.
(Hat tip: KurzweilAI.net)