Human population growth is currently experiencing something long thought impossible: population growth is fluctuating. World population is likely to peak much sooner than expected, exceeding 10 billion in the 2060s, before beginning to decline.
In wealthy countries, this is already happening: Japan's population is shrinking rapidly, at a net rate of 100 people per hour. Birth rates are falling sharply in Europe, the Americas and East Asia. Birth rates are also falling in many middle- and low-income countries.
This is a staggering change: Only a decade ago, demographers were predicting that our population would reach 12.3 billion, up from roughly 8 billion today.
For 50 years, some environmentalists have sought to save the environment by slowing world population growth. In 1968, a book called “The Population Bomb” predicted mass starvation and called for large-scale birth control.
Now we face a very different reality: population growth is slowing unchecked, the populations of wealthy countries are shrinking, and desperate but largely ineffective efforts to get people to have more children are setting in. How will a declining world population affect the environment?
Population decline is already happening
Parts of Europe, North America and northern Asia have been experiencing population declines for decades. Fertility rates have fallen steadily over the past 70 years and remain low, while life expectancy has increased, meaning the number of people aged 80 years or older in these regions will double within the next 25 years.
Until recently, China was the world's most populous country, accounting for one-sixth of the world's population, but it is now experiencing a population decline that is expected to accelerate rapidly in the future.
By the end of the century, China's population is projected to fall by two-thirds from its current 1.4 billion. This steep decline is the long-term effect of the one-child policy, which ended in 2016 but was too late to avert the decline. Japan, once the world's 11th most populous country, is expected to see its population halved by the end of the century.
What is happening now is known as the demographic transition. As a country moves from a primarily rural and agricultural economy to an industrial and service-based economy, birth rates will fall sharply. The combination of a falling birth rate and a falling death rate means the population will begin to decline.
Why? A big factor is women's choices: With greater options and freedom in education and careers, women are having children later in life and, on average, fewer children.
Why the sudden attention to population decline, when fertility rates in rich countries have been falling for decades? When the COVID-19 pandemic hit in 2020, birth rates in most countries plummeted, then recovered somewhat, but death rates rose. This combination hastened the onset of population decline more broadly.
A declining population will cause serious economic problems: fewer workers will be able to work and more elderly people will need support.
Rapidly declining countries may begin to restrict immigration to keep scarce workers in the country and prevent further aging and decline. The competition for skilled labor will intensify globally. Of course, migration does not change population, only where people live.
Are these just problems for wealthy countries? No. Brazil, a large middle-income country, is currently experiencing its slowest population growth on record.
By 2100, only six countries in the world are projected to have more births than deaths: Samoa, Somalia, Tonga, Niger, Chad and Tajikistan. The remaining 97 percent of countries are projected to have fertility rates below replacement level (2.1 children per woman).
Bad for the economy, but good for the environment?
Fewer of us will save nature, right? No, it's not that simple.
For example, energy use per person peaks between the ages of 35 and 55, then declines, and then increases again after age 70, as older people spend more time indoors and are more likely to live alone in larger homes. The tremendous growth in the elderly population this century could offset declines due to population decline.
Additionally, there are huge differences in resource use: if you live in the United States or Australia, your carbon dioxide emissions are almost twice as high as those of China, the largest emitter.
Richer countries consume more, so as more countries become wealthier and healthier while having fewer children, more of the world's population is likely to become emitters — unless, of course, they decouple economic growth from rising emissions and other environmental costs, as many countries are trying to do — but very slowly.
We expect to see more liberal immigration policies to increase the working age population. We are already seeing this, with immigration numbers exceeding projections for 2050.
When people migrate to developed countries, it can be economically advantageous for themselves and the host country. Environmentally, it can increase per capita emissions and environmental impacts, as the link between income and emissions is very clear.
And climate change upheaval looms: As the world warms, forced migration — people forced to leave their homes to escape drought, war, and other climate-related disasters — is projected to soar to 216 million within a quarter century. Forced migration could alter emissions patterns, depending on where people find refuge.
Apart from these factors, a declining world population could reduce overall consumption and ease pressures on the natural environment.
Environmentalists worried about overpopulation have long hoped for a decline in the world's population, and their wish may soon be realized — not through coercive birth control policies, but through the choices made by wealthy, educated women to have smaller families.
It's not yet an open question whether a declining population will ease pressure on the natural world – this is by no means guaranteed unless developed countries also cut emissions and change their consumption patterns.