If an asteroid would really collide with Earth as the IAU predicted, human beings could be erased in a short period. It would depend on where an asteroid would smash on Earth. If it was in one of the oceans, there would be a tsumani, a hundred times bigger than the one after Christmas 2004. Smaller islands would disappear like Atlantis, larger ones with high mountains such as Teneriffa would probably have a small chance of staying above sealevel.
If the asteroid should smash on land, it would be worse than the largest earthquake one could think of. The land around would be destroyed, everybody living there would die immediately killed by the impact of the explosion. There would be such a lot of dust in the air, that the sunlight wouldn't come through to the Earth and the plants would begin to fade and die because of their lack of oxygen. This would be the beginning of the end of life on Earth.