Well, contrary to previous predictions, we haven't run out of metals, haven't run out of oil, and haven't run out of food. So, let's all start worrying we might run out of water:
World water supplies will not be enough for our descendants to enjoy the sort of diet the West eats now, experts say.
Which is, as with all other environmental apocalypse predictions, complete and utter crap. In a world run by markets, as water for various uses becomes more scarce its price will rise, forcing its users to become more efficient or stop using it entirely. By allowing producers to keep more of their profits through reduced taxation and better manage their risks through properly-regulated futures markets, strong incentives will be created for water consumers to use technology and knowledge to come up with innovations that will reduce consumption by orders of magnitude. In the end, water will, like all the other commodities the chicken-little enviroweenies have latched onto during the past forty years, get cheaper.
In a perfect world, that is, which we of course do not live in. Because, you see, western agribusiness is not run by markets, it's run by government subsidies and various regulatory protection rackets. This removes market pressures from farming, causing it to be far less efficient than it otherwise would be. The US isn't as bad as some countries (Japan is probably the worst), but to this day we pay billions of dollars more for our food than we should.
But you won't hear Greenpeace start advocating the removal of farm subsidies. Not sexy enough, doesn't blame enough white people. Even if they did, our bucolic illusion of farmer Ted and his six kids having to sell his tiny family farm because of... well... we never actually get to the reasons, do we? We just see those little kids, some has-been rockers hold a few concerts, and away goes our tax money, straight into the coffers of gigantic and politically powerful farming corporations. Corporations that now have no reason to innovate or become more efficient, because we are all guaranteeing their bottom line.
We'll still manage to muddle through, because (to the great disappointment of technocrats all over the world) a commodity's supply cannot be increased by legislative fiat. Eventually the cost will rise beyond the point it can be hidden by even government accountants. Democracies around the world will play the politician shuffle until they get a set willing to do what needs to be done*, while socialists (*cough* France *cough*) and dictatorships will face yet another economic collapse.
It won't be pretty, but it won't be the end of the world either.
* And lose their jobs for it immediately. I mean, those guys will be responsible for killing the family farm!