Population Explosion

Although unlikely to end human life on earth, the continued population growth in much of the world threatens food supplies to the point where humans might overshoot their carrying capacity. This is made worse by the fact that the Earth is only currently able to support a human population at this level due to intense 'Green Revolution' farming. Peak oil would mean an end to cheep and easy High intensity agriculture, and might eventually lead to the collapse of civilizations as the first world nations close ranks to defend their food supply against the starving third world hordes. The other solution is Soylent Green.