At 2004 study told us that a GRB from a distance of just one kilo parsec could destroy half of Earth's ozone layer. It suggests that historical mass extinctions may have been caused by GRBs. NASA explains how gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer.
Recovery could take at least five years. With the ozone layer damaged, ultraviolet radiation from the Sun could kill much of the life on land and near the surface of oceans and lakes, and disrupt the food chain.
Possibility this century = 7
Possibility in 2012 = 7
Could Ancients Predict It? = 5