Field of Science

Demographic Transitions and the Future of Humanity

This week, the field of science bloggers are addressing the question "Are we doomed?"  It's a good question.  There is no shortage of evidence that we are, in fact, doomed.  But as an incorrigible optimist, my response is a cautious "maybe not?"

What am I referring to here?  In talking to friends concerned about the future of the world, many express a fear that the human population and its economies will continue to grow until they can no longer be sustained by the resources available on the planet.  At this point there will be a great "crunch", as billions die and the rest endure a life of scarcity and strife.  This fear is not new; it dates back to Thomas Robert Malthus, who wrote in his 1798 "Essay on the Principle of Population" that

The power of population is so superior to the power of the earth to produce subsistence for man, that premature death must in some shape or other visit the human race. The vices of mankind are active and able ministers of depopulation. They are the precursors in the great army of destruction, and often finish the dreadful work themselves. But should they fail in this war of extermination, sickly seasons, epidemics, pestilence, and plague advance in terrific array, and sweep off their thousands and tens of thousands. Should success be still incomplete, gigantic inevitable famine stalks in the rear, and with one mighty blow levels the population with the food of the world.

So the Maltusian prediction is yes, we are doomed. But according to contemporary demographic forecasters, Malthus was, surprisingly, wrong. The human population is not growing without bound. Rather, the growth rate is slowing, so that the total population level is headed for a peak—and relatively soon! A 2001 study entitled "The End of World Population Growth" put the chances at 85% that the human population will peak before 2100.  Here's a graph of their projections, with the most likely outcomes shaded darkest:

The numbers on the right-hand side are probabilities of being less than some number.  For example, there is an estimated 14.4% chance that the population will be less than 6 billion in 2100, and an 89.4% chance it will be less than 12 billion.  The thick white line is a UN prediction, not part of this study, provided for the sake of comparison.

This turnaround is remarkable, since the human population has been growing exponentially, with few declines, since the beginnings of recorded history.  So what's behind this unprecedented reversal?

It turns out that Malthus didn't know everything about human nature.  Population scientists have noticed a surprising, yet robust, pattern in human societies, which they call the "demographic transition".  This transition occurs in stages, which are linked to economic and social development.
  1. In pre-industrial societies, parents have many children, since the survival of each individual child is uncertain.
  2. As improvements are made to food supply, hygiene, health care, and infrastructure, more children survive. This leads to a period of rapid population growth.
  3. As the society becomes increasingly urbanized, children become less of an asset (for helping with farmwork) and more of an expense (they must be educated in order to participate in the economy). Increasing education also gives women options other than motherhood. Access to and acceptance of contraception increases. As a net result, birth rates fall.
  4. Eventually, birth rates decrease to levels comparable to or even less than the death rate. The population level then stabilizes or even contracts.
This transition can be seen clearly in the birth and death rates of Sweden:

Lines indicate number of births (blue) and deaths (red) per 1000 people per year.

Note that the death rate in Sweden now exceeds the birth rate.  This is also true of most European nations, as well as Canada, Russia, Iran, Japan, China, and many other countries.  The United States is an exception for now—but the New England states are an exception to the exception!   Many more developing countries have declining birth rates, so that their populations are predicted to stabilize within decades. 

The idea of the demographic transition, and its robustness across human societies, gives me hope.  Not only because it predicts the end of population growth, but also because it suggests a new paradigm of human existence.  A paradigm where quality of life is valued over quantity of life.  A paradigm where each individual is cared for, educated, and allowed to dictate the course of his or her own life.  A paradigm where the population is stabilized not by coercion, disease, wars or famine (as Malthus predicted) but by the free choices of happy and healthy people.

Of course, there are still major obstacles to overcome before we can live sustainably on this planet.  Even as our population stabilizes, irreparable damage to our environment continues.  Global consensus remains elusive on challenges such as climate change, deforestation, and biodiversity loss.  And conflict, inequality, and oppression squander much of our global human potential.

But the existence of the demographic transition suggests a goal for humanity: 

We must facilitate and manage the demographic transition across human societies, in an environmentally sustainable way.

If we achieve this goal, we will be on our way toward a healthy and happy, indefinitely sustainable human population.  So maybe, just maybe, we are not, in fact, doomed.