There is widespread concern about an 'overpopulation problem'. Let us be clear about what is meant by 'overpopulation'. It is not a problem for a lot of people to be alive. It is a problem if there are too many people for given resources to go around. So the important question is, "Is the human population likely to outstrip available resources?"
According to the US Census Bureau, the world population as of September 14th 2010 is 6,868,683,892. This number is growing; the UN's upper prediction is 10.6 billion for 2050. After that, the UN expects the population to begin to fall.
Let us assume population continues to rise beyond 2050 and reaches 40 billion, well beyond any UN estimate. Would we be overpopulated then, in relation to available resources? —
- Food. Without expanding farmland, we could grow enough food for 80 billion people using low-tech permaculture techniques only.
- Water. Our planet has about 1260 quintillion liters of water. This means that 40 billion people using 200 liters a day each would use, over the course of a year, less than 0.00025% of the world's water.
- Energy. The world used 15 terawatts of energy in 2008. If rising population and increasing technology increased this 100-fold to 1500 terawatts, we would still only need to convert less than 0.9% of the sunlight that falls on Earth. It is highly likely that we will have fusion reactors and space-based solar panels before our energy needs come anywhere near this level.
- Land. The planet's surface (including oceans) is about 510 million square kilometers. According to Wikipedia, one-eighth of this, 63,750,000km2, is habitable land. For a population of 40 billion people, this is 1593.75m2 habitable land per person, equivalent to a average population density of 628 people per km2. This is comparable to a fairly densely populated country like Taiwan.
 Doing more with less
100 years ago, 8000 square meters of land was needed to grow food for a person. It can now be done on a few hundred square meters. Why? Because human intelligence has figured out how to extract more resources from a fixed amount of material. The effect of human intelligence is always to enable us to : better solar cells can make more electricity from less sunlight, we can make a more powerful computer chip using less material than a few years ago, and more efficient vehicles can travel the same journeys with much less petrol.
Human intelligence is the key that unlocks all other resources. As Robert Anton Wilson has said, "You can starve in the middle of a field of wheat if your mind hasn't identified wheat as edible." The greater the population, the greater the store of human intelligence. A large population that is well networked and educated will concoct and communicate all kinds of technological solutions that enable us to do more with the resources we have. And so, paradoxically, an increased population can mean that we have more resources to go around.
 Space colonisation
There is ultimately an upper limit on the amount of people this planet can accomodate (though, as we have shown, the limit is not very limiting). Colonising space can be thought of as the ultimate solution to any question of overpopulation. Gerard K. O'Neill wrote a classic essay called The Colonization of Space in 1974. In it, he considers the ability of a series of space habitats orbiting the Earth and the Sun to absorb population increase. These colonies could be built from materials available in the asteroid belt and the Moon using the technology available in 1974. O'Neill's calculations show that they could house 20,000 times the world population at the time he wrote the essay - no less than 80 trillion people!