CYNTHIA ROSENZWEIG and DIANA LIVERMAN
2880 Broadway New York, NY 10025
The Pennsylvania State University
Department of Geography
University Park, PA 16802
Agriculture is the basic activity by which humans live and survive on the earth. Assessing the impacts of climate change on agriculture is a vital task. In both developed and developing countries, the influence of climate on crops and livestock persists despite irrigation, improved plant and animal hybrids and the growing use of chemical fertilizers. The continued dependence of agricultural production on light, heat, water and other climatic factors, the dependence of much of the world's population on agricultural activities, and the significant magnitude and rapid rates of possible climate changes all combine to create the need for a comprehensive consideration of the potential impacts of climate on global agriculture.
This chapter compares the effects of climate change on agriculture in temperate and tropical regions. These regions differ significantly in their biophysical characteristics of climate and soil, and in the vulnerability of their agricultural systems and people to climate change. The focus is on the impacts of the climate changes projected to occur as a result of higher levels of carbon dioxide in the atmosphere and associated global warming. The projected climate changes for the temperate and tropical areas differ in that climate models project larger temperature increases in temperate regions than in tropical regions. The projections of changes in the hydrological cycle are more similar but rather uncertain, showing a mixed picture of regional precipitation increases and decreases in both areas.
COMPARISON OF TEMPERATE AND TROPICAL AGRICULTURE
The tropics are defined as the geographical area lying between 23.5deg. N and 23.5deg.S latitude, while the temperate regions are found above these parallels. Climatologically, the tropics are characterized by high year-round temperatures and weather is controlled by equatorial and tropical air masses. Tropical precipitation is primarily convective. In the more humid tropical regions, annual rainfall is often above 2000 mm and falls in almost all months of the year. In the drier tropics, rainfall can fall below 50 mm, and be very seasonal. The remainder of the region lies between these precipitation regimes, with distinct wet and dry seasons. Agriculture is frequently limited by the seasonality and magnitude of moisture availability.
In the mid-latitude temperate zone, weather is controlled by both tropical and polar air masses. Precipitation here occurs along fronts within cyclonic storms. The temperate region also has many different climate regions with warmer and cooler temperatures and seasonal rainfall. Temperate agriculture is often characterized a predominantly limited by seasonally cooler temperatures.
Reported experiments have shown that even though yield per day is often higher in the tropics, total crop growth season is shorter (Haws et al., 1983). Leaf area expansion and phasic development are faster in the tropics because of higher temperatures during vegetative growth. Nevertheless, crop yields are consistently found to be higher in temperate regions than in the tropics (see Table 1) (FAO, 1990 Numerous factors contribute to this result. Soils in the humid tropics tend to be highly leached of nutrients and are therefore unproductive because of high temperatures, intense rainfall, and erosion. Soils in the drier tropics are often hampered by accumulations of salt and lack of water (Barrow 1987). Temperate soils are generally viewed as more favorable to agriculture than tropical soils because of higher nutrient levels. However, there are exceptions in both regions, with high productive volcanic and fluvial soils found in the tropics, and poorly developed and infertile soils in temperate regions.
Agricultural production is also severely limited in many humid tropical regions by the wide range of weeds, pests, and diseases that flourish in consistently warm and moist climates. The growth of some crops and varieties, which require long hours of daylight to reach maturity, is also limited by the invariable day lengths of the tropics. Solar radiation, which is critical to plant growth, and whose intensity is controlled by the angle of the sun, daylength, and cloudiness, is lower in winter and higher in summer in temperate zones. In the tropics, solar radiation is often limited by cloudiness during the rainy seasons.
Agricultural crops and cropping systems have been developed for, and adapted to, these varied regimes of climate, soil, diseases and pests (Haws et al., 1983). The main commercial agricultural crops and their adaptations include:
(a) Cassava and sugarcane, which only grow in tropical areas and have a crop duration of one year or longer. Cassava is drought resistant, but sugarcane requires irrigation in dry areas.
(b) Sorghum, groundnut, and sweet potato, which grow in both tropical and subtropical regions in relatively dry seasons.
(c) Rice, which is mainly grown in tropical and subtropical zones in the rainy season or with irrigation.
(d) Maize and field beans grow in both zones, preferably with seasons with enough rain
(e) Wheat, soybean, and potato are crops of the subtropical and temperate zones and grow in the tropics at high (cooler) elevations.
(f) Sugarbeet is grown only in the temperate zone.
A number of "luxury" agricultural crops, especially fruit (bananas, pineapples), stimulants (coffee, tea) and spices grow only, or best, in the tropics. Tropical regions are also important in providing winter season produce for temperate zones.
In temperate agriculture, plant breeding and fertilizer use produced dramatic yield increases for many crops early in the twentieth century. Similar increases occurred more recently in tropical regions for crops such as wheat, maize and rice, which benefited from the technological package or improved seeds, fertilizer, mechanization and pesticides known as the Green Revolution.
In both temperate and tropical regions, irrigation has been developed in areas where dry seasons exist and adequate water can be reserved from other seasons or brought in from adjacent regions. Irrigation is an important buffer against climate variability and climate change. About 20% of the world's cropland is irrigated, mostly in Asia, producing about 40% of the annual crop production.
Differences in farming systems, technology and economics also contribute to the yield differences in temperate and tropical regions. Agriculture in temperate regions is characterized by high levels of inputs (quality seed stock, fertilizer, herbicides and pesticides), and a high degree of mechanization and capitalization. However, there are wide variations in the use of technology, European agriculture being particularly intensive. In tropical regions, many farmers cannot afford inputs, and governments cannot afford to subsidize them. In some parts of the tropics, traditional technologies, such as multiple cropping and terracing, act to buffer the system against climate variability, conserve soil fertility, and increase yields.
In some senses, the tropics are more dependent on agriculture, and therefore more vulnerable to climatic change, than the temperate regions. As much as 75% of the world's population live in the tropics, and two thirds of these people are reliant on agriculture for their livelihoods. With low levels of technology, land degradation, unequal land distribution, and rapid population growth, many tropical regions are near or exceeding their capacity to feed themselves (Table 2).
Indeed, some authors have argued that the unequal social structures and international position of many tropical countries also increase vulnerability to climate change. Unequal land tenure, high numbers of landless rural dwellers, low incomes and high national debts exacerbate the negative impacts of climate variability, as some people have no extra land, job, savings or government assistance to see them through droughts or other climatic extremes (Jodha 1989). When the economic system is oriented towards export rather than subsistence agriculture, climatic change (as well as low export prices) may threaten the whole national economy and food system. Those regions that cannot feed their populations depend on cereal imports from the major cereal exporters such as the USA, France, Canada, Australia, Argentina, and Thailand (FAO 1990). All except Thailand would be defined as temperate agricultural producers.
THE INFLUENCE OF CLIMATE CHANGE ON CROP PRODUCTION
At the basis of any understanding of climate impacts on agriculture lies the biophysical sciences. The rates of most biophysical processes are highly dependent on climate variables such as radiation, temperature, and moisture, that vary regionally. For example, rates of plant photosynthesis depend on the amount of photosynthetically active radiation and levels of atmospheric carbon dioxide (C02). Temperature is an important determinant of the rate at which a plant progresses through various phenological stages towards maturity. The accumulation of biomass is constrained by the availability of moisture and nutrients to a growing plant.
Numerous studies have examined the impacts of past climatic variations on agriculture using case studies, statistical analyses and simulation models (e.g. Nix 1985; Parry 1978; Thompson 1975; World Meteorological Organization 1979). Such studies have clearly demonstrated the sensitivity of both temperate and tropical agricultural systems and nations to climatic variations and changes. In the temperate regions, the impacts of climate variability, particularly drought, on yields of grains in North America and the Soviet Union have been of particular concern because of their effects on world food security. In the tropics, drought impacts on agriculture and resulting food shortages have been widely studied, especially when associated with the failure of the monsoon in Asia or the rains in Sudano-Sahelian Africa. In the temperate regions, climatic variations are associated with economic disruptions; in the tropics, droughts bring famine and widespread social unrest (Pierce 1990).
THE BIOPHYSICAL IMPACT OF CLIMATIC CHANCES ASSOCIATED WITH GLOBAL WARMING
It is frequently assumed that global change will bring higher temperatures, altered precipitation, and higher levels of atmospheric CO2 (IPCC 1990a). What might these changes mean for the biophysical response of agricultural crops?
Interactions with thermal regimes. Higher temperatures in general hasten plant maturity in annual species, thus shortening the growth stages during which pods, seeds, grains or bolls can absorb photosynthetic products. This is one reason yield are lower in the tropics. Because crop yield depends on both the rate of carbohydrate accumulation and the duration of the filling periods, the economic yields of both temperate and tropical crops grown in a warmer and CO2-enriched environment may not rise substantially above present levels, despite increases in net photosynthesis (Rose 1989).
Because temperature and tropical regions differ in both current temperature and the temperature rise predicted for climate change, the relative magnitudes of combined CO2 and temperature effects will likely be different in the different regions. In the mid-latitudes, higher temperatures may shift biological process rates toward optima, and beneficial effects are likely to ensue. Increases in temperature will also lengthen the frost-free season in temperate regions, allowing for longer duration crop varieties to be grown and offering the possibility of growing successive crops (moisture conditions permitting). In tropical locations where increased temperatures may move beyond optima, negative consequences may dominate.
Both the mean and extreme temperatures that crops experience during the growing season will change in both temperate and tropical areas. Extreme temperatures are important because many crops have critical thresholds both above and below which crops are damaged. Prolonged hot spells can be especially damaging (Mearns et al, 1984). Critical stages for high temperature injury include seedling emergence in most crops, silking and tasseling in corn (Shaw, 1983), grain filling in wheat (Johnson and Kanemasu, 1983), and flowering in soybeans (Mederski, 1983). In general, higher temperatures should decrease cold damage and increase heat damage. Agro-climatic zones are expected to shift poleward as lengthening and warming growing seasons allow new or enhanced crop production (soil resources permitting) (Rosenzweig, 1985).
Changes in hydrological regimes. The hydrological regimes in which crops grow will surely change with global warming. While all GCMs predict increases in mean global precipitation (because a warmer atmosphere can hold more water vapor), decreases are forecast in some regions and increases are not uniformly distributed. The crop water regime may further be affected by changes in seasonal precipitation, within-season pattern of precipitation, and interannual variation of precipitation. Increased convective rainfall is predicted to occur, particularly in the tropics, caused by stronger convection cells and more moisture in the air.
Too much precipitation can cause disease infestation in crops, while too little can be detrimental to crop yields, especially if dry periods occur during critical development stages. For example, moisture stress during the flowering, pollination, and grain-filling stages is especially harmful to maize, soybean, wheat and sorghum (Decker et al., 1986).
The amount and availability of water stored in the soil, a crucial input to crop growth, will be affected by changes in both the precipitation and seasonal and annual evapotranspiration regimes. Some GCMs predict mid-continental drying the Northern Hemisphere (Manabe and Wetherald, 1986; Kellogg and Zhao, 1988) and other GCM predictions have been interpreted to suggest that the rise in potential evapotranspiration will exceed that of rainfall resulting in drier regimes throughout the tropics and low to mid-latitudes (Rind et al., 1990). Because the soil moisture processes are represented so crudely in the current GCMs, however, it is difficult to associate much certainty with these projections (IPCC, 1990a).
Global climate change is likely to exacerbate the demand for irrigation water (Adams et al., 1990). Higher temperatures, increased evaporation, and yield decreases contribute to this projection. However, supply of needed irrigation water under climate change in uncertain. Where water supplies are diminishing, such as the Ogallala Aquifer in the United States, extra demand might require that some land be withdrawn from irrigation (Rosenzweig, 1990).
Physiological effects of CO2. The study of agricultural impacts of trace gas induced climate change is complicated by the fact that increasing atmospheric CO2 has other effects on crop plants besides its alteration of their climate regime. These are often called "fertilizing" effects, because of their perceived beneficial physiological nature. Specifically, most plants growing in enhanced CO2 exhibit increased rates of net photosynthesis. The higher photosynthesis rates are then manifested in higher leaf area, dry matter production, and yield for many crops (Kimball, 1983; Acock and Allen, 1985; Cure, 1985). In several cases, high CO2 has contributed to upward shifts in temperature optima for photosynthesis (Jurik et al., 1984) and to enhanced growth with higher temperatures (Idso et al., 1987); other studies, however, have not shown such benefits (Jones et al., 1985; Baker et al, 1989).
CO2 enrichment also tends to close plant stomates, and by doing so, reduces transpiration per unit leaf area while still enhancing photosynthesis. The stomatal conductances of 18 agricultural species have been observed to decrease markedly (by 36%, on average) in an atmosphere enriched by doubled CO2 (Morison and Gifford, 1984). However, crop transpiration per ground area may not be reduced commensurately, because decreases in individual leaf conductance tend to be offset by increases in crop leaf area (Allen et al., 1985). In any case, higher CO2 often improves water-use efficiency, defined as the ratio between crop biomass accumulation or yield and the amount of water used in evapotranspiration. Increases in photosynthesis and resistance with higher CO2 have been shown to occur at less than optimal levels of other environmental variables, such as light, water, and some of the mineral nutrients (Acock and Allen, 1985).
Temperate crops may benefit more from increasing CO2 than tropical crops. In crop species with the C3 pathway characteristic of non-tropical plants (e.g., wheat, soybean, cotton) CO2 enrichment has been shown to decrease photorespiration, the rapid oxidation of recently formed sugars in the light, a process which lowers the efficiency of overall photosynthesis. C4 crops, which are particularly characteristic of tropical and warm arid regions (e.g., maize, sorghum, and millet), are more efficient photosynthetically under current CO2 levels than C3 plants (because they fix CO2 into malate in their mesophyll cells before delivering it to the RuBP enzyme in the bundle-sheath cells). Because of this CO2-concentrating and photorespiration-avoiding mechanism, experimental data show that C4 plants are less responsive to CO2 enrichment (Acock and Allen, 1985).
The physiological effects of high levels of atmospheric CO2 described above have been observed under controlled experimental conditions. In the open field, however, their magnitude and significance are still largely untested, and their importance relative to the predicted large-scale climatic effects uncertain. Greenhouse and field-chamber environments tend to be much smaller, less variable, and more protected from wind than field conditions. Furthermore, physiological feedback mechanisms such as starch accumulation or lack of sink (that is, growing, storing, or metabolizing tissue) for the products of photosynthesis may limit the extent to which the "fertilizing" CO2 effects may be realized. Finally, if trace gas emissions continue to grow unchecked, their climate warming effect is projected to continue even up to 2000 ppm (Manabe and Bryan, 1985), but the beneficial boost to photosynthesis appears to level off at about 400 ppm for C4 crops and about 800 ppm for C3 crops (Akita and Moss, 1973).
Soils. Climate change will also have an impact on the soil, a vital element in agricultural ecosystems. Higher air temperatures will cause higher soil temperatures, which should generally increase solution chemical reaction rates and diffusion-controlled reactions (Buol et al., 1990). Solubilities of solid and gaseous components may either increase or decrease, but the consequences of these changes may take many years to become significant (Buol et al., 1990). Furthermore, higher temperatures will accelerate the decay of soil organic matter, resulting in release of CO2 to the atmosphere and decrease in carbon/nitrogen ratios, although these two effects should be offset somewhat by the greater root biomass and crop residues resulting from plant responses to higher CO2.
In temperate countries where crops are already heavily fertilized, there will probably be no major changes in fertilization practices, but alterations in timing and method (e.g., careful adjustment of side-dress applications of nitrogen during vegetative crop growth) are expected with changes in temperature and precipitation regimes (Buol et al., 1990). In tropical countries, where fertilization level is not always adequate, the need for fertilization will probably increase.
Sea level rise, another predicted effect of global warming, will caused increased flooding, salt-water intrusion, and rising water tables in agricultural soils located near coastlines. This is particularly crucial in tropical countries such as Bangladesh, with large agricultural regions and high rural population located near current sea level.
Pests. Pests are organisms that affect agricultural plants and animals in ways considered unfavorable. They include weeds, and certain insects, arthropods, nematodes, bacteria, fungi, and viruses. Because climate variables (especially temperature, wind and humidity) control the geographic distribution of pests, climate change is likely to alter their ranges. Insects may extend their ranges where warmer winter temperatures allow their over-wintering survival and increase the possible number of generations per season (Stinner, et al., 1989). Pests and diseases from low latitude regions, where they are much more prevalent (see Table 3) may be introduced at higher latitudes. As a consequence of pest increase, there may be a substantial rise in the use of agricultural chemicals in both temperate and tropical regions to control them.
SPECIFIC STUDIES IN TEMPERATE AND TROPICAL REGIONS
Very few integrated regional studies have been completed on climate change impacts on agriculture. Parry et al. (1988a) report on integrated agricultural sector studies in high-latitude regions in Canada, Iceland, Finland, USSR, and Japan, concluding that warmer temperatures may aid crop production by lengthening the growing season, but that potential for higher evapotranspiration and drought conditions may be detrimental.
Adams et al. (1990) conducted an integrated study for the US linking models from atmospheric science, plant science, and agricultural economics. While the outcomes depend on the severity of climate change and the compensating effects of carbon dioxide on crop yields, the simulations suggest that irrigated acreage will expand and that regional patterns of US agriculture will shift with predicted global warming. With the more severe climate change scenario tested, the movement of US production into export markets is substantially reduced.
The Missouri, Iowa, Nebraska, and Kansas (MINK) study integrated both within the agricultural sectors and across other sectors (Rosenberg and Crosson, 1990). The study incorporated both the physiological effects of CO2 and adaptation by farmers to the climatic conditions of the 1930s. Even with the relatively mild warming (1.1deg.C) of the 1930s and with farmer adaptation and CO2 physiological effects taken into account, regional production declined by 3.3%. Given the IPCC (1990a) estimate of 2.5deg.C warming for doubled CO2 conditions, these results imply agricultural losses of about 10% for equilibrium warming of doubling of carbon dioxide-equivalent (Cline, 1991).
The second volume of Parry et al. (1988b) includes studies of the impact of climate changes on agriculture in Kenya, Brazil, Ecuador, India, and Australia. These case studies used the impacts of past climatic variations, rather than projections of future climate, to provide insights into the sensitivity of agriculture to climate change.
Liverman (1991) and Liverman and O'Brien (1991) have described how global warming may affect Mexican agriculture, using GCM output to project declines in moisture availability and maize yields at several sites in Mexico.
A number of conferences and publications have recently raised concern about the possible impacts of global warming in tropical regions, but they have not included explicit analyses of how global warming may affect tropical agriculture (Fundacion Universo Veintinuo 1990; HARC 1991; Suliman 1990; Universidad de Sao Paulo 1990).
GLOBAL ESTIMATES OF AGRICULTURAL IMPACTS
Global estimates of agricultural impacts have been fairly rough to date, because of lack of consistent methodology and uncertainty about the physiological effects of CO2. General studies of how climate change might affect agriculture include those of the National Defense University (1983), Liverman (1986), and Warrick (1988). Kane et al. (1989) broadly predicted improvements in agricultural production at high latitudes and reductions in northern hemisphere mid-continental agricultural regions. The IPCC (199Ob) concluded that while future food production should be maintained, negative impacts were likely in some regions, particularly where present-day vulnerability is high.
An international project of the US Environmental Protection Agency (EPA), "Implications of Climate Change for International Agriculture: Global Food Trade and Vulnerable Regions," has been established to estimate the potential effects of greenhouse gas-induced climate change on global food trade, focusing on the distribution and quantity of production of the major food crops for a consistent set of climate change scenarios and CO2 physiological effects. Other goals of the project are to determine how currently vulnerable, food-deficit regions may be affected by global climate change; to identify the future locations of those regions and the magnitudes of their food-deficits; and to study the effectiveness of adaptive responses, including the use of genetic resources, to global climate change.
As part of the EPA project, crop specialists are estimating yield changes at over 100 sites in over 20 countries (see Figure 1), under common climate change scenarios using compatible crop growth models. The focus is on staple food crops: wheat, rice, maize, and soybeans. The crop models are those developed by the International Benchmark Sites Network for Agrotechnology Transfer (IBSNAT, 1990)---a global network of crop modelers funded by the U.S. Agency for International Development. The choice of the IBSNAT crop models was based on several criteria. First, the models simulate crop response to the major climate variables of temperature, precipitation, and solar radiation, and include the effects of soil characteristics on water availability for crop growth. Second, the models have been validated for a range of soil and climate conditions. Third, the models are developed with compatible data structures so that the same soil and climate data bases could be used with all crops.
Preliminary national production changes for wheat based on IBSNAT crop model results are shown in Table 4 (Rosenzweig and Iglesias, et al., 1992). Results from individual sites have been aggregated according to rainfed and irrigated practice and contribution to regional and national production. The table shows national production changes for the three climate change scenarios with (555 ppm) and without (330 ppm) the physiological effects of CO2 on crop growth.
In general, these results show that the climate change scenarios without the physiological effects of CO2 cause decreases in estimated national production, while the physiological effects of CO2 mitigate the negative effects. Production declines occur in many locations, however, even with the compensating CO2 effects. Production changes tend to be less negative and even positive in some cases in countries in mid and high latitudes, while simulations in countries in the low latitudes indicate more detrimental effects of climate change on agricultural production. The UKMO climate change scenario (mean global warming of 5.2deg.C) generally causes the largest production declines, while the GFDL and GISS (4.0 and 4.2deg.C mean global warming, respectively) production changes are more moderate.
When embedded in a global agricultural food trade model, the Basic Linked System (Fischer et al., 1988), the production change estimates based on IBSNAT crop model results will allow for projection of potential impacts on food prices, shifts in comparative advantage, and altered patterns of global trade flows for a suite of global climate change, population, growth, and policy scenarios.
ADAPTATION TO CLIMATE CHANGE
The importance of farm and state level adaptations to climate change and variability has been demonstrated in a number of studies (e.g. Rosenberg et al 1989; Waggoner 1983; White 1974) Adaptations to climate change exist at the various levels of agricultural organization. In temperate regions, farm-level adaptations include changes in planting and harvest dates, tillage and rotation practices, substitution of crop varieties or species more appropriate to the changing climate regime, increased fertilizer or pesticide applications, and improved irrigation and drainage systems. Governments can facilitate adaptation to climate change through water development projects, agricultural extension activities, incentives, subsidies, regulations, and provision of insurance.
Similar adaptations could occur in tropical regions as shown in Table 5. In Mexico, preliminary results from the EPA International Agriculture project suggest that severe declines in maize yields under global warming could be mitigated if climate change is accompanied by increases in irrigation, fertilizer use and the use of drought resistant varieties. The major problem in the tropics, compared to most temperate most temperate regions, is the relative lack of resources, institutions, and infrastructure to promote such adaptations.
In general, the tropical regions appear to be more vulnerable to climate change than the temperate regions for several reasons. On the biophysical side, temperate C3 crops are likely to be more responsive to increasing levels of CO2. Second, tropical crops are closer to their high temperature optima and experience high temperature stress, despite lower projected amounts of warming. Third, insects and diseases, already much more prevalent in warmer and more humid regions, may become even more widespread.
Tropical regions may also be more vulnerable to climate change because of economic and social constraints. Greater economic and individual dependence on agriculture, widespread poverty, inadequate technologies, and lack of political power are likely to exacerbate the impacts of climate change in tropical regions.
In the light of possible global warming, plant breeders should probably place even more emphasis on development of heat- and drought-resistance crops. Research is needed to define the current limits to these resistances and the feasibility of manipulation through modern genetic techniques. Both crop architecture and physiology may be genetically altered to adapt to warmer environmental conditions. In some regions it may be appropriate to take a second look at traditional technologies and crops as ways of coping with climate change.
At the regional level, those charged with planning for resource allocation, including land, water, and agriculture development should take climate change into account. In coastal areas, agricultural land may be flooded or salinized; in continental interiors and other locations, droughts may increase. These eventualities can be dealt with more easily if anticipated.
As climatic factors change, a host of consequences will ripple through the agricultural system, as human decisions involving farm management, grain storage facilities, transportation infrastructure, regional markets, and trade patterns respond. For example, field-level changes in thermal regimes, water conditions, pest infestations, and most importantly, quantity and quality of yields, may lead to changes in farm management decisions based on altered risk assessments. Consequences of these management decisions could result in local and regional alterations in farming systems, land use, and food availability. Ultimately, impacts of climate change on agriculture may reverberate throughout the international food economy and global society.
At the national and international levels, the needs of regions and people vulnerable to the effects of climate change on their food supply should be addressed. In many cases, reducing vulnerability to current climate variability should also serve to mitigate the impacts of global warming.
It is important to ask, "What will or should agriculture be like in the next century?" Even if the answer is unknown, the flexibility gained in attempting to imagine the agricultural future should be a useful tool for adaptation to climate change.
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