Natural: not many forces of nature will be disastrous enough to wipe out civilization on a massive scale. In 1816, Tambora erupted and it caused the year without a summer. That one event lead to roughly 50,000 deaths by the global atmospheric impact to agriculture. Still, 50,000 was a tiny fraction of the global population at that time. The only one event that would eradicate most of the global population would be the eruption of a flood bassalt or meteorite impact.
Manmade: Lots of things will lead to these outcomes. Nuclear winter... most likely single event that could lead to complete destruction in a matter of days. Climate change... that depends. Urban runoff is a cause of increased flooding in areas. The destruction of the ozone layer is a very serious issue because ozone reduces the amount of UV radiation that the Earth receives and can't easily be fixed. I don't believe global warming is mostly manmade, but the threat of Greenland's glaciers melting off and entering the Atlantic Ocean threatens the gulf stream... not leading to destruction, but creates chaos for a region that represents 10 of Earth's population.
Famine: This will be the most likely to occur in my opinion. The agricultural land that feeds the people of Asia depends heavily on water collected from non-renewable sources. Once these fresh water reserves are depleted, there will be a mass reduction of the populations and then a new, reduced carrying capacity for the areas that would need desalinization just to provide for human consumption. On top of that, the use of agriculture destroys native vegetation and creates homogeneity instead of diverse ecosystems. The more diverse, the more resilient ecosystems become. This applies to gaps in the food chain leading to mass extinctions as well.
Disease: Not very likely, I think. In our day in age, there is the concern of transportation making the world smaller. If a plague breaks out, we have advanced medical research facilities and experts who make it less likely that another black plague will cause death on a massive scale. The reason why there was a population explosion in the last two centuries was because the death rate plummeted as a result of medical research and sanitation technologies.
Last edited by Darth_Yuthura; 04-19-2009 at 08:51 AM.