The easiest way to picture sustainability is to imagine a community of people living entirely off the fish in a single pond. A certain number of fish will grow to adulthood every year. If the people living on the shore only eat that exact number of new adult fish, the total fish population stays the same. The system replaces what is taken. The people are living off the interest.
But if the human population grows and they decide to eat more fish, the dynamics change. They are no longer just eating the interest or surplus; they are eating into the breeding population. Every year they do this, there are fewer adult fish left at the beginning of the season. Because there are fewer adults, the pond produces fewer new fish. Meanwhile, the human population has grown, so they do not eat less—they eat even more. In financial terms, they have started spending their savings or principal. This means the fish population drops faster and faster until one day, the pond is empty.
At that moment, the community faces starvation, and the crisis can appear sudden. They notice it is getting harder to catch fish, but they are still catching them right up until the day they cannot. Then the resource is gone. This is typically when severe shortages and conflict break out, as populations rarely face resource depletion quietly.
That is overshoot at the scale of a pond.
The first objection to lowering the population through fewer births is common: are there really too many people?
The answer is yes. Humans are in ecological overshoot. The Earth is carrying 8.3 billion people by liquidating forests, rivers, oceans, soil, groundwater, fossil fuels, mined nutrients, and wild habitat. That is not sustainability. That is a drawdown.
This chapter is for people who do not believe there are too many people on Earth, or simply do not know.
The key word for answering this question is sustainable. A population is sustainable only if it lives off the interest of the Earth, not the principal. Each year, only so many trees can regrow, fish can reproduce, and topsoil can form. A sustainable population lives within these limits. When humans take more than that, they are drawing down the principal. They may still eat. They may still build. They may still call the system successful. But the system is being paid for by using up the sources of future life and will eventually collapse.
As a reader, you have two paths: simple or detailed. I will walk you through why there are far too many humans on Earth right now, what constitutes a more sustainable number, and where the absolute limits lie. I will do it in a simplified form so it is easy to follow.
For those readers with deep curiosity, backgrounds in systems biology, ecology, population dynamics, and related fields, I will save the detailed data and science for you in the back of this chapter. I am using this structure so that no one gets lost in the numbers unless they want to dive in. Everyone is of course free to choose their own path.
Too Many People (Simplified Version)
When people hear about a world with fewer humans, they often imagine an empty, desolate wasteland. The reality of a planet operating within its ecological limits is the exact opposite. It is crowded, diverse, and constantly in motion.
On an intact native prairie, the sheer biomass is staggering. Millions of bison migrate in columns miles wide, physically churning the topsoil. The ground itself is heavily populated. Prairie dog networks stretch across thousands of acres, sharing the soil with burrowing owls and black-footed ferrets. Above ground, herds of pronghorn graze among wild prairie turnips and blooming coneflowers. A person walking through this environment has constant access to food, harvesting wild plums, ground cherries, and sunflower seeds straight from the stalk, while meadowlarks hunt for insects in the dense grass.
A functioning wetland is a massive concentration of protein and carbohydrates. The water runs clear, naturally filtered by thick beds of wild rice and sweet arrowhead roots—staple crops that grow entirely without cultivation. Beavers engineer the water flow, creating deep pools where northern pike and bluegill thrive. The shorelines are packed with painted turtles and red-winged blackbirds roosting in the cattails, a plant that provides edible starchy tubers year-round. The bird migrations are large enough to alter the light over the landscape, with flocks of snow geese and sandhill cranes creating a wall of sound that matches the dense local frog populations.
In an old-growth forest, the timber is so massive that the canopy completely dictates the temperature and moisture below. The soil is heavily layered with moss, chanterelle mushrooms, and wild ramps. The woods are highly active. Woodpeckers strip bark from dead trees to reach beetle larvae. Black bears forage in dense patches of wild raspberries and huckleberries to build fat reserves, while herds of elk move through the underbrush. Without centuries of industrial hunting, the wildlife lacks an innate fear of humans. Animals like red foxes, martens, or blue jays will readily approach a person to take a fallen chestnut or a handful of elderberries.
The food is geographically dense. The biodiversity is highly visible. This is what a planet looks like when it operates at peak capacity.
For at least 200,000 years, this was the baseline human experience. Our ancestors survived strictly by harvesting the natural surplus of these ecosystems. They lived off the Earth’s biological interest—the wild crops, the migrating herds, the seasonal fish runs that replenished on their own every year—without drawing down the principal.
But when we calculate how many humans the planet can actually feed this way, relying solely on natural cycles without fossil fuels, synthetic fertilizers, or industrial agriculture, the baseline is stark. The Earth can support a range of roughly 40 to 170 million people living sustainably off its interest.
When people hear a number like 170 million, let alone 40 million, it can be startling. It sounds small enough that some analyses attempt to find ways to push the sustainable limit higher. However, many estimates for how many people the Earth can hold are overestimates because they do not demonstrate long-term sustainability. The median carrying capacity study suggests 10 billion as the Earth’s limit. This is a misuse of the term carrying capacity. A true carrying capacity requires indefinite survival without drawdown. These studies usually assume the ongoing use of fossil fuels, unrestricted water sources, and maximized land allocation while minimizing environmental impacts. These models argue that we can feed 10 billion people temporarily, but there is nothing sustainable about it.
At population figures around 2 billion, some top-down ecological studies do prioritize sustainability and model a potential carrying capacity. They rely on major assumptions, showing what a sustainable world might look like if a long list of difficult structural challenges were successfully solved. They do not demonstrate that those problems can all be solved simultaneously, let alone provide a specific framework for doing so.
These papers are genuine efforts at a very difficult task. These examples are most useful as an optimistic goal of what a sustainable society could be, even without detailed instructions for how to get there. Therefore, I use the 40–170 million range as a sustainable baseline and this 2 billion carrying capacity figure as an unproven high-end estimate.
There is nothing inherently better or more successful about a planet holding two billion people instead of one hundred million. The closer a society lives to the absolute limits of its food and energy systems, the more likely it is to experience hardships when the inevitable hard years come. More people means less of a buffer. This will be a decision for future generations to make.
Anywhere within this range means humanity survives, creates, and thrives. The major difference is how much room is left for the rest of life, and how much labor humans must expend to survive. We can maintain advanced technology, innovation, arts, science, and a progressing civilization at 100 million or 2 billion.
People created oral and written language, built the Great Pyramid of Giza, learned to predict eclipses using the Saros cycle, developed advanced geometry, practiced surgery on fractured bones and skulls, built underground water systems, sailed across seas and around capes, made bronze, smelted iron, mastered the harp, trumpet, drums, and flute, fermented chiles, wine, cheese, and beer, unlocked the secrets of indigo dyes, and composed the Vedas before humanity crossed the 100 million population mark. And so long as we manage the demographic transitions rather than rushing off economic and ecological cliffs, we will not be starting from zero. Whether at 40 million or 2 billion, we will carry trillions of collective hours of experience, knowledge, and wisdom with us.
Today, we are nowhere near either of those numbers. We have over 8.3 billion people. We are carrying fifty times more weight than the planet’s natural interest can support. We will need about 100 to 250 years to return to a sustainable population level if humanity shifts to an average of one child per woman starting by 2027.
So how are eight billion of us alive and eating today? We are burning through our savings account. We like to think that modern farming is a massive victory of human intelligence—that we figured out how to make the planet grow more food. In reality, modern farming functions more like a global mining operation. We are watering our crops with deep underground water pockets that took tens of thousands of years to fill. We plow up healthy soil that takes centuries to form, leaving it bare much of the year, which allows wind and rain to wash parts of it away each season. To keep this degraded soil growing food, we mine minerals out of the ground and burn large amounts of oil, coal, and gas to manufacture synthetic fertilizer. We didn’t build this modern world on empty land; we displaced the habitats of billions of wild animals to plant giant fields of a single crop. We are running a massive resource deficit, and we are paying for it by emptying the bank account of the future.
When people realize this, they try to find hope in history. They point to the historical agriculture of old Japan or the Nile River in ancient Egypt, because these societies fed millions of people for centuries without engines or chemicals. They are often referred to as green paradises. When the Greek historian Herodotus visited Egypt over two thousand years ago, he was amazed by how the river naturally watered the soil. He wrote that the Egyptians “obtain the fruits of the field with less trouble than any other people in the world.”
He was right, to an extent. Ancient Egypt, like most early civilizations, was blessed with a major geological subsidy. The Nile River collected minerals and nutrients from across the African continent on its way north. The river flooded predictably and fertilized the local soil with those nutrients. The Nile River valley is an oasis in the desert.
The valley supported a population that averaged around 3 million people for about 5,000 years. To grow that much food without machines, these societies had to push their environments to the absolute edge, and they paid a price for it. When weather patterns shifted and the natural systems failed, the suffering was severe. The Egyptian population was entirely at the mercy of the river. When droughts hit and the Nile didn’t flood, populations collapsed. An ancient Egyptian governor named Ankhtifi carved a description of the reality of his era into his tomb, writing that the drought was so severe that “all of Upper Egypt was dying of hunger, to such a degree that everyone had come to eating his children.”
When the floods did arrive, survival required large-scale, coordinated human muscle. This type of farming is exhausting work. Free people, when they have a choice, rarely choose to labor to the point of physical collapse. To get a human being to stand in knee-deep mud planting rice fourteen hours a day, historically a society has had to use coercion.
Take old Japan between the 1600s and 1800s. The country was closed off from the world, and millions of poor farmers had to haul wild brush down from the mountains on their backs to fertilize the rice paddies. To squeeze enough food out of the soil to feed thirty million people, the ruling government took absolute control over human life. They passed laws making it illegal for farmers to eat the very rice they grew, ordering them to eat only “millet, vegetables, and other coarse food.” The rulers did not hide their perspective. Their official saying was: “Peasants are like sesame seeds: the more you squeeze them, the more you get out.” To avoid starvation under this intense pressure, families were forced into infanticide. Midwives routinely ended the lives of newborn babies just to keep family sizes small. They called it mabiki—“thinning out the seedlings.”
The historical models of intensive agriculture destroyed much of their surrounding ecosystems and relied on the forced labor of the vast majority of their populations. While forced labor historically involved buying and selling people, it also includes systems where individuals are forced to work without compensation under the threat of execution.
Modern agriculture continues to include elements of human trafficking, as well as unpaid labor under the threat of violence. In the tomato fields of Italy, undocumented immigrant workers labor under the threat of deportation, picking food for minimal compensation and occasionally dying of heatstroke. As Yvan Sagnet, an activist who used to pick those tomatoes, explains: “Slavery in the 21st century doesn’t need chains, because they exploit a continual sense of intimidation.” In Southeast Asia, the cheap palm oil found in half of our supermarket food is subsidized by child labor and workers trapped by debts they can never pay off. “I am not a free man anymore,” one worker in Malaysia stated to reporters. “I desperately want to see my mom and dad. I want to go home!”
In the United States, court cases have exposed criminal rings holding agricultural workers at gunpoint, forcing them to harvest onions with their bare hands. There are over 800 million people worldwide who work on farms that utilize known toxins. The majority receive wages below the local minimum wage, and hundreds of millions are not compensated at all. They are typically excluded from basic labor protection laws like overtime, and reports of wage theft are widespread throughout the industry.
This is modern agriculture.
I do not claim that 40–170 million people is the definitive population the Earth can sustain. It is simply the highest figure that can be modeled without drawing down the Earth’s finite resources and without exploiting hundreds of millions of people. Humans can likely find ways to increase the sustainable population limit with technological innovation and further research. However, that is not achieved by applying green technologies on top of a highly extractive system.
Whether you anchor on the 40–170 million range or the aspirational 2 billion range, humanity is billions of people past a sustainable population. We do not need a perfect, final number to know how to act right now. The goal of human civilization does not need to be to see how many breathing bodies we can squeeze onto a dying planet.
We also have the option to maximize the number of free humans living on a thriving Earth. We are nowhere near that balance, but we can get there. Have one kid. Let the population decline, let nature heal, and then learn how many free people a living Earth can truly support.