Created: 1995. Updated: 4 December, 2004

From Cornell University Alumni News

by Hillel J. Hoffmann


The Earth's limited supply of natural resources will only be able to sustain 2 billion humans by 2100, bad news for a world that already feeds 5.9 billion. The optimum human population, or carrying capacity, for the U.S. is projected to be 200 million, which is millions fewer than the current population. Disappearing stocks of fertile land, fresh water, fossil fuel energy, plants and animal life will control human population the old-fashioned way-through starvation and disease-if we cannot reduce our numbers voluntarily.


The Earth and its resources may be too small for all of us to share. Even if we learn how to make the most of a limited supply of land, energy, water, and biota, Cornell ecologists have calculated that by the year 2100, the planet will be able to provide for only 2 billion humans-almost 4 billion less than today's world population-with a modest but comfortable standard of living. Only 200 million humans can be sustained by the natural resources of the United States, making the current population 33 percent over its eco-budget.

"If we refuse to reduce our numbers ourselves," warns David Pimentel, PhD, professor of entomology and agricultural sciences, "nature will find much less pleasant ways to control human population: malnourishment, starvation, disease, stress and violence." The choice is simple, says Pimentel-total reproductive freedom now, or freedom from suffering in the not-so-distant future.

Cornell researchers presented their findings at the annual meeting of the American Association for the Advancement of Science in San Francisco. Their results were published in the May [1994] issue of Population and Environment. Joining Pimentel as co-authors were Marcia Pimentel, MS, retired senior lecturer in the Division of Nutritional Sciences, and Cornell undergraduates Rebecca Harman, Matthew Pacenza and Jason Pecarsky.

Optimum world population: 2 billion. Optimum U.S. population: 200 million. Ecologists call these magic numbers "carrying capacities." An area's carrying capacity (or "K") is the number of individuals of a given species that can be supported indefinitely by the local environment. K is defined by limiting factors-predation, limited space, limited amounts of food, competition from other species and other threats.

You may remember the ominous graphic representation of carrying capacity from your introductory biology course. the number of individuals erupts as the species exploits an area's supply of food and space, reproducing unchecked. As resources run out, the population first plateaus, then plummets sickeningly as individuals die, eventually stabilizing at a number just below the almighty K.

Where does Pimentel put us on the graph of Earth's human population? "Right now, while we still have relatively abundant fossil fuel energy, I would say that we're getting near the inflection-the point where the sharp rise in population begins to flatten off."

Sharp rise, indeed. Almost 5.8 billion people now live on the planet. The population is projected to reach nearly 8.4 billion by the year 2025 and 11 billion by 2100. Every day the total rises by a quarter-million. The relative abundance of young people in their reproductive prime gives the world population explosion surprising momentum. "Even if we adopt a zero population growth strategy tomorrow-a little over two children per couple-the world population will nearly double [by ?]," explains Pimentel. "It wouldn't stop growing for 60 years."

According to the Cornell team, the primary limiting factors that define the Earth's carrying capacity for humans are fertile land, fresh water, fossil fuel energy and a diversity of helpful natural organisms. All are essential in the production of food, and worldwide stocks of all four are being consumed faster than they can be replaced.

More than 25 million acres of arable and pasture land, the source of 98 percent of the world's food, are critically degraded and abandoned each year. An additional 12.5 million acres of new fertile land must be put into production to feed the 92 million new mouths added each year to the world population. One and a quarter acres of good crop land are needed to provide a diverse, nutritious diet of plant and animal products for one person. The current world average of crop land per capita is just over two-thirds of an acre.

Fresh water supplies are being overdrawn from surface and groundwater sources. The primary consumer: agriculture, which absorbs, without possibility of recovery, 87 percent of the fresh water used each year in the world. "It takes 1,400 pounds of water to produce one pound of food," says Pimentel.

Fossil energy from oil, gas and coal may be the first limiting resource to disappear. Essential for industrial production, fuel, construction, heating and cooling, packaging, delivery of clean water and fertilizer manufacture -- about 319 quads (that's 10,000,000,000,000,000 BTUs in air-conditioner-ese) of fossil fuel are used worldwide each year (one-fourth of which is burned in the United States). The Cornell team cites research suggesting that world supplies of oil and gas should last about 35 years at current pumping rates, coal and uranium stocks about 100 years.

The easiest natural resource to overlook is provided by the living organisms that share the planet with us. "Humans have no technologies that can substitute for the services provided by wild biota," says Pimentel. "There are about half a million species of animals, plants, and microbes that provide essential functions for humans in the United States." Large-scale food production would be impossible without pollinators, decomposers, scavengers and waste recyclers. Biodiversity also ensures a future gene bank for tomorrow's forestry and agriculture. Researchers estimate that 150 species are lost daily due to human activity. (See "Biodiversity: What's in it for Us?" November 1993, Cornell Magazine.)

In order to arrive at their optimum population figures, Pimentel and his colleagues make the optimistic assumption that humanity will make maximum use of Earth's finite resources. Even if humans make a transition to renewable energy sources, stop polluting and degrading their environment and accept a standard of living equal to one-half of that enjoyed by Americans today, the numbers stay the same.

Getting there will be painful, Pimentel admits, but achievable. Population reduction will require a growing proportion of elderly to be supported by a shrinking number of younger, more productive humans. "It will create economic and social stress when we have to makes these changes," he acknowledges. "But our alternative is far worse economic and social stress." If global human fertility can be reduced from the current rate of 3.3 children per female to 1 to 1.5 children per female, Pimentel estimates that the population can be reduced to carrying capacity in 100 to 150 years.

Population control presents ethical problems. "You have to take your choice," explains Pimentel. "People say that they should have the freedom to reproduce. I'm sympathetic to that view. But you're going to either lose some of that freedom, or your children and your grandchildren will lose some of their freedoms-freedom from starvation, freedom from disease."

Pimentel bristles at critics (rumored to include high- voltage media commentator Rush Limbaugh) who claim that technology will allow humanity to keep pace with a steep population curve. "Look at fishery production. We built bigger ships, larger nets, and now the fish populations of the oceans lakes and rivers are lower than they've been since 1970. Look at the Colorado River. As it flows south, California, Arizona, and Colorado take a big piece out of it to support their populations. By the time that river reaches Mexico, it's dry. What technology do we have available, short of manipulating the climate, that can double the flow of the Colorado River?"

And how does Pimentel respond to those who say that the endless, painful cycle of population booms and busts is nature's way? "I think that we're too intelligent to let nature control our numbers," he says, ultimately the optimist. "I think that we can do a more effective job of limiting our numbers than nature can."

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