SaulGoldie is right on. Follow any of these environmental and human disasters to their source, and you will find human overpopulation. As Bobert points out, there is no easy solution to this. But, don't worry - if we do not find a solution, nature will. As will all species in the history of Earth that have overexploited their niche, we will be rendered extinct (or at least insignificant) by a plague, famine, or some other population-crash-causing event. There is no easy solution, but not choosing a solution is, in fact, choosing a solution.