Obesity accounts  for a growing level and share of worldwide  noncommunicable diseases, including diabetes, heart disease, and certain cancers that can reduce quality of life and increase public health costs of already under-resourced developing  countries. The number of overweight children is projected to double by 2030.  Driven primarily by increasing availability of processed, affordable, and effectively marketed food, the global food system is falling short with rising obesity and related poor health outcomes. Due to established health implications and rapid increase in prevalence, obesity is now a recognized major global health challenge.

Agriculture

Agriculture is the human enterprise by which natural ecosystems are transformed into ones devoted to the production of food, fiber, and, increasingly, fuel. Given the current size of the human population, agriculture is essential. Without the enhanced production of edible biomass that characterizes agricultural systems, there would simply not be enough to eat. The land, water, and energy resources required to support this level of food production, however, are vast. Thus agriculture represents a major way in which humans impact terrestrial ecosystems.

For centuries scholars have wrestled with the question of how many people Earth can feed. In 1798 English political economist Thomas Robert Malthus published what would become one of the most famous pamphlets in social science, An Essay on the Principle of Population. Malthus proposed that because population tended to increase at a geometric (exponential) rate, while food supplies could only grow at an arithmetic rate, all living creatures tended to increase beyond their available resources.

“Man is necessarily confined in room,” Malthus argued. “When acre has been added to acre till all fertile land is occupied, the yearly increase of food must depend upon the melioration of the land already in possession. This is a fund; which, from the nature of all soils, instead of increasing must gradually be decreasing”. The resulting scarcity, he predicted, would limit human population growth through both “positive checks,” such as poverty, diseases, wars, and famines, and self-imposed “negative checks,” including late marriage and sexual abstinence.

In terms of global food production, however, Malthus has so far been proved wrong because his essay failed to take into account the ways in which agricultural productivity of cultivated lands, measured in terms of harvested (typically edible) biomass, could be enhanced. Agriculture involves the genetic modification of plant and animal species, as well as the manipulation of resource availability and species interactions. Scientific and technological advances have made agriculture increasingly productive by augmenting the resources needed to support photosynthesis and by developing plants and animals with enhanced capacity to convert such resources into a harvestable form. The outcome is that world food production has in fact kept up with rapid population growth. Gains have been especially dramatic in the past 50 years.

But these gains carry with them serious environmental costs. Large-scale agriculture has reduced biodiversity, fragmented natural ecosystems, diverted or polluted freshwater resources, and altered the nutrient balance of adjacent and downstream ecosystems. Agriculture also consumes major amounts of energy and generates greenhouse gas emissions that contribute to global climate change. However, these negative impacts must be weighed against human demand for food, as well as the fact that agriculture is the primary livelihood for 40 percent of the human population. In some countries, more than 80 percent of the population makes a livelihood from farming, so increasing agricultural productivity not only makes more food available but also increases incomes and living standards.

The future impacts of agriculture will depend on many factors, including world demand for food, the availability and cost of resources needed to support high levels of productivity, and technological advances to make agriculture more efficient. Global climate change is expected to alter temperature, precipitation, and weather patterns worldwide, thus changing many fundamental conditions that guide current agricultural practice.

Conventional Agriculture

As agriculture became increasingly dependent on technological inputs throughout the 20th century, it also underwent a structural shift, particularly in developed countries. Instead of raising a diverse mix of crops, farmers increasingly planted large holdings of one or a few crop varieties that had been developed for high yields. Monoculture makes it easier to cultivate large acreages more efficiently, especially using mechanized equipment and chemical inputs. However, these artificial ecosystems are vulnerable to outbreaks of pests and pathogens because they do not have natural protection from genetic diversity and they are typically nutrient-rich, thanks to abundant fertilizer use. Moreover, many pest species have adapted to spread rapidly in ecosystems where recent disturbances, such as plowing, have eliminated natural predators.

Agricultural pests include insects, mammals such as mice and rats, unwanted plants (weeds), fungi, and microorganisms such as bacteria and viruses. Humans have controlled pests with naturally-occurring substances such as salt, sulfur, and arsenic for centuries, but synthetic pesticides, first developed during World War II, are generally more effective.

Many of the first pesticides that were widely used for agriculture were organochlorines such as DDT (dichloro diphenyl trichloroethane), aldrin, dieldrin, and heptachlor. These substances are effective against a range of insects and household pests, but in the 1950s and 1960s they were shown to cause human health effects including dizziness, seizures, respiratory illness, and immune system dysfunction. Most organochlorines have been banned in the United States and other developed countries but remain in use in developing countries.

In her 1962 book Silent Spring, biologist and author Rachel Carson drew wide-scale public attention to the environmental effects of pesticides. Carson described how actions such as spraying elm trees with broad-spectrum pesticides to prevent Dutch elm disease severely affected many other parts of local ecosystems (Box 1).

Bioaccumulation of DDT and other organochlorines drastically reduced populations of bald eagles and other large predatory birds that fed at the top of the food chain. The pesticides disrupted birds’ reproductive systems and caused them to lay eggs with very thin shells that broke before young birds hatched.

Organochlorines were replaced in the 1970s with other pesticides that were less toxic and more narrowly targeted to specific pests. However, many of these newer options still killed off pests’ natural enemies, and when the insecticides were used repeatedly over time, pests became resistant to them through natural selection (many types of insects can develop through entire generations in days or weeks). Today hundreds of species of insects and weeds are resistant to major pesticides and herbicides.

In response some farmers have turned to methods such as releasing natural insect predators or breeding resistance into crops. For example, U.S. farmers can buy corn seeds that have been engineered to resist rootworms, corn borers, or both pests, depending on which are present locally, as well as corn that has been developed to tolerate herbicides. Others practice integrated pest management (IPM), an approach under which farmers consider each crop and pest problem as a whole and design a targeted program drawing on multiple control technologies, including pesticides, natural predators, and other methods.

In one notable case, Indonesia launched an IPM program in 1986 to control the brown planthopper, a notorious pest that lays its eggs inside rice plant stalks, out of range of pesticides. Outreach agents trained farmers to monitor their fields for planthoppers and their natural predators, and to treat outbreaks using minimal pesticide applications or alternative methods such as biological controls. Over the following decade, rice production increased by 15 percent while pesticide use fell by 60 percent. Yields on IPM lands rose from 6 to almost 7.5 tons of rice per hectare.

Plowing originally developed as a way to control pests (weeds), but created new issues in the process. Bare lands that have been plowed but have not yet developed crop cover are highly susceptible to erosion. The Dust Bowl that occurred in the United States in the 1930s was caused partly by poor agricultural practices. With support from the federal government, farmers plowed land that was too dry for farming across the Great Plains, destroying prairie grasses that held topsoil in place. When repeated droughts and windstorms struck the central and western states, hundreds of millions of tons of topsoil blew away. Today a similar process is taking place in northern China, where over-plowing and overgrazing are expanding the Gobi Desert and generating huge dust storms that scour Beijing and other large cities to the east.

Excessive plowing can also depress crop production by altering soil microbial communities and contributing to the breakdown of organic matter. To conserve soil carbon and reduce erosion, some farmers have turned to alternative practices such as no-till or direct-drill agriculture, in which crops are sown without cultivating the soil in advance. Direct drilling has been widely adopted in Australia, and some 17.5 percent of U.S. croplands were planted using no-till techniques as of the year 2000 (footnote 8).

No-till agriculture enhances soil development and fertility. It is usually practiced in combination with methods that leave crop residues on the field, which helps to preserve moisture, prevent erosion, and increase soil carbon pools. However, no-till requires an alternative strategy for weed control and thus frequently involves substantial use of herbicides and chemical means to control other pests.

Many subsistence farmers in traditional societies raise livestock along with their crops, either for their families’ use or for sale. But in industrialized nations, animal agriculture has been transformed in much the same way as crop production over the past century. Modern livestock farms are large and specialized and rely heavily on technology inputs. Like major plant crops, meat and dairy products are increasingly produced through a kind of monoculture in which farmers raise one or a few animal strains that have been bred to maximize output—hens that lay more eggs, dairy cows that produce more milk, or pigs that grow quickly and develop lean meat. Producers use technological inputs, such as antibiotics and hormone treatments, to make animals grow larger and more quickly.

To maximize efficiency, large-scale livestock farms confine animals indoors instead of letting them range outside (Fig. 10). Confining animals makes it easier to control the amount and type of feed they receive, administer medications and growth supplements, and artificially inseminate breeding females. But it also generates new management issues. Crowding stresses animals and promotes disease transmission, so many livestock farmers use antibiotics not only to treat sick animals but to prevent illnesses and promote growth. Many of these drugs are identical or similar to antibiotics used in human medicine, so their overuse threatens human health by promoting the development of drug-resistant bacterial strains that can infect humans through the food chain or via direct exposure to farm animals or wastes.

In addition, large farms accumulate massive quantities of animal waste. One cow can produce more than 40 pounds of manure per day. Manure liquefies when it is washed out of barns, so it is too heavy to transport economically over long distances. Many large farms store millions of gallons of manure onsite in tanks or lagoons (which may be lined or unlined, depending on local regulations), until it can be used on neighboring fields.

When manure leaks or spills from storage, it sends large pulses of nutrients into local water bodies, causing algal blooms that deplete dissolved oxygen in the water and kill fish when they die and decompose. Nutrient pollution also occurs when manure is applied too heavily to farmland, so that plants cannot take up all of the available nitrogen and phosphate before the manure leaches into nearby rivers and streams. Excess nutrients, mainly from agricultural runoff, are a major cause of “dead zones” in large water bodies such as the Chesapeake Bay and the Gulf of Mexico. Manure also pollutes water with bacteria, hormones, and other chemical residues from animal feed.

Large livestock farms also generate air pollution from manure, dust, and greenhouse gases produced in the digestive systems of cattle and sheep. Many people who live near animal feeding operations complain about smells and suffer physical symptoms such as burning eyes, sore throats, and nausea. A 2003 National Research Council study found that livestock farms produce many air pollutants that are significant hazards at scales ranging from local to global (Table 1). However, the report concluded that more analysis was required to develop accurate measurements of these emissions as a basis for regulations and that the United States lacked standards for quantifying odor, which could be caused by various combinations of hundreds of compounds.

Table 1. Potential importance of air emissions from animal feeding operations at different spatial scales. Source: Adapted from National Research Council, Air Emissions From Animal Feeding Operations: Current Knowledge, Future Needs (Washington, DC: National Academies Press, 2003), p. 6.

Emission	Global, national, and regional importance	Local Importance (property line or nearest dwelling)	Primary effects of concern
Ammonia (NH3)	Major	Minor	Acid rain, haze
Nitrous oxide (N2O)	Significant	Insignificant	Global climate change
Nitrogen oxides (NOX)	Significant	Minor	Haze, acid rain, smog
Methane (CH4)	Significant	Insignificant	Global climate change
Volatile organic compounds (VOCs)	Insignificant	Minor	Quality of human life
Hydrogen sulfide (H2S)	Insignificant	Significant	Quality of human life
Particulate matter (PM10)	Insignificant	Significant	Haze
Fine particulate matter (PM2.5)	Insignificant	Significant	Health, haze
Odor	Insignificant	Major	Quality of human life

World demand for meat and dairy products is increasing, driven by population growth and rising incomes in developing countries. Because of this growth and the trend toward raising animals on large-scale farms, the FAO calls livestock farming “one of the top two or three most significant contributors to the most serious environmental problems, at every scale from local to global.” According to FAO’s estimates, livestock production generates 18 percent of world greenhouse gas emissions (more than the transport sector), accounts for 8 percent of world water use, and is probably the largest sectoral water pollution source.

With global meat and dairy production predicted to roughly double between 2000 and 2050, these environmental impacts will have to be drastically reduced just to keep agricultural pollution from worsening.

Agriculture and Genetic Technology

Farmers have manipulated the genetic makeup of plants and animals since the dawn of agriculture. Initially they used selective breeding to promote qualities that made breeds readily usable for agriculture, such as animals that domesticated well and plants that were easy to harvest. Next, breeders focused on varieties that could be grown outside of their native geographic range—for example, overcoming natural photoperiod requirements (the amount of daylight that plants need to flower). In the twentieth century, plant geneticists selected for traits that would allow plants to use high levels of water and nitrogen to increase yields. Similarly, animal breeders worked to increase the amount of meat or milk that various domestic animal lines produced.

Today classical agricultural breeding is a highly quantitative science that uses genetic markers (specific DNA sequences) to select for desired characteristics. This approach enables scientists to manipulate the genetic makeup of crops with substantial precision, as long as genetic variation exists for particular traits. Agricultural breeders also use biotechnology to move genes across taxonomic barriers, combining genetic material from species that would not cross-breed naturally. For example, Bt corn has been modified by inserting a gene from the bacterium Bacillus thuringiensis that kills harmful insects so that farmers do not need to use insecticide.

Since the mid-1990s, the U.S. Department of Agriculture has approved 63 genetically engineered (GE) crops for unrestricted sale, including strains of corn, soybeans, cotton, potatoes, wheat, canola, and papaya. Most of these crops have been developed to tolerate herbicides or resist insects or fungi, while others have been engineered for specific product qualities such as longer shelf life. Products under development include grains, field crops, fruits, vegetables, trees, and flowers designed to achieve desirable growing properties such as cold or drought resistance or efficient use of nitrogen. The extent to which such strategies will be able to enhance agricultural productivity, however, remains to be seen.

An alternative use of biotechnology that some supporters advocate is to develop crops with improved nutritional content to combat nutritional disorders. One widely-publicized product is golden rice, a rice variety into which several “trans” or foreign genes have been added so that the plant produces beta-carotene (vitamin A) in its grains. Vitamin A deficiencies are widespread in societies that consume rice-based diets, causing thousands of cases of blindness and premature deaths among children in developing countries every year. Researchers are currently working to produce golden rice that contains the recommended daily allowance of vitamin A in a 100 to 200 gram serving, as well as to ensure the bioavailability of the beta-carotene contained within the modified rice grains. But not everyone is convinced by this approach: some experts argue that the same goals could be met more cheaply by promoting balanced, diverse diets in the target countries.

In addition to questioning whether agricultural and nutritional goals might be more effectively met using more traditional approaches, critics have raised many concerns about GE foods, including potential harm to nearby ecosystems and the possibility that GE crops or animals will hybridize with and alter the genetic makeup of wild species. For example, over-planting Bt-resistant crops could promote increased Bt resistance among pests, while genes from GE crops could give wild plants qualities that make them more weedy and invasive. Although most of these effects will probably be benign, it is hard to predict when and where GE species could have harmful effects on surrounding ecosystems.

A 2002 National Research Council report concluded that genetically modified plants posed the same broad types of environmental risks as conventionally-produced hybrids, like the strains introduced during the Green Revolution. For example, both kinds of plants could spread into surrounding ecosystems and compete with local species. But the report noted that either type of plant could have specific traits that posed unique threats and accordingly called for case-by-case regulation of new GE strains. The committee also observed that future generations of GE plants are likely to have multiple introduced traits and forecast that these products will raise issues that cannot be predicted based on experience with early herbicide- and pest-resistant crops.

Sustainable Agriculture

Growing concern about agricultural intensification in developed countries and its negative environmental impacts spurred an alternative movement in the 1970s to promote what advocates called sustainable agriculture. This perspective drew inspiration from sources that included organic farming (raising crops and animals with minimal synthetic inputs), the international environmental movement, and development advocates who criticized the Green Revolution for relying too heavily on pesticides and fertilizer. Ecology is a central pillar of sustainable agriculture, which treats farmed areas first and foremost as ecosystems, albeit unique ecosystems that have been disturbed and simplified by harvesting.

Few people would argue against the concept of sustainable agriculture, but there is no universally-agreed definition of what it means. Agricultural economist Gordon Conway describes sustainability as “the ability of an agroecosystem [an agricultural ecosystem and its social and economic setting] to maintain productivity in the face of stress or shock.” Farmers use countermeasures to respond to stresses and shocks. They may draw on resources that are internal to the system, such as plants’ natural pest resistance, or on outside inputs like herbicides and fertilizers.

Internal inputs typically rely on natural resources. Figure 15 shows the re-emerging practice of green manuring—tilling fresh plant material into soil to improve its physical and biological qualities. Outside inputs may be equally useful, but they usually cost more and may alter farming systems in unexpected ways—for example, introducing new species that compete with established crops.

Other formulations of sustainable agriculture, including legislation passed by the U.S. Congress in 1990, present it as a compromise between several sets of social goals, including but not limited to environmental conservation. Producing enough food, fuel, and fiber to meet human needs is a major objective, along with improving environmental quality, using non-renewable resources efficiently, and ensuring that farmers can earn reasonable livings from their products (footnote 17). In terms of methods, sustainable agriculture typically stresses treating soil as an ecosystem and using methods to keep it healthy, such as retaining organic matter and preserving diverse communities of soil organisms.

Many people equate sustainable agriculture with organic farming, which is practiced according to national legal standards in more than 60 countries, including the United States, the European Union, Britain, Canada, and Australia. Generally, organic standards bar the use of synthetic pesticides, herbicides, fertilizers, and genetically modified organisms for crop production and use of antibiotics, hormones, and synthetic feeds for animals. Organic agriculture typically has less severe environmental impacts than intensive farming with synthetic inputs. On average, organic farming conserves biodiversity, improves the structure and organic content of soil, leaches less nitrate into water bodies, and produces much less pesticide pollution.

As of 2002–2003, about 4 percent of utilized agricultural land in the European Union and up to 4 percent of farmed land for certain crops in the United States was farmed organically. Together, the United States and the E.U. account for 95 percent of global organic food sales.

Organic farming is not without its drawbacks. Output from organic farms is typically lower than from conventional agriculture for at least several years after shifting to organic production, because it takes time to restore soil productivity naturally and establish beneficial insect populations. Organic agriculture is more labor-intensive than conventional farming, so production costs are higher and farmers must receive higher prices to make a profit. And transitioning to organic production takes several years, so it is too expensive and difficult for small-scale farmers without access to technical assistance and transition funding.

With world population projected to rise from 6.5 billion in 2006 to roughly 10 billion by 2050, and growing demand for meat in developing countries (which increases demand for grain as livestock feed), world grain production may have to double in coming decades. If nations take the intensive route to this goal, using even more fertilizer, pesticides, and irrigation, nutrient pollution and freshwater depletion will increase well beyond current levels—the antithesis of sustainable agriculture (footnote 18).

One potential solution currently at the experimental stage is “precision agriculture”—using remote sensing to help farmers target fertilizer, herbicides, seeds, and water to exact locations on a field, so that resources are not over-applied or used where they are not needed. For example, satellite data could identify sectors within large cultivated fields that needed additional water or fertilizer and communicate the information to farmers driving machinery equipped with global positioning system receivers (reducing the need to apply inputs uniformly across entire fields).

More broadly, agriculture will have to become more efficient in order to double world grain production without further degrading the environment. No single innovation will provide a complete solution. Rather, feeding the world sustainably is likely to require a combination of many technological inputs and sustainable techniques.