Reproduced, with permission, from:
Smit, B., L. Ludlow, and M. Brklacich. 1988. Implications of a global climatic warming for agriculture: A review and appraisal. Journal of Environmental Quality 17 (4): 519-27.
B. SMIT,* L. LUDLOW, AND M. BRKLACICH
ABSTRACT
Recently it has been recognized that changes in the chemical composition of the atmosphere are likely to alter the earth's climate, and that these alterations may have severe implications for agriculture and other economic activities. This has stimulated research into the possible consequences of altered climatic regimes on several attributes or components of agri-food systems. Current consensus suggests that a global climatic warming, induced by increased concentrations of CO2 and other "greenhouse" gases, is likely, and hence the possible implications of warmer climates for agriculture has received considerable attention. Several analytical procedures have been employed in these studies and it is timely to assess the characteristics and achievements of these independent efforts. This paper classifies and reviews studies that examine the implications of climatic warming for agriculture. three approaches to assessment are recognized. Crop yield analysis identifies the effects of a specified change in climate on productivity levels for individual crops in particular locations. Spatial analysis examines the implications of climatic warming for the area and location of lands suitable for crop production. Agricultural systems analysis causes on the relationships among components of agri-food systems. Much remains to be learned about the effects of climatic warming on agriculture. The use of existing information to develop a comprehensive analysis is hampered by differences in analytical approaches and in climatic change scenarios, and by the virtual absence of information on the possible implications of climatic change on agriculture in developing nations. Nevertheless, current evidence suggests that a warmer climate could create a more favorable environment for wheat (Triticum aestivum L.) and grain corn (Zea mays L.) in Canada, Northern Europe, and the USSR, and restrict opportunities in the USA.
The possible implications for agriculture and food production of changes in atmospheric chemistry have prompted concern worldwide. This concern has stimulated considerable research into the direct impacts of ozone (O3, carbon dioxide (CO2), sulphur dioxide (SO2), and acid deposition on various aspects of agrifood systems. Some studies have examined the effects of these chemical changes on crop growth and yields (Heagle et al., 1983; Irving, 1983; Rogers et al., 1983), whereas others have assessed the subsequent implications for agricultural production (Forster, 1984; Heck et al., 1983; Ludlow and Smit, 1987). These studies suggest that adjustments to the chemical composition of the atmosphere may have serious consequences for agriculture in many regions of the world.
In addition to these direct impacts, there is a growing interest in the extent to which chemical changes might affect other atmospheric properties and how these adjustments, in turn, will alter agriculture. For example, the impacts on global climatic patterns of increases in infrared absorbing or "greenhouse" gases have been the focal point for many studies investigating the indirect effects of atmospheric alterations (Hare, 1983; Harrison, 1984; Liverman, 1986; Pittock and Nix, 1986; Rosenzweig, 1985; Williams et al., 1988). Although there is considerable uncertainty regarding the magnitude and regional effects of these changes (Schlesinger and Mitchell, 1985), existing evidence suggests that increases in greenhouse gases will contribute to warmer and drier climates in midlatitude regions such as the midwestern USA, southern Europe, and Asia, whereas the higher latitudes will in all likelihood be characterized by a warmer but somewhat wetter climate (National Research Council [NRC], 1982). Such changes in climate could have major implications for agriculture worldwide, and research is now being directed toward investigating these potential effects (Parry and Carter, 1988).
Studies have assessed the possible consequences of altered climatic regimes on various attributes or components of agri-food systems, ranging from those that assess the impacts of modified climates on crop yields to those that estimate the effects on global agricultural production and trade. A sufficient body of information now exists to warrant a review, which would contribute to the development of climatic impact assessment by providing a common ground for comparative studies, by summarizing current knowledge on this growing environmental concern, and by facilitating the development of an effective research agenda.
This article classifies and reviews studies that examine the implications of a greenhouse gas-induced global climatic warming on agriculture. Enrichment effects resulting from changes in atmospheric chemistry such as increases in plant photosynthesis and water-use efficiency (Kimball, 1983; Lemon, 1983; Rogers et al., 1983; Rosenberg, 1981) are not considered here; nor are second-order impacts, such as those that could occur from sea level rises (Postel, 1986) or increases in agricultural pests (Decker et al., 1986). Such cumulative impacts are beyond the scope of this article.
Of necessity, this review does not include all studies undertaken in this field, but at least provides examples of the major forms of analysis employed. This article presents an appraisal of the approaches with respect to the types of information they provide and their applications, summarizes results of impact assessments, and indicates the possible implications of warmer climates on the opportunities for crop production for major agricultural regions of the world.
Studies providing information on impacts of climatic change on agriculture vary in many respects. They differ according to their approach to assessment, the climatic change scenarios they specify, their geographic coverage, and the attributes of the agri-food sector they consider. this classification helps clarify they types of information available, and summarizes the possible effects of climatic warming in major regions of agricultural production.
Approach to Assessment
Although the provision of a better understanding of the relationship between climate and agriculture is a common link among virtually all studies investigating the impacts of climatic change on agriculture, no one approach to assessment has gained universal acceptance. In this review, the following three approaches are recognized:
Crop yield analysis represents the most common approach, and a variety of analytical techniques have been employed to examine the possible effects of climatic change on crop productivity levels (Bootsma et al., 1984; Lough et al., 1983). Spatial analysis investigates the consequences of altered climates on the location of the agricultural frontiers (Blasing and Solomon, 1984; Newman, 1980), whereas agricultural systems analysis focuses on the linkages among various components of the agri-food sector (Liverman, 1986; National Defense Univ. [NDU], 1983).
Climatic Change Scenarios
The climatic change scenarios employed in impact studies can be characterized by the methods utilized to estimate climatic modifications and by the nature of the climatic change. The magnitude and extent of any future climatic change is not known with certainty. A variety of climate variables such as land and sea surface temperatures, air temperatures, precipitation, atmospheric pressure, and cloudiness (Lamb, 1986) could be affected. Hence, it is not uncommon for impact studies to identify several scenarios of climate change. Many methods for identifying these scenarios have been employed (Lamb, 1986). Some studies rely on general circulation models (GCMs) to identify levels of future climatic change (Blasing and Solomon, 1984; Rosenzweig, 1985; Williams et al., 1988), whereas others use past climatic periods as analogues for future conditions (Lough et al., 1983; Palutikof et al., 1984). Climatic changes have also been identified by specifying parametric or incremental changes in climatic variables, such as temperature and precipitation (Bootsma et al., 1984; Parry, 1978), and by utilizing expert opinion (NDU, 1978).
The range of climatic properties considered tends to vary according to the purpose of the assessment and the analytical techniques used to specify the climate change scenario. Earlier studies (Parry, 1978; Ramirez et al., 1975) tended to consider a few macroclimatic variables, usually temperature and precipitation. More recently (Land Evaluation Group, 1986; Williams et al., 1988), the range of agroclimatic properties considered in the impact assessments has been expanded to include many of the other relevant factors that greatly influence crop growth and development, such as length of growing season and soil moisture.
Geographic and Sector Coverage
The geographic coverage and the number of agricultural activities considered are ultimately influenced by the intended application of the investigation. Geographic coverage can range from individual plants and farm operations (Bootsma et al., 1984; Terjung et al., 1984a, b) to major food producing regions (Blasing and Solomon, 1984; Rosenzweig, 1985). The range of agricultural activities can vary from particular crops (Parry, 1978; Waggoner, 1983) to a comprehensive assessment of many of the major primary production activities (Williams et al., 1988).
The various approaches and perspectives implicit in studies that examine the relationships between climatic warming and agriculture have provided a better understanding of the potential implications of warmer climates for selected components of food production systems. However, the variability in approach, design, and scope, which characterizes these partial analyses, has contributed to some confusion; the melding of these studies into a comprehensive assessment is both a needed and contentious task.
Table I (cont.) characterizes studies that assess the effects of global climatic warming on agriculture. To facilitate the review and appraisal process, studies summarized in Table I (cont.) are discussed initially according to their approach to assessment: crop yield analysis, spatial analysis, and agricultural systems analysis. This review also considers the climatic change scenario employed, the climatic properties considered, and the geographic and sector coverage of each study.
Crop Yield Analysis
Crop yield analysis estimates the effects of altered environments on crop productivity levels and has been employed widely in climatic impact assessments. It is appropriate to subdivide crop yield analysis into studies based on surveys of expert opinion and studies relying on crop productivity models.
Many assessments on the impacts of climatic warming on crop yields are made by experts, based on their accumulated knowledge of the expected climatic change and the climatic requirements of crops. The NDU (1980) study exemplifies crop productivity analysis employing expert opinion. This approach was utilized to estimate both the extent of expected climatic warming and the concomitant impacts on selected crop yields. Although this study has been widely distributed among policy makers in many countries, its validity and the reliability of its findings have been questioned (Stewart and Glantz, 1985). The major concerns stem from the use of averaging procedures to develop the climatic change scenarios and the yield responses. It is suspected that the approach has projected a future that will be inconsistent with the magnitude of change that is likely. More specifically, it is believed by many that the NDU study underestimates possible differences from the present climate. However, by employing rigorous techniques to illicit and utilize the opinions of experts, particularly in the aggregation of differing projections, expert opinions could be an effective and inexpensive means for estimating effects of climatic change on crop yields.
The application of crop productivity models to climatic impact assessment includes procedures based on statistical (regression) and/or simulation techniques, and to a large extent, follow the development of crop productivity models in general (Baier, 1979; Biswas, 1980). Regression models estimate yields as a function of multiple predictor variables. By using historical climatic and yield data for specific crops in particular areas, regression equations have been developed and used to predict the changes in yields expected due to alterations in climate (Lough et al., 1983; Santer, 1984). Among the reservations regarding the application of regression-based crop models are the lack of true independence among the predictor variables, especially those related to agroclimatic properties, such as length of growing season and soil moisture, and the extension of the statistical relationships beyond the range of conditions for which they were developed (Parry and Carter, 1988).
Simulation models have also been applied to assess the impacts of climatic change on crop yields (Terjung et al., 1984a, b; Waggoner, 1983). These models are developed by combining a set of mathematical equations based on experiments or knowledge of specific plant processes (photosynthesis, respiration, and transpiration) and their interactions with the environment (climate and soils). Simulation models are based on crop-climatic processes and hence can be applied to environmental conditions other than the current. Although the capacity of simulation models to capture and integrate specific plant processes is still quite limited, this approach to productivity modelling is ultimately better suited to estimating yield changes due to climatic modifications than empirical statistical regression models (Parry and Carter, 1988).
Many studies have been performed to assess the impacts of climatic warming on crop yields (Table I (cont.)). Results from these studies indicate that relative to current technologies or management practices, the yields of major crops in North America and Europe may diminish as a consequence of climatic warming. For example, grain corn (Zea mays L.) and wheat (Triticum aestivum L.) yields in the midwestern USA and Canada will tend to decrease as climate becomes warmer and drier (Benci et al., 1975; Bootsma et al., 1984; Ramirez et al., 1975; Williams et al., 1988). Current yields of these crops are also expected to decrease in most regions of Western Europe (Lough et al., 1983; Santer, 1985). Of course, changes in production inputs and producer response to climatic change (e.g., technological advances that might reduce the vulnerability of particular crops to heat and moisture stress and adoption of crops more suitable to an altered climate), could tend to counterbalance these potential declines in yields.
The gradual change in climatic norms due to elevated levels of greenhouse gases could allow time for changes in agricultural patterns and other management practices to occur. Studies that simultaneously consider climatic change and adjustments to technology and management suggest that a global climatic warming could contribute to increases in yields of major crops grown in many high-latitude regions (Table I (cont.)). In the USSR, for example, winter rye (Secale cereale L.) and spring wheat yields could increase substantially by increasing fertilizer applications and introducing new crop varieties (Pitovranov et al., 1988). In Canada it has been suggested that by switching from spring to winter wheat the problems associated with increased moisture stress under an altered climatic regime could be alleviated and yields could be enhanced (Parry et al., 1985).
So far, the crop yield approach to assessing the impacts of climatic warming on agriculture has focused on major crops currently produced in North America and Europe. Little information is available regarding the vulnerability to climatic change of many crops, including such important ones as soybean [Glycine max (L.) Merry and rice (Oryzo sativa L.). Despite the incompleteness of current evidence, crop yield studies provide useful information. Results indicate that increased moisture stress due to warmer and drier conditions could reduce the yields of current varieties of major crops in many regions of the world. If research is directed toward the development of drought-resistant crop varieties and water-conserving farm management practices, these yield reductions could possibly be abated in midlatitude regions. At higher latitudes, such as regions in Canada and the USSR, yields could increase by introducing field practices and crop varieties already existing at midlatitudes.
Spatial Analysis
Spatial analysis examines the implications of climatic warming on the area and location of lands suitable for agricultural production. One method for assessing these implications is the use of spatial analogues. This technique involves identifying analogue regions that currently experience conditions similar to those predicted in another region given a future climatic warming.
Spatial analogues can provide information on likely impacts of climatic warming on agriculture, such as the possibilities for introducing new crops into an area. For example, a study by the Land Evaluation Group (1986) found that under an environment of increased concentrations of greenhouse gases the temperature and precipitation levels in southern Ontario, Canada, would resemble those currently experienced in parts of the eastern USA, and many of the agroclimatic barriers that effectively limit the production of vegetable and horticultural crops in Ontario would be removed. This sort of spatial analysis provides an effective means for indicating the extent to which a climatic change might influence the biophysical opportunities for production, but it would of course be inappropriate to conclude that climatic change will cause an adjustment to cropping patterns. Cropping patterns are sensitive not only to climatic conditions, but also to soil characteristics and socio-economic conditions such as demands for crop products.
Another method for evaluating changes in the area and location of agricultural production is to examine the movements of agricultural margins under alternative climatic warming scenarios. the first step in such an analysis involves identifying a condition or set of conditions tat effectively constrain production at agricultural margins. These conditions may be biophysical, such as frost-free period, or socioeconomic, such as the extent to which climate implies a risk to production. Then, threshold or critical values for specific agricultural activities are established for each of the selected conditions. For biophysical conditions, the thresholds are usually derived from a particular crop requirement for maturation, whereas the socioeconomic thresholds identify acceptable levels of risk associated with temporal variability in climate. The area and location of lands suitable for an agricultural activity for a specified climate can be estimated by mapping these thresholds as isopleths. The sensitivity of suitable lands for agriculture to climatic warming can then be examined by repeating the analysis for two or more climatic scenarios.
Newman's (1980) examination of the implications of climatic warming on the location of the USA Corn Belt typifies spatial analysis employing thresholds for biophysical conditions. The prevailing thermal and moisture regimes effectively define the current margins of the USA Corn Belt, and Newman employed these boundaries to identify critical values for growing degree days (GDD) and potential evapotranspiration (PE). By increasing growing season temperatures by 1deg. C the critical GDD isopleth shifted northward and the critical PE isopleth eastward. These correspond to an estimated migration in the Corn Belt of about 175 km toward the northeast.
Blasing and Solomon (1984) also used spatial analysis to investigate the effects of climatic warming on the biophysical conditions that define the USA Corn Belt. This study employed the same threshold criteria as the Newman study, but specified a different climatic change scenario. A 3 deg. C increase in temperature and a corresponding increase in precipitation of 8 cm were identified as the climatic change scenario by using a GCM and assuming a twofold increase in atmospheric CO2. The analysis by Blasing and Solomon (1984) also reveals that the USA Corn Belt could shift towards the northeast although the magnitude of displacement is not as great as predicted by Newman.
The studies by Newman (1980) and Blasing and Solomon (1984) indicate that the USA Corn Belt could shift northeast, possibly into Canada (Table I (cont.)). The USA Wheat Belt is expected to show similar shifts if climate is to warm (Rosenzweig, 1985) (Table I (cont.)). These shifts in cropping patterns imply that the higher yielding USA corn varieties could replace Canadian corn varieties, and that Canada could switch from producing primarily the lower-yielding spring wheats to producing winter wheats. Consequently, opportunities for producing corn and wheat could increase in Canada, while prospects for producing these crops in the USA may become less favorable.
The location of agricultural production zones may also be interpreted from socioeconomic perspectives. As an example, Parry (1978) used failure frequency as the socioeconomic condition by which to measure the critical limits for oat (Avena sativa L.) cropping in Britain (Table I (cont.)). Any probability of crop failure >0.3 was considered to be "too risky" for continuing or viable oat production. The analysis revealed that a 1 deg. C increase in temperature would elevate this risk level isopleth 140 m. This would increase the land area in Britain that could physically and economically support oat production by about 2 million ha.
As yet, spatial analysis of agricultural zones has been applied only at the regional scale. However, spatial analysis could feasibly be conducted to study the impacts of climatic warming globally. Emanuel et al. (1985) has used this analysis to study the impacts of climatic warming on the worldwide distribution of natural vegetation zones. It has been suggested that similar types of spatial analysis may provide a basis for agricultural evaluation of climatic warming (Oram, 1985; Williams, 1985). Expanding these studies to all regions would provide insight into the opportunities for introducing new crops into many areas.
Agricultural Systems Analysis
Agricultural systems analysis focuses on assessing the impacts of climatic changes on multiple agricultural activities and on the functioning of the agri-food sector. Studies using agricultural systems analysis attempt to go beyond assessments based on crop yield and spatial analyses by providing information on many aspects of the agricultural sector, including total production, prices, trade patterns, and employment.
Historical analogues, which utilize responses to previous climatic changes as a means to suggest potential adjustments to future climates, represent one technique applied to agricultural systems analysis. For example, Butzer (1980) and Rosenberg (1982) have attempted to gain insight into the possible implications of a climatic warming on agriculture in North America's Great Plains by examining agricultural adaptations during the Dust Bowl of the 1920s and early 1930s. Although these sorts of analyses can provide a better understanding of responses to a specific set of conditions, their applicability to a future climatic warming is limited. The climatic changes experienced during the Dust Bowl are characterized by relatively abrupt adjustments, whereas climatic modifications stemming from increasing levels of greenhouse gases are occurring at a relatively slow rate. Socioeconomic conditions affecting agriculture during this era are considerably different from the current, let alone future technologies, management practices, and marketing conditions. The agricultural response to short-term deviations from climatic norms may be quite different from the response to gradual changes in the norms. The opportunities for direct application of historical analogues to estimating the evolution of agricultural systems under a warmer climate are limited.
Mathematical models represent another group of techniques that have been employed to trace the effects of climatic change on biophysical, economic, and/or social components of agricultural systems. At the farm level, studies by Williams et al. (1988) in Saskatchewan and the Land Evaluation Group (1986) in Ontario, Canada, used programming models to investigate the effects of climatic warming on farm income and profitability. Both studies employed output from GCMs in the development of scenarios for climatic change. The Saskatchewan study assessed the implications of climatic change on 29 farms representing major farming activities throughout the province. The Ontario study investigated only one type of farming, but the range of climatic change scenarios was expanded to consider adjustments to climatic variability as well as norms. Overall, both studies suggest that relative to current technologies and management practices, a climatic warming could increase the economic risks for production in the Canadian grain sector. Such analyses illustrate the need and opportunities to address issues related to the distribution of impacts stemming from climatic change (Waterstone, 1985).
The recently completed study funded by the International Institute of Applied Systems Analysis (IIASA) and the United Nations Environmental Programme (UNEP) is a major example of a study utilizing agricultural systems analysis at regional and national levels (Table I (cont.)). Various techniques were employed to assess the implications of future climatic warming for agriculture in the high-latitude regions of Canada (Saskatchewan), Iceland, Finland, the USSR, and Japan (Parry and Carter, 1988). In all studies, crop yield analysis was first employed to estimate the impacts of climatic warming on selected crops. These estimates were then applied to assess the downstream effects on regional and national economies. Mathematical models were used to determine impacts on aggregate production and income for Saskatchewan, the USSR, and Japan, whereas the studies conducted for Finland and Iceland relied on expert opinion.
Generally, the results of the IIASA/UNEP project suggest that effects of climatic warming on agriculture in high-latitude regions are beneficial. In Iceland the livestock-carrying capacity of pastures and rangelands are estimated to increase substantially (Bergthorsson et al., 1988). Total farm income for Finland and Japan are estimated to increase due to enhanced production of barley (Hordeum vulgare L.) and rice, respectively (Kettunen et al., 1988; Yoshino et al., 1988). In the USSR, wheat production could be enhanced greatly under a warmer climatic regime, and corn production could also increase but to a lesser extent. At the same time, oat and barley production could decrease slightly (Pitovranov et al., 1988). The Saskatchewan study reveals that prospects for spring wheat production could decrease, but the opportunities for introducing new crops or crop varieties afforded by warmer climate would increase the province's overall potential for agricultural production (Williams et al., 1988).
Global equilibrium models (GEMs) have been employed by the NDU (1983) and Liverman (1986) to assess agricultural systems for an international perspective. These models do not consider climate explicitly, but climatic warming can be considered by conducting crop yield analyses and using these findings as inputs to the GEM.
The NDU applied the grain, oilseed, and livestock (GOL) GEM to identify the effects of a climatic warming on regional production, trade, and prices of grain. The findings of the NDU (1983) study tend to concur with those studies that investigate the implications of climatic change for individual nations. That is, the altered climate would likely modify the structure of agricultural systems. The most notable adjustments could include a more favorable trading position for the Canadian and USSR grain sectors, reductions in grain exports from the USA, declines in the world price for wheat and coarse grains, and slight increases in the market price for rice.
The primary purpose of Liverman's (1986) research was to investigate the applicability of GEMs for climatic impact assessment. The International Futures Simulation (IFS) model was employed, and the study includes an appraisal of the effects on the international structure of agricultural systems, given a gradual climatic warming that is consistent with increasing levels of atmospheric CO2. The major finding is that global food stocks would decline and contribute to increases in food deficits, especially in southern Asia.
Agricultural systems analysis provides an opportunity to extend the utility of crop yield and spatial analyses, and to assess the subsequent implications for farm incomes, aggregate production, and international trading prospects. However, current knowledge regarding the possible impacts of climatic warming on agricultural systems is extremely fragmented, limited to a small set of future scenarios for climate and other conditions, and focused almost exclusively on high-latitude regions. There is considerable prospect for improving comprehension of agricultural sensitivities to climatic warming via the development of systems analysis to include other regions and a wide range of scenarios.
CONCLUSIONS
Despite the many research efforts, firm conclusions regarding the implications of future global climatic warming for agriculture are difficult to draw. Climatic impact assessments have not been conducted for many regions, particularly developing nations in Asia and Africa, and hence the response of crops and agricultural systems to climatic change in these areas is not known. Even in areas of North America and Europe, where considerably more research has been conducted, there is little certainty in knowledge about the implications of climatic warming for agriculture. Differences in the approaches in the agricultural activities covered and in geographic coverage make it difficult to compare results and develop a comprehensive appraisal of the potential consequences of climatic warming. Current knowledge indicates that the impacts of this warming on agriculture will vary from crop to crop, location to location, and system to system.
Evidence, although incomplete, suggests that conditions for producing crops such as grain corn and wheat may become more favorable in Canada, Northern Europe, and in the USSR, while at the same time opportunities for producing these crops in the midlatitude regions of the USA and Western Europe may diminish.
It is now widely accepted by environmental scientists that global climatic change is prompted in part by increasing levels of atmospheric CO2 and other "greenhouse" gases. This review and appraisal has focused on the agricultural implications of the consequent climatic warming, and has not investigated the other direct and indirect impacts of changes in the atmosphere that ultimately affect crop growth and agricultural production. Evidence elsewhere suggests that dry matter production and crop yields could increase in CO2-enriched environments (Kimball, 1983; Kramer, 1981; Lemon, 1983; Prior et al., 1987), and Sionit et al. (1987) and Kimball et al. (1987) illustrate the need to consider the combined influences of changes to climatic and atmospheric quality. There is an opportunity to extend this review and appraisal to consider the multiple and cumulative effects stemming from atmospheric change. Clearly, a better understanding of the effects of human activities on climate and of the impacts of altered climates on activities such as agriculture will better enable societies to prepare for and adapt to future climates.
ACKNOWLEDGMENT
This article benefited from reviewers suggestions and input from D. Bond.
Published in J. Environ. Qual. 17:519-527 (1988).
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