It's a frightening thought, and all too real. Another thing that could happen is that a disease sweeps across the planet and kills off a large percentage of humans. It is estimated that the black plague killed off 30 to 50 percent of Europe's population between 1347 and 1351. Nowadays with the entire world interconnected by airplanes a plague would spread vastly more quickly.

I sometimes wonder if to the Earth humans are a type of parasite that she must from time to time try to shake off with disease and natural disaster for her own self-protection.