David Pimentel is professor of insect ecology at the College of Agriculture and Life Sciences at Cornell University in Ithaca, New York.
Changes in the world's climate will bring major shifts in food production. In some places, temperatures will rise and rainfall will increase; in others, rainfall will decrease. In addition., coastal flooding will reduce the amount of land available for agriculture.
In general, food crops are sensitive to climate change. Such change, which affects soil temperature and moisture levels, also determines the vitality of both beneficial organisms and pests.
Due to the enormous uncertainties surrounding global climate change, estimates of cropland reductions vary widely--from 10 to 50 percent. But this much is clear: global warming is likely to alter production of rice, wheat, corn, soybeans, and potatoes--staples for billions of people and major food crops in both North America and Africa.
While climate change will have global impacts on agriculture, regional variations will be significant. Africa and North America exemplify the regional variations that may occur. These differences underscore the difficulty in proposing general strategies for adapting new agricultural technologies to deal with the climate change.
By 2030, according to one scenario, atmospheric carbon dioxide concentrations will be double pre-industrial concentrations, other greenhouse gases will increase substantially, and temperatures in North America and Africa will rise approximately 2 degrees Centigrade.
If these changes occur, projected average rainfall in central North America will be 10 percent lower than now; in eastern and northern Africa, it may be 10 percent higher. While more rain holds the promise of increasing African agricultural productivity, higher temperatures may offset this advantage by decreasing soil moisture. As a result, dry agricultural regions may continue to suffer the effects of inadequate water supplies, even if levels of rainfall increase.
Agricultural production will be affected by the severity--and pace--of climate change. If change is gradual, there will be time for political and social institutions to adjust. Slow change also may enable natural biota to adapt. However, even a minor change (for example, one-tenth of a degree per decade) could spark significant changes in the frequency of climate extremes, including heat waves, floods, and droughts.
Rapid climate change could jeopardize agriculture, forestry, and biodiversity worldwide. Compounding this problem is the fact that some African societies lack the capacity to adapt to these changes on their own.
Moreover, temperature and seasonal rainfall patterns vary from year-to-year and region-to-region, regardless of long-term trends in climate. Temperature and rainfall changes induced by climate change likely will interact with atmospheric gases, fertilizers, insects, plant pathogens, weeds, and the soil's organic matter to produce unanticipated responses.
Despite these uncertainties, an average global temperature rise of slightly more than one-half degree Centigrade would lengthen the frost-free growing season in the corn belt by two weeks. However, if temperatures continue to increase beyond a specific threshold, a crop's productive summer growing season could become shorter, thus reducing the yield.
Crops such as rice, potatoes, corn, wheat, and soybeans have optimal microclimate temperatures and an optimal growing season (see Table 1). Recognizing these optimal levels will enable farmers to alter their mix of crops in response to their region's, changing temperatures. However, turning to different crops will not guarantee that a farmer will produce the same amount of food or enjoy the same profits.
Rainfall is the major limiting factor in the growth and production of crops worldwide. Adequate moisture is critical for plants, especially during germination and fruit development. Many nations have constructed irrigation systems to pump water from rivers, lakes, and aquifers. In the United States, agriculture consumes 85 percent of the nation's "pumped" water; yet, only 12 percent of U.S. cropland is irrigated.
Crops transpire enormous amounts of water. For example, during the growing season, high-yield corn will transpire about 4.2 million liters of water per hectare. In California, the production of one kilogram of corn requires 1,400 liters of water; a kilogram of rice requires 4,700 liters; and a kilogram of cotton, 17,000 liters.
Large amounts of irrigated water require large amounts of fossil fuel. For example, to irrigate one hectare of corn by drawing water from a depth of only 30 meters requires more than twice as much energy as if the same amount of corn is rain-fed. Because of its high energy use, irrigation is costly. For example, irrigation costs for rice in Louisiana range from $68 to $239 per hectare, depending on the dimensions of the well and the source of power.
Unlike soybeans, some crops may be more tolerant of ultraviolet radiation. However, such crops also may be more susceptible to disease. For example, although wheat seems to tolerate ultraviolet radiation, "Red Hard" infection rates increased from 9 to 20 percent when experimental ultraviolet radiation was increased from 8 to 16 percent above ambient levels. Disease rates in rice also have increased when rice is exposed to higher ultraviolet radiation than normal.
Based on laboratory studies, doubling pre-industrial carbon-dioxide levels (as an isolated factor) should increase crop yields significantly. Underfield conditions, however, such carbon dioxide increases may not significantly increase the yields because other atmospheric gases surrounding the plant diminish the carbon dioxide's photosynthesis enhancement. In fact, studies indicate that yields on the farm may prove to be only one-quarter to one-third the levels reached in the laboratory.
While some plants benefit from high levels of carbon dioxide, others do not. Increased carbon-dioxide levels, for example, would benefit soybeans more than corn. In total, 16 of the world's 20 most important food crops would benefit from increased carbon-dioxide levels.
High levels of carbon dioxide also may compensate for other environmental deficiencies. For example, plants grown under high carbon dioxide can tolerate less water and still thrive. For crops such as wheat, high levels of carbon dioxide also may compensate for limited amounts of soil nitrogen. High levels of carbon dioxide, however, do not compensate for reduced availability of phosphorus or potassium.
Despite some advantages, the positive effects of carbon dioxide increases for soybeans and wheat may be offset by the related increases in moisture stress caused by low rainfall and high temperatures. Moreover, increased cloud cover due to higher global temperatures is projected to limit photosynthesis and result in reduced crop production.
Under the projected warming trend in the United States, farmers can expect a 25- to 100-percent increase in losses due to insects, depending on the crop. Because crop losses to insects in Africa are already high, the projected impact on different crops range from minus 30 percent for soybeans to plus 7 percent for rice. West Africa's warm, moist conditions are ideal for insects and crop diseases.
Under the warmer-but-drier conditions projected for North America, crop losses due to plant diseases are expected to decline as much as 30 percent. However, under the wetter conditions projected for Africa, losses to diseases will increase by more than 100 percent for some crops.
Weeds, which are better adapted to arid conditions than crops, will provide increased competition for moisture, nutrients, and light. Herbicidal controls are less effective under hot and dry conditions, but mechanical cultivation is more effective. Another problem with herbicides applied under arid conditions is that they accumulate in the soil, which can lead to serious environmental problems.
Overall, U.S. crop losses due to weeds are projected to rise from 5 to 50 percent for selected crops. Similarly, warm and moist conditions projected for Africa are expected to increase crop losses due to pests.
In addition, increased atmospheric carbon dioxide is expected to alter the nutritional makeup of crops, thereby affecting the severity of attack from insects and disease organisms.
While these impacts have not been quantified, studies indicate that certain caterpillars eat more but have a higher mortality on plants grown under high carbon-dioxide levels. Plant pathogens can be expected to react in a similar manner, because pathogens respond with increased vigor to improved nutrition in the plant. Therefore, increases in crop losses from insects and diseases because of nutritional changes in crops are projected to be minimal.
Although projected climatic conditions in Africa are different from those in North America, crop losses from pests are expected to increase because effective pest-control technologies are not readily available. There is no reason to think such technologies will be widely used in the near future.
Conservation. The high rate of soil erosion reflects the urgency of stemming soil loss, which is as threatening to sustained levels of food production as projected changes in climate. North America and Africa lose 16 to 30 tons of soil per hectare each year, and substantial amounts of nitrogen and other vital nutrients are washed or blown away with that soil.
Furthermore, optimum soil organic matter, topsoil depth, water-holding capacity, and soil biota all are reduced as soil erodes. Rainfall's beneficial impacts are reduced too due to increases in water runoff and reductions in the soil's water-holding capacity. Taken together or separately, these factors limit the soil's productivity and, as a result, can reduce crop yields from 15 to 30 percent.
When sound soil and water conservation practices are followed, erosion rates can be reduced to about one ton per hectare per year, which is approximately the rate at which soil reforms under normal agricultural conditions. Adopting such practices would increase crop yields 2 to 15 percent in North America and 5 to 15 percent in Africa (see Table 2).
Improving Crop Varieties. Some crop varieties have greater tolerance for moisture stress than current dominant crops and could be tested as substitutes for commonly used varieties. For example, switching from spring to winter wheat in Canada has been suggested as a practical way to deal with increased moisture stress.
If a crop such as corn can no longer be grown in a region because of low rainfall, crops such as sorghum, millet, cassava, sweet potato, or wheat, which have lower moisture requirements, could be substituted. However, the yield in kilograms of food produced per hectare would be lower because substitute crops usually produce less harvestable food than corn. They also may be less nutritious.
Plant breeding is expected to improve yields about 1 percent during the next half century. Biotechnology also could be used to develop new crop varieties that require slightly less water, but success will be difficult because drought tolerance is a multigenic character, and basic photosynthesis requires enormous amounts of water.
In North America, studies project about a 1-percent increase in crop yield due to the anticipated contributions of biotechnology during the next 20 years. However, no increase is projected for Africa because major advances in biotechnology are not expected to be applied there soon.
Crop Rotations. A return to crop rotations would substantially reduce soil erosion and water runoff and improve the control of insects, disease, and weeds. They are sound agricultural practices that should be widely used in agriculture.
Improved Pest Control. Because insects, diseases, and weeds destroy about 35 percent of potential crop production worldwide, use of appropriate technologies to reduce pest losses would increase crop yields. In addition to the prudent application of pesticides, increased use of nonchemical pest controls would help minimize crop losses. Nonchemical controls include crop rotations, biological controls, altering planting dates and fertilizer and irrigation applications, and soil management and tillage. These technologies could help minimize projected pest losses and thereby help maintain crop yields.
Irrigation. Irrigation could substitute for reduced rainfall, but only if abundant water and energy resources are available. Excessive groundwater withdrawal is a serious problem in the western United States, northern China, and parts of India and Mexico. Salinization and waterlogging of soil are other serious problems associated with irrigation.
All these factors are likely to limit the use of irrigation, even in the face of water shortages for agriculture. World net irrigated area per capita has been declining since 1978.
Other Technologies. Increasing soil organic matter, effectively using livestock wastes, increasing crop diversity, ridge-planting, and developing wind breaks could reduce the negative impacts of climate change on agriculture.
Individual crops vary in their response to climate change, however, making generalizations difficult. A crop-by-crop and region-by-region analysis is needed. However, moisture is vital in all crop production, and less moisture is certain to be a major limiting factor in North America. Also, heat stress may be a problem for crops such as corn and potatoes.
In eastern and central Africa especially, the picture is different due to projected higher rainfall. Increased yields of 10 to 30 percent are possible if rainfall increases and improved agricultural technologies are adopted. Even these projected increases, however, will not be sufficient to provide adequate food for Africa's growing population. Too, providing adequate food supplies in several African countries will be most difficult because of the self-serving economic policies and ineffective food aid and poverty programs.
Changes in temperature, moisture, carbon dioxide, insect pests, plant diseases, and weeds associated with global warming are projected to reduce food production in North America. The extent of alterations in crop yields will depend on each crop and its particular environmental requirements. However, improved agricultural technologies could partially offset decreased yields.
In Africa, the projected rise in rainfall associated with global warming is encouraging, especially since Africa already suffers from severe rainfall shortages. Therefore, the 10 percent increase in rainfall will help improve crop yields, but it will not solve Africa's food problems. Water shortages will persist, and serious crop losses from pests are expected to continue.
Additional research is needed on the potential impacts of climate change, including research on the temperature-induced degradation of soil, water, and biological resources and their potential impact on crop production.
Farmers in North America and Africa also need to adopt sound ecological resource management practices, especially soil and water conservation. These practices would benefit agriculture, the environment, farmers, and society, enabling agriculture to remain productive and offsetting some of the negative impacts of global warming.
1. The author wishes to thank Nick Brown, Fran Vecchio, Ariel Diaz, Veronique LaCapra, Seth Hausman, Olive Lee, John Williams, Scott Cooper, and Eric Newburger, all members of Cornell University's College of Agriculture and Life Sciences, for their help.
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