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Updated 11 November 2004

Consequences Vol. 1, No. 2, Summer 1995








Potential Impacts of Climate Change on Agriculture and Food Supply

by Cynthia Rosenzweig and Daniel Hillel

It seems obvious that any significant change in climate on a global scale should impact local agriculture, and therefore affect the world's food supply. Considerable study has gone into questions of just how farming might be affected in different regions, and by how much; and whether the net result may be harmful or beneficial, and to whom. Several uncertainties limit the accuracy of current projections. One relates to the degree of temperature increase and its geographic distribution. Another pertains to the concomitant changes likely to occur in the precipitation patterns that determine the water supply to crops, and to the evaporative demand imposed on crops by the warmer climate. There is a further uncertainty regarding the physiological response of crops to enriched carbon dioxide in the atmosphere. The problem of predicting the future course of agriculture in a changing world is compounded by the fundamental complexity of natural agricultural systems, and of the socioeconomic systems governing world food supply and demand.

What happens to the agricultural economy in a given region, or country, or county, will depend on the interplay of the set of dynamic factors specific to each area. Scientific studies, typically based on computer models, have for some time examined the effects of postulated climate and atmospheric carbon dioxide changes on specific agroecosystems--a now common term that defines the interactive unit made up of a crop community, such as a field of wheat or corn, and its biophysical environment. We have more recently gone a step farther by developing methods to study these systems in more integrated regional and global contexts. Both biophysical and socioeconomic processes are taken into account in these integrated studies, since agricultural production is a player in both worlds: it is very much dependent upon environmental variables and is in turn an important agent of environmental change and a determinant of market prices.

Climate change presents crop production with prospects for both benefits and drawbacks, some of which are shown schematically in Figure 1. To address any of them more clearly we must first define the main interactions that link a chain of processes together: food is derived from crops (or from animals that consume crops); crops in turn grow in fields, which exist in farms, which are components of farming communities, which are sectors in nation states, and which ultimately take part in the international food trade system. Understanding the potential impacts of global environmental change on this sequence of interlocking elements is a first step in modeling what will happen when any one of them is changed as a result of possible global warming, and a prerequisite for defining appropriate societal responses.

In this summary we look first at the possible biophysical responses of agroecosystems to the specific environmental changes that are anticipated as a result of the buildup of global greenhouse gases, and then at the range of adaptive actions that might be taken to ameliorate their effects. In subsequent sections we draw on our own and other modeling studies to show examples of regional and global assessments that have so far been made, including discussions of the effects of uncertainty, thresholds, and surprises, and the possible consequences of global warming on agricultural sustainability and food security. Finally we give our own views on two potentially misleading notions regarding climate change and agriculture.

Anticipated Responses of Agroecosystems

Effects of enhanced CO2 on crop growth

Plants grow through the well-known process of photosynthesis, utilizing the energy of sunlight to convert water from the soil and carbon dioxide from the air into sugar, starches, and cellulose--the carbohydrates that are the foundations of the entire food chain. CO2 enters a plant through its leaves. Greater atmospheric concentrations tend to increase the difference in partial pressure between the air outside and inside the plant leaves, and as a result more CO2 is absorbed and converted to carbohydrates. Crop species vary in their response to CO2. Wheat, rice, and soybeans belong to a physiological class (called C3 plants) that respond readily to increased CO2 levels. Corn, sorghum, sugarcane, and millet are C4 plants that follow a different pathway. The latter, though more efficient photosynthetically than C3 crops at present levels of CO2, tend to be less responsive to enriched concentrations. Thus far, these effects have been demonstrated mainly in controlled environments such as growth chambers, greenhouses, and plastic enclosures. Experimental studies of the long-term effects of CO2 in more realistic field settings have not yet been done on a comprehensive scale.

Higher levels of atmospheric CO2 also induce plants to close the small leaf openings known as stomates through which CO2 is absorbed and water vapor is released. Thus, under CO2 enrichment crops may use less water even while they produce more carbohydrates. This dual effect will likely improve water-use efficiency, which is the ratio between crop biomass and the amount of water consumed. At the same time, associated climatic effects, such as higher temperatures, changes in rainfall and soil moisture, and increased frequencies of extreme meteorological events, could either enhance or negate potentially beneficial effects of enhanced atmospheric CO2 on crop physiology.

Effects of higher temperature

In middle and higher latitudes, global warming will extend the length of the potential growing season, allowing earlier planting of crops in the spring, earlier maturation and harvesting, and the possibility of completing two or more cropping cycles during the same season. Crop-producing areas may expand poleward in countries such as Canada and Russia, although yields in higher latitudes will likely be lower due to the less fertile soils that lie there. Many crops have become adapted to the growing-season daylengths of the middle and lower latitudes and may not respond well to the much longer days of the high latitude summers. In warmer, lower latitude regions, increased temperatures may accelerate the rate at which plants release CO2 in the process of respiration, resulting in less than optimal conditions for net growth. When temperatures exceed the optimal for biological processes, crops often respond negatively with a steep drop in net growth and yield. If nighttime temperature minima rise more than do daytime maxima--as is expected from greenhouse warming projections--heat stress during the day may be less severe than otherwise, but increased nighttime respiration may also reduce potential yields. Another important effect of high temperature is accelerated physiological development, resulting in hastened maturation and reduced yield.

Available water

Agriculture of any kind is strongly influenced by the availability of water. Climate change will modify rainfall, evaporation, runoff, and soil moisture storage. Changes in total seasonal precipitation or in its pattern of variability are both important. The occurrence of moisture stress during flowering, pollination, and grain-filling is harmful to most crops and particularly so to corn, soybeans, and wheat. Increased evaporation from the soil and accelerated transpiration in the plants themselves will cause moisture stress; as a result there will be a need to develop crop varieties with greater drought tolerance.

The demand for water for irrigation is projected to rise in a warmer climate, bringing increased competition between agriculture--already the largest consumer of water resources in semiarid regions--and urban as well as industrial users. Falling water tables and the resulting increase in the energy needed to pump water will make the practice of irrigation more expensive, particularly when with drier conditions more water will be required per acre. Some land--such as the region of the U.S. supplied by the Ogallala aquifer (including parts of Nebraska, Oklahoma, Texas, Colorado, and New Mexico)--may be taken out of irrigation, following a trend that has already begun, with loss of considerable prior investment. Peak irrigation demands are also predicted to rise due to more severe heat waves. Additional investment for dams, reservoirs, canals, wells, pumps, and piping may be needed to develop irrigation networks in new locations. Finally, intensified evaporation will increase the hazard of salt accumulation in the soil.

Climate variability

Extreme meteorological events, such as spells of high temperature, heavy storms, or droughts, disrupt crop production. Recent studies have considered possible changes in the variability as well as in the mean values of climatic variables. Where certain varieties of crops are grown near their limits of maximum temperature tolerance, such as rice in Southern Asia, heat spells can be particularly detrimental. Similarly, frequent droughts not only reduce water supplies but also increase the amount of water needed for plant transpiration.

Soil fertility and erosion

Higher air temperatures will also be felt in the soil, where warmer conditions are likely to speed the natural decomposition of organic matter and to increase the rates of other soil processes that affect fertility. Additional application of fertilizer may be needed to counteract these processes and to take advantage of the potential for enhanced crop growth that can result from increased atmospheric CO2. This can come at the cost of environmental risk, for additional use of chemicals may impact water and air quality. The continual cycling of plant nutrients--carbon, nitrogen, phosphorus, potassium, and sulfur--in the soil-plant-atmosphere system is also likely to accelerate in warmer conditions, enhancing CO2 and N2O greenhouse gas emissions.

Nitrogen is made available to plants in a biologically usable form through the action of bacteria in the soil. This process of nitrogen fixation, associated with greater root development, is also predicted to increase in warmer conditions and with higher CO2, if soil moisture is not limiting. Where they occur, drier soil conditions will suppress both root growth and decomposition of organic matter, and will increase vulnerability to wind erosion, especially if winds intensify. An expected increase in convective rainfall--caused by stronger gradients of temperature and pressure and more atmospheric moisture--may result in heavier rainfall when and where it does occur. Such "extreme precipitation events" can cause increased soil erosion.

Pests and diseases

Conditions are more favorable for the proliferation of insect pests in warmer climates. Longer growing seasons will enable insects such as grasshoppers to complete a greater number of reproductive cycles during the spring, summer, and autumn. Warmer winter temperatures may also allow larvae to winter-over in areas where they are now limited by cold, thus causing greater infestation during the following crop season. Altered wind patterns may change the spread of both wind-borne pests and of the bacteria and fungi that are the agents of crop disease. Crop-pest interactions may shift as the timing of development stages in both hosts and pests is altered. Livestock diseases may be similarly affected. The possible increases in pest infestations may bring about greater use of chemical pesticides to control them, a situation that will require the further development and application of integrated pest management techniques.

Sea-level rise

Global warming is predicted to lead to thermal expansion of sea water, along with partial melting of land-based glaciers and sea-ice, resulting in a rise of sea level which may range from 0.1 to 0.5 meters (4 to 20 inches) by the middle of the next century, according to present estimates of the Intergovernmental Panel on Climate Change (IPCC). Such a rise could pose a threat to agriculture in low- lying coastal areas, where impeded drainage of surface water and of groundwater, as well as intrusion of sea water into estuaries and aquifers, might take place. In parts of Egypt, Bangladesh, Indonesia, China, the Netherlands, Florida, and other low-lying coastal areas already suffering from poor drainage, agriculture is likely to become increasingly difficult to sustain. Some island states are particularly at risk.


A wide variety of adaptive actions may be taken to lessen or overcome adverse effects of climate change on agriculture. At the level of farms, adjustments may include the introduction of later- maturing crop varieties or species, switching cropping sequences, sowing earlier, adjusting timing of field operations, conserving soil moisture through appropriate tillage methods, and improving irrigation efficiency. Some options such as switching crop varieties may be inexpensive while others, such as introducing irrigation (especially high-efficiency, water-conserving technologies), involve major investments. Economic adjustments include shifts in regional production centers and adjustments of capital, labor, and land allocations. For example, trade adjustments should help to shift commodity production to regions where comparative advantage improves; in areas where comparative advantage declines, labor and capital may move out of agriculture into more productive sectors. Studies combining biophysical and economic impacts show that, in general, market adjustments can indeed moderate the impacts of reduced yields.

A major adaptive response will be the breeding of heat- and drought-resistant crop varieties by utilizing genetic resources that may be better adapted to new climatic and atmospheric conditions. Collections of such genetic resources are maintained in germ-plasm banks; these may be screened to find sources of resistance to changing diseases and insects, as well as tolerances to heat and water stress and better compatibility to new agricultural technologies. Crop varieties with a higher harvest index (the fraction of total plant matter that is marketable) will help to keep irrigated production efficient under conditions of reduced water supplies or enhanced demands. Genetic manipulation may also help to exploit the beneficial effects of CO2 enhancement on crop growth and water use.

Recent studies by the National Research Council and other organizations have emphasized the ability of U.S. farming to adapt to changing conditions, since in the past technological improvements have indeed been developed and put into use when needed. The U.S. has substantial agricultural research capabilities and a wide range of adaptation options is currently available to farmers in this country. Hence, insofar as the U.S. is concerned, prospects for agricultural adaptation to climate change appear favorable, assuming water is available. Considerable investments may be needed, however, to utilize soil and water resources more efficiently in a changed climate. Other countries, particularly in the tropics and semi-tropics, are not so well provisioned with respect to both the research base and the availability of investment capital.

Limits to adaptation

The potential for adaptation should not lead to complacency. Agricultural adaptation to climatic variation is not now and may never be perfect, and changes in how farmers operate or in what they produce may cause significant disruption for people in rural regions. Indeed, some adaptive measures may have detrimental impacts of their own. For example, were major shifts in crops to be made, as from grain to fruit and vegetable production, farmers may find themselves more exposed to marketing problems and credit crises brought on by higher capital and operating costs. The considerable social and economic costs that can result from large- scale climatic extremes was exemplified by the consequences of the Mississippi River flood of 1993.

While changes in planting schedules or in crop varieties may be readily adopted, modifying the types of crops grown does not ensure equal levels of either food production or nutritional quality. Nor can it guarantee equal profits for farmers. Expanded irrigation may lead to groundwater depletion, soil salinization, and waterlogging. Increased demand for water by competing sectors may limit the viability of irrigation as an adaptation to climate change. Expansion of irrigation as a response to climate change will be difficult and costly even under the best circumstances. Mounting societal pressures to reduce environmental damage from agriculture will likely foster an increase in protective regulatory policies that can further complicate the process of adaptation.

Present agricultural institutions and policies in the U.S. tend to discourage farm management adaptation strategies, such as altering the mix of crops that are grown. At the policy level, obstacles to change are created by supporting prices of crops that are not well suited to a changing climate, by providing disaster payments when crops fail, and by restricting competition through import quotas. Programs could be modified to expand the flexibility allowed in crop mixes, to remove institutional barriers to the development of water markets, and to improve the basis for crop disaster payments.

Adaptation cannot be taken for granted: improvements in agriculture have always depended upon on the investment that is made in agricultural research and infrastructure. It would help to identify, through research, the specific ways that farmers now adapt to present variations in climate. Do farmers attempt to compensate for a less favorable climate by applying more fertilizer, more machinery, or more labor? Information of this nature is needed to assess potentialities for coping with more drastic climate change. Success in adapting to possible future climate change will depend on a better definition of what changes will occur where, and on prudent investments, made in timely fashion, in adaptation strategies.

Regional and Global Assessments

In studying the impacts of climate change, attempts are made to link state-of-the-art models developed by researchers in disparate disciplines--including climatology, agronomy, and economics--in order to project future food supplies. Present global circulation models, or GCMs, calculate the temporal and spatial transports and exchanges of heat and moisture throughout the Earth's surface and atmosphere. These models are used to predict changes in temperature, precipitation, radiation, and other climate variables caused by increases in greenhouse gases in the atmosphere. They are used as well to develop "practice climates" or climate change scenarios for use in impact studies. Crop models then predict the response of specific crops to alternative sets of climate and CO2 conditions. Results in terms of changed crop yields and water use are then subjected to an economic analysis based on a linked model system of international food trade. Such comprehensive, interdisciplinary research is needed to improve our understanding of the interactive biophysical and socioeconomic effects that may result from global environmental change. At the same time, however, the superposition of model upon model, each with its own range of inaccuracy, amplifies the overall range of uncertainty in the final result.

The GCM-based assessment of the IPCC contemplates a change in global surface temperature of 1.5 to 4.5°C by the year 2050, as a result of enhanced greenhouse gases. While global agricultural production may increase at the lower limit of the predicted range or decrease at the higher limit, global effects measured with current economic valuation techniques are generally predicted to be moderate. The reason is that the world economic system has been generally effective in fostering adaptation to current biophysical constraints on crop production and in realizing opportunities for improving crop production. This macroeconomic perspective, however, speaks only to the averaged global effect and not to specific regional and social impacts. Model studies done to date concur that there will be significant changes in regional agricultural patterns as a result of climate change. All regions are likely to be affected, but some regions will be impacted more adversely than others. The timing of regional effects--who gains or loses when and for how long--will also be complex, as is illustrated in Figure 2 in terms of modeled changes in country-by-country wheat yield.

Winners and losers

Modeled studies of the sensitivity of world agriculture to potential climate change have suggested that the overall effect of moderate climate change on world food production may be small, as reduced production in some areas is balanced by gains in others. The same studies find, however, that vulnerability to climate change is systematically greater in developing countries--which in most cases are located in lower, warmer latitudes. In those regions, cereal grain yields are projected to decline under climate change scenarios, across the full range of expected warming. Agricultural exporters in middle and high latitudes (such as the U.S., Canada, and Australia) stand to gain, as their national production is predicted to expand, and particularly if grain supplies are restricted and prices rise. Thus, countries with the lowest income may be the hardest hit.

Yet, not all impacts in developing countries may be negative. Inland areas located far from sources of precipitation may suffer increased aridity, whereas areas in the path of rain-bearing winds may benefit from increased rainfall. A point that needs to be stressed is that the ability of any country to take advantage of the opportunities and to avoid the drawbacks as climate changes will depend on the availability of adequate resources as well as on the quality of the research base. The presently inadequate capacity of agricultural research systems in the tropics and semi-tropics will need to be rectified, and this task can best be achieved through international cooperation.

Uncertainty, Thresholds and Surprises

Some observers believe that climate change will exert its influence so slowly--a fraction of a degree per decade--that the effects will be barely noticeable in the midst of other technological and economic changes. Others emphasize the need to study the potential for what are called "threshold effects"--i.e., the abrupt and disproportionate shifts in production that may be triggered when critical levels of certain factors are surpassed. Unexpected consequences or "surprises" may well accompany the buildup of greenhouse gases. Even if climate changes gradually, it will slowly affect the range of options available for agriculture in any given region. Under changing climate conditions, farmers' past experience will be a less reliable predictor of what is to come. These and other uncertainties must be taken into account explicitly in climate change impact studies.


The uncertainty inherent in predictions is a very important feature of climate change impact studies, and work has begun to develop explicit methods to deal with the concept. Earlier studies had often used "best estimate" scenarios that were based on the mid-points of the predicted range of expected change in temperature, precipitation, or other parameters. Including the entire range from the upper to the lower bounds of predicted effects is a more prudent and realistic approach, which may clarify the way that uncertainty can propagate throughout a modeled or a real system.

Other uncertainties derive from the fast pace and unpredictable directions of future social, economic, political, and technical changes. The world of the coming century will be different in many ways; unforeseeable developments in other sectors may change the way in which agriculture responds to climate change. Questions regarding population (i.e., for how many people need the world's agricultural system provide?) and technological change (can productivity continue to improve?) are particularly relevant and should be explored with upper and lower bounds of possible projections.


Some effects, such as the flooding of a river or the withering of a crop, come into play only after certain limiting conditions or thresholds have been crossed. The identification of thresholds in climate change impact research involves analyzing the effects of different levels of climate forcing on an agroecosystem to identify the critical conditions under which the response of crops will abruptly change. These critical levels can involve either natural or socioeconomic factors, and both should be considered. For example, in the biophysical domain threshold temperatures have been defined for many specific crop processes, notwithstanding the complexity of interactions among temperature, amount and duration of sunlight, nutrients, and water supply. Crop models have been developed accordingly to test the combined effects of environmental variables on crop growth and yield.

In the socioeconomic domain, defining critical levels of warming is even more challenging, due to the intricate interplay of supply, demand, and prices, and to the characteristic adaptability of agriculture as a managed human system. Here, determining critical levels of warming involves defining relative impacts on producers and consumers in diverse geographic and social groups.

The critical levels of climatic change that affect crop yields can be identified through computer-based sensitivity tests and crop models. Results of a crop modeling study that estimated the effects of a 2°C and 4°C temperature rise on yields of wheat, rice, corn, and soybeans are shown in Fig. 3. They were derived by first modeling the simulated effect on crop yields for a wide range of latitudes and then applying what was found to current production, nation by nation, to derive a result for the world as a whole. When only temperature effects were considered, aggregate crop yields showed an ever increasing drop in response to higher temperatures, with loss in yields approximately doubling from the +2 to +4°C cases. When the direct physiological effects of CO2 on crop growth and water use were included, the picture changed, but only for the lower temperature increase: a 2°C temperature rise increased aggregated crop yields on a global basis, while a 4°C rise led to an overall decrease, as is shown in the figure. The salutary effect of a 2° increase did not apply throughout the world: in some modeled locations in semi-arid and subtropical regions, even a 2° rise resulted in diminished yield. These results suggest the existence of a possible temperature threshold affecting global grain yields, given current crop varieties and crop management techniques.


An even more challenging task is to estimate the probability of coincidental events that might happen in conjunction with global warming, spanning the range between low probability catastrophic events (called "surprises") and higher probability gradual changes in climate and associated environmental effects. A seemingly small change in one variable--for example, rainfall--may trigger a major unsuspected change in another; for example, droughts or floods might possibly disrupt the transport of grain on rivers. Moreover, one "surprise" may then lead to another in a cascade, since biophysical and social systems are interconnected. Computer-aided studies based on what are called complex systems and chaos theory may provide conceptual and analytical tools for anticipating and preparing for surprises, in agriculture as in other systems.

Identifying potential surprises and communicating them to the public and to policy makers may help build the resilience that is needed to anticipate and mitigate harmful effects in timely fashion. Surprises related to global climate change may be both environmental and societal. Among the first of these are changes in patterns of atmospheric circulation and precipitation on the seasonal- to-interannual time scale, such as might result from varying patterns of El Niñ o events in the eastern equatorial Pacific. Such inter- seasonal variations (rather than the very gradual change in long- term averages) are likely to be the climatic effects that farmers actually feel in their year-to-year operations. Among the second are increases in the migration of people across national borders in consequence of famine.

Such events can be better accommodated if their causes and potential effects are anticipated in advance. Their study can be aided by efforts to integrate across conventional scientific disciplines, to support a variety of research approaches, and to consider results that lie outside the range of conventional wisdom and experience. Beyond the theoretical study of environmental surprise, it seems also worthwhile to increase the flexibility of social structures with a view to reducing vulnerability to abrupt perturbations. Such societal preparedness might include an intentional diversification of productive and technological systems (such as provision for reserve rangeland and supplementary irrigation for the eventuality of drought), the establishment of disaster coping and entitlement systems, and the creation of management systems that are capable of adapting to and learning from surprises. Adjustments in livestock populations represent one of the first lines of defense against the surprises that can result from short-term fluctuations in crop production.

Sustainability and Food Security

Agriculture is not a wholly benign actor on the environment, as it causes accelerated soil erosion by water and wind, through cultivation, and often introduces nitrates and other chemicals into water supplies through the application of chemical fertilizers and pesticides. The concept of "sustainable agriculture" endeavors to reduce chemical inputs and energy use in farming systems, in order to minimize environmental damage and to ensure longer-term productivity. Most agricultural assessments of global environmental change made to date have not focused explicitly on sustainability issues, and have neglected the considerable impacts of shifting agricultural zones, alterations in commercial fertilizer and pesticide use, and changes in the demand for water resources.

Climate change can impact agricultural sustainability in two interrelated ways: first, by diminishing the long-term ability of agroecosystems to provide food and fiber for the world's population; and second, by inducing shifts in agricultural regions that may encroach upon natural habitats, at the expense of floral and faunal diversity. Global warming may encourage the expansion of agricultural activities into regions now occupied by natural ecosystems such as forests, particularly at mid- and high-latitudes. Forced encroachments of this sort may thwart the processes of natural selection of climatically-adapted native crops and other species.

While the overall, global impact of climate change on agricultural production may be small, regional vulnerabilities to food deficits may increase, due to problems of distributing and marketing food to specific regions and groups of people. For subsistence farmers, and more so for people who now face a shortage of food, lower yields may result not only in measurable economic losses, but also in malnutrition and even famine.

Agriculture as a Greenhouse Gas Contributor

The role of climate as a determinant of agriculture has long been recognized. It is only in the last decade, however, that the reciprocal effect has come to light: the role of agriculture as a potential contributor to climate change. Clearing forests for fields, burning crop residues, submerging land in rice paddies, raising large herds of cattle and other ruminants and fertilizing with nitrogen, all release greenhouse gases to the atmosphere. The main gases emitted are CO2, CH4, and N2O. From about 1700 to 1900, the clearing of northern hemisphere forests for agriculture was the largest agent of change in the carbon cycle. Emissions from agricultural sources are believed to account for some 15% of today's anthropogenic greenhouse gas emissions. Land use changes, often made for agricultural purposes, contribute another 8% or so to the total. As a result, agriculture ranks third after energy consumption (which is also in part agricultural) and chlorofluorocarbon production as a contributor to the enhanced greenhouse effect.

Emissions of greenhouse gases from agricultural sources are likely to increase in the years ahead, given the necessity to expand food production in order to provide for the world's growing population. This imposes a task upon agricultural researchers to devise ways to continue improving yields while at the same time holding down emissions. Some possible improvements include reducing land- clearing and biomass burning in the tropics; managing rice paddies and livestock so as to reduce methane emissions; and improving fertilizer-use efficiency to reduce the conversion of nitrogen to gaseous N2O.

Much research is still needed to understand the processes by which greenhouse gases are emitted from different agricultural practices. Needed as well are efforts to disseminate the knowledge gained in order to apply it on the farm. Reductions in some gases are likely to be more easily achievable than in others, and appropriate strategies will vary by region. The task of reducing emissions will doubtlessly be complicated by accompanying changes in climate variables such as temperature and wind and precipitation, that interact with the processes through which greenhouse gases are released.

In Closing: An Opinion

We have attempted in this review to describe in general terms the possible agricultural consequences of the enhanced greenhouse effect. It seems also appropriate to address two specific notions that are often heard.

The first is the simplistic application of the concept of thresholds to the policy arena: namely, the setting of arbitrary levels for atmospheric trace gas concentrations, emission rates, or temperatures to serve as upper limits of acceptability for policy response. The term "threshold" is misleading whenever artificially contrived levels are specified rather than natural thresholds.

Proponents of this approach contend that such levels, if generally agreed upon, can serve as quantitative criteria or guideposts for directing national as well as international efforts to contain potentially harmful consequences of the greenhouse effect. The concept is, in fact, a double-edged sword, since it can be used to either justify or to delay societal action on the issue of global warming. The shadow side of such a policy is the implication, however unintended, that amounts under the specified levels are harmless, and that the consequences of the enhanced greenhouse effect do not become manifest or significant until these artificial levels are exceeded. Misconstrued, this concept can give license to continue "business-as-usual," with no need for societal action until the arbitrary level is about to be exceeded.

A more prudent principle, in our view, is the quite plausible assumption that global warming and its manifestations will be in some manner proportionate to the increase of trace gas concentrations and that the eventual consequences of any significant human alteration of the Earth's energy balance is potentially serious. This principle, were it accepted, would encourage responsible agencies to adopt a policy aimed at reversing current trends rather than implicitly sanctioning the continued enhancement of greenhouse gas emissions until such time as the warming effect becomes clearly evident.

The second notion, which can be equally misleading, is a blind faith in agriculture as a self-correcting process: that through forces of the market and self-preservation farmers can and will readily and fully adapt to climate change as it occurs. They will certainly make every effort to do so, but the efforts of farmers may well be constrained or even thwarted by factors beyond their control.

In the tropics, inadequate agricultural research, training, and credit now limit the capacity of farmers to adapt to climate change. In all areas of the world the necessary adjustments (such as substituting crops, introducing or intensifying irrigation, and modifying field operations such as tillage or pest control) may be too costly for many farmers to implement. Such changes may entail painful social dislocations as well as costly capital investments. Even for those who can afford such changes, the end result--measured in terms of production and income--will not necessarily compensate for the direct costs involved; heat-tolerant and especially drought-tolerant crops or varieties, for example, will likely have lower yielding potentials. Moreover, natural ecosystems such as forests may be less adaptable than agricultural systems to rapid change and may therefore prove more vulnerable to climate change with respect to such factors as species dieback and biodiversity.

Either of these potentially misleading notions, along with the convenient expectation by some plant scientists that the physiological effects of enhanced CO2 will be overwhelmingly positive, may lull decision makers and the public at large into complacency regarding global warming and--at the very least--could delay effective action. Global warming is, in our opinion, a real phenomenon that is likely to engender serious consequences.

Reviewed by Vernon Ruttan and William Easterling

Professor Vernon Ruttan is Regents Professor in the Department of Agricultural and Applied Economics and in the Department of Economics at the University of Minnesota in St. Paul. His research has been on the economics of technical change, agricultural development, and research policy. He has been elected a Fellow of the American Society of Arts and Sciences and to membership in the National Academy of Sciences.

Dr. William E. Easterling, a geographer, directs the Great Plains Regional Center for Global Environmental Change and is an associate professor of Agricultural Meteorology at the University of Nebraska in Lincoln. His research interests are in the interactions of agriculture, renewable natural resources, and climate in environments of stress.

For Further Reading

Agriculture, Environment, Climate and Health: Sustainable Development in the 21st Century, edited by V. W. Ruttan. University of Minnesota Press, Minneapolis, 1994.

"Agriculture in a Greenhouse World: Potential Consequences of Climate Change" by C. Rosenzweig and D. Hillel. National Geographic Research and Exploration vol. 9, pp. 208-221, 1993.

Climate Change: The IPCC Impacts Assessment. Prepared by the Intergovernmental Panel on Climate Change (IPCC). W. J. McG. Tegart, G. W. Sheldon and D. C. Griffiths. (editors). Australian Government Publishing Service. Canberra, 1990.

"Potential impact of climate change on world food supply," by C. Rosenzweig and M. L. Parry. Nature vol. 367, pp. 133- 138, 1994.

Preparing for an Uncertain Climate. U.S. Congress, Office of Technology Assessment. Vol. 1. OTA-O-567. U.S. Government Printing Office. Washington, D.C., 1993.

The Potential Effects of Global Climate Change on the United States, J. Smith and D. Tirpak (editors). Report to Congress. U.S. Environmental Protection Agency. Washington, D.C., 1988.


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