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CONSTRAINTS ON THE EXPANSION OF THE GLOBAL FOOD SUPPLY
by Henery W Kindall and David Pimentel,
from Ambio Vol. 23 No. 3, May 1994
The Royal Swedish Academy of Sciences

We examine whether and how global food production may be increased to provide for a world population expected to double by about 2050. Increasing current food production more than proportional to population growth is required so as to provide most humans with an adequate diet. We examine the possible expansion of food supplies to the year 2050, the inventory of presently utilized and potentially available arable land, rates of land degradation, and the limitations of water and biological resources. Serious degradation and loss of the world's arable land is taking place and expansion of irrigation, vital for food production, is becoming more costly and difficult. A business-as-usual scenario points to looming shortages of food. Additional stress from possible climatic alteration and enhanced ultraviolet radiation may make the provision of adequate food supplies extremely difficult to achieve. The nature of the changes that are required to make sufficient food available are identified.

INTRODUCTION

World population is projected to continue increasing well into the next century. A central question is whether and how global food production may be increased to provide for the coming population expansion. It would be necessary to increase current levels of food production more than proportional to population growth so as to provide most humans with an adequate diet. There are a number of actions that may be taken to help this food expansion, but there are also a number of constraints that make expansion of food output difficult. In this paper we examine the expansion of per capita food supplies required in the light of the current range of expectations of population growth, the inventory of currently utilized and potentially available arable land, rates of land degradation, and the limitations of water and biological resources. We make assessments of the prospects of achieving the needed growth of the global food supply to the year 2050, when the world's population is projected to have about doubled. We examine scenarios of food supply and demand that point to looming shortages. We do not analyze the problems of providing energy, capital, and other needs to support increasing numbers of people.

POPULATION AND FOOD

Numbers and Growth

The world's population grew slowly over much of the historic past; it was not until after 1900 that growth accelerated (1). The 1992 population was 5.5 billion. World population is now increasing at about 1.7% yr, corresponding to a doubling time of 40 years. Recently, a gradual decrease in the fertility rate has slowed in a number of countries (2, 3), most notably in China and India (4), which has led to upward revisions in population forecasts. The world population will grow by just under 1 billion people during the decade of the 1990s.

Figure 1 shows three such projections for world population (5). The United Nations has concluded that if the world's fertility rate were to fall to replacement levels during the period 1990-1995 and remained there, the world population would reach 7.8 billion in 2050. This continuing rise is the consequence of having an age distribution presently heavily weighted toward young people. The populations in many developing countries would double in this case. A population of 7.8 billion, under this implausible assumption, is only slightly less than the low projection for that year, which thus appears unrealistic. For the purposes of this paper we will employ the medium fertility estimate. The 2050 world population in this scenario is expected to be 10 billion.

Food: History and Supply

In the early 1960s, most nations were self-sufficient in food; now only a few are. In the period 1950-1984, the introduction of high-yield crops and energy intensive agriculture ushered in the Green Revolution, leading to increased crop production. World grain output expanded by a factor of 2.6 in this period (6, 7) increasing linearly, within the fluctuations. Except for parts of Africa, production exceeded population growth throughout the world. Per capita production has now slowed (8) and appears to be declining. Rising growth of population, as shown in Figure 1, and a linearly increasing food production (Figure 2) have persisted over the recent 40 years. Such circumstances have been of concern since Thomas R. Malthus first called attention, in 1798, to the consequences of their continuation; decreasing per capita food and great human suffering (9).

The success of the Green Revolution lay primarily in its increased use of fossil energy for fertilizers, pesticides, and irrigation to raise crops as well as in improved seed. It greatly increased the energy-intensiveness of agricultural production, in some cases by 100 fold or more. Plant breeding was principally aimed at designing plants that could tolerate high levels of fertilizer use and improving the harvest index (10). The Green Revolution was technologically suited to special circumstances: relatively level land with adequate water for irrigation and fertilizer, and in nations that could acquire the other needed resources. The Revolution has been implemented in a manner that has not proved to be environmentally sustainable. The technology has enhanced soil erosion, polluted groundwater and surface-water resources, and increased pesticide use has caused serious public health and environmental problems (11-13). Opportunities exist to reduce these negative environmental and social impacts. Research is underway at most of the International Crop Research Centers to make the Revolution more environmentally and socially sustainable.

Since 1980, there has been some improvement in world crop yields with the rate of increase in total grain production declining slightly. Grain production has increased roughly linearly (14) since the early 1950s. World area planted to grain is down 8% since 1981 (15). However, there are a number of important obstacles to a large, further expansion of the energy intensive practices that underlay the expansion based on the Green Revolution, including economics, technology adoption, and environmental degradation.

At the present time, 2 of 183 nations are major exporters of grain. the United States and Canada.

Food: Availability and Consumption

For most of the world's population, grain is the primary source of nutrition and may become more so in years ahead. It is thus a useful measure in estimating future food needs. The per capita consumption of foods and feed grains supplied per year is shown in Table 1. Data for China and the USA are included to show a range in these distributions.

Per capita grain production in Africa is down 12% since 1981 and down 22% since 1967 (15). Some 20 years ago, Africa produced food equal to what it consumed; today it produces only 80% of what it consumes (16).

Food from marine sources now provide between 1% and 2% of the world's supply of food (17, 18) and the amount, including the contribution from aquaculture, is unlikely to double within the next few decades (John Ryther pers. comm.).

In line with recent studies (19, 20), we estimate that with the world population at 5.5 billion, food production is adequate to feed 7 billion people a vegetarian diet, with ideal distribution and no grain fed to livestock. Yet possibly as many as two billion people are now living in poverty (V. Abernathy, pers. comm.), and over I billion in "utter poverty" live with hunger (7, 19-23). Inadequate distribution of food is a substantial contributing factor to this current situation.

It is clear from the above review that current food supplies, with present patterns of distribution and consumption, appear insufficient to provide satisfactory diets to all, although a recent FAO report indicates that chronic undernutrition in developing countries has improved somewhat (24).

It is generally agreed that, among a number of important global changes, economic and social well-being must improve for that large fraction of the world's peoples now in poverty. This includes more and better food. A doubling of the population would necessitate the equivalent of a tripling, or more, of our current food supply to ensure that the undernourished were no longer at risk and to bring population growth stabilization within reach in humane ways, without widespread hunger and deprivation. Improved nutrition may be achieved by dietary shifts and improved distribution as well as by an increased quantity of food, as discussed later in this paper.

AGRICULTURAL PRODUCTIVITY

Land Resources

Supply: The world's land devoted to food production and in forest and savanna is shown in Table 2.

Less than one half of the world's land area is suitable for agriculture, including grazing; total arable (crop) land, in use and potential, is estimated to comprise about 3000 million ha (25). However, nearly all of the world's productive land, flat and with water, is already exploited. Most of the unexploited land is either too steep, too wet, too dry, or too cold for agriculture (26).

There are difficulties in finding new land that could be exploited for agricultural production. Expansion of cropland would have to come at the expense of forest and rangeland, much of which is essential in its present uses. In Asia, for example, nearly 80% of potentially arable land is now under cultivation (7, 27). In the 1970s, there was a net annual gain in world cropland of nearly 0.7%. The rate of gain has slowed and, in 1990, the net annual gain was about 0.35% yr, largely as a result of deforestation. As much as 70-80% of ongoing deforestation, both tropical and temperate, is associated with the spread of agriculture.

For these reasons we estimate that the world's arable land could be expanded at most by 500 million ha, or a net expansion of roughly one-third. However the productivity of this new land would be much below present levels in land now being cropped.

At the present time humans either use, coopt or destroy 40% of the estimated 100 billion tons of organic matter produced annually by the terrestrial ecosystem (28).

Quality and Degradation: The loss of productive soil has occurred as long as crops have been cultivated. Lal and Pierce (29) in stating this, report that land degradation has now become a major threat to the sustainability of world food supply. This loss arises from soil erosion, salinization, waterlogging, and urbanization with its associated highway and road construction. Nutrient depletion, overcultivation, overgrazing, acidification, and soil compaction contribute as well. Many of these processes are caused or are aggravated by poor agricultural management practices. Taken together or in various combinations, these factors decrease the productivity of the soil and substantially reduce annual crop yields (30-32), and, more important, will reduce crop productivity for the long term (33).

Almost all arable land that is currently in crop production, especially marginal land, is highly susceptible to degradation. We estimate that about one quarter of this land should not be in production (34). This is depressing food production, as well as requiring increased fossil energy inputs of fertilizers, pesticides, and irrigation in an effort to offset degradation.

Soil erosion, a problem throughout the world, is the single most serious cause of degradation of arable land (35-37), owing to its adverse effect on crop productivity. The major cause is poor agricultural practices that leave the soil without vegetative cover to protect it against water and wind erosion.

Soil loss by erosion is extremely serious because it takes from 200 to 1000 years, averaging about 500 years, to form 2.5 cm (1 inch) of topsoil (38) under normal agricultural conditions (39-43). Throughout the world current soil losses range from about 20 to 300 t ha yr, with substantial amounts of nitrogen and other vital nutrients also lost (44). Topsoil is being lost at 16 to 300 times faster than it can be replaced (36).

Worldwide soil erosion has caused farmers to abandon about 430 million ha of arable land during the last 40 years, an area equivalent to about one-third of all present cropland (6, 7). Each year at least 10 million ha are lost to land degradation that includes the spread of urbanization (45). For example, Tolba (46) reported that the rate of soil loss in Africa has increased 20-fold during the past 30 years.

The estimated rate of world soil erosion in excess of new soil production is 23 billion t yr, or about 0.7% loss of the world's soil inventory each year (47). The continuing application of fertilizers (48) has so far masked much of the deterioration and loss of productivity from this process, so that world cropland yield is remaining roughly constant. This appears likely to continue in the next decades. Continued erosion at the current rate will result in the loss of over 30% of the global soil inventory by the year 2050, a truly severe damage and loss, obviously unsustainable over the long run.

Erosion reduces the availability of water (31), as well as nutrients to growing plants and diminishes organic matter and soil biota (29, 49). Reduction of the water available to growing plants is the most harmful effect of erosion.

Soil degradation is affecting 15% of the earth's cropland area (29). In developing countries, the degradation of soil is growing worse owing to increased burning of crop residues and dung for fuel. This reduces soil nutrients (50, 51) and quickly intensifies soil erosion.

Water: Resources and Irrigation

Supply and Use: Water is the major limiting factor for world agricultural production. Crops require and transpire massive amounts of water. For example, a corn crop that produces about 7000 kg ha of grain will take up and transpire about 4.2 million L ha of water during its growing season (52). To supply this much water to the crop, assuming no use of irrigation, not only must 10 million liters (1000 mm) of rain fall per ha, but it must be reasonably evenly distributed during the year and especially during the growing season.

Irrigation: irrigation is vital to global food production: About 16% of the world's cropland is under irrigation. This area contributes about one-third of crop production, yielding about 2 1/2 times as much per ha as nonirrigated land. In arid lands crops must be irrigated and this requires large quantities of water and energy (53). For example, the production of 1 kg of the following food and fiber products requires: 1400 liters of irrigation water for corn; 4700 liters for rice, and 17 000 liters for cotton (54). About 70% of the fresh water used by humans is expended for irrigation (55).

Much of the world's irrigated land is being damaged by salinization and waterlogging from improper irrigation techniques (56). It is sufficiently severe over 10% of the area to suppress crop yields (57). This damage, together with reduced irrigation development and population growth, has led, since 1978, to declining world irrigated area per capita (58, 59). Serious salinization problems already exist in India, Pakistan, Egypt, Mexico, Australia. and the United States. Because salt levels are naturally high in these regions, the problem of salt build-up is particularly severe. Recent research puts the current loss of world farmland due to salinization alone at 1.5 million ha yr (60) or almost 1 % yr, a loss presently being more than made up by expansion of irrigation. If the damage continues, nearly 30% of the world's presently irrigated acreage will be lost by 2025 and nearly 50% lost by 2050, losses increasingly difficult to make up.

Another damaging side effect of irrigation is the pollution of river and stream waters by the addition of salts.

Water Shortages: Pressures from growing populations have strained water resources in many areas of the world (59). Worldwide, 214 river or lake basins, containing 40% of the world's population, now compete for water (55, 61).

In many areas of the world, irrigation water is drawn from "fossil" aquifers, underground water resources, at rates much in excess of the natural recharge rates. The average recharge rate for the world's aquifers is 0.007% yr (62). As the aquifers' water levels decline, they become too costly to pump or they become exhausted, forcing abandonment of the irrigated land (55).

Africa and several countries in the Middle East, especially Israel and Jordan, as well as other countries, are depleting fossil groundwater resources. China has severe agricultural problems (13). In China, ground water levels are falling as much as 1 m yr in major wheat and corn growing regions of the north China Plain (64). Tianjin, China, reports a drop in ground water levels of 4.4 m yr (58, 59), while in southern India, groundwater levels are falling 2.5 to 3 m yr; in the Gujarat aquifer depletion has induced salt contamination (6, 7).

The prospect for future expansion of irrigation to increase food supplies, worldwide and in the US, is not encouraging because per capita irrigated land has declined about 6% since 1978 (57). Greatly expanded irrigation is a difficult, and probably unsustainable solution to the need for expansion of agriculture output (59) because of the rapidly accelerating costs of irrigation (57).

Greenhouse Effects

The continuing emission of a number of gases into the atmosphere from human activities, including chlorofluorocarbons (CFCs), methane, and, most important, carbon dioxide, is now thought likely to alter the global climate in the years ahead, a consequence arising from the greenhouse effect (65, 66). Worldwide changes in rainfall distribution are expected, including drying of some continental interiors as well as possible increases in climatic variability.

Increased variability in temperature and rainfall can, in many circumstances, be damaging to agricultural productivity. There are expected to be CO2-induced effects on productivity and growth of plants, including crops and weeds, and collateral effects on plant pathogens and insect pests. There may be decline or loss of ecosystems that are unable to accommodate a rapid climate change. The major impact will be caused by changes in rainfall and water availability to crops. Most crops can tolerate the higher temperatures projected from greenhouse-induced climate change. The detailed consequences are difficult to predict, in part because the expected global average temperature rise and changes in weather patterns have substantial uncertainties. The temperature rise expected from a doubling of the atmospheric CO2 level—which, in the absence of carbon emission controls, will occur a decade or so before the year 2100—is "unlikely to lie outside the range 1.5- to 4.5-C" (67). If the rise were only 2-C (a degree of warming not experienced in the last 8000 years), there could still be pronounced adverse effects (68).

The 1988 US experience is enlightening. It was the hottest year on record to that time which, accompanied by a mid-continent drought, resulted in a 30% decrease in grain yield, dropping US production below consumption for the first time in some 300 years. Similarly, Canadian production dropped about 37% (69).

Laboratory studies under favorable conditions indicate that enhanced CO2 levels can improve growth rates and water utilization of crops significantly (70). Under field conditions, the estimated increase in yields are projected to be only one-quarter to one-third of that observed in the controlled greenhouse conditions without taking into consideration other deleterious consequences of climate change that also may be present and yields may, in fact not improve at all (71).

Ozone Depletion

Ground-level ultraviolet enhancement arising from O3 loss in the upper atmosphere from the anthropogenic emission of chlorofluorocarbons can affect natural systems' productivity, alter pest balances, as well as affect the health of humans and surface and marine animals. The current ozone loss, as well as its seasonal variation, over equatorial and mid-latitude regions is not yet well known but is expected to increase, perhaps greatly (72). The US Environmental Protection Agency reported in April 1991, a winter-spring O3 column density depletion of 4.5-5% in mid-latitudes. More recently, there is evidence of a slow but steady ozone depletion over most of the globe; between 40- and 50-N the decline is as great as 8% per decade (73, 74). Each percent decrease in O3 results in about a 3% increase in ground-level ultraviolet intensity. Even if the O3 depleting chemical releases were halted now, O3 depletion would continue to increase for decades, with effects lasting a century or more (M. McElroy pers. comm.).

Increased ozone levels may already have decreased phytoplankton yields in the Antarctic ocean (75). Plant responses to ultraviolet radiation include reduced leaf size, stunted growth, poor seed quality, and increased susceptibility to weeds, disease, and pests. Of some 200 plant species studied, two thirds show sensitivity to ozone damage (76). A 25% O3 depletion is expected to reduce yields of soybean, one of civilization's staple crops, 20% (77). Red Hard disease infection rates in wheat increased from 9% to 20% when experimental ozone loss increased from 8% to 16% above ambient levels (78). Clearly, the potential exists for a significant decrease in crop yields in the period to 2050 from enhanced surface ultraviolet levels.

Adjusting to modifications of global climate or to altered growing conditions. caused by greenhouse gases or from enhanced ultraviolet, might stress management of agricultural systems greatly, especially if wholly new crops, and new procedures had to be developed for large areas of the world. Important uncertainties in the magnitudes of the effects expected may persist for a decade or so.

IMPROVING THE FOOD SUPPLY

Diet Modification

Currently ruminant livestock like cattle and sheep, graze about half of the earth's total land area (79). In addition, about one-quarter of world cropland is devoted to producing grains and other feed for livestock. About 38% of the world's grain production is now fed to livestock (79). In the United States, for example, this amounts about 135 million tons yr of grain, of a total production of 312 million tons yr, sufficient to feed a population of 400 million on a vegetarian diet. If humans, especially in developed countries, moved toward more vegetable protein diets rather than their present diets, which are high in animal protein foods, a substantial amount of grain would become available for direct human consumption.

Agricultural Technologies

There are numerous ways by which cropland productivity may be raised that do not induce injury over the long term, that is, are "sustainable" (26, 80-82). If these technologies were put into common use in agriculture, some of the negative impacts of degradation in the agro-ecosystem could be reduced and the yields of many crops increased. These technologies include:

Energy-Intensive Farming. While continuation of the rapid increases in yields of the Green Revolution is no longer possible in many regions of the world, increased crop yields are possible by increasing the use of fertilizers and pesticides in some developing countries in Africa, Latin America, and Asia (83). However, recent reports indicate a possible problem of declining yields in the rice-wheat systems in the high production areas of South Asia (J. M. Duxbury pers. comm.)

Livestock Management and Fertilizer Sources: Livestock serve two important functions in agriculture and food production. First, ruminant livestock convert grass and shrub forages, which are unsuitable for human food, into milk, blood, and meat for use by humans. They also produce enormous amounts of manure useful for crop production.

Soil and Water Conservation: The high rate of soil erosion now typical of world agricultural land emphasizes the urgency of stemming this loss, which in itself is probably the most threatening to sustained levels of food production. Improved conservation of water can enhance rainfed and irrigated crop yields, as discussed below.

Crop Varieties and Genetic Engineering: The application of biotechnology to alter certain crop characteristics is expected to increase yields for some crops, such as developing new crop varieties with better harvest index and crops that have improved resistance to insect and plant pathogen attack.

Maintaining Biodiversity: Conserving biodiversity of plant and animal species is essential to maintaining a productive and attractive environment for agriculture and other human activities. Greater effort is also needed to conserve the genetic diversity that exists in crops worldwide. This diversity has proven extremely valuable in improving crop productivity and will continue to do so in future.

Improved Pest Control: Because insects, diseases, and weeds destroy about 35% of potential preharvest crop production in the world (84), the implementation of appropriate technologies to reduce pest and disease losses would substantially increase crop yields and food supplies.

Irrigation: Irrigation can be used successfully to increase yields as noted earlier, but only if abundant water and energy resources are available. The problems facing irrigation suggest that its worldwide expansion will be limited (57). Owing to developing shortages of water, improved irrigation practices that lead to increased water in plant's root zones are urgently needed.

Constraints

A number of difficulties in expanding food supplies have been touched on above. Others are presented below:

There is a need to decrease global fossil-fuel use and to halt deforestation, in order to lessen carbon emissions to the atmosphere (85). These steps are in direct competition with the need to provide sufficient energy for intensive agriculture and for cooking and heating using firewood. A major decrease in fossil-fuel use by the industrial countries would require adoption of new technologies based on new energy sources, with improved conversion and end-use efficiencies, on a scale that would require 40 years at minimum to implement fully, even in favorable circumstances (86). Yet a three- or fourfold increase in effective energy services to the earth's peoples would be required to yield the improvements needed in the quality of life in a world of roughly doubled population. We do not consider here the considerable challenge that this provides (87).

Even assuming that sufficient fossil or other fuels were available in the future to support energy-intensive agriculture in developing countries, several constraints appear to make this difficult. These include: the high economic costs of the energy inputs to those countries that already have serious debt problems; the lack of rainfall and/or irrigation water preventing effective use of the inputs; and farmers in developing nations who are not educated in the use of intensive agricultural methods and who change their ways slowly.

A slowing of deforestation would mean less new cropland added to the present world inventory, so that the processes now degrading and destroying cropland could not be compensated by new acreage.

Population growth remains a basic problem. About 0.5 ha capita of cropland is needed to provide a balanced plant/animal diet for humans worldwide (88). For the 1990 population of 5.5 billion, only 0.27 ha capita is now available and this is likely to decline further. Moreover, the rate of population growth itself, especially in many developing nations, intensifies the problems of coping with shortages owing to the difficulty of providing the economic expansion required (89).

A major difficulty arises simply from the rate with which food supplies would have to be expanded to pace or to exceed population growth rates in those countries experiencing high growth rates. In order to stay even with population growth it will be necessary to expand food supplies, globally, by the rate of population increase. For many countries the rate of population expansion is in the range 2-3% yr. As an example, in order to achieve an increase of 50% in the per capita food production, by the end of a population doubling, the rate of expansion of agricultural production must be appropriately larger. If the population grows at 2% yr, the food production must increase at 3.2% yr, if it is 3% yr, the food production must grow at 4.8% yr.

During the Green Revolution the world grain yield expanded at 2.8% yr. As noted earlier, this rate of expansion has slowed and, it appears, is unlikely to be resumed (90) although some countries in Asia and Latin America are still gaining total annual increases in grain yield. In the US, which has one of the best records with corn, the rate of increase from 1945 to 1990 was about 3% yr. Since 1980, this rate has slowed. However, with wheat the record is not as good as with corn, the increase in world grain yield is less than 2% yr. If the historical record is any guide, no nation with a population growth rate above 2% yr has much hope of improving its per capita supply of food unless it receives very substantial external aid and support. Of course these rates of increase for both population and food production, if achieved. cannot be sustained indefinitely.

THE FUTURE

Introduction

Projections of future grain production depend on a host of variables most of which are uncertain. It is not possible to make useful forecasts. As an alternative we consider three scenarios, for the period to the year 2050. The first assumes a continuation of present trends, patterns, and activities. This is referred to as Business-As-Usual, or BAU. Population growth is assumed to follow the UN medium projection leading to about 10 billion people by 2050, soil erosion continues to degrade land productivity, salinization and waterlogging of the soil continues, and groundwater overdraft continues with supplies in some aquifers being depleted; there is a modest expansion of cropland at the expense of world forests, and a slight expansion of irrigation. In BAU, the consequences of the greenhouse effect and of ultraviolet injury are ignored, and the developed world does not provide significantly more aid to the developing world than at present, nor does the developing world continue, on balance, its current rate of economic development (91).

A pessimistic scenario considers qualitatively the possible consequences of climatic changes and ground-level ultraviolet radiation increase that could depress crop yields, coupled with the high UN population growth projection, leading to nearly 13 billion people in 2050. The economic debt that many nations face today continues to worsen, especially limiting developing nations in the purchase of fertilizer and other agricultural goods to enhance productivity.

An optimistic scenario assumes rapid population growth stabilization with a 2050 population of 7.8 billion, significant expansion of energy-intensive agriculture and improved soil and water conservation with some reclamation of now-abandoned land. In this scenario, the developed countries provide the developing nations with increased financial resources and technology and a more equitable distribution of food is achieved. There is a shift from high animal protein to more plant protein consumption in the developed countries, freeing more grain for the developing nations.

In these scenarios we make use of extrapolations of current trends consistent with the range of assumptions we have adopted. This procedure is inherently unsatisfactory owing both to the difficulty of determining trends from fluctuating historical data and because few trends continue for periods comparable to the interval of interest to us. Nevertheless, it does over a number of scenarios, shed light on the range of achievable futures.

Business As Usual (BAU)

Grainland declined from 718 million ha in 1980 to 704 million ha in 1991 (92), a decline we assume continues, leading to 620 million ha in 2050. There is 0.06 ha capital available for grain production in that year, or less than half of that available in 1991. This will create major obstacles to increasing grain food production, especially if land degradation continues. The rate of loss we assume is about half that projected for the next 25 years in The Netherlands report on the National Environmental Outlook 1990-2010, (93)

In BAU, we make the optimistic assumption that a modest expansion of irrigation will continue as it has recently. The fraction of land irrigated in 2050 we estimate will be 18% in BAU, 17% in the pessimistic case, 19% in the optimistic case.

Estimates suggest that degradation can be expected to depress food production in the developing world between 15% and 30% over the next 25-year period, unless sound conservation practices are instituted now, and that the "total loss in productivity of rainfed cropland would amount to a daunting 29%" due to erosion of topsoil during the same time period (94).

Despite the increased use of fertilizers. the rate of increase in grain production appears to be slowing (55, 95).Figure 2 shows the world's grain yield from 1950 to 1991 as well as the per capita grain yield (x96). In recent years, 1985 to 1991, the total growth rate has dropped below 1.4% yr, less than the current rate of world population growth. Based on past trends we estimate a 300% increase in the use of nitrogen and other fertilizer by the year 2050 and about 12% expansion of irrigated land, consistent with BAU.

In view of the constraints we have identified we conclude that an expansion of 0.7% yr in grain production is achievable in the decades ahead. With this rate of expansion, there would be a 50% increase in annual grain production by 2050 compared to 1991, with the world per capita grain production decreasing about 20%. These projections are shown in Figure 3. The 2050 per capita number is about the same as it was in 1960. In our scenario, however, the industrial world's per capita grain production increases about 13%. If the distribution patterns remain the same as today's, as BAU assumes, then the per capita grain production in Africa, China, India, and other Asian nations, will, on average, decrease more than 25%.

In BAU, most developing nations suffer reductions in per capita grain production. Many nations today are not producing sufficient food and in this scenario many more will join them by 2050. This conclusion is consistent with other assessments (15). One study concluded that if the African population continues to grow, and agricultural production remains roughly constant, the food produced would, in 25 years, support about 50% of its year 2000 population; for the Middle East about 60% of its population. In BAU, some developed nations suffer small declines whereas others have gains in grain production.

In general, it appears that Africa, as noted earlier, as well as China and India, will face particularly severe problems in expanding their food supplies in the coming decades. The people of these regions are likely to comprise almost two thirds of the developing countries', and over half of the world's, population, both in 2025 and 2050.

The US appears to have the potential of generating food surpluses for some years, a potential that it shares with parts of Europe, including Eastern Europe, Canada, and possibly other regions. The longer term prospects are unknown in view of difficulties which may appear later.

Pessimistic Scenario (PS)

Scenario PS adopts most of the assumptions in BAU, but includes several other factors which may decrease the rate of grain production in the years ahead. If the population growth rate continues only slightly lower than it is today to the year 2050, the world population will rise to about 13 billion, more than double the present population. A recent analysis (97, 98) of the consequences of climatic change on world food production, not including problems arising from the availability of irrigation water, concluded that decreases in global food production would

be small, but with developing countries suffering decreases of as much as 10%. We believe that, in the period to 2050, the greenhouse effect and ozone loss could together depress grain yields on a global basis by 10% to 20%. We base our estimates on current rates of cropland loss , continued decline in per capita irrigation (59), degradation of irrigated land , and continued decline on the rate of fertilizer use by some farmers in developing countries (26,99). A moderate combination of these adverse factors leads to grain production in 2050 about 15% below BAU. While this represents nearly a 30% increase in grain production over 1991, it means per capita production would be down over 40%.

There is, in this scenario, little hope of providing adequate food for the majority of humanity by the middle or later decades of the period we consider.

Optimistic Scenario (OS)

If rapid population growth stabilization can be effected, leading to a world population of 7.8 billion instead of 13 billion by the year 2050, then grain production adequate for the population might be achievable. This would require a near doubling of today's production. Soil and water conservation programs would have to be implemented to halt soil degradation and the abandonment of cropland. Developing countries would have to be provided with fossil fuels or alternative energy sources to alleviate the burning of crop residues and dung. Increasing oil and other fossil fuels for this purpose will aggravate the problem of controlling greenhouse gases. Irrigation would have to be expanded by about 20%. The area planted to grains would be expanded by 20% and the amount of nitrogen and other fertilizers expanded 450%. Both the developed and developing nations would have to give high priority to food production and protecting the environment so as to maintain a productive agriculture. The developed countries would have to help finance these changes and also provide technology to the developing nations. At the same time, with diet shifts in the developed world, the 2050 population of 7.8 billion might be fed an adequate diet.

If efforts were made to triple world food production, compared to today's yield, then all of the above changes would have to be made, plus increasing the level of energy intensiveness in the developing world's agriculture by 50- to 100-fold. This would include a major expansion in world irrigation. Such increases appear to be unrealistic. Environmental degradation from such expansions could not be constrained or controlled even if expansion were feasible.

SUMMARY AND CONCLUSIONS

The human race now appears to be getting close to the limits of global food productive capacity based on present technologies. Substantial damage already has been done to the biological and physical systems that we depend on for food production. This damage is continuing, and in some areas is accelerating. Because of its direct impact on global food production injury and loss of arable land has become one of the most urgent problems facing humanity. Of these problems, this is perhaps the most neglected.

Controlling these damaging activities and increasing food production must now receive priority and resources commensurate with their importance if humanity is to avoid harsh difficulties in the decades ahead.

Attempts to markedly expand global food production would require massive programs to conserve land, much larger energy inputs than at present, and new sources as well as more efficient use of fresh water. all of which would demand large capital expenditures. The rates of food grain growth required to increase the per capita food available, in the light of present projections of population growth, are greater than have been achieved under any but the most favorable circumstances in developed countries.

Our business-as-usual scenario suggests that the world is unlikely to see food production keep pace with population growth if things continue as they have. If they do continue then the world will experience a declining per capita food production in the decades ahead. This decline would include spreading malnutrition and increased pressure on agricultural, range, and forest resources.

Should climatic alteration from greenhouse warming and enhanced ultraviolet levels impose further stress on agricultural systems, the prospects for increased food production would become even less favorable than they are at present.

In our opinion. a tripling of the world's food production by the year 2050 is such a remote prospect that it cannot be considered a realistic possibility. If present food distribution patterns persist the chance of bettering the lot of the majority of the world's peoples vanishes. The likelihood of a graceful and humane stabilization of world population vanishes as well. Fertility and population growth in numerous developing countries will then be forced downward by severe shortages of food, disease, and by other processes set in motion by shortages of vital resources and irreversible environmental damage.

A major expansion in food supply would require a highly organized global effort-by both the developed and the developing countries-that has no historic precedent. As yet a major commitment from the developed nations to support the needed changes is missing, and inadequate commitment in both developing and developed nations has been made for population stabilization. Governments so far appear to lack the discipline and vision needed to make a major commitment of resources to increase food supplies, while at the same time reducing population growth and protecting land, water, and biological resources. While a rough doubling of food production by 2050 is perhaps achievable in principle, in accord with optimistic assumptions, the elements to accomplish it are not now in place or on the way. A large number of supportive policy initiatives and investments in research and infrastructure as well as socioeconomic and cultural changes would be required for it to become feasible. A major reordering of world priorities is thus a prerequisite for meeting the problems that we now face.

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107. The authors wish to express their appreciation to the following people for their help in the preparation of this work: V. Abernathy, M. grower, S. Chisholm, W. Dazong, M. ElAshty, P Faeth, M. Falkenmark, M. Giampietro, R. Goodland, K. Gottfried, S. Hams, D. Hornig, T. Mount, I. Oka, E. Oyer, M. Paoletti, M. Pimentel, P. Pinstrup Andersen, T Poleman, S. Postel, P. Raven, K. Robinson, T. Scott, L. Stifel and N. Uphoff.

108. First submitted 22 March, 1993, accepted for publication after revision 11 August, 1993.

Henry W. Kendall is J.A. Stratton professor of physics at the Massachusetts Institute of Technology. His principal research area is particle physics, with over 100 papers published. He is chairman of the Union of Concerned Scientists and he has co-authored articles or books on ballistic missile defense, alternate energy sources, controlling oil well fires, nuclear reactor safety issues, and other subjects. He is a 1990 Nobel laureate in physics. His address: 24-514 MIT, Cambridge, 02139, Massachusetts, USA. David Pimentel is professor of insect ecology and agricultural sciences at Cornell University. He has a PhD from Cornell University. His research spans the field of basic population ecology, genetics, ecological and economic aspects of pest control, biological control, energy use and conservation, genetic engineering, sustainable agriculture, soil and water conservation, and natural-resource management and environmental policy. He has over 400 scientific publications. He has served on many national and governmental committees including National Academy of Sciences, US Department of Agriculture, US Dept of Energy, US Department of Health, Education and Welfare, Office of Technological Assessment, US Congress, and the US State Department. His address: Dept. of Entomology, Comstock Hall. Cornell University, Ithaca, NY 14853-0999, USA.

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