Population control is sometimes championed as a way to fight climate change. Bernie Sanders recently raised this idea. The world population is growing. There are 7.7 billion people today. There will be nearly 11 billion in 2100, according to United Nations estimates. Extra people mean extra CO2 emissions. While two children per woman may seem reasonable, women in the poorest countries can average five or sex. Sanders and others believe this excess fertility is why the world population is growing. Fewer children will mean less CO2. Unfortunately, this perception is decades out of date.
We are already at “peak children.” Globally, there are 680 million children under the age of five. U.N. projections show a peak of 700 million about 2060. There are projected to be 650 million children in 2100, less children than there are today. Yet the overall population will grow by nearly 50%. How can that be? It is the compounding effect of people already born that will drive population growth over the next century, not excessive birth rates.
Trends in Fertility
Assume women have two children over their lifetime. They replace themselves and one other. Population size will be stable from generation to generation. (The actual replacement fertility rate for an affluent nation is 2.1 children per woman.) Throughout history, the total fertility rate has been six or seven children. One quarter of children died before their first birthday. Another sizable percentage died before age five. Of those who made it past five, many died before marrying and having children. High fertility rates ensured a couple of children would survive to continue the family.
As affluence emerged and health practices changed (first in Europe and then spreading elsewhere) more children survived into adulthood. We needed fewer births to perpetuate a family but it took a while for customs related to fertility to adjust downward. That lag between declining death rates and declining fertility rates is what led to the population explosion that began two centuries ago. That growth accelerated into the twentieth century and continues into its final stages today. As recently as the 1960s, the global total fertility rate was as high as five children per woman.
Today, the global total fertility rate is about 2.4 and dropping, but the rate is not evenly spread throughout the world. Europe, the Americas, China, Japan, and other regions have fertility rates below the replacement rate, well below it in many nations. A handful of smaller poor countries have fertility rates of five or six, but they also have some of the highest child death rates. The world average is fast approaching the 2.1 replacement rate. However, because the global fertility rate has usually been higher than the replacement rate for most of us now living, it means the cohort of people born one year is usually larger than the cohort of people born the previous year. So let us think about what that means for the future.
The Compounding Effect in Future Fertility
To keep things simple, assume going forward that we birth the same number of children each year, the death rate stays the same, and no one lives past one hundred years. Now say there are 100,000 people age 100. By next year, all will have died. The group that was 99 will now be 100. Because more of them were born, there will now be, say, 105,000 people 100 years old. After two years have passed, the new group of 100 hundred year old people will be the people who are 98 years old today. They will be a still larger group. That will continue for one hundred years.
At the other end of the age continuum, we have the largest number of children ever born in a year and the largest age cohort alive. A year later, this youngest age cohort will replace a smaller cohort a year older than them. But keep in mind that for the next forty-five years the number of women of childbearing age will be increasing each year. Therefore, each year the total fertility rate will need to fall slightly more below the replacement rate if the population is to stabilize. It can then rise to the replacement rate after forty-five years. This will continue until 2120 when each age cohort is the largest age cohort that has ever existed for that age. This is the primary driver of population growth going forward but there is at least one other key factor. (Clearly, I am oversimplifying to illustrate my point. We are not down to replacement rate fertility, so there is still some marginal population growth due to “excess” fertility.)
Furthermore, actual global death rates are not constant. They are dropping. People are living longer. So not only is each age cohort larger than the one before, it is living slightly longer than the one before. This too contributes to population growth over coming decades.
Therefore, we would need considerably less than replacement-rate fertility to limit global population growth substantially. In fact, there are credible projections of a population peak at fewer than nine billion. Demographers once assumed transition to low fertility and death rates would stabilize at about the replacement rate. Instead, the fertility rate has dropped well below replacement rate in nearly every affluent nation, in some cases nearly one child per woman. From the narrow view of CO2 emissions, this may look like a good thing. From a holistic standpoint, many social scientists are troubled. Depopulation can be as destructive to human well-being as overpopulation.
The Depopulation Problem
A vibrant society needs a critical mass of productive workers relative to its dependents (primarily children and the elderly.) This is the dependency ratio (dependents divided by workers multiplied by 100). With excessively low fertility rates, it is possible to have a great imbalance with a large number of seniors and too few workers to provide for society. We have already seen that current UN projections say we will have nearly the same number of children now as in 2100 but the overall population will be nearly 50% larger at eleven billion. That growth is a consequence of a burgeoning number of elders. A smaller population of nine billion may mean fewer total people but an even worse dependency ratio.
Modest help may come from lengthening the number of years people work or a higher participation rate in the work force for working age people but at some point that will be insufficient. Then consider the possibility of medical breakthroughs that cure cancer or dramatically reduce heart disease. That means more people living longer, intensifying the dependency ratio imbalance.
As an ever shrinking number of people (potentially a minority of the population) is expected to support everyone else, improvement in living standards will begin to stall and possibly reverse, making the world ripe for any number of societal dysfunctions. It will not take eighty years for this challenge to become real. Japan is already struggling with these issues. China is already headed down this road. One can envision China opening up to immigration from the remaining regions with the highest fertility rate and investing there resources in growth economies, but if every nation is headed to fertility rates well below the replacement rates, that is only a temporary fix. The dependency ratio for a world with eleven billion people is already seen as a challenge. The only way to get a peak population under nine billion people is to achieve fertility rates well below replacement rates substantially intensifying the dependency ratio imbalance just a generation or two down the road. (For more detail see the Brookings' piece, How will we cope when there are too few young people in the world?)
Decoupled Economic Growth
At the most basic level, climate policy needs to be about economic growth decoupled from CO2 emissions. Population growth is one issue driving this need for economic growth. But also consider people are advocating for a $15 living wage for every worker in the United States. That would put nearly every US worker in the top 10% of wage earners in the world. Meanwhile, despite astonishing improvements in human well-being around the world and the dramatic reduction in extreme poverty, there are still hundreds of millions of people in extreme poverty. A couple billion more have more stable lives but still live well below standards we would consider tolerable. If it is a matter of justice that everyone in the US has a $15 an hour living standard, then it is only just that all citizens of the world have something approximating that standard. The only way that happens is through economic growth. Measures like population control are shortsighted and potentially calamitous. The principle mission is decoupling economic growth from fossil fuel consumption, along with other disruptive measures like decoupling land use from agricultural production.