Nuclear war? Mars is a desolate wasteland that's bombarded with ionizing radiation from space (because it lacks both a sufficient atmosphere and a magnetic field to protect it).
Asteroid impact? Unless it eliminates all of our atmospheric oxygen, ozone, hydrogen and nitrogen and drops the average surface temperature to -55°C, we're still better off here.
Pandemic? If we can protect a colony from the environment of Mars, we should be able to keep at least that many people safe from a pandemic. (Just stick your Mars-bubble in Antarctica if nothing else. It's a lot like Mars except it has better temperatures, an atmosphere that you can breathe if you need to and the water is more readily available.)
[1] http://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinc...
Unfortunately, on top of that what makes the atmosphere truly deadly is a lack of Nitrogen one of the primary building blocks for organic chemistry.
I have to say that motivating with fear puts red flags up in my mind as I'm sure it does with the rest of the crowd here. If I were a politician, I'd see the mission to colonize space as an effective way to unite the world behind a single goal. Since I'm not, I have a different view which is that AI has the greatest potential to solve humanity's problems. But that's another discussion.