Raja Reddy and Harry F Hodges

Department of Plant and Soil Sciences, 117 Dorman Hall, Box 9555, Mississippi State University, Mississippi State, MS 39762, USA

Weather is the most important cause of year-to-year variability in crop production, even in high-yield and high-technology environments. There has been considerable concern in recent years about the possibility of climatic changes caused by human activities, because any change in weather will increase uncertainty regarding food production. Since the beginning of the industrial revolution, earth's population has increased dramatically, with accompanying large-scale burning of fossil fuels, the manufacture of cement, and intensive cultivation of lands not previously used for crops or livestock production. The largest population in human history will occur during the 21st century and thus dictate greater pertinence of climatic changes because the consequences may be so great and drastic.

Any observed or predicted changes in the global climate are of fundamental concern to man, because the present climate of the earth is so well-suited to support life. Alterations in our climate are governed by a complex system of atmospheric and oceanic processes and their interactions. In the context of crop production, relevant atmospheric processes consist of losses in beneficial stratospheric ozone concentration ([O3]) and increasing concentrations of the surface-layer trace gases, including atmospheric carbon dioxide ([CO2]), methane ([CH4]), nitrous oxide ([N2O]) and sulphur dioxide ([SO2]). Surface level [O3], [SO2] and [CO2] have direct impacts on crops, while [CO2], [CH4] and [N2O] are critical in altering air temperature. Products of atmospheric processes also result in increases in surface-level ultraviolet radiation and changes in temperature and precipitation patterns.

There is considerable uncertainty associated with determining the current average temperature of the earth and determining historical temperatures is an even more daunting task. Obtaining global or even regional averages is difficult, because both diurnal and seasonal temperatures vary considerably from place to place. For this reason, the possibility of climatic change is

©CAB International 2000. Climate Change and Global Crop Productivity (eds K.R. Reddy and H.F. Hodges)

somewhat controversial in the public view. However, the increasing atmospheric CO2 and other radiative gases in recent years are well documented and theoretical reasons for higher concentrations of these gases to cause global warming are not disputed.

Changes in temperature during geological and even recent historical times have occurred as evidenced by ice ages and the cool period (sometimes called the Little Ice Age) during the 16th and 17th centuries. Unfortunately, there is not a clear explanation for these shifts in temperature, but it seems unlikely that pre-industrial variations were related in any way to human activity.

Much of our knowledge of future climatic change comes from studies using climate models. Climate models are complex mathematical representations of many of the processes known to be responsible for the climate. The processes include interactions between atmosphere and land surfaces to attain topographical effects, ocean currents and sea ice. The models simulate global distributions of variables such as temperature, wind, cloudiness and rain. These models have evolved in complexity over time as knowledge of atmospheric physics has increased and computational technology has improved. Today's three-dimensional atmospheric models are linked with mixed-layer ocean models that allow the annual seasonal cycle of solar radiation to be included. As understanding and computational power have increased, more detailed topography of land surfaces, vegetation and atmospheric interactions have been described more completely and smaller grids are being used. As such mechanistic details have increased, the simulation of current climates has improved both seasonally and spatially; however, relatively large discrepancies sometimes still occur.

Two climate models were run to the year 2100 for the National Assessment Program in the USA. One showed regional temperature increases of 5°C in winter and 3°C in summer by the year 2060, while the other predicted even greater increases. The models did not agree on specific regional climate changes, e.g. one had precipitation increases and the other decreases in the southeastern USA during the summer. It appears reasonable to conclude that since the concentration of radiative gases is clearly increasing in the atmosphere and the theoretical reasons for causing warming are not disputed, the conditions to induce warming are in place. Many atmospheric scientists agree with this assessment.

Agriculture provides a sizable contribution to the radiative gases that appear to be the driving forces in climatic change. The primary sources of these gases are the fossil fuel used in agricultural activities, soil carbon (C) loss because of tillage operations associated with crop culture, burning crop and forest residues, raising livestock and consequent manure-handling operations, manufacture and utilization of N fertilizer, and growing of flooded rice. Rice production in flooded paddies and lagoon storage of barnyard manure cause the production of relatively large quantities of CH4, while various aspects of fertilization result in the release of N2O. Methane and N2O cause considerably more radiative forcing (21 and 310 times, respectively, per unit mass of gas) than does atmospheric [CO2].

A large amount of C is stored in the soil and is relatively labile. It is subject to management as agricultural practices may result in the gain or loss of C from the soil. Less tillage usually results in more soil C accumulation and the resulting desirable attributes associated with soil conservation and sustainable crop production. Recent advances in herbicide technology make less tillage more economically feasible, because the primary reason for tillage in crop production is to control weeds. Crop production that utilizes improved herbicides allows reduced cultivation and also results in the use of less fossil fuel than do soil tillage operations that require high energy. In addition, less tillage usually results in secondary benefits such as better water infiltration, greater soil aggregate stability, lower susceptibility to erosion, and improved plant water relations with a lower incidence of crop drought injury. Organic C compounds bind soil aggregates and cause them to resist the breakdown but repeated tillage induces their degradation.

Animal manure represents a relatively small overall contribution of N2O, CH4 and CO2 to the atmosphere. However, modern methods of livestock production that result in concentrations of animals have caused considerable concern among the public for finding ways to minimize undesirable odours, contamination of water sources, and atmospheric pollution by subsequent radiative gases from manure. Even though production of livestock in large concentrations accentuates the problem in a local community, the overall effect of producing a similar number of livestock animals that are evenly distributed over a wide area probably would not result in a very different overall impact, except for the production of methane in lagoons.

Methane production in flooded rice is correlated with biomass production during vegetative growth, but in areas where two crops per year are grown some management practices can be used to reduce CH4 production and emission without yield loss. Methane emission is highly sensitive to water management; however N2O emissions may result from practices that minimize CH4 production. Additional information is needed to find ways to minimize both CH4 and N2O emissions during flooded rice production.

On a global scale, the C cycle consists of large C reservoirs having flows from one reservoir to another. The largest of these reservoirs is the ocean, followed by soil, atmosphere and, finally, living organic matter. Natural processes in the oceans and plant biomass are responsible for most CO2 absorption and emission. The most important natural processes are the release of CO2 from oceans, aerobic decay of plant materials, and plant and animal respiration. Green plants, through photosynthesis, sequester a great deal of C and at the same time return about 50% of that sequestered C to the atmosphere through respiration. The remaining 50% becomes biomass that is eventually oxidized slowly through microbial decomposition and also released to the atmosphere.

Soil is the major repository site of C where organic matter decomposition takes place. Organic matter in the soil consists of both living and non-living components. The living component is composed of plant roots, soil microorganisms and animals, while the non-living component includes remnants of microorganisms and plant and animal materials. New non-living organic matter typically decomposes rapidly, leaving a residual quantity that becomes more recalcitrant with time so that some organic C may remain in soil for thousands of years. Plant materials produced in a C-rich environment may have a higher percentage of lignin and therefore be more resistant to decomposition. Little is known about the effects of high CO2 environments on the rate of decomposition of the materials produced and therefore on litter and soil organic matter accumulation. The rate of decomposition depends on temperature, moisture, chemical composition of the decomposing material, the soil chemical environment and land use. Thus, the amount of C in soil depends on the balance between the input of photosynthetically fixed C and the loss of C through biomass decomposition. Agricultural practices modify both of these processes.

Substantial quantities of C are temporarily stored in plant materials. Natural ecosystems, particularly forests, store C for relatively long periods in tree trunks. The percentage of the earth's terrestrial surfaces covered with natural forests has decreased from 46% in pre-industrial times to only about 27% today. However, tree plantations are becoming more important in today's society and provide C storage for intermediate periods in vegetative structures. Fast-growing trees are managed to provide useful pulp and structural fibres and are grown on approximately 130 Mha.

The major changes in the earth's atmosphere are the concentrations of CO2, which have increased by about 25% since the beginning of the industrial revolution. Carbon dioxide enhances photosynthesis and depresses plant respiration; these effects are expected to increase plant growth as well as affecting various other processes. However, a number of plant physiological processes are also affected by changes in temperature, ozone, ultraviolet radiation, nutrients and water, all of which are variable factors often associated with climatic change. This book addresses the way the most important food and fibre crops react to these physical and chemical changes and how we might expect the hypothesized changes to impact humanity's ability to live in the changing environment.

This book examines the case for man-induced climatic changes, the role of agriculture in these apparent changes, and the impact of those changes on agriculture. As a consequence of changes in food and fibre production, society will surely be affected. Some regions will likely be affected negatively, while other regions may benefit. Since trade and commerce are important today, the causes of economic shifts may not be transparent to the public but, nevertheless, the changes will occur. Most of the crops of major economic consequence are considered and the impacts of environmental changes are reviewed. Crops that are important for human food (rice, wheat, soybean, vegetable crops and root and tuberous crops) are examined. Crops that are used primarily as animal foods (maize, sorghum, productive forage crops and rangelands) are also reviewed. Crops grown primarily for fibres, such as cotton and tree crops, are included as well as some desert-grown species harvested for making beverages and other uses. The impact of environmental factors on various physiological processes and on the yield of the harvestable components are evaluated. Plants with C4 and C3 type photosynthesis and plants with crassulacean acid metabolism (CAM) are compared for their sensitivities to likely environmental changes.

Crop/weed and crop/insect pest interactions and the relative importance of pests on crop production in a changing climatic environment are discussed. The role that conventional breeding procedures have played in the past and this role in a changing environment, along with recent innovations in trans-genic techniques, are contemplated. Transgenic techniques have only recently been applied to the solution of abiotic problems. Introductory studies suggest that dramatic and often unexpected responses to environmental stresses may be obtained by altering certain genes. Tolerance to more than one environmental stress may be increased as a result of changing a single gene. The importance of developing cultivars tolerant of various environmental stresses will increase and both transgenic and conventional breeding procedures will be essential.

Global warming will probably have a negative impact on tropical regions and any other areas where high temperature or inadequate rain often limits crop productivity. Regions where cold temperatures are the primary factor limiting crop production will probably benefit most from warming. Farmers in temperate regions, where most food is produced, will find ways of altering production practices to avoid the occurrence of particularly temperature-sensitive crop growth stages during brief periods of extremely high temperatures. The relative importance of particular crops in certain areas will likely change due to global warming and economic factors will determine these changes.

The importance of altering cultural practices and engineering techniques will also continue to be urgent to a thriving human culture. In temperate regions, the greatest risk to crop production associated with global climatic change is being caused by changes in frequency of extreme events. Unexpected early or late frosts can destroy the production of a crop in an otherwise favourable season. Increased incidences of drought or floods can likewise drastically alter crop production potential. These considerations and conditions will favour the industrialized regions of the world and place people in regions less able to change at an even greater disadvantage than they are today. Ways of extending the advantages of science and technology to disadvantaged people remain one of society's most daunting challenges.

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