I thought of a gamma ray burst too. Such bursts if strong enough from a nearby star, would destroy the earth's protective ozone layer. Without the ozone layer, the Sun's harmful ultraviolet radiation would kill life on the surface and the surface of water. Life as we know it would either cease to exist, or suffer radical chromosomal damage.
If Al Gore and his band of Chicken Littles really wanted to protect life on this planet, they should work on a ozone layer recovery process.