The suddenness is a problem because there isn't enough time to adapt? Implying that we'll adapt to climate change? Watch out for heresy there ;)

But asteroid or comet impacts. Comets in particular can come so fast and undetectably that we won't have time to react. We could prevent a comet impact if a few humans decided to build such a defense system, and we might be wiped out if those few humans decide not to do it. Sounds equivalent to nuclear war.