Well global warming is something that takes a very long time. It's not something that we can cause just because we litter and cause pollution massively in one year. We could dump all of our trash and spray things that cause depletion of the O-zone, we could burn harmful chemicals and all drive diesel rigs for 5 years straight, but that won't cause the Ice caps to melt because we did that. It takes time...LOTS of time. an Asteroid on the other hand that is heading straight for earth could cause a major Cataclysm, bigger than the Polar Ice caps melting could cause. If the Scientist are tracking the Asteroid and it is in fact heading right for us, then it probably wouldn't miss. With the technology that we have they could probably pin-point exactly where it will hit.