Populations

I am going to express the unpopular position now.  The number one threat to continued human existence is the run-away population explosion that we have seen since the Industrial Revolution, and driven forward at an amazing rate since the Green Revolution.  We have been well aware of it since at least the 60s.  America's birth rate is still too high, and our life-expectancy continues to grow our numbers too quickly, but nothing as compared to the developing world.  There, you can not avoid seeing exactly what the population explosion is doing.  It is the main driving force behind our increase in resource consumption, green house gas emissions, and poverty levels.

More people are being born into a state of abject poverty from which they have no hope of ever getting out of than ever before in human history.  More people with the same or less resources = poverty for a vast majority, e.g. the US must create more than 300,000 new jobs every month just to employ all the people coming into the market, but we can't even manage half that amount.