Scientists Say Earth Could Eventually Have Just 4 Billion People

What if the biggest environmental solution on Earth had nothing to do with electric cars, solar panels or recycling, but with the number of people living on the planet? A new paper from 11 researchers across 10 countries has proposed a future global population of roughly 4 billion people by 2200, less than half the number expected to live on Earth later this century.

The researchers say they are not calling for forced population control or government limits on family size. Their proposal relies on voluntary access to contraception, better education for girls and women having greater control over when and whether they have children. Critics, however, argue that turning population decline into an environmental objective raises uncomfortable questions about who bears the burden of reducing humanity’s footprint.

Researchers Say 4 Billion People Could Be Better For Earth

The paper, published in the journal Sustainable Development and led by Mark Keegan of Columbia University’s Teachers College, makes a long-term case for a much smaller human population. The researchers argue that global population could eventually decline to around 4 billion by the year 2200 while avoiding coercive population-control policies.

The number is striking because the world currently has around 8.3 billion people. United Nations projections cited in the debate suggest the global population could reach approximately 10.3 billion during the 2080s before beginning a gradual decline. The researchers’ proposal would take that eventual decline much further.

Their model does not involve removing people or imposing a global birth quota. Instead, the population would gradually shrink because fertility remained below replacement level for generations. The researchers describe their approach as “a pro-nature and pro-human strategy,” arguing that a smaller population could reduce pressure on ecosystems while improving access to reproductive choices.

That framing has become one of the most controversial parts of the paper. Critics say there is a major difference between supporting voluntary family planning and declaring that the planet would be better off with billions fewer people. The researchers insist the demographic change they envision would happen through individual choices rather than coercion.

The Number They Actually Want Is 1.75

The immediate target behind the 4 billion projection is a global fertility rate of about 1.75 children per woman by 2050. That figure would put fertility below the replacement level needed to maintain a stable population over the long term, meaning the effects would accumulate gradually across generations.

Global fertility has already moved substantially in that direction. The source material notes that the average number of children per woman has fallen from around 2.75 to 2.2 over the past 25 years. The researchers believe extending that trend could bring the world toward their proposed target without requiring governments to dictate family size.

They also point to a large unmet demand for contraception. UNFPA estimates that 259 million women who want to avoid pregnancy are not using a modern contraceptive method. The researchers argue that giving those women greater control over reproduction could naturally lower fertility in countries where birth rates remain high.

Education is the other major part of their argument. The authors write, “When women are free to decide how many children to have, fertility rates tend to fall toward replacement level or below, often rapidly.” Their position is that expanding opportunity and reproductive choice could produce demographic change without the coercive policies associated with some population-control programs of the past.

Iran Shows How Fast Birth Rates Can Fall

Iran is one of the strongest examples cited by the researchers. According to the figures presented in the source material, the country’s fertility rate dropped from 4.08 children per woman in 1992 to 1.94 in 2001.

That means the decline happened in only nine years. The change is significant because it demonstrates how quickly fertility can shift when social conditions, reproductive access and family expectations change.

South Korea experienced another dramatic decline. Its fertility rate fell from 4.07 children per woman in 1972 to 1.93 in 1984. Costa Rica followed a slower trajectory, dropping from 4.16 in 1972 to 1.98 in 2009.

The researchers use these examples to argue that fertility does not necessarily need to be controlled through direct government restrictions. Their critics point out that demographic transitions are complicated and can involve economic pressures, cultural changes, government policy and changing attitudes toward marriage and parenthood. A falling fertility rate therefore cannot automatically be attributed to one intervention.

The Researchers Found Population Was Linked To Environmental Pressure

The environmental side of the paper is based on eight measures covering the period from 1970 to 2020. The researchers examined wildlife populations, carbon emissions, energy consumption, freshwater use, pesticide use, plastics, mineral extraction and overall ecological footprint.

For six of those eight measures, they found that population size had a stronger relationship with environmental pressure than consumption per person. That finding forms a major part of their argument that population deserves a much larger place in climate and environmental policy discussions.

Their projections become particularly stark when they consider what could happen if global population reaches the higher end of current projections. The researchers argue that a population of 11.4 billion by 2100 could place substantially more pressure on wildlife and natural resources.

They write that such a scenario “might deplete [the Living Planet] Index to half its current value and produce a world with nearly double the current CO2 emissions, energy and pesticide use, and quadrupled plastics waste.” The projection is not a prediction that those exact outcomes must occur, but it shows how the researchers connect population growth with rising environmental demand.

A Major Study Puts The Focus On The Richest People

There is, however, another piece of research that complicates the population argument. A 2025 study published in Nature Climate Change found that the wealthiest 10% of people were responsible for about two thirds of global warming since 1990, while the richest 1% accounted for roughly one fifth.

That finding raises a difficult question about whether population size is the most useful measure of environmental responsibility. Every person counts equally in a population total, but people do not consume equal amounts of energy, goods or natural resources.

A person living in a low-consumption economy can have a dramatically smaller environmental footprint than a wealthy consumer in a high-income country. That difference is one reason critics argue that reducing population could distract attention from the economic systems and consumption patterns responsible for a large share of environmental damage.

The researchers behind the population paper acknowledge the problem themselves. “Ever-growing wealth and consumption are not ecologically sustainable, and the sooner our societies grapple with this inconvenient truth, the better,” they write. Their critics argue that if consumption among wealthy populations is responsible for such a large share of warming, that should be addressed before population reduction becomes a central environmental goal.

The Study Also Has A Problem With Correlation

Another criticism concerns the way the environmental evidence is interpreted. The study examines relationships between population and environmental measures over several decades, but a correlation between two trends does not automatically establish that one directly caused the other.

Population growth can increase demand for food, housing, energy and materials. At the same time, technological development, industrial policy, economic growth and consumption patterns can dramatically change the environmental impact associated with each person.

That distinction becomes particularly important when comparing countries. The environmental footprint of someone living in a low-income country can be vastly different from that of someone living in a wealthy, high-consumption economy. A global population figure cannot capture those differences by itself.

There is also a historical reason why population-reduction proposals attract intense scrutiny. Governments have previously used forced sterilization, coercive birth policies and other measures to control fertility. The researchers explicitly reject such methods, but critics argue that the history of population control makes the language surrounding demographic targets especially important.

The World Is Already Facing Falling Birth Rates

The debate is also happening at a time when fertility is already falling across large parts of the world. Several countries are dealing with birth rates far below replacement level, creating concerns about aging populations, shrinking workforces and the financial pressure of supporting older generations.

South Korea is one of the most dramatic examples. Its fertility rate has continued to fall far below the levels seen during the period highlighted by the researchers. Other countries, including Italy, have also faced prolonged demographic decline.

That has produced an unusual situation in which some governments are actively trying to encourage people to have more children. Policies aimed at supporting families, reducing the financial burden of raising children and reversing population decline have become increasingly important in countries facing demographic pressure.

The researchers argue that population decline does not necessarily mean economic collapse. They point to the period from 1970 to 2020, when global GDP per person increased substantially even as fertility and demographic patterns changed.

But a much larger population decline would create a different test. A world moving toward 4 billion people would have fewer workers, fewer consumers and potentially much older populations in many regions. Whether the environmental benefits would outweigh those pressures remains an open question.

The 4 Billion Target Is Still 174 Years Away

There is one detail that can easily get lost in the controversy: the researchers are talking about the year 2200. That is 174 years from now.

The proposed figure is therefore not a plan that could be implemented by one government or one institution. It is the eventual result of a demographic scenario in which fertility remains below replacement level for generations.

That distinction matters because the researchers’ recommendations themselves are relatively familiar. Expanding access to contraception and education for girls are already widely supported goals in international development. The controversy begins when those policies are connected to an explicit target for reducing the global population.

The researchers believe that a smaller population would place less pressure on wildlife, natural habitats, energy systems and the climate. Critics believe the environmental debate should put far greater emphasis on consumption and wealth, particularly given the disproportionate contribution of high-income populations to global warming.

The final number, 4 billion, is therefore doing more than describing a possible future. It has become the dividing line in a much larger argument about whether humanity’s environmental problems are primarily caused by having too many people, consuming too much, or both.

For now, the world is moving toward lower fertility anyway. The question is whether that demographic shift should simply be allowed to unfold, or actively encouraged as part of the environmental strategy.