How Many People Can Live on Planet Earth?


Advocate for the environment, natural historian and broadcaster David Attenborough says, “Today, we are living in an era in which the biggest threat to human well-being, to other species, and to the Earth as we know it, might well be ourselves.”

IFSPP wants the U.S. to take the lead in combatting global climate change. That’s why we support higher mileage requirements for cars and trucks and increased funding for mass transit; replacing coal-fired power plants with solar, wind and other alternative energy sources; and higher efficiency standards for heating, cooling and insulating new buildings. But in recent decades, four-fifths of the increase in U.S. greenhouse gas emissions has come from U.S. population growth, as more people drove more cars, built more houses, ate more food, and did all the other things that generate carbon.2 Unless we stop population growth, America will continue to generate too much CO2, methane and other greenhouse gases.

Some environmentalists argue that Americans only need to focus on fighting pollution and reducing our consumption in order to curb environmental destruction. They are right to argue for decreased consumption and increased vigilance against polluters, but wrong to assume that such efforts can take the place of stabilizing our population.

A growing population can swamp improvements in consumption or pollution abatement. In fact, we have seen this happen regarding national energy use and carbon emissions in the past few decades, as greater efficiency in per capita energy use has failed to keep pace with increased numbers (more “capitas”). Total energy use and total carbon emissions have risen, due to population growth.

Ecologists use a formula to measure environmental impact, developed in part by John Holdren, the Obama administration’s chief science advisor: I = P x C x T. Here I, total environmental impact, is a function of three factors: P = total population, multiplied by C = consumption per person, multiplied by T = the technology used to facilitate that consumption. All three factors are equally important in creating overall environmental impacts. None can be neglected if we hope to limit our impacts and create a sustainable society. As President Clinton’s Council on Sustainable Development put it:

“Managing population growth, resources, and wastes is essential to ensuring that the total impact of these factors is within the bounds of sustainability. Stabilizing the population without changing consumption and waste production patterns would not be enough, but it would make an immensely challenging task more manageable. In the United States, each is necessary; neither alone is sufficient.”3

One of the Council’s ten main recommendations for creating a sustainable society was: “Move toward stabilization of U.S. population.”

Some American environmentalists argue that overpopulation is solely a global problem, not a national one, and that it requires an exclusive focus on global solutions. They are right that worldwide population growth is an immense environmental problem, but wrong to think that addressing it is best done by ignoring U.S. population growth.

The U.S. government should finance and encourage family planning efforts in developing nations to help them slow their population growth. We should stand for the rights of women in international forums, and encourage female literacy and economic empowerment in poor countries. Securing these rights and furthering these interests are both the right things to do, as a matter of justice toward women, and they have proven successful at reducing fertility rates around the world.

However, Americans also need to attend to our own house. The U.S. is the third largest nation in the world, and our population is growing rapidly. Between 1990 and 2000, the population grew by 36 million people. Our most direct and important responsibility regarding global population growth is to end population growth within our own borders.

In addition, while many progressives like to think of ourselves as “citizens of the world,” concerned for the well-being of all humankind, those of us who remain citizens of the U.S. have further particular responsibilities. As Americans, we believe we have a special responsibility to preserve wild species and wild landscapes right here in our own country. Not just because the wild world is something that’s nice and pretty, but because healthy ecosystems are essential for healthy human populations too.

Our children and grandchildren will blame us, rightly, if we fail to preserve opportunities for them to get to know and appreciate wild nature. They will blame us, rightly, if we fail to preserve clean air, clean water, sufficient topsoil to grow food, and all the other resources essential for their well-being. In other words, we have a duty to future generations of Americans to create a sustainable society. Continued population growth makes achieving that goal impossible. So we must end U.S. population growth.

However, in order to stabilize America’s population, we must reduce immigration, since today it is high immigration rates that are driving continuing rapid population growth in the country. During much of the previous century, population increase was fueled primarily by high native birth rates, but in recent decades, the total fertility rate of American women has fallen dramatically: from 3.5 children per woman in the 1950s, to 1.7 children in the 1970s, to 2.05 children today. According to a Pew Hispanic Center study, 82 percent of population growth between 2005 and 2050 will be due to new immigrants arriving and their descendants.

With a total fertility rate slightly below 2.1 children per woman, today the U.S. is well positioned to transition to slower population growth in coming decades. If we can encourage slightly lower birth rates among American citizens, we could stabilize our population sometime later in this century. If we do not reduce immigration, however, our population will balloon over the next hundred years, and continue growing with no end in sight.

Skeptical? Consider four numbers: 330 million, 377 million, 571 million and 854 million. U.S. population in 2019 is approaching 330 million. The other three numbers are population projections for the year 2100, according to a study by the U.S. Census Bureau.4 Each of the three projections holds fertility rates steady, while varying immigration levels, so varying annual immigration rates accounts for the different projections.

Under a zero immigration projection, the U.S. population continues to grow throughout the 21st century, increasing to 377 million people, 47 million more than our current population. Under a “middle” projection, with immigration a little less than 1 million annually, we instead add nearly 250 million people, growing to 571 million people.

Under the highest scenario, with more than 2 million immigrants annually, our population grows by nearly half a billion people by the end of the century! Obviously, according to the Census Bureau, immigration makes a huge difference to future U.S. population numbers.

A booming population has numerous harmful ecological effects beyond the sprawl and increased greenhouse gas emissions we have already discussed. It increases water use. It accelerates deforestation. It furthers crowding, which in turn makes it harder for young Americans to connect with nature, furthering “nature deficit disorder.”

As Sen. Gaylord Nelson, the founder of Earth Day, asked in a speech in Madison, Wisconsin, in March 2000, “With twice the population, will there be any wilderness left? Any quiet place? Any habitat for songbirds? Waterfalls? Other wild creatures? Not much.”

Population growth also increases our dependence on fossil fuels, making the U.S. more likely to resort to deepwater oil drilling and more susceptible to disasters such as the 2010 Deepwater Horizon oil disaster. Indeed, it is hard to think of a single environmental problem that is not made significantly worse by population growth, or that could not be more effectively met if we could stabilize or reduce our population.

As the Clinton Council on Sustainable Development wrote, “The sum of all human activity, and thus the sum of all environmental, economic and social impacts from human activity, is captured by considering population together with consumption.”5

As President Jimmy Carter’s Council on Environmental Quality put it, in a report years earlier, “The United States should ... develop a U.S. national population policy that includes attention to issues such as population stabilization.”6

As the great conservationist Aldo Leopold put it, 50 years before that:

“If there is any question of ‘superiority’ involved at all, it is whether we will prove capable of regulating our own future human population density by some qualitative standard, or whether, like the grouse, we will automatically fill up the large biological niche which Columbus found for us, and which Mr. Edison and Mr. Ford, through ‘management’ of our human environment, are constantly making larger. I fear we will. The boosters fear we will not, or else they fear there will be some needless delay about it.”7

American environmentalists face a choice. Ultimately, our environmental goals can only be accomplished if the population of the U.S. stops growing. This will only occur if immigration is substantially reduced, preferably by bringing immigration numbers in line with emigration numbers.

We must choose between sustainability and continued population growth. We cannot have both.


Further reading
Reproduction and the carbon legacies of individuals

1 Quoted in Stewart Udall, “The Quiet Crisis,” 1966.

2 Carbon Dioxide Information Analysis Center, U.S. Department of Energy, “National Fossil Fuel CO2 Emissions – All Countries.”

3 President’s Council on Sustainable Development, “Towards a Sustainable America: Advancing Prosperity, Opportunity, and a Healthy Environment for the 21st Century,” 1999.

4 Frederick Hollmann, Tammany Mulder, and Jeffrey Kallan, “Methodology and Assumptions for the Population Projections of the United States: 1999 to 2100.” Population Division Working Paper 38, table F (Washington, DC: U.S. Census Bureau, 2000).

5 President’s Council on Sustainable Development, “Towards a Sustainable America.”

6 U.S. Council of Environmental Quality and U.S. Department of State, “Global 2000 Report to the President,” 1981.

7 Aldo Leopold, “Game Methods: The American Way,” 1931.