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View the documentPaper 15 World food production: the past, the present and the future
View the documentPaper 16 Climate change and the future of agriculture

Paper 15 World food production: the past, the present and the future

T. W. Tanton and R. F. Stoner
Institute of Irrigation Studies, University of Southampton

Summary: The paper examines why growth in agricultural production is now slowing and whether present-day intervention measures are capable of reversing this trend. It is argued that the present methodology used to identify potential development projects is unsatisfactory, and as a result many projects fail because they do not address the key issues that limit productivity of the farms. It is further argued that if we are to halt the decline we must therefore adopt a more systematic approach to discovering the key bottlenecks in the agricultural sector. This would then allow the most appropriate intervention measures to be identified. An alternative planning framework is suggested for the agricultural sector that could lead to the development of a better method for identifying effective intervention measures.

Past success

In the past 30 years world food production has undergone an unprecedented rate of growth, largely enabling agriculture to meet the needs of the rapidly growing population of most countries outside Africa. This success story is in contrast to the trends in the preceding years, when increases in production could not meet the expanding demand for food.

This success story has resulted from a number of key factors which combined to give an unprecedented increase in production:

· New lands were brought into production by the rapidly expanding population. In all countries this has traditionally been the main way of producing more food to feed a growing population and, in most cases, when land is plentiful this provides an economic solution. Unfortunately, with the increased pressure on land, new farmers are forced on to more marginal land, resulting in a rapid degradation of much of the world's soil resource, our short-term success being partly at the expense of long-term sustainability.

· The introduction of short straw, disease tolerant varieties of wheat and rice, capable of responding well to artificially high levels of fertility.

· The rapid expansion in area of irrigated land, with its potential to produce consistently high yields, has played a significant role in increasing agricultural production during this period; the area of irrigated land now being some 220 million hectares.

· Increased use of agrochemicals.

· Increased mechanization.

Although there are other contributing factors that have been important for increasing the production of specific crops and in solving problems occurring in unique locations, there can be little doubt that the above mainly account for the unprecedented success of agriculture in the last 30 years. The relative importance of each of the above factors differs from country to country depending on local conditions, and where one or more of these variables have proved limiting or inapplicable they have been unable to benefit from the so-called 'green revolution'. For example, sub-Saharan Africa has not been able to benefit significantly since wheat and rice are not the staple food crops, and surface water supplies suitable for irrigation are restricted.

Clearly, despite the overall success story we have had significant failures and we need effective intervention measures to overcome the production constraints in many countries, particularly in Africa. We must also consider if this unprecedented rate of growth in agricultural production can continue.

Where are we now?

A nation's land and water resources are finite and in many countries these resources are becoming fully utilized and, hence, the potential for further extensive expansion of agriculture is limited, with farmers already developing the more marginal lands and utilizing poor quality water for irrigation.

Improved agricultural inputs, the so called 'green revolution' technologies, have enabled farmers to utilize the land more intensively and to obtain better crop yields the technology being particularly important in those countries dependent on wheat and rice. From a slow beginning in the early 60s the technology has been increasingly adopted, and in most countries farmers are now aware of the benefits to be gained from it. During this period nearly all farmers throughout the world have progressively adopted the practices which they find to be beneficial in their own environment, have modified others such as the recommended levels of fertilizer to their own socio-economic circumstances, and rejected others. As a result in many countries the unprecedented rate of growth is now slowing, and in others it has stopped, despite the fact that in many countries yields are still at a fairly modest level.

Pakistan is a good example of a nation that has benefited significantly from all these intervention measures, enabling it to feed its rapidly growing population (growing at 3.1% per year). Unfortunately, this impressive rate of growth has slowed in the past 10 years, with the yield of the major food crops remaining fairly static; increased production resulting from an increase in the irrigated area (Table 1). Nevertheless, yields remain relatively low and if present trends continue Pakistan will move from having a grain surplus to an annual deficit of 23% by the year 2000, and a 30% shortfall by the year 2012. Large deficits are also predicted for other crops'. In the years of rapid expansion growth was largely achieved by a progressive increase in irrigated area and by the gradual introduction of new varieties and the use of fertilizer. Production is now levelling off as water resources become fully utilized and as farmers both plant improved varieties and apply levels of fertilizer that they find attractive. Although there is some scope for further production from irrigating more land, the availability of water is likely to pose a major obstacle to further large-scale development of land. As with many countries, if they are to increase agricultural production to meet the needs of a growing population they must achieve more intensive production on existing farmland and use water resources more wisely. The potential to double or more these existing low yields exists, and the technical know-how to achieve this is available and relatively simple, unfortunately as in many other cases the technology proves to be inapplicable because of constraints in the farming system. The result is that we do not know what intervention measures might bring about a significant increase in production, nor do we have a rational approach to identify what the true bottlenecks are and how we might overcome them.

The example is fairly typical of many countries which achieved rapid growth in agricultural production in the past but which are predicted to have severe difficulties in meeting the growing needs of their population unless they find appropriate intervention measures for their farming systems. Unfortunately, all too often many governments try to overcome the problem by developing investment strategies to increase production which are founded on political will or unreliable analysis, with little or no analysis of the sector as a whole. A clear case of this is to be seen in Egypt where scarce financial and water resources are being used to irrigate impoverished sand to grow wheat in the desert.

Table 1 Wheat and rice production in Pakistan

Can investment strategies remove the bottle-necks in agricultural production of the world?

The key intervention measures outlined above provided a simple broad-brush approach to agricultural development with new widely applicable technologies being readily transferred from country to country. Although the approach has had great success it is clear that it has been far more effective in some countries than others, depending on the relevance of the technology to the local environment. The development sector appears to feel that because of the general success of this broad-brush approach a similar strategy can be followed for the implementation of a wide range of possible intervention measures. The result has been a tendency to take the success of a project in one country as a basis to justify the implementation of projects of a similar nature across the globe, with agencies tending to follow each other in investing in projects of a similar nature at any one particular time, the so-called 'bandwagon effect'. Listed below are some of the areas that have been favoured for investment at various times during the past 20 years, they come and go, often having little long term impact:

- Fertilizer subsidies
- Integrated rural development projects
- Extension
- T and V extension systems
- Irrigation
- Dryland agriculture
- Small-scale projects
- Large-scale projects
- Rural credit banks
- On-farm water management
- Financial restructuring of the economy
- Adaptive research Sector-based education.

and more recently:

- The environment
- Women
- Good government
- Project-based training.

There appear to be a number of ways new bandwagons are initiated. They may be based on a very successful project in a specific location or result from a combined political will for projects in a specific area; they can arise from a desperation to try to do something new that might work. There is nothing wrong with any of these and it is well worth trying new ideas on an experimental basis. The problem occurs when the need to spend large amounts of money with the least administrative costs results in the experiments becoming large projects which are rapidly replicated as the bandwagon begins to roll.

On mentioning these reservations to a banker in the aid sector the reassuring reply was that there is no need to worry because all projects undergo thorough economic analysis and have to be 'bankable', giving an internal rate of return of at least 12%. Although the discounted cash flow techniques used for economic assessments provide an accepted procedure for comparing the viability of projects, the results can only be justified if the assumptions on which they are based can be justified. Unfortunately, all too often this is not the case, and on close examination the economic viability of a project is often pivotal on a number of professional judgments which the evaluation team has had to make. Given that in most developing countries we do not know with any level of certainty what the underlying bottlenecks in the agricultural sector are, nor the magnitude of the effects, many of our judgments must be considered open to question. Further, in the development business we are all being asked to formulate and evaluate projects in an ever decreasing amount of time, giving little opportunity for us to understand truly the complexity of the agricultural sector of a country for which we are formulating investment polices. We therefore put forward for discussion the idea that the level of uninformed judgment that is required to formulate many agricultural development initiatives largely invalidates much of our economic analysis, with the result that in many countries, despite vast investment in the agricultural sector, production continues to stagnate. We all start out as optimists, not realizing that optimists and pessimists are the same thing except that the latter are better informed.

The international hotels of all developing countries are full of 'experts' in the development industry, all trying to improve the performance of the agricultural sector, but with none of us fully understanding the key issues that are causing the bottlenecks in the system. We readily fall back on the well-used phrase "in my opinion" to make up for lack of facts; the result is that in all too many of our decisions are too subjective. As a result many development initiatives are based on preconceived ideas with little understanding of the multifaceted and complex systems which interact to control production on the farms. Hence, projects end up not addressing the real constraints that exist in a given agricultural system, simple because these constraints have not been recognised.

On asking a wide range of staff connected with a large irrigation system, from the minister to the smallest farmer, "why are yields so low and where should money be invested to increase productivity?", every person identified a different cause and a different solution depending on our personal perspective of the project. In reality we had all identified many symptoms but not necessarily the underlying cause of the problems.

This week's conference has brought together a large group of informed expertise from within engineering and natural resources with a brief to combine the expertise of the two disciplines to enable us to explore where irrigation fits into the agricultural sector and where it should be going in the light of such important issues as global warming and desertification. Depending on our outlook we expound the virtues of different development strategies such as the need to protect the environment, the benefits of small schemes, of large schemes, the role of NGOs and of the cost/benefit of irrigation development as compared with investment in dryland agriculture. The reality is that all have their appropriate time and place; the skill is determining when and where it is.

It is argued therefore that if agriculture is to meet the needs of the world in the next 30 years we will need to develop a more systematic approach to identifying the key constraints in the agricultural systems of the world to enable us to target our aid much more effectively. Such an approach would supersede the need for debates like i' Are large irrigation projects better than small? Should irrigation be favoured at the expense of dryland agriculture?".

A look at possible future investment strategies

Value for money is essential when aid money is targeted at the poorest people on earth. Unfortunately, in recent years it has become more and more difficult to target aid effectively in the agricultural sector. All too frequently money has been spent on projects that have not removed the key constraints in the agricultural sector and it is clear that if aid is to be more cost effective in the future it must be targeted at the key bottlenecks.

There appear to be two main potential investment strategies which could focus investment into effective projects.

· To identify a number of key constraints which are known to limit production in many countries, but to which we do not readily have solutions, and concentrate a large proportion of the aid money into developing solutions. This is a high-risk strategy and projects would need to have the potential for very high returns. Such a strategy might result from a feasibility study to evaluate the technical possibility of genetically engineering cereals to enable them to live with symbiotic bacteria for nitrogen fixation.

· If such a programme were estimated to have only a 50% chance of success it could be argued that the potential benefits could justify spending a large proportion of the world's total aid budget on it. Such high-risk high-gain strategies are unlikely to prove attractive to donors for a number of reasons but they do have many attractions, not least the size of the potential benefits.

· If the managers of a car plant decide that they want to increase production they do not just make investment in the different areas of the production process and hope that it will remove the bottlenecks that will allow increased production. They carry out detailed analysis of the production system to enable them to identify the constraints and the investment needs that will allow them to increase production. Further production constraints are progressively identified until a level of investment is identified that will allow them to reach production targets, i.e. until marginal costs exceed marginal benefits.

Within the agricultural sector of most countries such an approach is not considered. Indeed in many countries the agricultural production system is often directed from within two separate ministries, irrigation and agriculture. There is often both an irrigation sector five-year plan and an unrelated agricultural sector plan. Detailed examination of a number of such plans from several countries indicates that there is normally no rational basis for the plan but that they are made up of a mixture of the aspirations of the civil servants and the wishes of the politicians. In reality at the end of the planning period they prove to have been little more than a work of fiction. These poorly thought out plans are used to identify the investment opportunities for development agencies and form the basis for requests for aid and/or soft loans.

We clearly need to move towards a more rational planning process in a similar systematic fashion to that used in manufacturing industry. The World Bank Œries out regular sector reviews, as do many aid agencies, but these fall far short of a full analysis of a country's agricultural sector in which the key bottlenecks are identified and quantified.

Many agricultural researchers have recognized that if they are to produce research results that meet the farmers' needs they must adopt a systematic approach to identify on-farm constraints, thus enabling them to identify their research priorities. The analytical techniques that they have developed are proving powerful tools and it is logical that such a systematic approach should be extended to enable us to identify the true bottlenecks in a nations agricultural sector; thus enabling investment to be targeted accurately.

This approach implies a rigorous, quantitative and systematic effort, first to understand the farming operation, and then proceeding to define and develop an appropriate investment strategy. Farming systems need to be analysed in their entirety to establish which of the multitude of variables impinging on farm activities are the main causes of low productivity. This appears to be the only way to identify which constraints need to be removed, if it is possible to remove them, and how.

For this analysis the farm population needs to be disaggregated by such factors as agroclimatic zone, size, tenure systems and the availability of water. The effect of social issues need to be quantified. The economics of the farming practices of the disaggregated farms need to be fully understood. Analysis is largely possible through statistical and survey techniques developed by farming systems researchers and others. Such farm-level models, however, have severe limitations if used in isolation and the analytical approach has to be extended to examine the constraints that exist in domains outside the farm. It is at this level that the approach is likely to need further refinement.

Such a systematic approach to analyse the agricultural sector of a country would bring together the skills of all the disciplines involved in effective agriculture development to produce a comprehensive reference document on the state of a nation's agriculture. Such documents would adopt a systematic approach to analyse the agricultural sector, starting from identifying issues that affect the production of individual crops and management of a farm's resources, to studies of the potential to utilise further the nation's resources in the political and economic climate of a country. It is expected that such a document would be updated annually and act as reference work to provide analysed data for the further development of agricultural sector plans; both for the effective identification of projects and also to improve project formulation.

Although such a systematic approach is needed in many countries it is suggested that the methodology should be fully developed and tested in one country. If successful it would then act as a blueprint for use in other appropriate environments.


It was pointed out that a conclusion made at a conference held at Wye College two years ago was that targeting aid at the emergent commercial farmers was more likely to produce substantial benefits than attempting to target the poorest but ODA policy has continued to overlook this conclusion. It was argued that it is not a question of choosing between the top down scientifically driven strategy or the site-specific bottom-up approach analysing constraints on farmers but both should be pursued. In discussion over possibilities of further dramatic increase in yields in South Asia it was argued that good use of water was the key. It was stated that we are on the verge of breaking major scientific barriers to higher yield, such as the photosynthetic barrier, but crop yields of up to 6 tonnes/ha, though easily achievable, were rarely being attained in the developing world, well below the 10-20 tonnes/ha set by the photosynthetic limit. It was added that the major issue is not so much the upper limits to achievable yields but the yield gap between average farmers and poor farmers. Under-yielding is a problem common to many industries besides agriculture. It was argued that there are common socio-political problems which donors attempt to address through influencing policy. The concept of 'yield per unit of water evaporated' was suggested as a useful criterion. It was recognised that improvements in water-use efficiency could be made but also that excess water passes into the groundwater and is re-used. Despite the importance of fish in diets (up to 90% of animal protein consumed in Bangladesh) many of the benefits of increased rice production are lost through decline in fish catches due to irrigation practices. It was suggested that proponents of irrigation needed to take a wider perspective but also pointed out that experience in India shows that aquaculture and rice irrigation can go hand in hand.

Paper 16 Climate change and the future of agriculture

Martin Parry

Environmental Change Unit, University of Oxford

Summary: The purpose of this paper is to review our current knowledge of the potential effects of climate change on agriculture and world food supply. Attention is first directed to the types of climate change, particularly reductions in soil water availability, likely to be most critical for agriculture. Secondly, a number of types of effects on agriculture are considered: 'direct' effects of elevated CO2, shifts of thermal and moisture limits to cropping, effects on drought, heat stress and other extremes, effects on pests, weeds and diseases, and effects on soil fertility. Thirdly, a summary is presented of likely overall effects on crop and livestock production. The conclusion is that, while global levels of food production can probably be maintained in the face of climate change, the cost of this could be substantial. There could occur severe negative effects at the regional level. Increases in productive potential at higher latitudes are not likely to open up large new areas for production. The gains in productive potential here are unlikely to balance possible reductions in potential in some major grain-exporting regions at mid-latitudes.


This review considers, firstly, those systems, sectors and regions of agriculture that are most sensitive to anticipated changes of climate that may result from emissions of greenhouse gases, primarily carbon dioxide. Secondly, it summarises present knowledge about the potential socioeconomic impact of changes of climate on world agriculture. Thirdly, it considers the adjustments in agriculture that are most likely to occur. Finally, it establishes research priorities for future assessments of impact.

The most significant aspects of climatic change

The potentially most important changes of climate for agriculture include: changes in climatic extremes; warming in the high latitudes; poleward advance of monsoon rainfall; and reduced soil water availability (particularly in mid-latitudes in midsummer, and at low latitudes).

Climatic extremes

It is not dear whether changes in the variability of temperature will occur as a result of climate change. However, even if variability remains unaltered, an increase in average temperatures would result in the increased frequency of temperatures above particular thresholds. Changes in the frequency and distribution of precipitation are less predictable, but the combination of elevated temperatures and drought or flood probably constitutes the greatest risk to agriculture in many regions from global climate change.

Warming in high latitudes

There is relatively strong agreement among GCM predictions that greenhouse-gas induced warming will be greater at higher latitudes (IPCC, 1990). This will reduce temperature constraints on high-latitude agriculture and increase the competition for land here (Parry and Duinker, 1990). Warming at low latitudes, although less pronounced, is also likely to have a significant impact on agriculture.

Poleward advance of monsoon rainfall

In a warmer world the inter-tropical convergence zones and polar frontal zones might advance further poleward as a result of an enhanced ocean-continent pressure gradient. If this were to occur then total rainfall could increase in some regions of monsoon Africa, monsoon Asia and Australia, though there is currently little agreement on which regions these might be (IPCC, 1990). Rainfall could also be more intense in its occurrence, so flooding and erosion could increase.

Reduced soil water availability

Probably the most important consequences for agriculture would stem from higher potential evapotranspiration, primarily due to the higher temperatures of the air and the land surface. Even in the tropics, where temperature increases are expected to be smaller than elsewhere and where precipitation might increase, the increased rate of loss of moisture from plants and soil would be considerable (Parry, 1990; Rind et al., 1989). It may be somewhat reduced by greater air humidity and increased cloudiness during the rainy seasons, but could be pronounced in the dry seasons.

Types of effect on agriculture

There are three ways in which increases in greenhouse gases (GHG) may be important for agriculture. Firstly, increased atmospheric carbon dioxide (CO2) concentrations can have a direct effect on the growth rate of crop plants and weeds. Secondly, GHG-induced changes of climate may alter levels of temperature (as well as temperature gradient), rainfall and sunshine and this can influence plant and animal productivity. Finally, rises in sea level may lead to loss of farmland by inundation and to increasing salinity of groundwater in coastal areas.

Effects of CO2 enrichment

· Effects on photosynthesis

Carbon dioxide is vital for photosynthesis, and the evidence is that increases in CO2 concentration would increase the rate of plant growth (Cure, 1985; Cure and Acock, 1986). There are, however, important differences between the photosynthetic mechanisms of different crop plants and hence in their response to increasing CO2. Plant species with the C3 photosynthetic pathway (e.g. wheat, rice and soybean) tend to respond more positively to increased CO2 because it tends to suppress rates of photorespiration. However, C4 plants (e.g. maize, sorghum, sugar-cane and millet) are less responsive to increased CO2 levels (Figure 1). Since these are largely tropical crops, and most widely grown in Africa, there is thus the suggestion that CO2 enrichment will benefit temperate and humid tropical agriculture more than that in the semi-arid tropics. Thus, if the effects of climate changes on agriculture in some parts of the semi-arid tropics are negative, then these may not be partially compensated by the beneficial effects of CO2 enrichment as they might in other regions. In addition we should note that, although C4 crops account for only about one-fifth of the world's food production, maize alone accounts for 14% of all production and about three-quarters of all traded grain. It is the major grain used to make up food deficits in famine-prone regions, and any reduction in its output could affect access to food in these areas (Morison, 1990).

The C3 crops in temperate and subtropical regions could also benefit from reduced weed infestation. Fourteen of the world's 17 most troublesome terrestrial weed species are C4 plants in C3 crops. The difference in response to increased CO2 may make such weeds less competitive. In contrast, C3 weeds in C4 crops, particularly in tropical regions, could become more of a problem, although the final outcome will depend on the relative response of crops and weeds to climate changes as well (Morison, 1990). Many of the pasture and forage grasses of the world are C4 plants, including important prairie grasses in North America and central Asia and in the tropics and subtropics. The carrying capacity of the world's major rangelands are

Figure 1 Typical photosynthesis response of plants to CO2

Little is also known about possible changes in yield quality under increased CO!. The nitrogen content of plants is likely to decrease, while the carbon content increases, implying reduced protein levels and reduced nutritional levels for livestock and humans. This may, however, also reduce the nutritional value of plants for pests, so that they need to consume more to obtain their required protein intake.

· Effects on water use by plants

Just as important may be the effect that increased CO2 has on the closure of stomata. This tends to reduce the water requirements of plants by reducing transpiration (per unit leaf area) thus improving what is termed 'water-use efficiency' (the ratio of crop biomass accumulation to the water used in evapotranspiration). A doubling of ambient CO2 concentration causes about a 40% decrease in stomata! aperture in both C3 and C4 plants which may reduce transpiration by 23 46% (Morison, 1987; Cure and Acock, 1986). This might well help plants in environments where moisture currently limits growth, such as in semi-arid regions, but there remain many uncertainties, such as to what extent the greater leaf area of plants (resulting from increased CO2) will balance the reduced transpiration per unit leaf area (Allen et al., 1985; Gifford, 1988). In summary, we can expect that a doubling of atmospheric CO2 concentrations from 330 to 660 ppmv might cause a 10-50% increase in growth and yield of C3 crops (such as wheat, rice and soybean) and a 0-10% increase for C4 crops (such as maize and sugar-cane) (Warrick et al., 1987). Much depends, however, on the prevailing growing conditions. Our present knowledge is based on experiments mainly in field chambers and has not yet included extensive study of response in the field under suboptimal conditions. Thus, although there are indications that, overall, the effects of increased CO2 could be distinctly beneficial and could partly compensate for some of the negative effects of CO2-induced changes of climate, we cannot at present be sure that this will be so.

Effects of changes of climate

· Changes in thermal limits to agriculture

Increases in temperature can be expected to lengthen the growing season in areas where agricultural potential is currently limited by insufficient warmth, resulting in a poleward shift of thermal limits of agriculture. The consequent extension of potential will be most pronounced in the northern hemisphere because of the greater extent here of temperate agriculture at higher latitudes. There may, however, be important regional variations in our ability to exploit this shift. For example, the greater potential for exploitation of northern soils in Siberia than on the Canadian Shield may mean relatively greater increases in potential in northern Asia than in northern N America (Parry, 1990). A number of estimations have been made concerning the northward shift in productive potential in mid-latitude northern hemisphere countries (see, for example, Figure 2). These relate to changes in the climatic limits for specific crops under a variety of climatic scenarios, and are therefore not readily compatible (Newman, 198(); Blasing and Solomon, 1983; Rosenzweig, 1985; Williams and Oakes, 1978; Parry and Carter, 1988; Parry et al., 1989). They suggest, however, that a 1°C increase in mean annual temperature would tend to advance the thermal limit of cereal cropping in the mid-latitude northern hemisphere by about 150-200 km, and to raise the altitudinal limit to arable agriculture by about 15() 200 m.

While warming may extend the margin of potential cropping and grazing in mid-latitude regions, it may reduce yield potential in the core areas of current production, because higher temperatures encourage more rapid maturation of plants and shorten the period of grain filling (Parry and Duinker, 1990). An important additional effect, especially in temperate mid-latitudes, is likely to be the reduction of winter chilling (vernalization). Many temperate crops require a period of low temperatures in winter either to initiate or accelerate the flowering process. Low vernalization results in low flower bud initiation and, ultimately, reduced yields. A 1°C warming has been estimated to reduce effective winter chilling by between 10 and 30%, thus contributing to a poleward shift of temperate crops (Salinger, 1989). Increases in temperature are also likely to affect the crop calendar in low latitude regions, particularly where more than one crop is harvested each year. For example, in Sri Lanka and Thailand, a 1°C warming would probably require a substantial re-arrangement of the current crop calendar which is finely tuned to present climatic conditions (Kaida and Surarerks, 1984; Yoshino, 1984).

· Shifts of moisture limits to agriculture

There is much less agreement between GCM-based projections concerning GHG-induced changes in precipitation than there is about temperature - not only concerning changes of magnitude, but also of spatial pattern and distribution through the year. For this reason it is difficult to identify potential shifts in the moisture limits to agriculture. This is particularly so because relatively small changes in the seasonal distribution of rainfall can have disproportionately large effects on the viability of agriculture in tropical areas, largely through changes in growing period when moisture is sufficient and thus through the timing of critical episodes such as planting, etc. However, recent surveys for the IPCC have made a preliminary identification of those regions where there is some agreement amongst 2 x CO2 experiments with general circulation models concerning an overall reduction in crop-water availability (Parry, 1990; Parry and Duinker, 1990). It should be emphasized that coincidence of results for these regions is not statistically significant. The regions are:

Figure 2 Grain maize limit under the GISS transient response Scenario in the 1990s, 2020s, and 2050s (relative to the limit for the current climate)

Decreases of soil water in December, January and February:

Africa :

north-east Africa, southern Africa

Asia :

western Arabian Peninsula; SE Asia

Australasia :

eastern Australia

N America :

southern USA

S America :

Argentine Pampas

Decreases in soil water in June, July and August:

Africa :

north Africa; west Africa

Europe :

parts of western Europe

Asia :

north and central China; parts of Soviet central Asia and Siberia

N America :

southern USA and Central America

S America :

eastern Brazil

Australia :

western Australia

- Regions affected by drought, heat stress and other extremes

Probably most important for agriculture, but about which least is known, are the possible changes in climatic extremes, such as the magnitude and frequency of drought, storms, heat waves and severe frosts (Rind et al., 1989). Some modelling evidence suggests that hurricane intensities will increase with climatic warning (Emanuel, 1987). This has important implications for agriculture in low latitudes, particularly in coastal regions.

Since crop yields often exhibit a nonlinear response to heat or cold stress, changes in the probability of extreme temperature events can be significant (Mearns et al., 1984; Parry, 1976). In addition, even assuming no change in the standard deviation of temperature maxima and minima, we should note that the frequency of hot and cold days can be markedly altered by changes in mean monthly temperature. To illustrate, under a 2 × CO2 equilibrium climate, the number of days in which temperatures would fall below freezing would decrease from a current average of 39 to 20 in Atlanta, Georgia (USA), while the number of days above 90°F would increase from 17 to 53 (EPA, 1989). The frequency and extent of area over which losses of agricultural output could result from heat stress, particularly in tropical regions, is therefore likely to increase significantly. Unfortunately, no studies have yet been made of this. However, the apparently small increases in mean annual temperatures in tropical regions (c. 1-2°C under a 2 × CO2 climate) could sufficiently increase heat stress on temperate crops such as wheat so that these are no longer suited to such areas. Important wheat producing areas such as N India could be affected in this way (Parry and Duinker, 1990).

There is a distinct possibility that, as a result of high rates of evapotranspiration, some regions in the tropics and subtropics could be characterised by a higher frequency of drought or a similar frequency of more intense drought than at present. Current uncertainties about how regional patterns of rainfall will alter mean that no useful prediction of this can at present be made. However, it is clear in some regions that relatively small decreases in water availability can readily produce drought conditions. In India, for example, lower-than-average rainfall in 1987 reduced food grains production from 152 to 134 million tonnes, lowering food buffer stocks from 23 to 9 million tonnes. Changes in the risk and intensity of drought, especially in currently drought-prone regions, represent potentially the most serious impact of climatic change on agriculture both at the global and the regional level.

· Effects on the distribution of agricultural pests and diseases

Studies suggest that temperature increases may extend the geographic range of some insect pests currently limited by temperature (EPA, 1989; Hill and Dimmmock, 1989). As with crops, such effects would probably be greatest at higher latitudes. The number of generations per year produced by multivoltine (i.e. multigenerational) pests would increase, with earlier establishment of pest populations in the growing season and increased abundance during more susceptible stages of growth. An important unknown, however, is the effect that changes in precipitation amount and air humidity may have on the insect pests themselves and on their predators, parasites and diseases. Climate change may significantly influence interspecific interactions between pests and their predators and parasites. Under a warmer climate at mid-latitudes there would be an increase in the overwintering range and population density of a number of important agricultural pests, such as the potato leafhopper which is a serious pest of soybeans and other crops in the USA (EPA, 1989). Assuming planting dates did not change, warmer temperatures would lead to invasions earlier in the growing season and probably lead to greater damage to crops. In the US Corn Belt increased damage to soybeans is also expected due to earlier infestation by the corn earworm.

Examination of the effect of climatic warming on the distribution of livestock diseases suggests that those at present limited to tropical countries, such as Rift Valley fever and African Swine fever, may spread into the mid-latitudes. For example, the horn fly, which currently causes losses of $ 730.3 million in the US beef and dairy cattle industries might extend its range under a warmer climate leading to reduced gain in beef cattle and a significant reduction in milk production (Drummond, 1987; EPA, 1989). In cool temperate regions, where insect pests and diseases are not generally serious at present, damage is likely to increase under warmer conditions. In Iceland, for example, potato blight currently does little damage to potato crops, being limited by the low summer temperatures. However, under a 2 x CO2 climate that may be 4°C wanner than at present, crop losses to disease may increase to 15% (Bergthorsson et al., 1988). Most agricultural diseases have greater potential to reach severe levels under warmer and more humid conditions (Beresford and Fullerton, 1989). Under warmer and more humid conditions cereals would be more prone to diseases such as Septoria. In addition, increases in population levels of disease vectors may well lead to increased epidemics of the diseases they carry. To illustrate, increases in infestations of the Bird Cherry aphid (Rhopalosiphum padi) or Grain aphid (Sitobian avenae) could lead to increased incidence of Barley Yellow Dwarf Virus in cereals.

Effects of sea-level rise on agriculture

Greenhouse gas-induced warming is expected to lead to rises in sea level as a result of thermal expansion of the oceans and partial melting of glaciers and ice caps, and this in turn is expected to affect agriculture, mainly through the inundation of low-lying farmland but also through the increased salinity of coastal groundwater. The current projection of sea-level rise above present levels is 20±10 cm by c. 2030, and 30±15 cm by 2050 (Warrick and Oerlemans, 1990).

Preliminary surveys of proneness to inundation have been based on a study of existing contoured topographic maps, in conjunction with knowledge of the local 'wave climate' that varies between different coastlines. They have identified 27 countries as being especially vulnerable to sea-level rise, on the basis of the extent of land liable to inundation, the population at risk and the capability to take protective measures (UNEP, 1989). It should be emphasised, however, that these surveys assume a much larger rise in sea levels than is at present estimated to occur within the next century under current trends of increase of GHG concentrations. On an ascending scale of vulnerability (1 to 10) experts identified the following most vulnerable countries or regions: 10, Bangladesh; 9, Egypt, Thailand; 8, China; 7, western Denmark; 6, Louisiana; 4, Indonesia. The most severe impacts are likely to stem directly from inundation. South East Asia would be most affected because of the extreme vulnerability of several large and heavily populated deltaic regions. For example, with a 1.5 m sea-level rise, about 15% of all land (and about one-fifth of all farmland) in Bangladesh would be inundated and a further 6% would become more prone to frequent flooding (UNEP, 1989). Altogether 21% of agricultural production could be lost. In Egypt, it is estimated that 17% of national agricultural production and 20% of all farmland, especially the most productive farmland, would be lost as a result of a 1.5 m sea-level rise. Island nations, particularly low-lying coral atolls, have the most to lose. The Maldive Islands in the Indian Ocean would have one-half of their land area inundated with a 2 m rise in sea level (UNEP, 1989).

In addition to direct farmland loss from inundation, it is likely that agriculture would experience increased costs from saltwater intrusion into surface water and groundwater in coastal regions. Deeper tidal penetration would increase the risk of flooding and rates of abstraction of groundwater might need to be reduced to prevent re-charge of aquifers with sea water. Further indirect impacts would be likely as a result of the need to relocate both farming populations and production in other regions. In Bangladesh, for example, about one-fifth of the nation's population would be displaced as a result of the farmland loss estimated for a 1.5 m sea-level rise. It is important to emphasize, however, that the IPCC estimates of sea-level rise are much lower than this (about 0.5 m by 2090 under the IPCC 'Business-As-Usual case').

Summary of plant, pest and sea-level effects

Potential impacts on yields vary greatly according to types of climate change and types of agriculture. In general, there is much uncertainty about how agricultural potential may be affected. In the northern mid-latitudes where summer drying may reduce productive potential (e.g. in the US Great Plains and Corn Belt, Canadian Prairies, southern Europe, south European former USSR) yield potential is estimated to fall by c 10-30% under an equilibrium 2 x CO2 climate (Parry, 1990). However, towards the northern edge of current core-producing regions (e.g. the northern edge of the Canadian Prairies, northern Europe, northern states of the former Soviet Union and Japan, southern Chile and Argentina) warming may enhance productive potential, particularly when combined with beneficial direct CO2 effects. Much of this potential may not, however, be exploitable owing to limits placed by inappropriate soils and difficult terrain, and on balance it seems that the advantages of warming at higher latitudes would not compensate for reduced potential in current major cereal producing regions.

Effects at lower latitudes are much more difficult to estimate because production potential is largely a function of the amount and distribution of precipitation and because there is little agreement about how precipitation may be affected by GHG warming. Because of these uncertainties the tendency has been to assert that worthwhile study must await improved projection of changes in precipitation. Consequently very few estimates are currently available of how yields might respond to a range of possible changes of climate in low-latitude regions. The only comprehensive national estimates available are for Australia where increases in cereal and grassland productivity might occur (except in western Australia) if warming is accompanied by increase in summer rainfall (Pearman, 1988).

The impacts described above relate to possible changes in potential productivity or yield. It should be emphasised that such potential effects are those estimated assuming present-day management and technology. They are not the estimated future actual effects, which will depend on how farmers and governments respond to altered potential through changes in management and technology. The likely effects on actual agricultural output and on other measures of economic performance such as profitability and employment levels are considered in the next section.

Effects on production and land use

To date (1992) six national case studies have been made of the potential impact of climatic changes on agricultural production (in Canada, Iceland, Finland, the former Soviet Union, Japan and the United States) (Parry et al., 1989; Smit, 1989; EPA, 1989). These studies are based on results from model experiments of yield responses to altered climate and the effects that altered yields might have on production. Other countries have conducted national reviews of effects of climate change, basing these on existing knowledge rather than on new research. [he most comprehensive of these are for Australia and New Zealand (Pearman, 1988; Salinger et al., 1990). Brief surveys have also been completed in the UK and West Germany (SCEGB, 1989). Several other national assessments are currently in progress but not yet complete. This section provides a summary of results from the most detailed of these surveys. These provide us with an array of assessments for three world regions: the northern and southern mid-latitude grain belts and northern regions at the current margin of the grain belt. We shall take these regions in turn. Unless otherwise stated the estimated effects are for climates described by 2 x CO2 GCM experiments. No national assessments have been completed using climates described by transient response GCM experiments. Some of the estimates relate to the effect only of altered climate, others the combined effect of altered climate and the direct effect of increased atmospheric CO2.

Effects on production in the northern and southern mid-latitude grain belts

· United States

Increased temperatures and reduced crop water availability projected under the GISS and GDFL 2 x CO2 climate experiments are estimated to lead to a decrease of yields of all the major unirrigated crops (EPA, 1989) (Figure 3). The largest reductions are projected for the south and south-east. In the most northern areas, however, where temperature is currently a constraint on growth, yields of unirrigated maize and soybeans could increase as higher temperatures increase the length of the available growing season. When the direct effects of increased CO2 are considered, it is evident that yields may increase more generally in northern areas but still decrease in the south where problems of heat stress would increase and where rainfall may decrease. Production of most crops is estimated to be reduced because of yield decreases and limited availability of suitable land. The largest reductions are in sorghum (-20%), corn (-13%) and rice (-11%), with an estimated fall in net value of agricultural output of $ 33 billion. If this occurred, consumers would face slightly higher prices, although supplies are estimated to meet current and projected demand. However, exports of agricultural commodities could decline by up to 70%, and this could have a substantial effect on the pattern of world food trade.

· Canada

On the Canadian prairies, where growing season temperatures under the GISS 2 x CO2 equilibrium climate would be about 3.5°C higher than today, average potential yields could decrease 10 to 30%. Spring wheat yields in Saskatchewan are estimated to fall by 28% (Williams et al., 1988). Since Saskatchewan at present produces 18% of all the world's traded wheat, such a reduction could well have global implications. Assuming (unrealistically) that the present-day relationship between production and profit in Saskatchewan holds in the future, average farm household income is estimated to fall by 12%, resulting in a reduction in expenditure by agriculture of Can$ 277 million on the goods and services provided by other sectors, leading to a Can$ 250 million (6%) reduction in provincial GDP in sectors other than agriculture and a 1% loss of jobs. In Ontario, precipitation increases of up to 50% would be more than offset by increases in evapotranspiration with consequent increased moisture stress on crops (Smit, 1987). Maize and soybean would thus become very risky in the southern part of the province. In the north, where maize and soybean cannot currently be grown commercially because of inadequate warmth, cultivation may become profitable but this is not expected to compensate for reduced potential further south and, if there were no adjustment of current land use and farming systems, the overall cost in lost production is reckoned at Can$ 100 million to 170 million.

Figure 3 Estimated maize yields in the USA under the GISS and GFDL 2 x CO2 with and without the direct effects (DE) of CO2 (a) dryland and (b) irrigated

· Japan

Under a warming of 3.0-3.5°C and a 5% increase in annual precipitation (the GISS 2 x CO2 climate), rice yields are expected to increase in the north (Hokkaido) by c. 5%, and in the north-central region (Tohoku) by c. 2%, if appropriate technological adjustments are made (Yoshino et al., 1988). The average increase for the country overall is c. 2-5%. Cultivation limits for rice would rise about 500 m and advance c. 100 km north in Hokkaido. Yields of maize and soybeans are both estimated to increase by about 4%. Sugar-cane yields in the most southern part of Japan could decrease if rainfall was reduced. The northern economic limit of citrus fruits would shift from southern Japan to northern Honshu Island (Yoshino, personal communication, 1989). Net primary productivity of natural vegetation is expected to increase by c. 15% in the north, c. 7% in the centre and south of Japan (Yoshino et al., 1988).

· Australia and New Zealand

In Australia and New Zealand national assessments have been based on a thorough review of existing knowledge and on use of expert judgement rather than on model experiments (Pearman, 1988; Salinger et al., 1990). Overall, it is reckoned that wheat production in Australia could increase under a 2 x CO2 climate, assuming a quite simple scenario of increased summer rainfall, decreased winter rainfall and a general warming of 3°C. Increases are expected in all states except Western Australia, where more aridity might cause a significant reduction in output (Pittock, 1989). More generally, the major impact of production would probably be on the drier frontiers of arable cropping. For example, increases in rainfall in subtropical northern Australia could result in increased sorghum production at the expense of wheat. Increased heat stress might shift livestock farming and wool production southward with sheep possibly replacing arable farming in some southern regions. Many areas currently under fruit production would no longer be suitable under a 3 C warming, and would need to shift southwards or to higher elevations in order to maintain present levels of production. All of these changes would also be affected by changes in the distribution of diseases and pests.

Effects on production in northern marginal regions

Some of the most pronounced effects on agriculture would be likely to occur in high-latitude regions because GHG-induced warming is projected to be greatest here and because this warming could remove current thermal constraints on farming. Inappropriate terrain and soils are, however, likely to limit the increase in extent of the farmed area and, in global terms, production increases would probably be small (Parry, 1990). A summary of available information is given below.

· Iceland

With mean annual temperatures increased by 4.0°C and precipitation 15% above the present average (consistent with the GISS 2 x CO2 climate), the onset of the growing season of grass in Iceland would be brought forward by almost 50 days, hay yields on improved pastures would increase by about two-thirds and herbage on unimproved rangelands by about a half (Bergthorsson et al., 1988). The numbers of sheep that could be carried on the pastures would be raised by about 250% and on the rangelands by two-thirds if the average carcass weight of sheep and lambs is maintained as at present. At a guess, output of Icelandic agriculture could probably double with a warming of 4°C.

· Finland

Assuming in Finland an increase in summer warmth by about a third and precipitation by about half (consistent with the GISS 2 x CO2 climate) barley and spring wheat yields increase about 10% in the south of the country but slightly more in the north (due to relatively greater warming and lower present-day yields) (Kettunen et al., 1988). The area under grain production in Finland might increase at the expense of grass and livestock production as a consequence of raised profitability, with the greatest extension being in winter crops such as wheat rather than spring crops such as barley or oats.

· Northern states former Soviet Union

The only other region for which an integrated impact assessment has been completed is in north European former Soviet Union. In the Leningrad and Cherdyn regions, under climates that are 2.2-2.7°C warmer during the growing season and 36-50% wetter (consistent with the GISS 2 x CO2 climate), winter rye yields are estimated to decrease by about a quarter due to faster growth and increased heat stress under the higher temperatures (Pitovranov et al., 1988a and b). However, crops such as winter wheat and maize, which are currently low yielding because of the relatively short growing season in these regions, are better able to exploit the higher temperatures and exhibit yield increases in Cherdyn of up to 28% and 6% respectively with a 1°C warming.

The differential yield responses described above are reflected in substantial changes in production costs incurred in meeting production targets. Thus, while production costs for winter wheat and maize in the Central Region around Moscow are estimated to be reduced by 22% and 6% under a 1°C warming and with no change in precipitation, they increase for most other crops, particularly quick-maturing spring-sown ones which are the dominant crops today. This would suggest that quite major switches of land use would result and the land allocation models used in the study indicate that, to optimise land use by minimising production costs, winter wheat and maize would extend their area by 29% and 5% while barley, oats and potatoes would decrease in extent.

· Summary of potential effects in mid-latitude regions

The effects of possible climatic changes on regional and national production have not yet been investigated in any great detail, nor for more than a few case studies. The effects are strongly dependent on the many adjustments in agricultural technology and management that undoubtedly will occur in response to any climatic change. So numerous and varied are these potential adjustments that it is extraordinarily difficult to evaluate their ultimate effect on aggregate production. In this section we have therefore considered the effects on production that are likely to stem directly from changes in yield, unmodified by altered technology and management. Adjustments in technology will be considered, briefly, in the last section of this paper. In summary, it seems that overall output from the major present-day grain producing regions could well decrease under the warming and possible drying expected in these regions. In the USA grain production may be reduced by 10-20% and, while production would still be sufficient for domestic needs, the amount for export would probably decline. Production may also decrease in the Canadian prairies and in the southern former Soviet Union. In Europe production of grains might increase in the UK and the Low Countries if rainfall increases sufficiently, but may fall in southern Europe substantially if there are significant decreases in rainfall as currently estimated in most GCM 2 × CO: experiments (Parry, 199()). Output could increase in Australia if there is a sufficient increase in summer rainfall to compensate for higher temperatures. Production could increase in regions currently near the low-temperature limit of grain growing: in the northern hemisphere in the northern Prairies, Scandinavia, north European (former) Soviet Union; and in the southern hemisphere in southern New Zealand, and southern parts of Argentina and Chile. But it is reasonably clear that, because of the limited area unconstrained by inappropriate soils and terrain, increased high-latitude output will probably not compensate for reduced output at mid-latitudes. The implications of this for global food supply and food security are considered in the next section.

Implications for global food security

Although, on average, global food supply currently exceeds demand by about 10-20, its year-to-year variation (which is about ±10%) can reduce supply in certain years to levels where it is barely sufficient to meet requirements. In addition, there are major regional variations in the balance between supply and demand, with perhaps a billion people (about 15% of the world's population) not having secure access to sufficient quantity or quality of food to lead fully productive lives. For this reason the working group on food security at the 1988 Toronto Conference on 'The Changing Atmosphere' concluded that:

"While averaged global food supplies may not be seriously threatened, unless appropriate action is taken to anticipate climate change and adapt to it, serious regional and year-to-year food shortages may result, with particular impact on vulnerable groups .

Statements such as this are, however, based more on intuition than on knowledge derived from specific study of the possible impact of climate change on the food supply. No such study has yet been completed, although one is currently being conducted by the US Environmental Protection Agency and is due to report in 1992. The information available at present is extremely limited. It has for example, been estimated that increased costs of food production due to climate change could reduce per caput global GNP by a few percentage points (Schelling, 1983). Others have argued that technological changes in agriculture will override any negative effects of climate changes and, at the global level, there is no compelling evidence that food supplies will be radically diminished (Crosson, 1989). Recent reviews have tended to conclude however that, at a regional level, food security could be seriously threatened by climate change, particularly in less developed countries in the semi-arid and humid tropics (Parry, 1990; Parry and Duinker, 1990). Analyses conducted for the IPCC, designed to test the sensitivity of the world food system to changes of climate, indicate what magnitudes and rates of climatic change could possibly be absorbed without severe impact and, alternatively, what magnitudes and rates could seriously perturb the system (Parry, 1990; Parry and Duinker, 1990). These suggest that yield reductions of up to 20% in the major mid-latitude grain exporting regions could be tolerated without a major interruption of global food supplies. However, the increase in food prices (7% under a 10% yield reduction) could seriously influence the ability of food-deficit countries to pay for food imports, eroding the amount of foreign currency available for promoting development of their non-agricultural sectors. It should be emphasized that these analyses are preliminary and more work is necessary before we have an adequate picture of the resilience of the world food system to climatic change.


Our assessment of possible effects has, up to this point, assumed that technology and management in agriculture do not alter significantly in response to climatic change, and thus do not alter the magnitude and nature of the impacts that may stem from that change. It is certain, however, that agriculture will adjust and, although these adjustments will be constrained by economic and political factors, it is likely that they will have an important bearing on future impacts. On balance, the evidence is that food production at the global level can, in the face of estimated changes of climate, be sustained at levels that would occur without a change of climate, but the cost of achieving this is unclear. It could be very large. Increases in productive potential at high mid-latitudes and high latitudes, while being of regional importance, are not likely to open up large new areas for production. The gains in productive potential here due to climatic warming would be unlikely to balance possible large-scale reductions in potential in some major grain-exporting regions at mid-latitude. Moreover, there may well occur severe negative impacts of climate change on food supply at the regional level, particularly in regions of high present-day vulnerability least able to adjust technically to such effects. The average global increase in overall production costs could thus be small (perhaps a few percent of world agricultural GDP). Much depends however, on how beneficial are the so-called 'direct' effects of increased CO2 on crop yield. If plant productivity is substantially enhanced and more moisture is available in some major production areas, then world productive potential of staple cereals could increase relative to demand with food prices reduced as a result. If, on the contrary, there is little beneficial direct CO2 effect and climate changes are negative for world agricultural production could increase significantly, these increased costs amounting to perhaps over 10% of world agricultural GDP.

Although we know little, at present, about how the frequency of extreme weather events may alter as a result of climatic change, the potential impact of concurrent drought or heat stress in the major food-exporting regions of the world could be severe. In addition, relatively small decreases in rainfall or increases in evapotranspiration could markedly increase both the risk and the intensity of drought in currently drought-prone (and often food-deficient) regions. Change in drought-risk represents potentially the most serious impact of climatic change on agriculture both at the regional and the global level. The regions most at risk from impact from climatic change are probably those currently most vulnerable to climatic variability. Frequently these are low-income regions with a limited ability to adapt through technological change. This paper has emphasised the inadequacy of our present knowledge. It is clear that more information on potential impacts would help us identify the full range of potentially useful responses and assist in determining which of these may be most valuable.

Some priorities for future research may be summarised as follows:

· Improved knowledge is needed of effects of changes in climate on crop yields and livestock productivity in different regions and under varying types of management; on soil-nutrient depletion; on hydrological conditions as they effect irrigation-water availability; on pests, diseases and soil microbes, and their vectors; and on rates of soil erosion and salinisation.

· Further information is needed on the range of potentially effective technical adjustments at the farm and village level (e.g. irrigation, crop selection, fertilizing, etc.); on the economic, environmental and political constraints on such adjustments; and on the range of potentially effective policy responses at regional, national and international levels (e.g. re-allocations of land use, plant breeding, improved agricultural extension schemes, large-scale water transfers).


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The discussion opened with a question on comparison between the level of climatic change predicted and annual variation as well as recent changes in the Sahel. In response the author said that 1976 was a 1 in 800 year event in the UK but with the predicted change it would be a 1 in 20 event. The 200-300 km southward move of the desert in the Sahel in the 1960s and 1970s is the degree of change predicted for a 1°C rise in temperature. No generalisations about changes in climatic variability are possible. It was also agreed that the regional estimates are not yet reliable enough and that standard errors should be added to the estimates. In about 10 years it is expected that sufficiently good models will have been developed to be reasonably sure of the signs of climatic change. It was suggested that climatic change might be an opportunity in that better farmers might respond to the change with output that replaced that of less efficient fanners. It was said that the issue is one of reducing vulnerability and increasing resilience in agricultural systems.