The Real Perils of Human Population Growth

David Pimentel, Marcia Pimentel

About forty years ago, the world population was only 3.5 billion, or about half of the present population of 6.7 billion people. Most of us seem to ignore or be unaware of the magnitude of this rapid expansion and the vast changes that it is causing throughout the world. Indeed, the daily and even the annual impacts of this growth go unnoticed. Yet the impacts of the growing world population on land, water, energy, and biota resources are real and indeed overwhelming.

What resources are required to secure a quality life for future generations worldwide? Will there be sufficient cropland, water, energy, and biological resources to provide adequate food and other essential human needs? Balanced against the future availability of these basic resources are the escalating needs of an ever-growing population.

Clear scientific evidence suggests worldwide problems of food availability already have emerged. According to the World Health Organization, nearly 60 percent of the world population now is malnourished—the largest number and proportion of malnourished people ever reported in history. Further, many serious diseases, like malaria, HIV/AIDS, and tuberculosis are increasing, not only because of worldwide malnutrition but also because the increasing density and movement of human populations facilitate the spread of diseases.

More humans than ever before cover the earth with their urbanization, highways, and other activities. This imperils the availability of land resources. Cropland is a vital resource, with more than 99.7 percent of human food calories coming from the land and less than 0.3 percent from oceans and other aquatic ecosystems. Globally, an average of only 0.22 hectares of cropland per capita is now available for crop production. In contrast, 0.5 hectares per capita is available to support the diverse food systems of the United States and Europe. At present, cropland in the United States now occupies 17 percent of the total land area, but relatively little additional cropland is available to support the future expansion of U.S. agriculture. As a result, valuable forest areas are being permanently destroyed and replaced with cropland. This is causing many long-range global problems, including contributing to global warming.

Each year more than 10 million hectares of valuable cropland are degraded and lost because of soil erosion. In addition, an added 10 million hectares are being destroyed by salinization resulting from improper irrigation. Combined, world soil erosion and salinization account for the major losses in productive cropland.

Along with the loss of cropland and irrigated land, per-capita fertilizer use worldwide is declining, and all these changes are suppressing food-crop production, especially in developing countries. Adding to the 22 percent decline in per-capita use of fertilizers is the rapid increase in fertilizer prices. This decline is a concern because fertilizer nutrients are critical for crop production. The recent doubling of fertilizer prices had major impacts on farmers, especially struggling farmers in developing countries.

The major changes now occurring in world cereal-grain production should alert us to the problems ahead. Cereal grains comprise about 80 percent of world’s human food intake, making their sustained availability vital to human survival. Disturbing reports from the United Nations Food and Agriculture Organization indicate per-capita availability of basic cereal grains has been decreasing for the past twenty-four years. Thus far, despite all the advances in biotechnology and agricultural technologies, per-capita grain production has continued a slow decline since 1984, as harvests have to be divided among an increasing number of people.

Cropland without adequate water is unproductive. Adequate quantities of freshwater, which support the very survival of every human, plant, and animal on Earth, are not available in many regions of the world. A human requires slightly more than one liter of water each day. In contrast, producing the food needed to feed a human each day requires more than 1,600 liters of water.

Indeed, more than 70 percent of all available freshwater is used in world agriculture. For example, to produce one hectare of corn requires 5 million liters of water during the growing season (more than 600,000 gallons per acre). Further, the 17 percent of arid cropland that is now irrigated provides about 40 percent of the world’s food. This confirms the importance of water and the role of irrigation in world food production.

As human populations continue to increase, more freshwater will be required for consumption by people, crops, and livestock. Water is being removed in some aquifers in the Western United States ten times faster than the recharge rate. Existing water conflicts within and between countries will escalate.

Over time, energy from many sources has assisted humans in their activities, from growing crops to sustaining major transportation systems. Prior to 1850, Americans were primarily dependent on forests for their fuel. Today, Americans are 96 percent dependent on fossil energy. Nearly 20 percent of all the fossil energy used in the United States is devoted to supplying food. In the last century, agriculture in industrialized nations has used enormous quantities of fossil energy to produce fertilizers and pesticides, to manufacture and run farm machinery, and to power irrigation systems. All of these fossil-energy based additions to agricultural production have greatly increased total energy requirements. For example, to produce one hectare of corn or rice currently requires approximately 1,000 liters of oil equivalents. But in contrast, in developing countries, expensive fossil energy has been replaced by human and animal power in order to provide the needed energy for crop production. About 1,200 hours of human manual labor are required to produce a hectare of grain.

Americans hold the dubious honor of being the world’s leaders in fossil energy use. An average American consumes about 9,500 liters of gasoline energy-equivalents each year. Because of this high fossil-energy use and inadequate domestic energy sources, the United States now imports nearly 70 percent of its oil at an annual cost of $700 billion.

Yes, oil is a finite energy resource. The world supply of oil has peaked, and the remaining oil will now decline as use continues. Reliable projections are that by 2040 the world supply of oil will be more than 60 percent depleted. As oil resources diminish worldwide, costs increase, as evidenced until the current economic slowdown.

In the United States, supplies of natural gas and coal are expected to last fifty to one hundred years, depending on how fast they are substituted for oil and how fast the U.S. population grows. However, the processing of coal into oil and gas will contribute to damaging air pollution and global climate change unless technology is developed to help overcome these serious consequences.

Currently, the U.S. population uses about 100 quads of energy each year. (A quad is a large unit equivalent to 1015 BTUs.) This rate of consumption will continue to increase as the population continues to grow and further diminish fossil energy reserves. Now, as world fossil-energy sources are being depleted, all renewable energy sources must be investigated and priority given to their development and use. Depending on the geographic region, the most reliable of the potential renewable sources are wind power, photovoltaics, solar, thermal, and biomass energy. Even if all of these solar-based technologies become fully operational, they are projected to provide only about half—of the current U.S. consumption of fossil energy—or nearly 50 quads of energy. Furthermore, these renewable energy sources would occupy another 17 percent of additional land area, most
ly in order to collect the solar energy. Some of this required land would compete with vital cropland, pasture, and forest land. Nuclear energy will probably have to be developed further in the world. At present, France has one of the better models for the relatively safe use of nuclear energy.

Optimists suggest ethanol produced from corn grain and cellulosic biomass, like grasses, could replace much of the oil used in United States (see Stephen Paley, George Oister, and Richard Hull, “Can We Survive?,” Free Inquiry, February/March 2008 and April/May 2008). But consider that when 20 percent of the U.S. corn crop was converted into 6 billion gallons of ethanol in 2007, it replaced only 1 percent of U.S. oil consumption. If the entire corn crop were converted into ethanol, it would replace a mere 7 percent of oil consumption—and not make the United States independent of foreign oil! More critical is the fact that it takes 140 percent more fossil energy to produce one liter of ethanol than is contained in the ethanol produced.

Using food crops such as corn grain, canola, soybeans, and sugarcane to produce biofuels also causes major nutritional and economic concerns. Nearly 60 percent of humans in the world are currently malnourished, so the need for grains and other basic foods is critical. Jacques Diouf, director general of the U.N. Food & Agriculture Organization, reports that biofuel production is increasing human starvation worldwide. Therefore, growing crops for fuel squanders land, water, and energy resources vital for the production of food for people. The president of the World Bank reported that at peak, biofuels increased world food prices 75 percent.

Unfortunately, the environmental impacts of corn ethanol are serious and diverse. These include severe soil erosion of valuable cropland, the consumption of large amounts of water, plus the heavy use of costly nitrogen fertilizer and pesticides that pollute soils and waterways. In addition, large quantities of carbon dioxide are produced and released into the atmosphere because of the significant amounts of fossil-fuel energy that are needed in ethanol production. Then, during the subsequent fermentation process, about 25 percent of the carbon from the sugars and starches is released as carbon dioxide into the atmosphere, thereby contributing to global warming.

Cellulosic biomass is touted as the replacement for corn in making ethanol. Unfortunately, cellulosic biomass contains less than one-third the amount of starches and sugars found in corn and requires major fossil-energy inputs to release the more-tightly bound starches and sugars it contains for ethanol conversion. About 170 percent more energy (oil and gas) is required to produce ethanol from cellulosic biomass than in the ethanol produced.

Over many decades, the pervasive use of chemicals throughout the world has expanded, impacting the survival of animals, plants, and microbes while also presenting serious constraints to ensuring sustainability. Exposure to a variety of chemicals is documented to contribute to a variety of serious human diseases. These include cancer, birth defects, immune system defects, reduced intelligence, behavioral abnormalities, decreased fertility, altered sex hormones, altered metabolism, and specific organ dysfunctions.

Air pollution, especially that caused by the increased burning of fossil fuels by humans, is predicted to have negative impacts on the environment. The widespread activities of agriculture, public health, and various aspects of the broader environment will suffer. Some problems and damages associated with global warming are already apparent, especially in agriculture. These changes include altered rainfall patterns, increased length of growing seasons, and increased pest problems.

Recent studies of environmental refugees throughout the world reveal their numbers are increasing. These movements of human populations contribute to major changes in population distribution and global insecurity. Understandably, these refugees are fleeing income disparity as well as shortages of food, cropland, freshwater, fossil energy, and other diverse essential resources they need for survival. This pattern will increase over time as the world population increases. According to the United Nations, the rapid growth in the world population serves as a potential breeding ground for terrorists and threatens global peace and security.

The present world population of 6.7 billion is projected by the United Nations to increase to 9 billion and may rise to as many as 11 billion by 2050. Even if a worldwide policy of two children per couple (instead of the current 2.8 children) were agreed on tomorrow, the world population will continue to expand for about seventy years before stabilizing at about 13 billion people. China, with a present policy of one child per couple, will add about 8 million to its population this year because of its young-age structure. Population momentum depends on the young-age structure of the current world population and propels the speed of growth. Note that 40 percent of the world population is under the age of twenty.

To be able to ensure a reasonable standard of living, Americans will have to reduce their population and their consumption of goods and energy by one-half. When the United States runs out of oil, natural gas, and coal, it will have to rely only on renewable energy. Such renewable energy sources will be able to provide only about half of the oil equivalents now used per capita each year—slightly more than 5,000 liters of oil equivalents instead of the current 9,500 liters per capita. But as the population continues to grow and resources decline, several problems will increase.

Clearly, the current energy-population imbalance will impose drastic changes in energy, land, and water use and result in major changes in the American lifestyle. Achieving energy conservation and efficiency of all energy sources is paramount. Other major changes should include: smaller automobile size with double the gasoline efficiency; significant reductions in living space; reduction in heating, cooling, and light-energy usage; improvement in the movement of goods by energy-efficient methods; and heightened consumption of locally produced goods.

To halt the escalating imbalance between expanding population numbers and the earth’s essential natural resources, humans must control their numbers. At the same time, they must make efforts to conserve cropland, freshwater, energy, biodiversity, and the other life-supporting environmental resources. People in developed countries could contribute by reducing their high consumption of all natural resources, especially fossil fuels.

Continued rapid population growth damages the lives of all individuals and their offspring. Personal well-being, based on health as well as personal freedoms, is directly related to population numbers. If humans do not control their numbers, nature will.

Further Reading

  • Pimentel, D., ed. Biofuels, Solar and Wind as Renewable Energy Systems: Benefits and Risks. Springer: Dordrecht, the Netherlands, 2008.
  • Pimentel, D. and Pimentel, M. Food, Energy and Society. 3rd ed. Boca Raton: CRC Press (Taylor and Francis Group), 2008.
  • World Health Organization. Malnutrition Worldwide. World Health Organization, 2005. Accessed at http://www.mikeschoice.com/reports/malnutrition_worldwide.htm.

David Pimentel

David Pimentel is a professor of ecology and agricultural science at Cornell University. He has published more than six hundred scientific papers and twenty-eight books and has served on many national and government committees.

Marcia Pimentel

Marcia Pimentel is senior lecturer in the Division of Nutritional Sciences, College of Human Ecology at Cornell University. She is a nutritionist specializing in foods and nutrition, world food supplies, human ecology, and the environment. She has published more than sixty scientific papers and the following books: Food, Energy and Society (1996) and Dimensions of Food (2006).


About forty years ago, the world population was only 3.5 billion, or about half of the present population of 6.7 billion people. Most of us seem to ignore or be unaware of the magnitude of this rapid expansion and the vast changes that it is causing throughout the world. Indeed, the daily and even the …

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