“For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled.”

Richard Feynman


LIVING WITHIN OUR ENVIRONMENTAL MEANS: Natural Resources And An Optimum Human Population by Rachel F. Preiser

by Rachel F. Preiser

In a recently published report, “Natural Resources and an Optimum Human Population,” Cornell University professor Dr. David Pimentel has presented some sobering statistics indicating the insufficiency of world resources to sustain a rapidly-expanding human population. On February 21, 1994 at the annual meeting of the American Association for the Advancement of Science (AAAS) in San Francisco, Dr. Pimentel indicated that even if humans succeed in using rapidly diminishing resources more efficiently, the planet can sustain a “quality” standard of living for only two billion people while still maintaining environmental integrity. The report also concluded, “For Americans to continue to enjoy a high standard of living and for Society to be self-sustaining in renewable energy and food and forestry products, given U.S. land, water, and biological resources, the optimum U.S. population is about 200 million.”

Land and Water Supply

In order for people to live in “relative prosperity,” the world population—currently 5.6 billion and growing at a rate of 1.7 percent—will have to be reduced to 2 billion. The magnitude of this problem becomes apparent when one considers that the world population is expected to double in the next 41 years. According to Dr. Pimentel’s statistics, the U.S. population— now 260 million—would have to be reduced by about 60 million people, even though actual trends suggest that the U.S. population will double over the next 63 years.

Of the 260 million Americans, 32 million live in poverty; a problem that is not merely one of distribution. If the U.S. population continues to use cropland, water, and fossil energy at present rates while permitting population growth along current trends, shortages of essential goods and services will be experienced across the entire population. Technological development has enabled humans to push productivity from our land, water, energy resources almost to its environmental limits. However, expanding production to environmental capacity has its price in increasing the environmental degradation that ultimately makes this level of production and the population growth it has encouraged unsustainable.

The world food supply comes almost entirely (98 percent) from the land: food and fiber crops are grown on 12 percent of the earth’s total land area, while the remaining 24 percent of the land is used as pasture to graze livestock that provide meat and milk products. Forests cover an additional 31 percent of the earth’s land area, while the remaining 34 percent is too cold, dry, stony or steep to be suitable for agriculture or grazing.

About 15 million hectares (1 hectare is about 2.5 acres) of new land are required each year to support the earth’s steadily expanding population. Unfortunately, more than 10 million hectares of arable land are severely degraded and must be abandoned each year due to water and wind erosion, salinization, and water logging. Since topsoil formation proceeds at a painstakingly slow rate of about 2.5 cm every 500 years, arable soil is being degraded at a rate that far exceeds our environment’s replacement capacity. The 15 million hectares of new land required each year to sustain growing population are thus being taken largely from the world’s forests; however, the consequent deforestation is producing a shortage of the raw materials used to make paper products essential to a “quality” standard of living. In short, efforts to compensate for the deficiency of land resources by redistributing limited resources can and will continue to be felt in the decreasing availability of food and other products on which our present standard of living depends.

The increase in human population and the demands on agricultural production to sustain world population growth not only create a strain on the earth’s limited arable land resources, but also produce a tremendous draw on its water resources. While 24 percent of reservoir water is depleted yearly due to evaporation alone, 87 percent of the world’s fresh water supply—and 85 percent of the U.S. fresh water supply—is consumed by agricultural production.

In addition to the human population’s indirect demands on the water supply to fulfill its growing agricultural needs, an individual requires three liters (1 liter is about 1.06 quarts) per day of fresh water for drinking and a minimum of 90 liters per day for cooking, washing, and other domestic needs. Because, like topsoil, groundwater resources are renewed at an extremely slow rate of about one percent per year, the largest threat to the surface and ground water supply is the inequality between consumption of fresh water and its replacement by the environment. This problem is further compounded by pollution due to sewage, pesticides, and chemical wastes, which makes some portion of this limited resource unsuitable for human drinking and agricultural use. Under such circumstances, the average U.S. citizen uses about 400 liters of water per day, or more than four times as much water as Europeans use.

Irreplaceable Biodiversity

In addition to crop and livestock species on which we depend for food and other essential products, humans depend in more subtle, yet equally vital ways on a rich reservoir of other species in our agroecosystems. There are no technologies to substitute for many of the services that wild biota provide, including pollination of crops and wild plants, recycling of manure and other organic wastes, degrading of chemical pollutants, and purification of water and soil. Bees, for example, play an essential and irreplaceable role in the pollination of nearly $30 billion worth of U.S. crops annually.

Yet worldwide approximately 150 species of plants and animals are lost per day due to deforestation, pesticide use, and pollution. To preserve essential natural biological diversity, about one-third of the terrestrial ecosystem would need to be set aside to provide food, shelter, and protection for these valuable species, an allotment made impossible by the demands annual worldwide population growth continues to make on land and water resources.


Energy is necessary to drive all forms of human activity: it is indispensable to the technologies on which we increasingly rely to produce an adequate supply of food and other basic commodities essential to a “quality life.” Although worldwide, humans utilize about 50 percent of all solar energy captured by photosynthesis, this source is inadequate to meet production needs. This has created a heavy reliance on fossil fuels to meet the demands of an expanding population for energy to drive industrial production, transportation, construction, heating, and the packaging of goods.

Developed nations consume close to 80 percent of the world’s fossil energy, 25 percent of which is used by the U.S. alone. Fossil energy dependence in different U.S. economic sectors has increased 20 to 1000-fold in the past 40 years, suggesting how heavy reliance on this finite resource has become. At current rates of consumption, the known and discoverable potential oil reserves in the U.S. will last only 10-15 more years. Although coal supplies have a better prognosis, they currently account for a comparatively small part of fossil fuel consumption.

In 1850, with a population of 23 million, the U.S. derived 91 percent of its energy from the sun in the form of wood biomass. Today 93 percent of U.S. energy use comes from fossil fuels, while solar biomass energy provides a mere 3.5 percent of energy needs. Under current conditions of scarcity, it is essential that the U.S., as well as other developed nations, reduce their consumption of fossil energy, returning to a reliance on renewable solar energy. Dr. Pimentel’s report suggests that, by cutting energy use in half while exercising strict control over population growth, the U.S. would have sufficient fossil energy reserves (particularly of coal) to make the necessary transition to a renewable energy economy over the next 100 years—if we start now. Unfortunately, the shortage of uranium and the problems of nuclear waste disposal prevent nuclear energy from being a suitable “renewable” replacement for disappearing fossil fuel supplies.

If the U.S. population were to decrease according to the report’s prescriptions, it might be possible to return to a greater dependence on solar-based biomass energy, alleviating some of the pressure on our rapidly dwindling fossil fuel supply. Unfortunately, since most of the biomass produced by the U.S. consists in agricultural crops and forest products essential to supply food, fiber, pulp and lumber, a very limited portion of biomass is available as an energy source. In addition, the shortage of arable land severely limits the possibility of expanding our biomass production for energy. Dr. Pimentel’s report suggests that even to increase our collection and harnessing of solar energy five-fold would require the use of 10 percent of the U.S. land area for solar systems and then would provide for only 40 percent of our current energy consumption.

What Can We Do?

While humans can attempt to compensate for shortages in some prime resources (like arable land) by manipulating the distribution of other prime resources (like water), the overall possibility of such supplementation is restricted by environmental degradation due to the demands of an increasing population and unsustainable agricultural practices. This report indicates the necessity of developing alternative strategies for living within our environmental means.

According to Dr. Pimentel, significant quantities of fossil energy might be conserved if (as a part of our move to greater reliance on renewable energy sources) we make better use of manure instead of fossil-based fertilizers to enhance our agricultural soils while also reducing use of fossil-based pesticides. Along with these revisions of agricultural techniques, Dr. Pimentel suggests that a return to crop rotation in growing crops like corn would stem soil erosion and conserve soil and water resources as well. While saving energy these changes would also promote a more sustainable agricultural use of the environment.

Nevertheless, even the most diligent conservation efforts will be effective only if they are accompanied by stopping population growth. If returned to a self-sustaining renewable energy system, the earth will be able to support a population of approximately two billion people living in relative prosperity. Dr. Pimentel recognizes that “a drastic demographic adjustment to two billion humans will cause serious social, economic and political problems,” but insists that “continued rapid population growth will result in even more severe social, economic and political conflicts—plus catastrophic public health and environmental problems.” At current growth rates, the earth’s population could reach an environmentally impossible 12 to 15 billion in the year 2100. In order to Prevent the disastrous worldwide poverty and privation that such a population increase would produce, the report recommends profound revisions in our relationship to the environment and an end to population growth.

The report concludes with both a warning and promise.

“Decision making tends to be based on crises; decisions are not made until catastrophe strikes. Thus, decisions are ad hoc, designed to protect or promote a particular aspect of human well-being instead of examining the problem in a holistic manner. Based on past experience, we expect that leaders will continue to postpone decisions concerning human carrying capacity of the world (Fornos, 1987), maintenance of a standard of living, conservation of resources, and the preservation of the environment until the situation becomes intolerable, or worse still, irreversible.

Starting to deal with the imbalance of the population-resource equation before it reaches crisis level is the only way to avert a real tragedy for our children’s children. With equitable population control that respects basic individual rights, sound resource management policies, support of science and technology to enhance energy supplies and the environment, and with all people working together, an optimum population can be achieved. With such cooperative efforts we would fulfill fundamental obligations to generations that follow—to ensure that individuals will be free from poverty and starvation in an environment that will sustain human life with dignity.”