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We need a global debate on the best way to respond to these demographic changes.Photo illustration by Arthimedes / Shutterstock
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In their 1968 book The Population Bomb, biologists Paul Ehrlich and his wife Anne foretold a Malthusian future of famine and disease if humanity failed to control its growth. The Ehrlichs’ warning made sense. At the time, the global population sat at about 3.5 billion, and its rate of growth was 2.1 percent—close to the highest it had ever been. Such a reproductive pace could have outstripped our ability to compensate. 

Except that was it—1968 was the peak. We’ve since slowed the speed of our planetary conquest, so much so that a demographic transition is taking place. We have just over 7.8 billion people on the planet now, but populations in many countries are aging and even experiencing negative growth. In fact, we’re poised to drop below current levels by the end of the century—if goals for educating women and providing contraception are met. At least that’s how Stein Emil Vollset, the first author on a new paper on population change published in The Lancet, sees things. 

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Vollset, a Professor of Health Metrics at the University of Washington, leads the Future Health Scenarios forecasting team at the Institute for Health Metrics and Evaluation. Every year, the Institute publishes The Global Burden of Disease Study, which includes information on incidence and prevalence of disease, the years lived with disability, the years of life lost, and the combined measure of non-fatal and fatal disease burden for over 350 diseases and injuries for 204 countries, as well as subnational estimates. It also estimates the health burden due to about 70 risk factors, like air pollution, smoking, or lack of clean water. “To do this, we need to know the population of every country by age and sex,” Vollset said. “We’ve been working for more than a year and a half relatively intensely to try to forecast populations. We have done that for 195 countries, and it’s what’s presented in this Lancet paper.” 

Nautilus caught up with Vollset, a reserved and thoughtful Norwegian man whose office walls are filled with books and colorful graphs, to discuss the intriguing findings. 

Stein Emil Vollset

How do you see the planet’s human population moving in the run up to 2100?

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We forecast that we will increase by 2 billion until the mid 2060s, to 9.7 billion. From 2064 to 2100, we’ll decline by 1 billion, to 8.8 billion. We also project that sub-Saharan Africa will increase from 1 to 3 billion. From now to 2100, we see a decline from 4 to 3 billion in Asia, driven by China and India and other countries.

You also forecast that, by 2035, China will overtake the United States as the largest economy. However in 2098, the U.S. will regain the lead. What explains that flip?

The flip is driven solely by our forecasts of the size of the working age population, people 20-64 years old, in the two countries, combined with our forecasts of GDP per worker.  Even though both countries will see fertility below replacement levels, the working age population in the U.S. will grow whereas China’s will decline. This is because the U.S. will maintain immigration, but China will not increase it to the extent necessary to prevent workforce decline.

The United Nations’ population division in New York has been forecasting for 70 years, and they estimate that by 2100 the global population will be near 11 billion. Why is your estimate so much lower? 

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To forecast population, you need to look at three factors—fertility, mortality, and migration. Of the three, fertility, the number of children women have, is the key—that’s the major determinant of the population in 2100. And we have a different fertility model than the U.N. We use trends in women’s education and in access to contraceptives. Historically, there has been a very strong relationship between the number of children per woman and education and contraceptives access. Our model is based on the assumption that that relationship will continue into the future. It also means that we have to forecast female education and access to contraceptives before we can do the fertility projections. When we do this, our model diverges from the U.N.‘s because it does not use the same variables, the access and education.

Why is fertility more of a key determinant of population change than mortality?

Of course, if an asteroid hit the globe, you’d have enormous fatalities. But barring cataclysmic events like that, we don’t really foresee dramatic changes in mortality. We are in a good position to assess that because we have data on disease burden from the Global Burden of Disease Study, which mapped all diseases both by mortality and by non-fatal burden, the suffering diseases and injuries cause without killing you. And since fertility is a sum of individual choices, it can change relatively quickly. For example, the Baby Boom after the Second World War resulted in huge numbers of children. But especially in many high income countries, we have since seen relatively strong declines in replacement fertility. 

Did COVID-19 have a significant impact on your mortality forecast? 

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We don’t think COVID-19 will affect global population size, even if it’s serious and many people die. Actually, one byproduct of the mortality forecasts is that our models can predict life expectancy for all countries until 2100. There is an increase at the global level, but it will slow down, especially in the last half of this century. We will have less growth in life expectancy in the coming 80 years than we have seen since 1950.

How many countries have low replacement fertility, meaning their population growth has really slowed or stopped growing? 

Close to half of the countries today have low replacement fertility. If a country is below 2.1 children per woman, in the long run, if there is no immigration the population will decline. By 2100 close to all of them will—183 of the 195 countries.

Is the idea that the more educated women become, the more they use contraception—and that drives down fertility?

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There are, of course, differences based on culture. It’s also based on the economy, religion, and other factors like urbanicity. But, generally, education is associated with later marriage. Women are more likely to enter, and stay, in the workforce. And, this leads to delayed and more spaced childbirths, which results in smaller family size. The relationship between female education and fertility is long established, but our estimates are also based on our own very large global analysis using the Global Burden of Disease database. Our model of fertility shows a strong relationship between education and fertility—fertility rates are lower when women complete more years of education. We estimate that in a population where all women have 16 years of education and 95 percent have access to modern contraception, women will have an average of 1.4 children.

 Why do you think women’s education and access to contraception will become more widespread by 2100? 

Based on historical trends, we predict that women’s education and access to contraception will continue to increase this century. But, because of the uncertainty involved in basing projections on historical trends, we constructed additional scenarios with slower or faster growth of education and family planning access. First and foremost, more or less investment in these things will lead to faster or slower growth rates. But, these are forecasts. We don’t know the future. We’ve done the best we can, and with new methods.

To see that there is a population decline potentially coming—isn’t that great?

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There is absolutely good news in this, and maybe this could incite a discussion. How many people do we think there should be on this planet with current technologies? Of course, we could have technological breakthroughs when it comes to how we create energy. So, we could sustain a larger population. But, as long as we emit carbon, it’s probably good news. It’s also less stress on the food systems. 

Are there drawbacks to this level of population decline?

Sure. The inversion of the population pyramid. We are not the first to predict that we will get more and more older people and less young people, which means less people in the workforce to sustain care and healthcare. They are needed for nursing facilities for the elderly, and to take care of that growing elderly population. This will really come and hit us much harder than it has so far in the coming 50 and 80 years. This will be accentuated in many countries. 

Some countries, like Japan, have actually been doing well despite population decline and decreases in the working-age population. 

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That’s true. But it remains to be seen how it will go in the future. We should look into how they organize the way that they take care of the large population of elderly people. That shift toward the elderly will happen relatively fast in some countries.

How should we as a global society respond to the forecasts you’ve made? 

We should plan for a continued population growth of 2 billion until the mid-2060s. We need to tackle continued strong population growth in Africa and parts of the Middle East, as well as widespread population declines and accentuated inversion of the population pyramid (more old people and fewer young people) in other parts of the world. Societies can prepare for inversion of the population pyramid if they consider the different scenarios, but it will require long term planning. Demographic changes are relatively slow. 

We need a global debate on the best way to respond to these demographic changes. There are a number of options for responding to smaller workforces and changing age structures, such as support for women to both participate in the workforce and have children, paid or subsidized childcare services, expansion of social support services for older adults, and liberal immigration policies. We need to be on the watch for restrictions on female reproductive health rights and access. That’s something that we warn against in the paper. If this can inspire the discussion, a lot has been gained.

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No doubt. Are there any visualizations of your forecasts people can easily check out?

There is a visualization on our website where you can look at countries and see how the population has changed since 1950, and how we project it to continue. Also, you can switch to fertility. The things I’m talking about, they are all there. 

Kiki Sanford holds a Ph.D. in Molecular, Cellular and Integrative Physiology from U.C. Davis, and is a specialist in learning and memory. She is also the founder and host of the radio show This Week in Science

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