by Loren Cobb
Many of the arguments over the shape of the future turn on the central question of population growth. Over the past ten thousand years the world population has been increasing at an accelerating rate, with only a few fluctuations. How then will it be possible to advance civilization or even merely to maintain it in the face of the depletion of finite resources? To find an answer to this question for, say, the rest of this century requires before all an understanding of future trends in population growth.
For those of us who like to ponder the future, these projections provide a way to see through the fog of uncertainty. No projection can be understood, however, without the historical context from which it arises, so it occurred to me to try to put together into one simple graph all of the following elements:
On the right you can see the complete graph , showing world population from the year 500 CE, when it was just 200 million people, through the present and then beyond, in three demographic projections made by the United Nations going out to the year 2150. The line segments colored in green, orange, and red depict the "low", "high", and "medium" projections of the United Nations, respectively. 
Observe how comparatively short a stretch is based on reliable census data (the blue line segment). During this interval, from 1950 to 2005, the rate of population growth is thought to have been higher than at any time in the entire history of homo sapiens.
The black line segment shows my calculations of mean historical estimates, interpolated and smoothed. The overall uncertainty in these values is only about three or four times the width of the line segment (more in earlier years, less in the recent past).
If you mask the future projections and look only look at the past 1500 years, then it becomes perfectly clear why many have viewed the population explosion with great alarm. In the context of a world with finite surface area and finite resources, any reasonable person observing this history of accelerating growth would expect global society to crash and burn in a spectacular Malthusian catastrophe.
For those who do not know of him, the Rev. Thomas Malthus was the dour British clergyman who, in 1798, predicted that if the vices of humankind did not limit population growth, then "... epidemics, pestilence, and plagues advance in terrific array... Should success be still incomplete, gigantic inevitable famine stalks in the rear, and with one mighty blow levels the population ..."
Despite all the gloomy books on the population explosion, there are good scientific reasons why a Malthusian catastrophe is unlikely to happen. In fact, I argue that it is far more likely that we will have an orderly population implosion (the green line segment in the graph), than a catastrophic explosion (the red line segment) followed by famine, disease, and societal collapse. An in-between result is also possible, in which population stabilizes for several centuries at about 10 billion people (the orange line segment).
Predicting the future
The three UN scenarios, like all modern demographic projections, are based on explicit assumptions about future trends in health and an indicator known as "total fertility". Total fertility is informally defined as the expected number of children that each woman will have in her lifetime, based on age-specific birth rates for the current female population.
Total fertility is what economists would call a leading indicator of population growth: changes in fertility in any one year predict surprisingly well how population growth rates will change over the next generation or two.
When a society maintains a total fertility of about 2.2 children per woman for an extended period of time, then its population size will eventually level off to zero population growth. Fertility rates higher than 2.2 portend a growing population.
Conversely, when fertility rates drop below 2.2 then the population will eventually begin to decline. "Eventually" can be quite a while; it may take a generation or two to happen. This is because the cohort of children already born will increase the size of the population of mothers when they reach child-bearing age, so that even if they have fewer children per woman the total number of children born can continue to increase for a generation or two.
The high projection
The red line segment shows the "high" population projection of the professional demographers who work for the United Nations. The fundamental assumption behind this projection is that fertility rates for all countries will slowly converge to 2.35 children per woman. Countries that now have a higher fertility rate (India and Pakistan, for example), will gradually slow their rate of population increase. Conversely, the countries of Europe under this assumption will gradually increase their rates of population growth. The net effect is the line segment in red.
Since 2.35 average children per woman is higher than the replacement rate, this scenario yields exponential growth, but at a greatly reduced rate compared to the present. Still, in the context of a world with finite resources this scenario remains of grave concern. Disaster may not happen as quickly as many suppose, but this growth rate cannot be maintained for very long without a movement of the human race beyond the confines of this planet.
The medium projection
The orange line segment shows the "medium" population projection, whose fundamental assumption is that fertility rates for all countries will slowly converge to 1.85 children per woman. This fertility rate is below replacement, so the world's population under this assumption reaches a maximum of about 9 billion people in the year 2075. After slowly crossing this maximum, the curve begins a very gradual exponential descent.
If "Peak Oil" projections of world petroleum production are correct, then a population maximum of 9 billion in the year 2075 implies trouble ahead. This is because the peak of petroleum production is expected to occur within the next decade (if it has not occurred already), yet world population will continue to increase in this scenario for the next 70 years.
All of these projections also assume that life expectancy will gradually rise all over the world, as a result of improvements in medical science. This increase has a perceptible impact only in the "medium" scenario, in which it is sufficient to counteract the exponential decline beginning in 2200. The medium scenario, therefore, results in an approximate population equilibrium.
The low projection
The green line segment shows the "low" population projection, whose fundamental assumption is that fertility rates for all countries will slowly converge to 1.35 children per woman, much less than replacement level. Under this assumption the world's population will reach a maximum of about 7.4 billion people in 2050. After slowly crossing this maximum, the curve begins a pronounced exponential descent.
The world's population today, in late 2006, is about 6.5 billion people. Under the UN's "low" projection, the total increase over the next 44 years will be less than a billion people. Put another way, under these assumptions the world's population will gain only another 13% before it begins its decline in about 2050.
How realistic are these assumptions?
Projections beyond 2050 or so are of course very problematic, even when based on the best available demographic theories. Perhaps the biggest uncertainty concerns the technology of genetic and reproductive engineering, which is rapidly developing into a new "revolution" comparable to the agricultural and industrial revolutions. We touched on the topic of transhumanism once before, in TQE 114, and I am sure it will come up again.
But in our lifetimes, and those of our children, what will happen? Between now and 2050, will fertility rates change as assumed by these UN projections? My personal belief is that they will fall even faster than the "low" UN projection, converging eventually to an average of about one child per woman. I base this belief on the empirical fact that fertility rates appear to be continuing to plunge throughout all high- and medium-income countries and in many low-income countries too without any hint of reaching an equilibrium.
There may be excellent economic reasons for this continuing decline, based on rational decisions made by families in the face of improving odds that their children will survive to healthy adulthood, the emancipation of women and their entry into the workforce, and the steadily increasing costs of child-rearing and education.
For all of these reasons, my best guess is that world population will peak at about 7.0 billion in 2040 or so, and that the rate of decline will be uncomfortably steep for a century or two after that. In other words, I believe that even the lowest UN projection is too high. Population implosion is on the horizon.
Despite all the positive effects that lower population will have on the environment, I have to point out that even this scenario carries with it considerable stress for society, caused by a burgeoning elderly population. The average age of retirement will rise steadily, I think, and every human society will have to create new categories of jobs and occupations that can be filled by the elderly with dignity and respect.
I would like to be able to reassure you that the coming population implosion will provide a solution for that part of global warming that is caused by industrial activity. That there will eventually be fewer people on the planet will help, of course, but it seems likely to me that these reductions will be too little and too late to avoid serious climate changes. More on this later.
Meanwhile, on the economic front, the reality of a steadily declining population in countries like Italy and Japan will soon produce an apparent paradox: even when these countries are in a mild recession or a period of zero economic growth, their per capita incomes will still be rising!
Sincerely your Friend,
1. How the graph was made
2. Data sources
Please send comments on this or any TQE, at any time. Selected comments will be appended to the appropriate letter as they are received. Please indicate in the subject line the number of the Letter to which you refer!
Loren, I know nothing about this subject and am fascinated by (and grateful for) your article. A couple questions: has anyone done work on the effect of religious beliefs on birth rates and how that interaction alters over time? Also, is there not in the UN demographers' estimates (and yours) the assumption that developing countries will become developed countries, where people (especially women) have some control over their birth rates? How do they (and you) justify that assumption? I've read of African cultures, for example, where women have as many sons as possible so that where their husbands die, their neighbors can't steal all their belongings. That suggests to me that competition for resources might raise the birth rate, not lower it.
— (Name withheld on request).
The shortage of oil, the superabundance of human beings and the consequent environmental degradation have prepared us for apocalyptic visions. It is becoming sadly plain that that we may be too late in recognising what is happening to the world's climate. As sea levels rise, the automatic consequence must be mass migrations from low-lying sea-board areas by many millions of people. These areas are normally more prosperous than their corresponding hinterlands, who may well not be either willing or able to support the unhappy migrants. Possibly there will be civil wars; certainly there will be some extreme deprivation and probably mass deaths as a result. The demands from poor countries for help from the richer ones are unlikely to be willingly met. The ultimate consequence may be a rolling wave of migration from costlands to hinterlands, and onward across progressively more sophisticated countries, a tragedy affecting us all in the end.
What have we, as Friends, to say to a such a world, bereft, as it probably will be, both of compassion enough and of means enough to handle the problem?
Loren, although I have not always been in agreement with the views that you or Jack hold, I'm glad that some reasonable reflection on Malthus and the "population explosion" is happening.
Even assuming such an "explosion" exists, the decline of population in educated, industrialized populations and demographic groups is much more alarming than the overall population increase. And much of that decline is due to concerns about population growth. This reduction could cause serious problems in technological development, as well. Peace,
Josh Humphries, Roanoke Monthly Meeting & Baltimore Yearly Meeting.
Thank you for the article discussing the range of world population estimates. Given the range of outcomes resulting from plausible assumptions, it is difficult to put much faith in any single estimate.
There are two interesting issues in Europe that are related: First, the fall in the birth rate, greater female labor force participation, and the implications for traditional family roles. Second, the changing ethnic composition of the European population as a result of immigration and different birth rates for different ethnic groups. Both of these are excellent topics for students to consider.
Stephen A. Baker, Capital University, Columbus, Ohio.
You mentioned only "improvements in medical science" as a cause of rising life expectancy all over the world. I trust you meant to include improvements in public health and preventive medicine under this rubric. (Unfortunately, the term "medical science" usually brings to mind doctors and researchers in white coats working to save lives by developing new surgical operations or new medicines and antibiotics.)
The practice of curative medicine by doctors and hospitals may extend the lives of some "well-healed" individuals for a few months, or even years; but if the practitioners of curative medicine, together with all the hospitals in the world, suddenly disappeared from the earth, while public health, preventive medicine, and famine relief systems continued (and other factors remained equal), I think there would hardly be a noticeable dip in the semi-log total world population graph.
However, that graph would show a precipitous fall if all the famine relief and public health and preventive medicine activities suddenly stopped.
I think it would be fair to say that improvements in agriculture, nutrition, famine relief, and public health (including control of epidemics) have been vastly more important in extending the average expectation of human life all over the world, than have the improvements in curative medical science.
I am reminded of the graphs of populations of rabbits and foxes living in a confined space with plenty of rabbit food. They look like two opposite sine waves. As the foxes eat the rabbits, the population of foxes goes up and the rabbit population goes down. Then as the foxes die off from hunger, the rabbit population goes up again, and so on ad infinitum.
We might hope that the green line in your semi-log graph, after peaking and falling a little way, would turn around again and begin to oscillate. To this humanist, that seems more acceptable than the idea of a "population implosion."
(Name withheld on request).
Found on the Web
"I read every issue of Forbes, in order to get an idea of the world-view of the prototypical 'Rich Person' ... For the same reason, but in search of information about a very different world-view, I read The Quaker Economist, and am often astonished at what I find there. Sometimes I agree, sometimes I don't, but I always learn something. Unlike Forbes, it's free." Ozarque, 8 Jan 2006.
Copyright © 2006 by Loren Cobb. All rights reserved. Permission is hereby granted for non-commercial reproduction with attribution. Students take note: copying any part of this essay into your written work without attribution is plagiarism.