Click for next page ( 79


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 78
7 Food, Water, and Changing Climate As scientists work to improve their predictions of change in the earth's climate, a long-stancling question assumes ever- greater currency: How many people can the planet support without using up our natural resources and forever undermin- ing the earth's ability to support people in the future? In other worcls, what is the carrying capacity of the earth? Today, assuming equitable distribution to the 5 billion peo- ple all over the world, the earth certainly provides enough food for an adequate diet. This fact, however, conceals a distressing paradox: In recent years, although the earth produced record amounts of grains, half a billion people were seriously mal- nourished. One reason is the unrelenting poverty that prevents millions from purchasing adequate food even when it is avail- able. Another reason is that in many cases starvation reflects not the absence of food but rather poor distribution due to politics or civil war, as in Sudan and Ethiopia during the 1980s. Societal excuses notwithstanding, in some parts of the world, the ability to provide food increasingly fails to keep pace with population growth. In sub-Saharan Africa, for example, the rate of popula- tion growth is 40 percent faster than the rate of growth in food production. 78

OCR for page 78
FOOD, WATER, AND CHANGING CLIMATE 79 Scientists, economists, and philosophers have been fasci- nated by the notion of carrying capacity since Thomas Malthus suggested in 1812 that rates of increases in food production lagged so far behind population growth that starvation was in- evitable. After years of study and debate, the definition and applications of this concept are still evolving. Malthus thought that ultimately a shortage of food would be the limiting fac- tor on population growth, but his predictions ctid not take into account the remarkable advances in our ability to produce food. THE GLOBAL HARVEST Taking the world as a whole, Lester Brown, in the WorId- watch Institute's annual State of the World assessments, reports that grain production per person has climbed an impressive 40 percent between 1950 and 1984. The "green revolution" of the 1960s-which introduced new varieties of rice and wheat and intensified use of pesticides, fertilizer, and irrigation is respon- sible for a major share of this increase. But from 1984 through 198S, grain production per person fell. While per capita grain production rebounded in 1989, it was still below 1984 levels. Such fluctuations do not suggest long-term trends or imply that environmental deterioration, rather than economic forces or iso- latecl years of adverse weather, is solely responsible. They do illustrate that the earth's ability to supply food to the growing population cannot be taken for granted. Discussions of how many people the planet can support often begin with some widely accepted numbers. Sometime in 1987, for instance, the worId's population crossed the 5-billion mark. Demographers project that by the end of the coming century our numbers will increase to 10 billion or more before they begin to stabilize, with more than 95 percent of the growth occurring in the developing world. Paul Ehrlich, Gretchen Daily, Anne Ehrlich, and Peter Vi- tousek, all at Stanford University, and Pamela Matson, at NASA Ames Research Center, are part of the Stanford Carrying Ca- pacity Group. They attribute carrying capacity not only to

OCR for page 78
80 tudes as we! THE FACES OF GLOBAL ENVIRONMENTAL CHANGE the earth's physical and biological capabilities to provide re- sources necessary for food, clothing, and other essentials, but to humanity's ability to develop new technologies and atti 1. Through cultural evolution, they explain, hu- man beings may quickly shift their demand for and ability to extract different resources. At the same time, natural and human-induced changes with global environmental change as a primary example alter the distribution and abundance of ~ ~ , . ~ ~ ~ ~ ~ ~ ~ 1 _ ~ ~ ~ ~ 1 ~ available resources. coon araws parncu~ar ar~ennon Because ~ production is sensitive to changes in the environment, particu- larly to changes in temperature and precipitation, and yet basic human nutritional requirements are relatively inflexible. The ability of the earth to produce food depends heavily on elements of what can be considered our species' capital: groundwater, the genetic diversity of nonhuman species, and productive soil. These natural assets, which population ecolo- gist Ehrlich describes as part of "humanity's one-time inheri- tance," once seemed limitless. Now, he explains, groundwater in many places is being pumped faster than it is being recharged. Untold species and millions of genetically distinct populations that potentially provide the genetic resources for new crops dis- appear each year as tropical forests are cleared. Fertile soil, which is generated at rates on the order of inches per millen- nium, is losing its productivity in many parts of the world because of erosion or salinization, a process in which salts re- main as irrigation water evaporates from the soil surface. Such direct human impact on carrying capacity is especially evident on marginal land in arid and semiarid regions Darticularlv in Africa. -A-- -, ~ A 1988 study by Robert Chen and colleagues for the Alan Shawn Feinste~n World Hunger Program at Brown University estimated that even if food were equitably distributed (with nothing diverted to livestock), the amount of food produced On 1985 an all-time record-could have provided a minimal veg- etarian diet to about 6 billion people, a number we will exceed by the end of the century. The same global harvest, allowing a diet with about 15 percent of the calories from animal products,

OCR for page 78
FOOD, WATER, AND CHANGING CLIMATE 400- 300 Oh o 200 y 100 ~ O l l 1950 1960 1970 81 ,~ ~4 1 1 1 1980 1990 2000 YEAR World grain production per capita, 195~1988. (Reprinted, by permission, from State of the World 1989. Copyright (I) 1989, by Worldwatch Institute, all rights reserved.) could feed some 4 billion people. A diet consisting of 35 per- cent animal products, similar to that consumed by most North Americans and West Europeans today, could be provided to only about 2.5 billion people less than half of today's population. These estimates assume that 40 percent of the food harvested will not be available to human use because of wastage and con- sumption by pests. But as many economists have pointer! out, if the poorest people had more wealth, the increased demand might well stimulate production of more food. So the issue of whether there would be enough food is never quite resolved. Based on current projections for population growth and increases in per capita income, world demand for food in the middle of the next century couIct easily be 2 to 2.5 times the level of the mid-19SOs, according to calculations by William Easterling Ill and Pierre Crosson, both of Resources for the Future, and Martin Parry, of the Atmospheric Impact Research Group at the University of Birmingham in the United Kingdom. Crosson and his colleague Norman Rosenberg, also at Resources for the

OCR for page 78
82 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE Future, believe there is room for optimism. They predict that if food production grows at or even slower than the current rate, there would still be enough food for the 10 billion people by the time they arrive. They temper this conclusion with the caveat that the ability to produce enough food can be sustained only if techniques that are less environmentally damaging than the current monoculture crops and heavy applications of chemical fertilizers and pesticides are developed and used. The expansion of food production to feed the worId's grow- ing population is not likely to be accomplished by increasing the worId's cultivated area. Even though only half of the potentially arable land is being farmed, expansion onto new land is lim- ited because remaining land may be geographically inaccessible (uncultivated land is most scarce in many developing coun- tries where population is growing fastest), infested by pests that transmit parasitic diseases such as trypanosomiasis (sleeping sickness), or covered by soils so thin or acidic that agriculture cannot be sustained. In fact, the land area planted in grain worldwide has actually declined by about 7 percent since 1981, owing mainly to abandonment of cleteriorated land, conversion of cropland to nonfarm uses, especially in clensely populated re- gions, and "set-asides" in the United States (a practice in which farmers are compensated for retiring cropland to limit overpro- duction). The primary prospects for expanding food production are the potential for increasing yields on existing agricultural land through more intensive cropping, increased fertilizer use, and the development of more productive strains, and increased reliance on anct clevelopment of new methods for harvesting food from the oceans. THE EFFECT OF GLOBAL WARMING ON FOOD PRODUCTION Whether the projected global warming will be good or bad for agriculture depends on the specific location and on how much warming occurs. Little is certain as scientists try to sort

OCR for page 78
FOOD, WATER, AND CHANGING CLIMATE 83 out complexities such as how changes in temperature and pre- cipitation patterns may affect agricultural productivity and also how crop yielcl may change as plants respond to increased con- centrations of carbon dioxide, which can stimulate growth and reduce water consumption. Crop yields are certain to decline in some regions and increase in others. But researchers, lacking knowledge about the regional distribution of climate changes, can only estimate where and by how much these shifts in pro- ductivity will occur. Scientists trying to simulate potential effects of global warm- ing on agriculture rely on the same general circulation climate models (GCMs) that other scientists use to study global pro- cesses (see chapter 61. For the purposes of the agricultural mod- els, the GCMs project some important changes in climate. In particular they find that warming will be greatest in the high latitudes, that soils may tend to be ctrier in mid-continental re- gions in summer, and that globally the hydrologic cycle will intensify more rain, more evaporation as the earth's sur- face warms. The models show that with an effective doubling of preindustrial carbon dioxide concentrations (that is, with a combination of all trace greenhouse gases that equals the heat- trapping effect of a doubling of the concentration of carbon dioxide), evaporation on a global basis will increase by 7 to 12 percent. The atmosphere cannot store large amounts of water vapor, anct so precipitation will increase. The increases will not be uniformly distributed, however; nor will the proportions of rain, snow, or dew necessarily remain the same. In a summary of the causes, impacts, and uncertainties as- sociated with the greenhouse effect, Stephen H. Schneider, of the National Center for Atmospheric Research in Boulder, Col- orado, and Rosenberg suggest that if analyses of the effects of temperature changes on evaporation and runoff of water from the land surface are correct, "the greatest impact of greenhouse warming on natural resources will occur because of changes in the seasonality and amounts of precipitation and of evapotran- spiration." Parry, with colleagues Timothy Carter, also of the University

OCR for page 78
84 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE of Birmingham, and Nicolaas Konijn, of Agricultural University in the Netherlands, synthesized results of a multinational study of the impacts of climatic variations on agriculture. The study, sponsored by the International Institute for Applied Systems Analysis and the U.N. Environment Programme, used the God- dard Institute for Space Science (GISS) climate mode} to study the effects on crop yielcis of warming due to an effective dou- bling of carbon dioxide. in the study, Parry and colleagues compared average yields expected under current climate conditions in several Northern Hemisphere regions with average yields that might be expected with effective doubling of carbon dioxide warming. For sim- plicity, they assumed the same technology and management used today, which they acknowledge is not a realistic assump- tion. They did not consider the fertilizing and moisture-saving effects of added carbon dioxicle on the plants. Parry, Carter, and Konijn report that if summer dryness becomes more frequent in mid-latitudes as predicted for the Northern Hemisphere, decreases in yields might occur in the productive areas of North America anct the USSR. In general, they suggest, the higher temperatures would favor higher yields of cereal cror)s now Frown in regions where current tempera- lures limit the growing season. For climate conditions produced by effective carbon dioxide doubling as projected by the GISS model, for instance, wheat yields increase by about one third in the central European region of the Soviet Union, where there is currently a short, coo! growing season. The yields of barley, on the other hand, which thrives under coo! temperate conditions, drop by about 4 percent. Where cereal production is already prone to drought, increased evaporation rates predicted by the climate mode! could limit any increase in yields that would be expected due to higher temperatures. This could be the case in Saskatchewan, for instance, where increases in yields of spring- sown wheat plants could be lessened by one fifth to one third. The direct effects of carbon dioxide on plant growth and use of water complicate efforts to predict how future climate change induced by rising concentrations of greenhouse gases may affect r- - ~

OCR for page 78
FOOD, WATER, AND CHANGING CLIMATE 85 agriculture, forests, and other ecosystems. As carbon dioxide concentrations increase, rates of photosynthesis increase in most plants. At the same time, with rising concentrations of carbon dioxide plants partially close their stomates, the pores through which water vapor and carbon dioxide pass. Because plants use less water (transpiration) per unit leaf area when exposed to elevated levels of carbon dioxide, water use efficiency may increase. So far, effects of carbon dioxide enrichment have been studied primarily under controlled experimental conditions. If the positive direct effects occur in the field, the combinations of increased growth and improved water use efficiency may help offset the negative effects of climate change on crops. WATER SUPPLY, IRRIGATION, AND THE HYDROLOGIC CYCLE One of the more generally accepted conclusions of the gen- eral circulation climate models is that as average global tem- peratures increase, the hydrologic cycle will speed up, increas- ing global precipitation. This does not mean, however, that the added precipitation will fall where or when it is needed. As temperature and precipitation patterns change, so will soil moisture and the timing and magnitude of runoff, with possibly adverse effects for many of the world's important agricultural areas. One likely consequence of these changes would be that demand for water, especially for irrigation, would increase in some regions. The task of estimating future changes in water supply is difficult because the resolution of global climate models is too coarse to represent the complexity of regional or local changes. Many water problems such as floods and drought occur on these spatial scales. Despite their imperfections, however, the models tend to agree on several key points. One point on which models tend to be in agreement in- volves changes in soil wetness, which may be just as impor- tant as changes in atmospheric temperature. The soil moisture regime determines the types and extent of vegetation that can

OCR for page 78
86 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE thrive in a given location. Some studies, such as the ones con- ducted by Syukuro Manabe and Richard Wetherald, both of the NOAA Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, predict that as the concentrations of greenhouse gases increase, soil will become dryer in summer over vast ex- panses of the middle and high latitudes including the U.S. Great Plains, Westem Europe, northern Canada, and Siberia. Mode! results suggest that there would be significant chang- es in runoff patterns under a changing climate. Runoff is sen- sitive to changes in precipitation and to evaporation, which is strongly affected by temperature. In many regions of the world, runoff comes as snow melts. With higher temperatures, relative amounts of rain and snow are likely to shift, as will the timing and speed of snowmelt. Peter H. Gleick, of the Pacific Institute for Studies in Development, the Environment, and Security in Berkeley, California, identifies in the 1989 publication Greenhouse Warming: Abatement and Adaptation a seasonal effect for basins in the western United States In which changes in runoff patterns may alter the likelihood of flooding and the availability of water during such times as the peak irrigation season. Similar changes are predicted for China, Canada, and Europe. One implication of such findings is that if global warming becomes a reality, crop irrigation requirements are certain to increase, but farmers may find it difficult to expand the area of irrigated cropland, or even to maintain present irrigation levels. Agriculture already accounts for three quarters of the fresh water used globally. A 1989 National Research Council study states that in the United States, agriculture accounts for 85 percent of all consumptive uses of water; of this amount, 94 percent is used for irrigation. But if water supplies diminish, other uses such as industry, drinking water, and sanitation would also compete for the available fresh water. Dean F. Peterson and Andrew Keller, both then at Utah State University, computed how three levels of climate change 3C (5.5F) warming, 3C warming with a 10 percent increase in precipitation, and 3C warming with a 10 percent decrease in precipitation-would affect irrigation requirements. In all three

OCR for page 78
FOOD, WATER, AND CHANGING CLIMATE 87 scenarios, irrigation increased because of the longer growing season, shifts in crops and more multiple cropping (more than one crop grown in a growing season), and greater potential evapotranspiration. Irrigation is the underpinning of the world food production system. For millennia, farmers have relied on irrigation to in- crease yields of crops and to free them from the uncertainties in the timing and amount of rainfall. In some areas, irrigation makes farming possible; in others, it augments rainfall, with of- ten dramatic results. In the Unitec! States, only 13 percent of the cropland is irrigated, but this land accounts for nearly one third of the value of crops produced. In 1985, the 270 million hectares (667 million acres) of the worId's irrigated cropland provided nearly one third of the harvest. These predicted changes in pre- cipitation and runoff would affect the availability of water for irrigation and hence food production. EXPLORING AVENUES FOR ADAPTATION The history of civilization is punctuated by swings of climate that have tried the ingenuity of people drawing their livelihood from the land. Sometimes societies can take steps to moderate the severe effects of climate swings by changing pricing struc- tures or providing assistance to farmers. In other cases, historians believe that climate change, com- bined with a lack of adaptation on the part of society, has led to the decline of civilizations. In a joint publication of the U.S. EPA and the U.N. Environment Programme, Martin Parry cites accounts of the Norse settlers in Greenland during the period between 1250 and 1500, the beginning of the Tithe Ice Age. The Norse settled along the GreenIanct coast around 985. By the thirteenth century the 6000 Norse inhabitants in two settlements faced a constellation of stressful circumstances: hostile Inuit, a declining European market for walrus ivory, and sequences of coo! summers and stormy winters. The Norse did not opt to ex- ploit the seas, as the Tnuit did with such success, but continued to raise livestock despite the reduced capacity of the pastures.

OCR for page 78
88 THE FACES OF GLOBAL ENVIRONMENTAL CHANGE Between 1350 and 1450, the Norse abandoned the settlements, while the Tnuit continued to survive there. The experience of the Norse settlers is an extreme example, Parry explains, of how societies "can fad! to identify and im- plement appropriate policies of response, not only to climatic change but to the synergistic effects of a number of concurrent events." This historical lesson, he suggests, shows the value of designing policies that respond to the host of difficult en- vironmental problems facing us today. The effects of climatic disruption can reverberate throughout an entire society, from the fortunes of specific farmers and regions, to national and global food supplies, to trade imbalances and the global economy. As the initial steps in analyzing the impacts of climate change on agricultural yields, the studies mentioned above largely assume that farm policy, management, and technol- ogy remain as they are today. But these systems are far from static. it is inconceivable that farmers will not react. Their liveli- hood is defined by continual adjustment to changing patterns of weather and consumer demand. Farmers in the Midwestern United States, for instance, after experiencing the 1988 drought that reduced corn harvests nationwide by almost 40 percent, took special care to sow their 1989 spring crops early in case the drought persisted. Besides adjustments in planting and harvest dates, other important adaptations at the farm level include changes in tiliage practices; crop varieties, species, and rotations; and fertilizer, herbicide, and pesticide applications. Farmers may also improve existing irrigation efficiency or in- stall new irrigation facilities. At the regional level, agricultural market, transportation, finance, and water resources infrastruc- tures are very likely to change, as are national farm policy and international trade agreements. An analysis by Easterling, Parry, and Crosson finds that if growing seasons in coo! regions become longer and warmer, farmers could increase yields substantially by substituting va- rieties used today for varieties that already thrive elsewhere under higher temperatures. Under conditions of effective car- bon dioxide doubling, if late-maturing rice now grown in central

OCR for page 78
FOOD, WATER, AND CHANGING CLIMATE 89 Japan, for instance, were planted in northern Japan, yields might increase by 26 percent. The efficacy and the cost of these adjustments depend in part on the severity of the climate change experienced. As Cynthia Rosenzweig, a researcher at Columbia University and NASA's Goddard Institute for Space Studies, points out in testimony presented to the U.S. House of Representatives Committee on Agriculture, "Farmers can make some adjustments to less severe climate change by planting earlier, substituting better-adapted crop varieties and species, and Increasing demand for water for irrigation. More severe climate change will likely require ma- jor adaptations, including expansion of irrigation infrastructure, farm abandonment and rural dislocation, In some regions." A special burden may fall on the half billion poorest and hungriest farmers of the world. Robert Kates, director of the Alan Shawn Feinste~n World Hunger Program at Brown Uni- versity, notes that they are increasingly finding themselves re- stricted to ecologically marginal land and water resources as their numbers increase and traditional access to important sea- sonal uses of land or water are lost to development, dams for electricity production, large farms for export crops, or even wildlife and forest conservation. Forced onto marginal land, they add to its degradation. And they may be the ultimate victims of global change, having neither the resources to take advantage of climates more favorable for agriculture nor the resources to cope with a less productive climate. Although critical uncertainties exist about the magnitude and timing of predicted warming and agricultural systems are sure to adjust in many ways, climate change raises long-term concerns about agricultural productivity, depletions of major resources (especially land and water), viability of rural com- munities, and the environment. The concurrent projections of population increases and vulnerability of carrying capacity de- scribed early in this chapter can only add to the already enor- mous challenges currently facing global agriculture.