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Sustainable Society:  A society that balances the environment, other life forms, and human interactions over an indefinite time period.







Natural Resources

and an

Optimum Human Population


David Pimentel, Rebecca Harman, Matthew Pacenza, Jason Pecarsky and Marcia Pimentel


According to the Washington, DC-based Population Crisis Committee, the world's human population, currently more than 5.5 billion, is projected to reach nearly 8.4 billion by the year 2025 and may reach a disastrous 15 billion by 2100. There are now 1.2-2 billion people living in poverty —malnourished, diseased and experiencing short life spans.

The Population Reference Bureau estimates that the average American consumes about 23 times more goods and services than the average world citizen. Americans also bum 10,000 liters (2600 gallons) of oil-equivalents per year —seven times the world average. Clearly, achieving a US standard of living is impossible for the rest of the world, based both on projections of future resource availability and on population growth. The affluent standard of living now enjoyed by Americans (made possible by our abundant supplies of fertile cropland, water and fossil energy) is projected to decline if the US population doubles during the next 63 years.

If the US were to move quickly to a renewable energy economy, with sustainable use of energy, land, water and biodiversity, and a relatively high standard of living, our research indicates that the optimum US population would be about 200 million, significantly less than the current population of 256 million.

Math for a Small Planet

Approximately 0.5 hectares (1.25 acres) of cropland is needed to provide one person with a diverse, nutritious diet of plant and animal products.

The US supply of crop-land is at this level now, but the world average is only 0.28hectares (.69 acres). Each year, more than ten million hectares (24.7 million acres) of once-productive land are degraded and abandoned.

Simultaneously, an additional 5 million hectares (12.3 million acres) of new land must be put into production to feed the 92 million humans added yearly to the world population. Most of this 15 million hectares needed for expansion comes from the destruction of the world's forests. The urgent need for agricultural land accounts for 80 percent of the deforestation now occurring worldwide.

Humans do not have any technologies that can substitute for the services provided by wild biota. In the US, there are approximately 500,000 species of plants, animals and microbes that provide essential functions (pollinating crops, recycling manure, purifying water and soil) and serve as a vital reservoir of genetic material. Yet the world is losing roughly 150 species per day from such human activities as deforestation, pollution, pesticide application and urbanization. In his book, Fundamentals of Ecology, E. P. Odum reports that, if sufficient natural biological diversity is to be maintained to ensure a quality environment, about one-third of the terrestrial ecosystem should be preserved as natural vegetation.

Limits to Solar Energy Growth

The United Nations Environment Program estimates that the US accounts for approximately 25 percent of the world's annual consumption of fossil fuels. All other developed nations combined use another 55 percent and developing nations, with approximately 75 percent of the world's population, consume only 20 percent.

In 1991, US Department of Energy estimates of national oil reserves plummeted. Instead of the 35-year supply forecasted in 1987, reserve predictions were lowered to 10-13 years at 1991 pumping rates. In 1850, when its population was only 23 million, the US was dependent on wood biomass (a form of solar energy) for 91 percent of its energy. Today, we depend on fossil fuels for 93 percent of our energy needs. With the imminent decline in fossil-fuel supplies, a transition should be made to renewable energy sources.

In the US, the annual amount of solar energy captured by vegetation —agricultural crops, forests, lawns, gardens and wild growth— is 54 quads (one quad equals one quadrillion British Thermal Units). Americans currently use 40 percent more fossil energy than the total amount of solar energy captured by plants.

Although the conversion of biomass crops, like corn, into fuel energy appears promising at first glance, 72 percent more energy is used in the production of ethanol than the energy it provides. Furthermore, the land area needed to provide the raw material is enormous; about six hectares (15 acres) of corn grain is needed to provide the ethanol fuel for one US car for one year.

Providing one billion kwh of solar power to a city of 100,000 for a year would require 2700 hectares (6669 acres). Only ten percent of the needed area could be supplied by mounting photovoltaic units on the roofs of buildings. If the current level of seven quads of solar energy collected and used annually in the US could be increased five-fold without adversely affecting agriculture, forestry or the environment, these 35 quads would still represent only about 40 percent of the current US energy consumption.

Producing this 35 quads would require 90 million hectares (222 million acres) or nearly ten percent of US land area. Worldwide, if 500-600 million hectares (1235-1482 million acres) were devoted to solar energy production, approximately 200 quads of energy might be available each year, while maintaining agricultural and forestry production. This is roughly two-thirds of the total annual world use of solar and fossil fuels combined —369 quads.

Combined with active conservation efforts, a satisfactory standard of living would be possible for everyone, but only if the human population were much smaller than the present 5.5 billion.

Assuming that 0.5 hectare per capita is necessary for an adequate food supply, and assuming that soil conservation programs were implemented to counteract erosion, it would be possible to sustain a global population of approximately 3 billion.

Alternatively, with a self-sustaining renewable energy system producing 200 quads of energy per year and providing each person with 5000 liters (1300 gallons) of oil-equivalents per year (half the current US consumption, but an increase for most people in the world), a population of 1-2 billion could be supported in relative prosperity. This adjustment could be made over a century or more.

Granted, such a drastic demographic adjustment will cause serious social, economic and political problems, but to continue rapid population growth will result in more severe social, economic and political conflicts, plus catastrophic public-health and environmental problems.

Does human society want 10-15 billion humans living in poverty or 1-2 billion living with abundant resources? Dealing with the imbalance of the population-resource equation before it reaches a crisis level is the only way to avert a real tragedy for our children's children. Citizens of the US and the world must support their leaders in making these critical decisions for the future. To do nothing to control population is to condemn future humans to a lifetime of absolute poverty, suffering, starvation, disease and violence.

Based on past experience, we expect that leaders will continue to postpone decisions on the human carrying capacity of the world until the situation becomes intolerable or, worse, irreversible. Nature will impose the ultimate control on the human population.
* Used with permission of the Earth Island Journal.
Originally published in the "Earth Island Journal", Summer 1994, Vol. 9 Issue 3.p26-30.
Copyright 1995, Earth Island Journal.
See at < http://www.earthisland.org/journal/naturres.html >.
Adapted from a longer article published in the May 1994 issue of Population and Environment.

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