See the anti-congestion argument for more details. The sad fact is, there are just too many people in the world. As of October 20, 2010, the current world population is over 6.5 billion people. Our current agriculture methods deplete nutrients from the soil at an astonishing rate. Eventually, the soil will be devoid of nutrients and crop yields will drop. We pollute the air, water, and soil and destroy the environment around us.
According to estimates, there will be 8 to 10 billion people in the world by 2050. Simply put, the world cannot support that many people. Resources would be spread too thin, and environmental pollution would likely be at dangerous levels. Sanitation would probably be poor and disease would be widespread. Society might even beak down and there could be fighting in the streets over food, water, and other resources.
Whether voluntarily or not, the population on Earth would eventually drop to a sustainable level. A rampant disease, war, or starvation would wipe out many people.
To lower the population voluntarily, we would have two options:
- Enforce laws that say each family can only have one child. This would seem very unfair, and most people would complain. This is the cost-effective option.
- The other option is to ship massive numbers people off the Earth and onto another planet like Venus or Mars, or put them inside a gigantic orbiting space colony. But why jam them all in one place. It is better to scatter and maintain communication in other ways, see the anti-congestion argument. This option is more attractive because we would not only reduce the population, but we would also colonize space and ensure the long-term survival of our species (until the sun nears the end of its life. Then we have a BIG problem). To assume confinement to the Solar system is extremely naive, see interstellar colonization.