I think the greatest challenge facing the world today is the rampant growth of the human population. We are presently increasing at a rate equivalent to adding one new Germany (80 million people) each year. Unfortunately, our approach to this problem has always been backwards: We try to accommodate the increasing population instead of controlling it. As a result, we are already approaching some significant limits. Consider oil. At the present rate of consumption, we will effectively run out of oil in just 40 years. Of course, we will find a bit more, but the rate at which we use this oil will also be steadily increasing as people around the world try to improve their living conditions and the population continues to grow. Imagine a world with 10-12 billion people and no oil! Think of what will be happening to the environment as we draw closer to this limit.