Resource and Environmental Constraints

The leading resource and environmental constraints faced by the world's farmers include soil loss and degradation; water logging and salinity; the coevolution of pests, pathogens and hosts; and the impact of climate change. Part of my concern is with the feedback of the environmental impacts of agricultural intensification on agricultural production itself (Tilman et al., 2001).

Soil. Soil degradation and erosion have been widely regarded as major threats to sustainable growth in agricultural production in both developed and developing countries. It has been suggested, for example, that by 2050, it may be necessary to feed "twice as many people with half as much topsoil" (Harris, 1990, p. 115). However, attempts to assess the implications of soil erosion and degradation confront serious difficulties. Water and wind erosion estimates are measures of the amount of soil moved from one place to another rather than the soil actually lost. Relatively few studies provide the information necessary to estimate yield loss from erosion and degradation. Studies in the United States by the Natural Resources Conservation Service have been interpreted to indicate that if 1992 erosion rates continued for 100 years, the yield loss at the end of the period would amount to only 2 to 3 percent (Crosson, 1995a). An exceedingly careful review of the long-term relationship among soil erosion, degradation and crop productivity in China and Indonesia concludes that there has been little loss of organic matter or mineral nutrients and that use of fertilizer has been able to compensate for loss of nitrogen (Lindent, 2000). A careful renew of the international literature suggests that yield losses at the global level might be roughly double the rates estimated for the United States (Crosson, 1995b).

At the global level, soil loss and degradation are not likely to represent a serious constraint on agricultural production over the next half-century. But soil loss and degradation could become a serious constraint at the local or regional level in some fragile resource areas. For example, yield constraints due to soil erosion and degradation seem especially severe in the arid and semiarid regions of sub-Saharan Africa. A slowing of agricultural productivity growth in robust resource areas could also lead to intensification or expansion of crop and animal production that would put pressure on soil in fragile resource areas -- like tropical rain forests, arid and semiarid regions and high mountain areas. In some such areas, the possibility of sustainable growth in production can be enhanced by irrigation, terracing, careful soil management and changes in commodity mix and farming systems (Lal, 1995; Smil, 2000; Niemeijer and Mazzucato, 2000).

Water. During the last half-century, water has become a resource of high and increasing value in many countries. In the arid and semiarid areas of the world, water scarcity is becoming an increasingly serious constraint on growth of agricultural production (Seckler, Molden and Barker, 1999; Raskin et al., 1998: Gleick, 2000). During the last half-century, irrigated area in developing countries more than doubled, from less than 100 million hectares to more than 200 million hectares. About half of developing country grain production is grown on irrigated land. The International Water Management Institute had projected that by 2025, most regions or countries in a broad sweep from north China across east Asia to north Africa and northern sub-Saharan. Africa will experience either absolute or severe water scarcity.

Irrigation systems can be a double-edged answer to water scarcity, since they may have substantial spillover effects or externalities that affect agricultural production directly. Common problems of surface water irrigation systems include water logging and salinity resulting from excessive water use and poorly designed drainage systems (Murgai, Ali and Byerlee, 2001). In the Aral Sea basin in central Asia, the effects of excessive water withdrawal for cotton and rice production, combined with inadequate drainage facilities, has resulted in such extensive water logging and salinity, as well as contraction of the Aral Sea. that the economic viability of the entire region is threatened (Glazovsky, 1995). Another common externality results from the extraction of water from underground aquifers in excess of the rate at which the aquifers are naturally recharged, resulting in a falling groundwater level and rising pumping costs. In some countries, like Pakistan and India, these spillover effects have in some cases been sufficient to offset the contribution of expansion of irrigated area to agricultural production.

However, the lack of water resources is unlikely to become a severe constraint on global agricultural production in the next half-century. The scientific and technical efforts devoted to improvement in water productivity have been much more limited than efforts to enhance land productivity (Molden, Amarasinghe and Hussain, 2004), so significant productivity improvements in water use are surely possible. Institutional innovations will be required to create incentives to enhance water productivity (Saleth and Dinar, 2006). But in 50 to 60 of the world's most arid countries, plus major regions in several other countries, competition from household, industrial and environmental demands will reallocate water away from agricultural irrigation. In many of these countries, increases in water productivity and changes in farming systems will permit continued increases in agricultural production. In other countries, the reduction in irrigated area will cause a significant constraint on agricultural production. Since these countries are among the world's poorest, some will have great difficulty in meeting food security needs from either domestic production or food imports.

Pests. Pest control has become an increasingly serious constraint on agricultural production in spite of dramatic advances in pest control technology. In the United States, pesticides, have been the most rapidly growing input in agricultural production over the last half-century. Major pests include pathogens, insects and weeds. For much of the post-World War II era, pest control has meant application of chemicals. Pesticidal activity of Dichlorodiphenyl-trichloroethane (DDT) was discovered in the late 1930s. It was used in World War II to protect American troops against typhus and malaria. Early tests found DDT to be effective against almost all insect species and relatively harmless to humans, animals and plants. It was relatively inexpensive and effective at low application levels. Chemical companies rapidly introduced a series of other synthetic organic pesticides in the 1950s (Rutlan, 1982; Palladino, 1996). The initial effectiveness of DDT and other synthetic organic chemicals for crop and animal pest control after World War II led to the neglect of other pest control strategies.

By the early 1960s, an increasing body of evidence suggested that the benefits of the synthetic organic chemical pesticides introduced in the 1940s and 1950s were. obtained at substantial cost. One set of costs included the direct and indirect effects on wildlife populations and on human health (Carson, 1962; Pingali and Roger, 1995). A second set of costs involved the destruction of beneficial insects and the emergence of pesticide resistance in target populations. A fundamental problem in efforts to develop methods of control for pests and pathogens is that the control о results in evolutionary selection pressure for the emergence of organisms that are resistant to the control technology (Palumbi, 2001). When DDT was introduced in California to control the cottony cushions scale, its predator, the vedelia beetle, turned out to be more susceptible to DDT than the scale. In 1947, just one year after the introduction of DDT, citrus growers were confronted with a resurgence of the scale population. In Peru, the cotton bollworm quickly built up resistance to DDT and to the even more effective -- and more toxic to humans -- organo-phospate insecticides that were adopted to replace DDT (Palladino, 1996, pp. 36-41).

The solution to tlie pesticide crisis offered by the entomological community was Integrated Pest Management (IPM). IPM involved the integrated use of an array of pest control strategies: making hosts more resistant to pests, finding biological controls for pests, cultivation practices and also chemical control, if needed. At the time Integrated Pest Management began to be promoted in tlie 1960s, it represented little more than a rhetorical device. But by the 1970s, a number of important Integrated Pest Management programs had been designed and implemented. However, exaggerated expectations that dramatic reductions in chemical pesticide use could be achieved without significant decline in crop yields as a result of Integrated Pest Management have yet only been partially realized (Gianessi, 1991; Lewis et al„ 1977).

My own judgment is that the problem of pest and pathogen control will represent a more serious constraint on sustainable growth in agricultural production at a global level than either land or water constraints. In part, this is because die development of pest and pathogen resistant crop varieties and chemical methods of control both tend to induce the evolution of more resistant pests or pathogen. In addition, international travel and trade are spreading the newly resistant pests and pathogens to new environments. As a result, pest control technologies must constantly be replaced and updated. The coevolution of pathogens, insect pests and weeds in response to control efforts will continue to represent a major factor in directing the allocation of agricultural research resources to assuring that agricultural output can be maintained at present levels or continue to grow.

Climate. Measurements taken in Hawaii in the late 1950s indicated that carbon dioxide (CO2,) was increasing in the atmosphere. Beginning in the late 1960s, computer model simulations indicated possible changes in temperature and precipitation that could occur due to human-induced emission of CO2 and other "greenhouse gases" into the atmosphere. By the early 1980s, a fairly broad consensus had emerged in the climate change research community that energy production and consumption from fossil fuels could, by 2050, result in a doubling of the atmospheric concentration of CO2, a rise in global average temperature by 2. 5 to 4. 5 C (2. 7 to 8. 0 F) and a complex pattern of worldwide climate change (Ruttan, 2006, pp. 515-520).

Since the mid-1980s, a succession of studies has attempted to assess how an increase in the atmospheric concentration of greenhouse gases could affect agricultural production through three channels: a) higher CO2 concentrations in the atmosphere may have a positive "fertilizer effect" on some crop plants (and weeds); b) higher temperatures could result in a rise in the sea level, resulting in inundation of coastal areas and intrusion of saltwater into groundwater aquifers; and c) changes in temperature, rainfall and sunlight may also alter agricultural production, although the effects will vary greatly across regions. Early assessments of the impact of climate change on global agricultural suggested a negative annual impact in the 2 to 4 percent range by the third decade of this century (Parry, 1990). More recent projections are more optimistic (Mendelsohn, Nordhaus and Shaw, 1994; Rosenzweig and Hillel, 2003). The early models have been criticized for a "dumb farmer" assumption--they did not incorporate how farmers would respond to climate change with different crops and growing methods. Efforts to incorporate -- how public and private suppliers of knowledge and technology might adjust to climate change are just beginning (Evenson, 2003). But even the more sophisticated models have been Unable to incorporate the synergistic interactions among climate change, soil loss and degradation, ground and surface water storage and the incidence of pests and pathogens. These interactive effects could combine into a significantly larger burden on growth in agricultural production than the effects of each constraint considered separately. One thing that is certain is that a country or region that has not acquired substantial agricultural research capacity will have great difficulty in responding to anticipated climate change impacts.

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