Image from "National Geographic"
If the demographic projections by the United Nations will turn out to be true, the world population should reach over 11 billion people by 2100. Some think that it will be a disaster, others see it as a good thing as it would bring more economic growth. But is it really possible to reach such numbers? Can we really think that women would be so stupid to continue making children even in the midst of the crisis caused by declining natural resources and worsening ecosystem disruption? (unless the Pope himself were to tell them to stop)?
Yet, some models tell us the human population could keep increasing even after the collapse of the world's economy. There exists something called the "demographic transition" and it is a historical observation that may be extrapolated into the future. The data show a sort of "U-shaped fertility curve" that makes the poor and the very rich to be more fertile than those who are in the middle. When applied to the scenarios of "The Limits to Growth" of 1972, this idea generated a curious behavior, with the impoverishment of the population causing an increase in the birthrate that causes the population to continue increasing for a few decades after the collapse.
But, as it is always the case, extrapolating past trends into the future is extremely dangerous. In particular, it is at least improbable that the post-collapse world will be like running the same movie in reverse. The demographic transition has been observed to occur in growing economies, it won't simply change sign and reverse itself in contracting economies. To see how it works we can look at the demographic trends in Russia.
Now, I have a highly positive opinion of the intelligence of Russian people and, in particular, of Russian women. But I just can't think that people in other regions of the world would behave very differently. Those who maintain that people will make more children as they become poorer seem to assume that most people, and women in particular, are not more intelligent than an average rabbit. But, as it is always the case, extrapolating past trends into the future is extremely dangerous. While is true that most people are not very effective at the task of acting in order to benefit humankind as a whole, most people are perfectly able to understand what's good for themselves and their immediate families. And, in times of trouble, they normally react by planning in order to optimize the number of their surviving offspring. Humans apply what's called the "K-strategy" in reproduction: they concentrate the resources available on fewer children in order to maximize their probability of success.
Of course, we are dealing with phenomena of which we know little. After that the world's economy peaks, all the bets are off. But it may not be a coincidence that whole regions of the world, such as Southern Europe have started a population decline, despite having traditionally been demographically active. Among these Greece, Italy, Spain, and Portugal. Outside Europe, Japan has also started to decline, inverting a tendency of growth that had been ongoing from the 1920s. It is not yet possible to say if these inversions are to be understood as long-term trends. But, if this is the case, they are evidence that an economic crisis has nearly immediate effects on population.
A global population decline would have at least a positive effect in the sense that it would reduce the threat of climate change and the pressure on the ecosystem. But it may turn out to be a disaster if we enter the rapidly descending slope that I called the "Seneca Cliff". Will that lead to the "near term human extinction" that some think as a likely future? It cannot be excluded, but it is also true that there may be life on the other side of the cliff if we are smart enough to understand the future. It is up to us to prepare for it.