A future of overpopulation is one of a number of hoary old objections to progress and longer, healthier lives. It has been raised over and over again throughout recent history, but like all other Malthusian concepts, it was wrong then, and it's just as wrong now. Common Malthusianism - the idea that a given resource (such as living space or food) will run out in the future based upon extrapolation of present trends - stems from fundamental misunderstandings about economics, human action and change. We create change in response to our environment; our self-interest leads us to constantly strive at the creation of new resources where old resources are becoming scarce and expensive. This is the path to profit for the individual - and progress for all. One needs a certain amount of willful blindness to avoid seeing the process in action now and in recent history.
The ideas of Malthus were just as wrong as the ideas of those who warn of overpopulation today, and for just the same reasons. The simple answers to any claim of overpopulation with increasing longevity are much as follows:
1) Population growth declines and reverses with increasing wealth, longevity and technological progress:
Decelerating population growth appears to be an inevitable result of growing wealth. Early on in a country's developmental curve, children can be regarded as 'producer goods' (as economists would say). Parents put their children to work on the farm to generate food and revenue. Very little effort is put into caring for the child: no expensive health plans, special classes, trips to Disneyland, X-Men action figures, or mounting phone bills. As we become wealthier, children become 'consumer goods'. That is, we look on them more and more as little people to be enjoyed and pampered and educated, not beasts of burden to help keep the family alive. We spend thousands of dollars on children to keep them healthy, entertain them, and educate them. We come to prefer fewer children to a vast mob of little ones. This preference seems to be reinforced by changing tastes resulting from improved education.
UPDATE 05/28/2010: Researchers Leonid Gavrilov and Natalia Gavrilova have produced detailed population models to demonstrate that even large increases in human life span produce comparatively small changes in population size. You can find a summary near the end of a more recent Fight Aging! post, or at PubMed:
For example, we applied the cohort-component method of population projections to 2005 Swedish population for several scenarios of life extension and a fertility schedule observed in 2005. Even for very long 100-year projection horizon, with the most radical life extension scenario (assuming no aging at all after age 60), the total population increases by 22% only (from 9.1 to 11.0 million). Moreover, if some members of society reject to use new anti-aging technologies for some religious or any other reasons (inconvenience, non-compliance, fear of side effects, costs, etc.), then the total population size may even decrease over time.
2) It is self-evident from even a few back of the envelope calculations that the Earth can support tens of billions in comfort using the technology of today - and never mind the rest of the solar system once the cost of getting into orbit has been sufficiently reduced.
So it turns out that if 5% of the United States were converted into urban area with a population density of 6,000/km2, and 45% were converted into suburban area with a population density of 2,000/km2, with the remaining 50% left for rural area, parks, and farms, there would be enough room for 3 billion in the urban areas, and 9 billion in the suburban areas, for a total population of 12 billion. This is in the US alone. This scheme could be extended to the other countries and continents for a total population of around 100 billion. Everything between the Arctic and Antarctic circles are potential targets for colonization. This is about 130,000,000 km2 of land area (the circumpolar regions have about 20,000,000 km2 of land).
3) What some presently view as "overpopulation" is more accurately described as crushing poverty amidst the potential for plenty and resources left unused. This is the result of despotism, corruption, economic ignorance, short-sighted greed and the inhumanity of man unto man - it is not a matter of counting heads.
Here, then, is a short guide for kleptocrats and egalitarians who want to keep their countries poor. All of these policies have stood the test of time as techniques for creating and maintaining poverty. The list is by no means exhaustive, but it will give would-be political leaders a good idea of how to start their countries on the road to ruin.
Malthusianism and cries of overpopulation in the face of a future of healthy life extension are forms of relinquishment. It is a call for death and suffering to continue on a massive scale; a certain type of person prefers any present horror to the uncertainty of change. Fortunately, such people have usually been swept aside in the past by the urge of the many to better their lives, one step at a time. May that long continue to be the case.