NORMAN J. ROSENBERG
Senior Fellow and director, Climate Resources Program, Resources for the Future, 1616 P. Street, N.W. Washington, D.C. 20036, U.S.A.
Preparing agriculture for adaptation to climate change requires advance knowledge of how climate will change and when. The direct physical and biological impacts on plants and animals must be understood. The indirect impacts on agriculture's resource base of soils, water and genetic resources must also be known. We lack such information now and will, likely, for some time to come. Thus impact assessments for agriculture can only be conjectural at this time. However, guidance can be gotten from an improved understanding of current climatic vulnerabilities of agriculture and its resource base, from application of a realistic range of climate change scenarios to impact assessment, and from consideration of the complexity of current agricultural systems and the range of adaptation techniques and policies now available and likely to be available in the future.
Projections by the United Nations and other sources indicate that world population could grow to about 8.5 billion by 2025 (Keyfitz, 1989) and 11 billion by the end of the coming century (UNFPA, 1990). Global agricultural production of food, feed and fiber will need to be increased several times from present levels if that growing population is to be fed and clothed and if worldwide standards of nutrition are to improve.
Agriculture is today, as it has always been, vulnerable to losses caused by unfavorable weather events and climatic conditions. Often the linkages between weather and production losses are obvious - drought shriveled wheat, flooded corn fields, soybean plants blown over by wind, bony animals wandering aimlessly over parched range. Sometimes, however, the linkages are more subtle and less direct. For example, temperature and humidity conditions can combine to favor the outbreak of devastating plant diseases. The white mold fungus that attacks the bean crop from New York State to western Nebraska provides an example (Coyne et al., 1974). Insect outbreaks are also controlled by climatic conditions. In the Northern Great Plains, for example, grasshopper populations grow during warm dry periods. One reason is that a particular fungus that preys on the grasshoppers needs moist conditions for spore germination and infection (Capinera and Horton, 1989).
The vulnerability of agriculture to unfavorable climatic conditions has been demonstrated over and again. A recent example is the 1988 drought centered in the U.S. Northern Plains and Midwest. Corn production was reduced by 30% and soybeans by 20% below 1987 levels (Riebsame et al., 1990). Unusually severe and frequent frosts during the 1970's and 1980's in Florida contributed importantly to the loss of much of that state's orange juice industry to Brazil (Miller and Glantz, 1988).
The drought that began in Sahelian Africa in the late 1960's and continues on and off since then, has contributed to widespread malnutrition and displacement of populations. The drought has also exacerbated the problems caused by overgrazing and is thought to have contributed to the desertification reported in the region (UNEP, 1986).1 These examples are not meant as illustrations of problems caused by climate change. Rather, they are intended to show some of the many ways in which natural swings in climate can upset established farming and grazing patterns and disrupt the economies and societies that depend on them.
We can conceive of many ways in which climate could possibly change to the direct detriment of agriculture. Warming of the troposphere because of increasing atmospheric concentrations of CO2, CH4, N20, and the CFCs - the greenhouse effect - might, per se, be among the least worrisome. Of greater concern might be the possible effects of greenhouse warming on climatic patterns, especially the distribution of precipitation. The emphasis of this paper, as of most of its predecessors, is on direct climatic change effects on the growth and yield of grain crops. However, for perspective it is important to acknowledge the obvious fact that the production of grains and all other crops depends not only on climate but also on a natural resource base of soil, water and genetic resources. These may also be directly or indirectly affected by climate change as well as by human interventions unrelated to climate or climate change. Some of the linkages of climate change and the resource base are explored in the following section, after which the direct effects on cropping are dealt with. In this paper examples drawn relate primarily to grain production as it is affected by current climate variability and as it might be under conditions of climatic change.
So far in human history the natural resource base has not imposed serious constraints on the expansion of global agricultural capacity, although it has, of course, done so in specific local situations, viz. Holland with too much water and the Sahel with too little. Whatever loss of natural resource productivity may have occurred until now has been more than offset globally by advances in agricultural technology. In the future natural resource limitations may be more constraining, however. Rising global population and income likely will generate a several-fold increase in demand for food and fiber by the middle of the next century at the same time that the lands and fresh water resources available for expansion of agriculture may be of lower quality than those currently in use (Crosson and Rosenberg, 1989). Virtually all of the increase in agricultural output, therefore, will have to come from technological advances which increase the yield of agricultural output per unit of land and water. The pressure to maintain these advances will be high and rising. The international agricultural research establishment has shown great resourcefulness in the last 40 years, and may well be up to the task. Nonetheless, the prospective situation suggests we should give substantially more attention to protecting the future productivity of the natural resource base than has so far been necessary.
Climate may also be considered a natural resource that affects and interacts with others. for example, the rate of soil development is determined by climate (among other factors) (Jenny, 1941). The rates at which soils, once formed, and eroded depends (among other factors) on the intensity of precipitation or the strength of the wind. Vulnerability of soil to erosion depends (among other factors) on the moisture content when the rains begin to fall or the winds begin to blow. Soil moisture status depends on water holding capacity, antecedent rainfall and evapotranspiration.
The availability of moisture to sustain crops and pastures obviously depends on the amount and timeliness of precipitation and the balance between precipitation and evapotranspiration. This balance also determines how much water is left to run off the land into the streams and reservoirs from which it is drawn to irrigate other lands.
World agriculture today is based on a very limited number of species that have been domesticated from among the millions that have existed at one time or another (Wilson, 1989). Serious climatic changes could conceivably lead to the extinction of wild races of the main cultivated species (e.g. potatoes, maize and others) that can be used to strengthen current cultivars against pests and to improve resistance to climatic stresses. Species whose potential uses for food or industrial crops are not yet recognized could also disappear if their habitat is destroyed or climate changes occur too rapidly for migration (Wilson, 1989).
It is reasonable to conclude that the sustainability of agriculture depends on maintaining the availability and quality of the natural resource base that supports agriculture currently and that will be needed to support it in the future. This requires that the resources be managed so as to reduce disruption from all causes including detrimental weather and possible climate change. The changes in weather and climate that may occur because of global warming may require higher levels of resource surveillance and management than are applied today and/or the development of new adaptation techniques and strategies for agriculture in the face of changing climate and resources.
To understand what kinds of adaptation are needed, however, requires information on the nature of the climate and other changes we await and also on the impacts of these changes. In the forthcoming pages I show that we know too little about the specific regional climate changes that may occur in the future because of greenhouse warming. Hence their impacts on agriculture can only be conjectural at best. Human history is, however, replete with examples of successful agricultural adaptation to stressful climatic conditions and these can provide general guidance on how best to approach adaptation (Rosenberg, 1982). Additionally, there are lessons to be learned from human adjustment to hazards of all sorts (Burton et al., 1978; Jodha, 1989).
Leaving aside short term climate changes caused by volcanic activity and longer term changes attributable to cycles of solar activity or orbital factors, greenhouse warming appears the most likely potential cause of climate change in the coming decades (Schneider and Rosenberg, 1989). On the global scale we expect warming of the troposphere, particularly in the high latitudes, increased precipitation (but not uniformly distributed) and increased evapotranspiration (also not uniformly distributed) (Schneider and Rosenberg, 1989). We expect significant differences between regions in the degree of warming and the kinds of hydrological change that occur.
However, these changes cannot yet be predicted with a degree of certainty that allows us to tailor specific strategies and adaptations for any particular region.
In this complex of uncertainties lies one certainty with important implications for agriculture regardless of region. We know that the carbon dioxide that contributes to greenhouse warming also affects plant growth directly - through stimulation of photosynthesis and reduction of transpiration.
It is a well-known and demonstrable fact that plants, when exposed to increased concentrations of carbon dioxide, respond with an increased rate of photosynthesis.2 Such increases in photosynthesis normally lead to larger and more vigorous plants and to higher yields of total dry matter (roots, shoots, leaves) and often of harvestable product as well. The behavior described here is demonstrated particularly by plants of the C3 category which includes most of the world's small grains, legumes, root crops, cool season grasses, and trees. Another category of plants, the C3 or tropical grasses such as corn, sorghum, millet, and sugar cane, are naturally more efficient photosynthesizers than are the C3 plants. They too respond, but less markedly, to increases in atmospheric carbon dioxide. However, the C4 plants show another interesting response to increased carbon dioxide in the atmosphere: their consumption of water by transpiration is reduced because of partial closure of the leaf stomata (pores) induced by high carbon dioxide concentration. This effect is also demonstrated by the C3 plants. This reduction in transpiration is not accompanied by any significant loss in photosynthesis. In the Chaudhuri et al. (1990) experiment with wheat (see footnote 2), for example, water requirement for grain was reduced by elevated CO2 with the greatest reduction in almost all cases with the first increment of added CO2. As a result of such responses to elevated levels of carbon dioxide the C4 plants have a higher water use efficiency (production per unit of water consumed) primarily because of reduced transpiration. The C3 plants experience a still greater improvement in water use efficiency because of reduced transpiration and increased photosynthesis. If the findings from laboratory, greenhouse and open chambers upon which the above statements are based can be extrapolated to the open field, we may expect important benefits to agriculture from the increasing concentration of carbon dioxide in the atmosphere - increased photosynthesis in many important species and decreased water consumption in most species. This direct `fertilization' effect of carbon dioxide should offset detrimental climatic changes to some degree. In cases where the climatic changes are clearly beneficial and increase the potential for production - for example, through increased warmth in the northern parts of the temperate zone or increased soil moisture in the semi-arid regions - the fertilization effect increases the likelihood that the greater production potential can be realized.
In the 1970's and early 1980's debate was vigorous (e.g. Lemon, 1976; Rosenberg, 1981) on the question of whether these laboratory-demonstrable CO2 effects on photosynthesis and transpiration do now or will occur in the future in the field where temperature, moisture and nutrients are the factors that normally limit plant productivity. Since then further laboratory, controlled environment and open-top chamber experiments have shown that CO2-enrichment of the atmosphere actually reduces in the impacts of moisture and salinity stress on plants (Rosenberg et al., 1990). A summary by Allen et al. ( 1990) shows that high temperature stress is also alleviated by CO2-fertilization. The effects on nutrient stress remain ambiguous at this writing.
It is possible, although certainly not yet proven, that some portion of the overall increase in crop yields that has occurred in the last hundred years has been due to the increase in atmospheric concentration of CO2. Direct field evidence of this `CO2-fertilization effect' eludes us since it is not yet possible to identify a CO2 signal in the noisy records of annual crop yields. However, as I speculated in Climatic Change a decade ago (Rosenberg, 1981), increased capture of CO2 by the terrestrial biosphere (including agricultural ecosystems) should be occurring as result of rising CO2 levels in the atmosphere. Recent findings are consistent with this view. For example, Kohlmaier et al. (1987) interpret the increasing amplitude of the CO2 concentration wave at Mauna Loa as suggesting an increasing global biomass. This could be due, at least in part, to CO2-stimulation of plant growth. Tans et al. (1990) indicate that the oceanic uptake of atmospheric CO2 is smaller than previously believed, which strongly suggests that more carbon has been and is being captured by vegetation on land. A more recent study in which measurements of atmospheric and surface water CO2 concentrations were made in the North Atlantic (Watson et al., 1991) indicates, however, that the uncertainties about oceanic uptake of CO2 are still large enough to make the Tans et al. assertions questionable.
CO2-enrichment of the atmosphere could also lead to some troublesome effects. Because of the special benefit that C3 plants derive from elevated CO2, C3 weeds may become more competitive in fields of C4 plants. Additionally, the C:N ratio increases in leaves of plants fertilized with CO2. Some short-term studies show that herbivory increases as insects consume more vegetation to satisfy their nutritional needs (Lincoln et al., 1984). More recent studies suggest more complicated outcomes. Over longer periods, the population of insects feeding on CO2-fertilized plants would decline in response to the diminished proportion of nitrogen in the plant tissues. Pest populations would likely decline as would the populations of their predators (Fajer et al., 1989). The long-term ecological changes that might follow from changing nutrient content of vegetation are difficult to predict. However, their impacts on agriculture would probably be less profound than they would be in ecosystems that are unmanaged.
4.1. The Climate Change Scenarios
Since reliable predictions of impacts on agriculture (or any other sector) can be made only on the basis of reliable predictions of regional scale climate changes, it would seem that attempts at impact-prediction are premature. There may, however, be virtue in exploring at least the range of impacts that can be anticipated if we can define a set of reasonable scenarios of climate change and if we can develop appropriate analytical tools that take use beyond the scenario into the workings of the agricultural sector.
Climate change scenarios can be derived in a number of ways. We hear most about scenarios derived from the results of general circulation models (GCMs). These models are parameterized to represent the dynamics of the atmosphere under current conditions. They are then rerun at doubled or quadrupled atmospheric concentrations of CO2 meant to represent future conditions. Differences that develop between runs in temperature, rainfall, evapotranspiration and other climatic factors are taken as predictors of climate change (Schlesinger and Mitchell, 1985). The GCM scenario approach has been used in a number of comprehensive studies (Parry et al., 1988; Smith and Tirpak, 1989).
Another approach to scenario building is based on paleo-climatic reconstructions for various regions. The reasoning is that when, at various times in the past, the global climate was warmer (or colder) than it is today, particular regions were either wetter or drier than they are today (e.g. Kellogg and Schware, 1982). The paleoclimatic approach is not often used in impact studies to illustrate the kinds of climatic changes that are possible.
A third approach makes use of the historic weather record. Periods when climate was significantly different from today are identified and used in various ways to provide `analogues' of climate change. For example. in one recent study weather records from the 1930's `dust-bowl' period were run through various crop growth, forest succession and hydrological models (Rosenberg and Crosson, 1991). Since a number of GCM's predict a warmer and drier future climate for mid-continental North America, the analog, while obviously imperfect as a scenario of the future, provides detail on spatial and temporal variability in climate, as well as information on important climate elements such as solar radiation, humidity and wind. These three elements are often neglected since it is difficult to obtain information on how they might be changed on a regional scale by greenhouse warming.
These are serious problems associated with the use of each of these techniques of scenario development. One problem with the use of GCM's in impact studies stems from the fact that, for any particular region, available GCM's differ quite considerably in their predictions of future temperature and precipitation patterns (Grotch, 1988). Grid size in all current GCMs is larger than many U.S. states. Hence uniform climate changes are imposed over large regions and the natural spatial variability in climate, which contributes to much of the variability in yields, is lost.
There are problems, as well, with the use of historical weather records. There is no assurance, for example, that temperature and precipitation will be linked in the future under greenhouse warming in the same way as during any prior period for which an historic record exists. Nor is there reason to believe that the interseasonal, interannual, and intra-regional variability in climate which can be captured through the use of historic weather records will be similar to the climate of the same region as it is changed by greenhouse warming. Despite the recognized difficulties and ambiguities in selection of climate change scenarios, attempts to assess the possible impacts of climate change on agriculture proceed and their results can be useful and instructive if due caution is observed.
While they do not really provide scenarios of the kind that can be used as input to numerical models. another approach - that of `societal analogues' deserves mention here. In these the stresses caused by events or phenomena not necessarily climatic provide insights about how climate change might affect specific societal and economic sectors such as agriculture. One excellent example is Glantz and Ausubel's (1988) application of lessons learned in the U.S. High Plains from the mining of the Ogallala aquifer. Falling ground water levels, higher pumping costs and the possible return of irrigated land to dryland agriculture provide insights as to what might happen were the region to become more arid because of higher temperature and lowered rainfall. Another example is Rosenberg's (1982) study showing the expansion during the period 1920-1980 of the zone in North America in which hard red winter wheat is grown on a large scale. The area doubled in size as the crop moved across climatic gradients larger in some cases than the changes predicted by GCM's for a doubling of atmospheric CO2.
4.2 From Scenario to Production
Most early impact studies were based on statistical regression techniques in which the annual crop yields were related to deviations of monthly mean temperature and precipitation from the long-term normals. These deviations were derived from the results of early GCM runs. Bach (1979) used this approach to examine the effects of climate warming on corn yields in North America. Corn production, he concluded, could be expected to decline by 11% per deg.C increase in mean maximum summer temperature by 1.5% for each 10% reduction in summer precipitation. Using a similar regression approach Newman (1982) concluded that the North American Corn Belt would shift on an axis from southwest to northeast by about 175 km for every 1deg.C increase in temperature. Newman's conclusions were supported, in a sense, by Blasing and Solomon (1982) who examined the potential effects of various scenarios of climatic warming and drying on the location of dryland and irrigated corn production in North America. They assumed a mean monthly temperature increase of 3deg.C combined with a slight decrease in summer precipitation. This change was found to result in a northeastward shift in the Corn Belt. In none of these studies discussed were the possible direct effects of CO2-enrichment on crops considered. Nor were adaptations through changes in farm operations considered.
Waggoner (1983), also using a regression-based approach, found that an assumed 1deg.C warming combined with a 10% decrease in precipitation would significantly reduce yields of the major field crops grown in the midwestern United States. North Dakota spring wheat yields, for example, declined by 12%. Waggoner did take account of the potential direct effects of rising atmospheric carbon dioxide levels on crops by adjusting yields upwards by 5%. This adjustment offset some but not all of the yield reduction caused by the presumed climate change.
The impacts of warming need not all be threatening. Rosenzweig (1985) identified possible shifts in the geography of the North American wheat crop that would occur under a climate change scenario predicted by a Goddard Institute for Space Studies (GISS) GCM run with doubled atmospheric carbon dioxide. In general, the climate changes predicted by the GISS model would encourage a northward expansion of wheat production. Wheat would remain cultivable in the southern part of the present wheat belt although more frequent heat stress could lower yields there. Rosenzweig, in this study, considered neither the direct effects of carbon dioxide nor adaptations in farming, both of which could moderate the impacts of the climate change.
A report to the US Congress by the Environmental Protection Agency (Smith and Tirpak, 1989) deserves special attention here. Regions studied included the Great Lakes, Southeastern United States, Great Plains and California. Various simulators of crop growth were used including CERES-maize (Jones and Kiniry, eds., 1986). CERES-wheat (Ritchie and Otter, 1985), and SOYGRO (Jones et al., 1988). CO2-fertilization effects and opportunities for simple adaptation were considered in these modeling exercises. The climate change scenarios were based on two particular 2 x CO2 GCM experiments - those of GISS and GFDL (see Smith and Tirpak, 1989 for details). The major findings of these studies were as follows: without the direct effects of CO2 yields of wheat, soybeans and corn declined in the Great Lakes, Southeast, and Great Plains regions, except in the northernmost latitudes where the frost-free season was lengthened.
Decreases in yield stem mostly from the shortened life-span of crops caused by higher temperatures. Differences in climate scenario led to important differences in predicted yields ranging from mild improvements to severe losses. Locational differences within region were important. Irrigation moderated yield losses as did CO2, least in the southeast and most in the north. However, significant increases in irrigation requirement were noted especially with the more severe GFDL climate change scenario. Adaptations tested such as longer season varieties of corn in Illinois, did not fully compensate for loss of yield.
Parry et al. (1988) summarized the results of an ambitious study conducted through the International Institute of Applied Systems Analysis (IIASA) of climate change impacts on agriculture that concentrated on the cold and semi-arid margins of world agriculture. Various types of process models were applied as were other techniques of agro-geography and agro-climatology. Various scenarios of climate change were developed for the study areas but the results of a particular 2 x CO2 GISS experiment were applied to all the regions studied. The IIASA study identified existing sensitivities of agricultural systems to climate variability and potential sensitivities to climate change. The most likely first order effects identified were: changes in length of the potential growing season and in plant growth rates leading to changes in the required growing season; changes in mean yield; changes in yield variability and in certainty of expectable yields; changes in yield quality; and changes in sensitivity of plants to different levels of fertilizer, pesticide and herbicide applications. Spatial shifts in comparative advantage and of crop potential could result from any or all of these. Climate change may also affect water balance altering irrigation and flood control requirements for agriculture and may affect soil erosion, soil fertility conditions and the incidence of pests and disease.
Findings of the IIASA and USEPA studies figure prominently in the report of the Intergovernmental Panel on Climate Change (IPCC).3 This new analysis relies primarily on scenarios drawn from the results of GCM 2 x CO2 equilibrium experiments. Its findings, based primarily on estimated impacts as they relate to changes in productive potential against a baseline of present day technology and management, are as follows:
- it has not yet been determined conclusively whether, on average, global agricultural potential will increase or decrease.
- severe negative effects are possible at the regional level, particularly in regions of high present day vulnerability least able to adjust technically to such effects.
- two broad sets of regions appear most vulnerable: a) some semi-arid tropical and subtropical regions (possibly western Arabia, the Maghreb, western W. Africa, Horn of Africa, southern Africa and eastern Brazil and b) some humid tropical and equatorial regions (possibly southeast Asia and central America).
- changes that lead to water shortages in regions that are currently net exporters of grains (southern Europe, southern USA, parts of S. America and western Australia) may cause them to lose productive potential.
4.3. Some Comments on Models
A few comments on the matter of models used to predict impacts of climate change on agriculture may be useful here. Regression equations of the sort described above are instructive but not generally reliable for extrapolation beyond the range of the data used in their formulation (Katz, 1977; Rosenberg, 1982). Also they cannot be used to deal with CO2-enrichment effects at the process level. Process models that simulate plant growth, yield and water use offer the best alternative to the regression model. But simulators have their own limitations. For example, most simulators control the rate of plant development (phenology) by reference to growing degree days or other index of heat accumulation. With higher temperatures as input the thermal requirements for maturity are achieved earlier and opportunity for production and accumulation of photosynthesis is curtailed. This is not unreasonable since plants do mature sooner in hot years. However, for this reason simulated effects of greenhouse warming on summer crops may be too severe. Fall sown crops, on the other hand, benefit in simulation from the milder winters, break dormancy sooner and mature before the hot, dry weather sets in. Hence, they often show only moderate losses to even severe warming. Thus far most simulators have not dealt effectively with the episodic events that are most critical in determining yield such as disease outbreaks, pollen sterility due to extreme temperatures at critical times, etc. Modelers are attempting to overcome these limitations. One distinct advantage of the process models is their ability to account directly for CO2-fertilization effects. Another distinct advantage is that they can be used to evaluate adaptations - i.e. use of different varieties, planting dates, tillage practices, etc.
4.4. Other Methodological difficulties
Only a small sample has been given of an instructive literature that contributes to our understanding of the possible impacts of climate change on agriculture. This sample also illustrates some of the methodological weaknesses that characterize the current 'state-of-the-art' in climate change impact assessment. Primary among these weaknesses are:
- uniform climate changes (e.g. +2 deg.C; -20% precipitation) are imposed on very large regions determined by GCM grid-cell size. As a result the very important spatial and temporal variety in climate is ignored.
- the uniformly changed climate is imposed on the world of today, rather than on the world as it may be when the climatic changes are large enough to have an impact. The question asked, then, is 'how would the world of today be impacted if the climate were to change instantaneously?' This may not be the right question!4
- the complexity of real agricultural systems and the interlinkage of agriculture with the rest of the economy is usually ignored. We learn that corn yields in a particular place may increase or decrease by some percentage or that, under current circumstances corn would not be growable here but might be grown there. We fail to learn how farmers would respond to change using currently available technology or how prices might he affected if demand and supply change in other regions.
- the full range of possible adaptations to climate change - those that are autonomous in the sense that they can he done within the existing institutional and policy regime and those that might require changes in current institutions and policies for their development are not fully explored.5
A methodology that aims to overcome some of these problems has been applied in a U.S. Department of Energy sponsored study of a region in the central United States comprised of the states of Missouri, Iowa, Nebraska and Kansas (the MINK region, hereafter). This methodology is fully explained in Rosenberg and Crosson (1991).
4.5. Economic Integration
Difficult as it is to reckon agricultural sensitivities to the impacts of climate change, it may be yet more difficult to come up with meaningful numbers on economic impacts. In one of the few studies that consider economic impacts on agriculture, Adams et al. (1989) used the outputs of Rosenzweig's and other crop modeling studies included in the EPA report to investigate the impacts of the two climate scenarios mentioned above on the economics of United States agriculture. Using a spatial equilibrium agricultural model of the United States, they found that the GISS scenario caused an 18% decline in crop prices and $9.9 billion increase in the economic surplus. The GFDL scenario on the other hand caused prices to rise 28% and a $10.5 billion decrease in the economic surplus. Results of this analysis indicate that consumers would be harmed more than farmers would be benefited by the latter climate change scenario.
Crosson (1989) commended the efforts of Rosenzweig and Adams though he found certain deficiencies in both. He points out the crudeness of the modeled climate scenarios vis-á-vis the sophistication of the impact assessments. Crosson also notes the potentially misleading conclusions that can be drawn from results obtained by imposing a future climate on the world of today. Finally, he stresses the importance of taking stock of climatically-induced changes in comparative advantage within and outside the United States in the calculation of crop price responses to a particular regional climate change.
In the MINK study (Rosenberg and Crosson, 1991 ) with no adjustments and no CO2-enrichment a regional input-output model shows that the decline in regional production of corn, sorghum, wheat, hay and soybeans would be $4.4 billion (1982 $s) or 1.4% of the total regional production to final demand, if all the decline were to fall on exports. If the decline in feedgrains - corn and sorghum - were to fall only on exports the loss would be $3.1 billion or 1%, but if the decline falls on animal producers the loss would be as high as $30 billion (or 10% of the regional production) because of impacts on the meat packing industry, the largest manufacturing activity in the region. Higher CO2 reduces the losses in all cases.
5. Some Possible Adaptations
The most plausible of the projected climate changes and direct effects of CO2 (i.e. general warming, altered precipitation and evapotranspiration, CO2-fertilization effect on photosynthesis and improved water use efficiency) indicate a number of possible response strategies. These include the use of cultivars that require either longer or shorter growing seasons. In high latitude regions shortness of the growing season is the primary limiting factor. Warming would allow the use of more productive long season cultivars. Such cultivars should be readily available from lower latitude locations
In the MINK study and the IIASA and USEPA studies that preceded it some simple adaptations to the shortened growing seasons are tested. The simulators show in general that longer seasons varieties and earlier planting dates can help overcome yield losses. Wilks (1988), in modeling the response of North American corn and wheat to a 2 x CO2 climate change reached a similar conclusion. He found that by selecting the most appropriate planting dates and cultivars for the changed climate, yield reductions could be kept to a minimum.
Photoperiod limitations (if any) may be overcome by means of traditional plant breeding procedures. In regions where mid-and late-summer temperatures and/or water stress become severe enough to interfere with the plant's reproductive cycle shorter season varieties may be introduced. A shift from spring-sown to fall-sown small grains may be anticipated in higher latitude regions. Less severe winters will ease the winter-kill limitation on fall-sown wheat while increasingly severe summers will injure the later-maturing spring wheat.
Similarly the development or importation of more drought and heat resistant strains of major crop species can be expected to occur in response to warming and/ or desiccation. Soybeans avoid drought by defoliation and refoliate when (if) rains return. Sorghum resists drought by its tolerance of high temperatures and ability to roll its leaves to reduce transpiration. These crops may replace dryland corn where that heat and drought sensitive crop becomes uneconomic. This is already happening where rain-fed agriculture is practiced at the hot and dry southern and western edges of the corn belt (Wilhite, 1979).
Greater emphasis will likely be placed on moisture conserving tillage methods in dryland agriculture. Various forms of minimum-tillage, conservation tillage, stubble mulching and fallowing are already gaining in popularity in the U.S. midwest and elsewhere because of lower energy and labor requirements than conventional tillage. In the MINK study simple improvements in tillage improved water conservation and helped reduce loss of simulated yields in summer row crops. The rate of adaptation of these techniques would likely increase if farmers were to gain a perception that the climate is becoming drier and hotter.
Drying in the mid-continental regions would reduce runoff from range, forest and agricultural lands. The supply of water for all competing uses would decline, but the effect would probably be greatest on agriculture since the quantities used in irrigation are great. Municipal and industrial users can outbid agriculture if supplies are short.
Additionally, environmental concerns encompassing such issues as water quality, instream needs of fish and wildlife and recreation have potent political support and if water supplies are short can draw them away from agriculture. At the same time, dryland agriculture might no longer he economic in certain areas and demand for irrigation water would be increased.
An average increased demand for irrigation water of approximately 15% (for a mixture of alfalfa, corn and winter wheat) was found by Allen and Gichuki (in Smith and Tirpak, 1989) for the Great Plains states from Texas to Nebraska in response to the two GCM scenarios they applied. Demands may be greater during peak periods and growing seasons may be lengthened by various adaptations. Similarly, in the MINK study irrigation demands increase by about 25% for corn and sorghum and 10% for wheat exposed to the climate of the 1930s. It is important to recognize, as well, that in climatic circumstances that create a greater demand for irrigation water, runoff to streams and reservoirs will likely be reduced.
Declining (or limited) supplies and increasing demand for water can only result in reduced irrigated production and/or greatly increased costs of production. Where irrigation currently depends on non - or poorly - replenished ground water supplies, further expansion of irrigation would only hasten the exhaustion of the aquifers. The expansion of irrigated acreage in regions of declining or unreplenishable supplies would clearly be unsustainable. Irrigation development is likely to occur instead in regions or nations where surface and groundwater supplies are not yet heavily exploited and/or in regions where climate change increases supplies. In general, in the U.S. at this time, even in the absence of climate change, competing demands for water are likely to severely limit further development of irrigation (Frederick and Kneese, 1990).
On the other hand, it is true that improvements in irrigation efficiency can compensate somewhat for increased demand or decreased supply. The 'oil shocks' of the 1970's and the declining water tables in regions irrigated from poorly rechargeable aquifers has prompted development, improvement and adaptation of many water-saving practices.
Irrigation efficiency (the ratio of water entering and remaining within the root zone to that applied to the field) can be improved considerably with laser-leveling of fields, use of surge irrigation in furrows, adaptation of sprinkler systems to operate at low pressures and other techniques. Thus far, however, irrigators in the High Plains region of the United States have emphasized new field practices (e.g. chiseling compacted soils, stubble mulching, minimum tillage) and management strategies (e.g., irrigation scheduling, monitoring soil moisture status, planting drought tolerant crops) rather than replacement or improvement of irrigation systems in their attempts to conserve the limited ground water supplies from which they draw (Kromm and White, 1990).
As noted above, water use efficiency (photosynthate produced per unit of water transpired) is improved by CO2-enrichment of the atmosphere. There are other ways to improve water use efficiency in agriculture. Windbreaks, for example, reduce evaporative demand in the air over the plants they shelter. The sheltered plants remain better hydrated and, thus, better able to carry out photosynthesis (Rosenberg, 1979). A wide array of intercropping techniques - multi-cropping, relay cropping, and others - provide greater overall production per unit of land occupied and can be conducive to improved water use efficiency because of the microclimatic conditions they create (Crosson and Rosenberg, 1989).
A small number of adaptive strategies that are possible with currently available technology have been described above. But there remains time (probably measurable in decades) during which other adaptation techniques and policies can be readied. Research and development efforts over the coming decades may make current techniques more efficient or more widely applicable and may provide new techniques as well.
Biotechnology offers the prospect of developing 'designer-cultivars' for coping with specific drought, heat, insect or disease problems much more rapidly than is possible today (Goodman et al., 1987). Species not previously used for agricultural purposes may be identified and others already identified may be quickly domesticated. A report of the Council on Agricultural Science and technology (CAST, 1984) identified a number of candidates for new food and industrial crops.
Most of the world's grain crops are annuals that must be grown each year from seed. Workers in the USDA/ARS (Rawlins, 1988) have proposed that crops should be grown for biomass rather than for harvestable grain or other edible parts which constitute only a small fraction of the total photosynthate produced. Perennial plants would be harvested periodically to provide feedstocks for the industrial synthesis of foods and other products, making for a more efficient use of land, water and nutrients. Farms in this scheme would be biomass producing entities connected by pipeline to the processing plants, much like oil wells linked to a refinery.
Even this visionary (if unappetizing!) approach leaves agriculture in the open air where weather can cause trouble. On the other hand, problems created for agriculture by an incorrectly termed 'greenhouse effect' might be solved, in part, through the use of real greenhouses. Controlled climate environments are already very important in providing high value products such as ornamental plants, flowers and vegetables. Facilities in which improved techniques of water management, nutrient recycling, CO2-fertilization, high intensity lighting, and integrated pest management might be used to provide high protein, concentrated carbohydrate foods. This might help offset some portion of lost agricultural productivity, if that is to be the net effect of greenhouse warming.
In agriculture, as opposed to other sectors such as forestry, water resources and power generation, capital investments are relatively small and are made for relatively short periods of time - sowing annual crops, establishing perennial pastures, planting 20-30 year fruit orchards, purchasing 10-15 year tractors, investing 10 years in breeding a new cultivar, etc. This short time-horizon makes rapid adaptation simpler for agriculture than it would be where the investments are much larger and commitments are for much greater periods of time. One important exception relates to the expansion of irrigation as an adaptation. The planning and construction of dams and water distribution systems have much longer time horizons than most of the adaptations described above. It seems more likely, however, given the uncertainty about regional climate changes, that more emphasis will be devoted to increasing the efficiency of existing water supply systems than to developing new ones as a response to climate change.
6. Why Adapt?
The adaptations described above represent only a partial list of possible technological 'fixes' for coping with the climatic impacts on agriculture in a greenhouse warmed world. How can we provide for a steady stream of these technological advances at the rate at which they may be needed. The obvious answers are: (1) to develop policies and programs that protect biological resources from disappearance; (2) to develop policies and programs that create incentives for stewardship of land and water resources and (3) to assure that agricultural research establishments are adequately funded and led (Crosson and Rosenberg, 1989). But with all the remaining uncertainties about climate change, does it make sense to invest now in research on adaptation?
Let's assume for the moment that GCM's are able to provide reliable regional scale forecasts of climate change and that the analytical tools are available that permit us to calculate the impacts of the climate changes on the relative costs of production of agricultural commodities among countries. How does a nation armed with this knowledge react?
There will be winners and losers of comparative advantage. Even the winners will have transition problems and costs. But the losers are faced with three choices (Easterling et al., 1989):
- The first choice is to accept the loss of comparative advantage and increase food imports. This requires that people, land and resources be moved out of agriculture into more productive sectors.
- The second choice is to resist the climate-induced decline in comparative advantage. In order to do so the national policy must be to spur research on development of adaptive measures. It would be necessary to hold resources in agriculture but to redistribute them to new kinds of agriculture and new localities.
- The third (and least desirable) choice would be to protect the agricultural sector by restricting imports. This can only be done if the nation is willing to accept higher economic and environmental costs.
These choices all point to the need for maintenance of strong agricultural research establishments where they already exist and for creation of new ones where they are needed. It will take a steady stream of new technologies just to keep up with increasing population and demand for food in the future even if climate does not change. How much more so, if one of the resources on which agriculture depends - climate - makes current production techniques less appropriate!
7. Relevance to the Developing World
What of the developing world? In the preceding sections I have taken a relatively optimistic view of current and future agricultural capability to cope with climate change - at least the more modest sorts that have been predicted. The implicit assumption has been that the resources available to agricultural research establishments in the developed world (more particularly in the U.S.) will be adequate to the task and that new tools, such as biotechnology, may make rapid adaptation easier in the future than it is today. But now we must also address the question of means and resources. Will the developing world have the capacity to cope as easily?
There is no reason to believe that the developing countries will be exposed as a group to worse climatic changes than will the developed countries. What is certain, however, is that their margins of survival are smaller and that the impacts of climatic change might be more immediate and profound where the infra-structure including research capacity is smaller. N. S. Jodha, an Indian agricultural economist argues (Jodha, 1989) that farmers in the less developed countries use well-tried coping techniques in times of stress and that these provide a sort of arsenal from which to draw when the evidence of climate change becomes strong enough to convince them and their governments of the need. Jodha provides many examples of these responses from the Indian experience. There are exceptions of course. In climatically marginal areas, particularly in the semiarid tropics, even a slow, small change in climate can accentuate climatic risks. Not everywhere in the developing countries are the climates marginal, however.
Because the developing countries are preoccupied with raising their currently low standards of living, they have shown relatively little interest in mitigating global warming. However, as Jodha has shown, these countries, without necessarily having any greater interest in an adaptive strategy, nonetheless have accumulated substantial experience in adaptation, particularly in agriculture. Of course, the developing countries, like the developed countries, will need more knowledge of the prospective impacts of climate change and of possibilities for effective adaptive responses. There is a mutuality of interest here between developing and developed countries that may foster cooperative efforts in devising strategies for adaptation, even if agreement on strategies for mitigation remains elusive.
Increased agricultural production will be needed to serve the growing human population and provide an improved standard of nutrition worldwide.
Agriculture is vulnerable today, as it always has been, to unfavorable climatic conditions. Global warming could alter this vulnerability directly - for better or worse - depending on the region and the ways in which the climate changes. The natural resource base on which agriculture depends - soils, water and genetic resources - could also be directly affected by climate change. The sustainability of agriculture depends on management that avoids its disruption from all causes, including detrimental weather and climate. Changes that occur because of greenhouse warming (or other causes), especially if change is rapid, may create erosion, disease or their problems that require higher levels of resource surveillance and management than are applied today and/or development of new adaptation techniques and strategies in the face of changing climate and resources. To understand the kinds of adaptations that will be needed requires information on the nature of the climate and other changes we await and the impacts of these changes.
To what will we have to adapt? Specific changes in regional climates are not yet predictable. Direct effects of carbon dioxide enrichment of the atmosphere are generally beneficial as proven in controlled environment experiments, although their manifestation in the field is not yet established. Thus we still lack information on what the precise impacts of changing climate will be for agriculture in any one region or for the world as a whole. But the range of impacts can be anticipated if a set of reasonable scenarios of climate change are considered systematically and if appropriate analytical tools are developed for this purpose.
Most impact assessments share a number of methodological weaknesses. These stem from: (1) the imposition of future climates on the world of today, (2) the imposition of uniform climate changes over very large regions, (3) the lack of spatial and temporal variety in climate change and (4) lack of consideration for the complexity of agricultural systems and enterprises and their ability to respond to climate change with currently available technological and policy tools, let alone possible future technologies and policies. Advances in methodology offer to overcome these inadequacies.
Sustained and well-supported agricultural R & D programs will be needed if a steady stream of appropriate technologies and adaptations such as those described in preceding pages is to be available when and where needed as a response to climatic change. No less important is the development of technological methods and social and economic policies that encourage stewardship of the natural resources on which agriculture (and so much else) depends. Crosson and Rosenberg (1989) discuss incentives that encourage such stewardship.
Likely the greenhouse effect will alter global and regional climates enough by 2030 (perhaps even earlier) so that the research and development recommended in this paper will have proven necessary (and hopefully effective). It has been argued that research of the sort proposed above as preparatory to adaptation for climate change will draw resources away from other needed activities. This, in a broad sense, may be correct. One may assert, however, with reasonable confidence that research that improves our basic understanding of how crops and other ecosystems and resources respond to climate and its variations and changes would always have wider application. With or without climate change, research that aims to deal with climate variability should provide the means to strengthen the resiliency of agricultural systems.
This review is drawn from a report contributing to the National Academy of Sciences Committee on Science, Engineering and Public Policy (COSEPUP) study on the Policy Implications of Greenhouse Warming (NAS/NRC, 1991). I am indebted to Pierre Crosson, my colleague at RFF, for instructive discussions on the changing condition of and demands on agriculture's natural resource base. Both he and Peter Morrisette reviewed this manuscript critically. I am also indebted to two anonymous reviewers, the responses to whose challenging questions and useful suggestions have, hopefully, sharpened the arguments presented in this paper. Thanks too, to Angela Blake and Laura Katz who provided stenographic and bibliographic assistance.
1 How large these areas may actually be and whether or not the `process' of desertification is reversible is the subject of an emerging debate (see Crosson and Rosenberg, 1989; Rhodes, 1990). A recent report based on data from polar-orbiting meteorological satellites links the north-south latitudinal movement of the southern Saharan boundary, as evidenced by changes in vegetation index, quite directly to annual precipitation in the region (Compton et al., 1991).
2 Although many such studies are conducted at CO2 concentrations intended to roughly double the preindustrial concentration (600 to 660 vppm), the photosynthetic response is demonstrable at levels between the current ambient (approx. 355 vppm) and those levels (Kimball et al., 1990). For example, in a recent experiment Chaudhuri et al. (1990) grew winter wheat over three growing seasons at 340 (ambient), 485, 660, and 825 vppm of CO2 at both high and low levels of water supply. Yields of wheat increased in both water regimes and at all levels of CO2 above the ambient.
3 IPCC Working Group II (Impacts) Section A. Draft report on Agriculture and Forestry (2nd rev. 3/15/90).
4 Crosson's comments (1989) on the EPA-sponsored studies by Rosenzweig (1989) and Adams (1989), the IPCC draft report as well as earlier commentary (e.g. Rosenberg, 1982) recognize the need for a more futuristic approach to impact assessment such as is described below.
5 One unidentified reviewer of this paper points out other weaknesses in most impact assessments - among them the limited range of climatic variables included in the scenarios and the limited focus on grain crops.
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(Received 30 May 1990; in revised form 25 November 1991)