The most spectacular of them is the possible collision of Earth with some celestial body - comet or asteroid. Depending on the size of such a body, the consequences may be different, including extinction of humanity as a species , as well. It suffices to look through ordinary binoculars at the surface of the Moon, pockmarked with thousands of impact craters. It is also certain that similar collisions will happen in the future. Therefore, every year of delay may turn out to be the critical one.
Earth also becomes overpopulated .
Nothing indicates that the number of its inhabitants may undergo any significant decrease - except as a result of some severe war or global cataclysm, which could endanger the whole Earth's biosphere. Expansion into space offers here a possibility for a peaceful and gentle unburdening of the natural environment , while, at the same time, making survival of our species and civilization possible, even if the damage turns out to be irreversible. The civilization which crosses the space barrier and makes its existence independent of the fate of a particular planet will survive. Others are doomed. Unfortunately, in recent times, the understanding of that fact by people diminishes.