6. Depends on your definition of 'killer', I suspect
I had no idea anyone was talking about the destruction of the planet. But with so much relying on satellite comms and more than a few powergrids hanging on by their fingernails on a good day, we might end up with a few extra corpses: The '03 blackout in the NE was mainly caused by a tree in Ohio, but claimed - at least in part - 11 lives. the '89 solar max took out a large chunk of Canada, although I don't know of any deaths from that. We don't need another 1859-ish storm to cause havoc: A repeat of 1959 would fuck us up nicely.