another possibility... (file under: 2012 - the end of the world.)

Wired News Blog:
" We are caught in the cone of gamma radiation from one dying star or another about a dozen times a week. Most of them are safely millions of light-years distant -- but it has been calculated that if such an implosion occurs within six thousand light-years of us, and happens to emit its beam of gamma radiation in our direction, it would strip the protective ozone layer off our planet and leave all life on the surface exposed to deadly ultraviolet radiation for up to five years. It would also fill the upper atmosphere with nitrogen oxides that absorb the sun's heat and could easily push the Earth into an ice age. "