Cosmic Rays - scientists have yet to work out where most high-energy cosmic rays come from. Either an increase striking our atmosphere, or a weakening of our shields, will let in more cosmic rays. They are already zipping through your body every second, and they could end up being our silent killers.
As one example of their danger, airlines do not let pregnant women work on their inter-continental routes, because of the increased exposure to cosmic rays at those heights.
Possibility this century = 8
Possibility in 2012 = 6
Could Ancients Predict It? = 3
TOTAL = 17