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.
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).
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 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 (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.
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.