Hmm...What if, for whatever reason, the acidity or salinity levels of Earth's waters rose and made it toxic? There's a growing concern that our agricultural land, as it stands right now, is becoming infertile. Petroleum fertilizers are said to be the big cause. Additionally I make overpopulation a large problem. It's not so much that there's just too many people, as it is that there's not enough food/water to go round for that particular volume. Therefore, they spread out. (E.g.) The problem is (#of people > sustainable food/water supply) so relocating people to sustain themselves elsewhere would be the solution. As for minerals, David Cohen wrote an article in 2007 about this for newscientist.com, titled "Earths Natural Wealth: An Audit". It says, "There is speculation that key elements needed for modern industry, including antimony, zinc, tin, silver, lead, indium, gold, and copper, could be exhausted on Earth within 50-60 years." Now imagine if consumption were incredibly high. I mean world population has risen from 3 billion to over 6.5 billion in 50 years. There's got to be a limit somewhere down the line, who's to say what that number is or when it will occur?