CIESIN Reproduced, with permission, from: Parry, M. L. 1990. Climate change and world agriculture. London: Earthscan Publications.


There are three ways in which the Greenhouse Effect may be important for agriculture. First, increased atmospheric CO2 concentrations can have a direct effect on the growth rate of crop plants and weeds. Secondly, CO2-induced changes of climate may alter levels of temperature, rainfall and sunshine that 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. These three types of impact will be considered in turn.


Effects on photosynthesis

If increases in atmospheric CO2 were occurring without the possibility of associated changes in climate then, overall, the consequences for agriculture would probably be beneficial. CO2 is vital for photosynthesis, and the evidence is that increases in CO2 concentration would increase the rate of plant growth. Photosynthesis is the net accumulation of carbohydrates formed by the uptake of CO2, so it increases with increasing CO2. A doubling of CO2 may increase the photosynthetic rate by 30 to 100%, depending on other environmental conditions such as temperature and available moisture.[1] More CO2 enters the leaves of plants due to the increased gradient of CO2 between the external atmosphere and the air space inside the leaves. This leads to an increase in the CO2 available to the plant for conversion into carbohydrate.[2] The difference between photosynthetic gain and loss of carbohydrate by respiration is the resultant growth.

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 (the first product in their biochemical sequence of reactions has three carbon atoms) use up some of the solar energy they absorb in a process known as photorespiration, in which a significant fraction of the CO2 initially fixed into carbohydrates is reoxidized back to CO2.[3] C3 species tend to respond positively to increased CO2 because it tends to suppress rates of photorespiration (Figure 4.1). This has major implications for food production in a high-CO2 world because some of the current major food staples, such as wheat, rice and soya bean, are C3 plants.

However, in C4 plants (those in which the first product has four carbon atoms) CO2 is first trapped inside the leaf and then concentrated in the cells which perform the photosynthesis.[3] Although more efficient photosynthetically under current levels of CO2, these plants are less responsive to increased CO2 levels than C3 plants (Figure 4.1).

The major C4 staples are maize, sorghum, sugarcane and millet. 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 and that, if the effects of climatic 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 per cent 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.

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.[5] The difference in response to increased C02, 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 climatic changes as well.

The different response of C3 and C4 crops may encourage changes in areas sown. It may, for example, accelerate the recent trend in India toward wheat, rice and barley and away from maize and millets, a trend that has largely been driven by the promise of greater increases in yield. It may tend to reverse the current trend in temperate areas away from perennial rye grass (a C3 crop) towards silage maize (C4) as the major forage crop; and in the USA it might encourage a tendency to switch from maize to soybean (C3) for forage.

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.[6] The carrying capacity of the world's major rangelands are thus unlikely to benefit substantially from CO2 enrichment Much, of course, will depend on the parallel effects of climatic changes on the yield potential of these different crops.

The actual amount of increase in usable yield rather than of total plant matter that might occur as a result of increased photosynthetic rate is also problematic. In controlled environment studies, where temperature and moisture are optimal, the yield increase can be substantial, averaging 36 per cent for C3 cereals such as wheat, rice, barley and sunflower under a doubling of ambient CO2 concentration (Table 4.1). Few studies have yet been made, however, of the effects of increasing CO2 in combination with changes of temperature and rainfall.

Little is also known about possible changes in yield quality under increased CO2. 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, however, may 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, small openings in leaf surfaces through which CO2 is absorbed and through which water vapour is released by transpiration. 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 per cent decrease in stomatal aperture in both C3 and C4 plants[8] which may reduce transpiration by 23-46 per cent.[9] 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 how much the greater leaf area of plants due to increased CO2 will balance the reduced transpiration per unit leaf area.[10]

In summary, we can expect a doubling of atmospheric CO2 concentrations from 330 to 660 ppmv to cause a 10 to 50 per cent increase in growth and yield of C3 crops (such as wheat, soybean and rice) and a 0 to 10 per cent increase for C4 crops (such as maize and sugarcane).[7] Much depends, however, on the prevailing growing conditions. Our present knowledge is based on a few experiments mainly in glass-houses and has not yet included extensive study of response in the field under subtropical 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 on growth-rates

In high mid-latitude regions (above 45deg.), at high latitudes (above 60deg.) and at high altitudes, temperature is frequently the dominant climatic control on crop and animal growth. It determines the potential length of the growing and grazing seasons, and generally has a strong effect on the timing of developmental processes and on rates of expansion of plant leaves. The latter, in turn, affects the time at which a crop canopy can begin to intercept solar radiation and thus the efficiency with which solar radiation is used to make plant biomass.[ll]

In general, plant response to temperature follows that indicated in Figure 4.2. Development does not begin until temperature exceeds a threshold; then the rate of development increases broadly linearly with temperature to an optimum, above which it decreases broadly linearly. [12]

However, the effect of this development on plant biomass depends on whether the growth habit of the plant is determinate (that is, it has a discrete life cycle which ends when the grain is mature, such as in cereals), or whether it is indeterminate (that is, it continues to grow and yield throughout the season, such as in grasses and rootcrops). Temperature increase shortens the reproductive phase of determinate crops, decreasing the time during which the canopy exists and thus the period during which it intercepts light and produces biomass (Figure 4.2b). The canopy of indeterminate crops, however, continues to intercept light until it is reduced by other events such as frost or pests, and the duration of the canopy increases when increased temperatures extend the season over which crops can grow (e.g., by delaying the first frosts of autumn) (Figure 4.2c). An increase in temperature above the base but not exceeding optimum temperatures should therefore generally lead to lower yields in cereals and higher yields of root crops and grassland, though higher temperatures may also lead to higher rates of evaporation and therefore reduced moisture availability that can also be expected to affect yields. These effects on moisture are discussed later.

Effects on growing seasons

One of the most important effects of an increase in temperature, particularly in regions where agricultural production is currently limited by temperature, would be to extend the growing season available for plants (e.g. between last frost in spring and first frost in autumn) and reduce the growing period required by crops for maturation. An example is given, for the Canadian prairies, in Figure 4.3. Here the length of growing season is estimated to increase by about 10 days per deg.C increase in mean annual temperature. At the same time the maturation time for spring wheat is reduced by about 3 days per deg.C, with the result that the probability of the crop not maturing before first autumn frost is reduced by as much as 5 per cent per deg.C.[13] However, the length of time during which the crop is producing dry matter (from heading to ripening in Figure 4.2c) is also reduced with consequently reduced average grain yield. In the Canadian prairies warming therefore implies less frost damage but lower average yields of spring wheat.

The effects of warming on length of growing season and growing period will vary from region to region and from crop to crop. For wheat in Europe, for example, the growing season is estimated to lengthen by about 10 days per deg.C and in central Japan by about 8 days per deg.C. [14, 15] In general the conclusion is that increased mean annual temperatures, if limited to two or three degrees, could generally be expected to extend growing seasons in high mid-latitude and high-latitude regions. Increases of more than this could increase evapotranspiration rates to a point where reduced crop-water availability begins to limit the growing season. The effects of these changes in growing season on agricultural potential are discussed in the following chapter.

Effects on yield

Whether crops respond to higher temperatures with an increase or decrease in yield depends on whether they are determinate or indeterminate, and whether their yield is currently strongly limited by insufficient warmth. In cold regions very near the present-day limit to arable agriculture any temperature increase, even as much as the 7-9deg.C indicated for high latitudes under a doubling of CO2 (see Chapter 2), can be expected to enhance yields of cereal crops. For example, near the current northern limit of spring-wheat production in the European region of the USSR yields increase about 3 per cent per deg.C, assuming no concurrent change in rainfall (Table 4.2). In Finland, the marketable yield of barley increases 3-5 per cent per deg.C and in Iceland hay yields increase about 15 per cent per deg.C [17, 18]

Away from current temperature-constrained regions of farming and in the core areas of present-day cereal production such as in the Corn Belt of North America, the European lowlands and the Soviet Ukraine, increases in temperature would probably lead to decreased cereal yield due to a shortened period of crop development. [19] In eastern England, for example, a 3deg.C rise in mean annual temperature is estimated to reduce winter-wheat yield by about 10 per cent although the direct effect of a doubling of ambient atmospheric CO2 might more than compensate for this (Figure 4.4).

In other mid-latitude regions much would depend on possible changes in rainfall. For example, in the Volgograd region, just east of the Ukraine, spring wheat yields are estimated to fall only a small amount with a 1deg.C increase in mean temperature during the growing season, though they could increase or decrease substantially if the temperature change was accompanied by an increase or decrease of rainfall (Table 4.3).

Yields of root crops such as sugar beet and potatoes, with an indeterminate growth habit, can be expected to see an increase in yield with increasing temperatures, provided these do not exceed temperatures optimal for crop development. [12]

Effects on livestock

A rise in temperature could also have a significant effect on the performance of farm animals, in addition to the effects that might flow from altered yields of grassland and forage crops. Young animals tend to be less tolerant of a wide range of temperature than adults (Figure 4.5). A rise in summer temperatures, especially in regions with a continental climate characterized today by summer temperatures near the threshold tolerated by livestock (such as the south-central USA and USSR) could be detrimental to production.[l2]

Effects on moisture availability

Changes of temperature would also have an effect on moisture available for crop growth, whether or not levels of rainfall remained unchanged. In general, and at mid-latitudes, evaporation increases by about 5 per cent for each deg.C of mean annual temperature. Thus if mean temperature were to increase in the east of England by 2deg.C potential evaporation would increase by about 9 per cent (assuming no change in rainfall). The effect of this would be small in the early part of the growing season, but after mid-July the soil moisture deficit would be considerably larger than at present and, for some crops, this implies substantially increased demand for irrigation.[22] Of course, the amount of water available for plant growth is affected by a combination of climatic and non-climatic variables such as precipitation, temperature, sunshine, windspeed as well as soil porosity, slope, etc. These are considered in the following section.


In most of the tropical and equatorial regions of the world, and across large areas outside the tropics, the yield of agricultural crops is limited more by the amount of water received by and stored in the soil than by air temperature. Even in the high mid-latitudes such as in southern Scandinavia too little rain can restrict growth of cereal crops during the summer when evapotranspiration exceeds rainfall. In all these areas the amount of dry matter a crop produces is roughly proportional to the amount of water it transpires . [11] This, in turn, is affected by the quantity of rainfall but not in a straightforward manner: it also depends on how much of the rainfall is retained in the soil, how much is lost through evaporation from the soil surface, and how much remains in the soil that the crop cannot extract.

The amount of water transpired by the crop is also determined by air humidity, with generally less dry matter produced in a drier atmosphere[11] Thus, changes in both rainfall and air humidity would be likely to have significant effects on crop yields.

Reliability of rainfall, particularly at critical phases of crop development, can explain much of the variation in agricultural potential in tropical regions. Thus, many schemes used to map zones of agricultural potential around the world have adopted some form of ratio of rainfall to potential evaporation, r/Eo, to delimit moisture-availability zones, which are then overlaid on temperature and soils maps to indicate agro-ecological zones.[23] The regions are distinguished largely on the basis of the length of growing season determined by the r/Eo ratio. In Kenya, for example, average plant biomass is estimated to vary by more than an order of magnitude between agroclimatic zones that lie within 100 km of each other.[24] These are characterizations of the effect of differences in average rainfall on agricultural potential, but it is important to note that a high degree of inter-annual variability of rainfall, particularly in the drier zones, can lead to very marked variation in crop yield between wet and dry years, so that changes in rainfall over time as well as over space are also likely to have a similar effect on crop yields.

A strongly positive relationship between rainfall and crop yield is generally found in the major mid-latitude cereal-exporting regions of the world, such as the US Great Plains and Soviet Ukraine. For example, in the dry steppe zone of the Volga Basin (USSR), a 0.5 or 1deg.C warming, with no change in rainfall, is estimated to have little effect on spring-wheat yields, while a 20 per cent decrease in rainfall (at current temperatures) could reduce yields by more than a tenth (Table 4.3).

Relatively few studies have been made of the combined effects of possible changes in temperature and rainfall on crop yields, and those that have are based on a variety of different methods. However, a recent review of results from about ten studies in North America and Europe noted that warming is generally detrimental to yields of wheat and maize in these mid-latitude core cropping regions. With no change in precipitation (or radiation) slight warming ( + 1deg.C) might decrease average yields by about 5 + 4 per cent; and a 2deg.C warming might reduce average yields by about 1() + 7 per cent.[7] In addition, reduced precipitation might also decrease yields of wheat and maize in these breadbasket regions. A combination of increased temperatures (+2deg.C) and reduced precipitation could lower average yields by over a fifth.


Important effects from changes of climate need not only stem from changes in average temperature and rainfall, but also from changes in the frequency of extreme climatic events that can be damaging and costly for agriculture. The balance between profit and loss in commercial farming often depends on the relative frequencies of favourable and adverse weather; for example, on the Canadian prairies a major constraint on profitable wheat production is related to the probability of the first autumn frost occurring before the crop matures.[25]

Among semi-commercial and subsistence farmers the probability of yield in a given year being more than a minimum necessary to feed the household may be more important than the average over several years.[26] Levels of risk such as these may well be altered quite markedly by apparently small changes in mean climate, particularly the risk of successive extremes, which can quickly lead to famine in food-deficit regions.

To illustrate, suppose that extremely dry summers (of a kind that can cause severe food shortage in a given region) occur at present with a probability of P = 0.1. The return period of the occurrence of a single drought is, therefore, 10 years, while the return period for the occurrence of two successive droughts is 100 years (assuming a normal distribution of frequencies). A change in climate can lead to a change in P, either through altered variability which will change P directly, and/or through a change in mean conditions that must also change P if drought is judged relative to an absolute threshold. Alternatively, P may change through changes in some critical impact threshold as a result of altered land use, increasing population pressure, and so forth. If P becomes 0.2, then the return period of a single drought is halved to 5 years. The return period for two successive droughts, however, is reduced by a factor of four to only 25 years.[26, 27] Thus, not only is agriculture often sensitive to climatic extremes, but the risk of climatic extremes may be very sensitive to relatively small changes in the mean climate.

The sensitivity of marginal farmers to climatic change may be especially great. The reason for this is that, near the margins of cultivation, the probability of critical levels of warmth or moisture required to avoid crop failure or a critical crop shortfall tends to increase not linearly but quasi-exponentially towards the margin of cultivation (Figure 4.6). Marginal areas are thus commonly characterized by a very steep "risk surface", with the result that any changes in average warmth or aridity, or in their variability, would have a marked effect on the level of risk in agriculture.

For the reasons given above, much of the impact on agriculture from climatic change can be expected to stem from the effects of extreme events. Consider, first, the significantly increased costs resulting from increased frequency of extremely hot days causing heat stress in crops. In the central USA the number of days with temperatures above 35deg.C, particularly at the time of grain filling, has a significant negative effect on maize and wheat yields.[29, 30, 31] The incidence of these very hot days is likely to increase substantially with a quite small increase in mean temperature. For example, in Iowa, in the US Corn Belt, an increase in mean temperature of only 1 .7deg.C may bring about a three-fold increase in the probability of 5 consecutive days with a maximum temperature over 35DEG.C.[32] At the southern edge of the Corn Belt, where maize is already grown near its maximal temperature-tolerance limit, such an increase could have a very deleterious effect on yield.

The increase in risk of heat stress on crops and livestock due to global warming could be especially important in tropical and subtropical t regions where temperate cereals are currently grown near their limit of heat tolerance. For example, in northern India, where GCM experiments indicate an increase in mean annual temperature of about 4deg.C, wheat production might no longer be viable.

An important additional effect of warming, especially in temperate regions, 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 to accelerate the flowering process. Low vernalization results in low flower-bud initiation and, ultimately, reduced yields. A 1deg.C warming could reduce effective winter chilling by between 10 and 30 per cent.[33]

Changes in rainfall could have a similarly magnified impact. For example, if mean rainfall in the Corn Belt in March (which is about 100 mm [4 inches]) decreased by 10 per cent (an amount projected by some GCMs under a 2 x CO2 climate) this would raise the probability of less than 25 mm [1 inch] being received by 46 per cent. For cattle, crops and trees a 1 per cent reduction in rainfall could mean that drought-related yield losses increase by as much as a half.[34]


No comprehensive study has yet been made of the impact of possible climatic changes on soils. Higher temperatures could increase the rate of microbial decomposition of organic matter, adversely affecting soil fertility in the long run.[3] But increases in root biomass resulting from higher rates of photosynthesis could offset these effects. Higher temperatures could accelerate the cycling of nutrients in the soil, and more rapid root formation could promote more nitrogen fixation. But these benefits could be minor compared to the deleterious effects of changes in rainfall. For example, increased rainfall in regions that are already moist could lead to increased leaching of minerals, especially nitrates. In the Leningrad region of the USSR a one-third increase in rainfall (which is consistent with the GISS 2 x CO2 scenario) is estimated to lead to falls in soil productivity of more than 20 per cent. Large increases in fertilizer applications would be necessary to restore productivity levels.[16]

Decreases in rainfall, particularly during summer, could have a more dramatic effect, through the increased frequency of dry spells leading to increased proneness to wind erosion. Susceptibility to wind erosion depends in part on cohesiveness of the soil (which is affected by precipitation effectiveness) and wind velocity. The only study completed on this subject suggests that in Saskatchewan (on the Canadian prairies) the frequency of moderate and extreme droughts would increase three-fold under a 2 x CO2 climate if mean May-August temperatures increased by 3.5deg.C and precipitation increased by 9 to 14 per cent, which is consistent with the GISS 2 x CO2 climate. They would increase 13-fold if increases in temperature are not accompanied by increases in precipitation.

Estimated changes in the potential for wind erosion under the latter scenario vary from +24 to +29%.[13]


Studies suggest that temperature increases may extend the geographic range of some insect pests currently limited by temperature.[35] Figure 4.7 shows the results from one of the first of these studies - an assessment of the effects of climatic change on the potential distribution of the European Corn Borer (Ostrinia nubilalis) in Europe.[36] The European Corn Borer is a major pest of grain maize in many parts of the world. It is multivoltine (multigenerational) and, depending on climatic conditions, can produce up to four generations per year. Using degree-day (thermal) requirements, the potential distribution of the European Corn Borer in Europe has been mapped under present (1951-80) temperatures. With a 1deg.C increase in temperature a northward shift in distribution of between 165 and 500 km is indicated for all generations. In addition to favourable climatic conditions the distribution of any pest is dependent on the availability of a host plant. As indicated in Figure 5.3 the potential limit of grain maize cultivation is also likely to shift northwards with increasing temperatures providing suitable conditions for the European Corn Borer. This example serves to highlight the need to examine crop-pest interactions in any climate impact assessment.

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.[19] Assuming planting dates did not change, warmer temperatures would lead to invasions earlier in the growing season (i.e. at more susceptible stages of plant development) 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, which could result in serious economic losses.

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 USA causing serious economic losses.[19] The geographic distribution and activities of other diseases already important in the USA may also expand. The horn fly, which currently causes losses of $730.3 million in the 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.[19, 37] In the 1960s and 1970s a combination of the increased resistance of ticks to insecticides and the high costs of dipping threatened the profitability of the Australian beef industry. Prolonged summer rainfall and an extended developmental season, or, conversely, prolonged dryness leading to increased nutritional stress in the host, are likely to cause heavy infestations.[38] If such climatic conditions were to prevail in the future it is likely that ticks could become an increasing problem.

One of the major threats of climatic change is the establishment of "new" or migrant pests as climatic conditions become more favourable to them. In New Zealand, for example, the swarming of locusts in the North Island in recent years may be an indication of a more widespread problem in the future.[39] In a similar fashion, anomalously warm conditions in 1986-1988 led to locust swarms reaching new limits in southern Europe.[40]

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 4deg.C warmer than at present, crop losses to disease may increase to 15 per cent. [18]

Most agricultural diseases have greater potential to reach severe levels under warmer conditions. Fungal and bacterial pathogens are also likely to increase in severity in areas where precipitation increases.[41] 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 could 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.


It is possible that some of the impact of climatic changes on agriculture would stem not directly from the effects of altered temperature, precipitation, radiation, etc. on crops and animals, nor even indirectly from effects on pests, diseases and soils, but through potential effects on natural and semi-natural plant communities.

For example, if warming were to induce a northward shift of the boreal forest in northern regions of America, Europe and Asia, it is possible that extensive grazing, livestock rearing and cultivation of quick-maturing crops (farming types currently located at the southern limit of the boreal zone) would be encouraged to shift northwards to exploit regions vacated by forestry. A geographic shift of agriculture in these marginal regions would thus be the combined result of changes in potential for farming and changes in potential for forestry, with the outcome perhaps determined by the comparative advantage of one use over the other; and this might further be influenced by future policies of conservation.

An illustration of the possible extent of poleward shift of the boreal zone in the northern hemisphere is given in Figure 4.8. This is based on an estimation of the levels of effective temperature sum above a threshold temperature of 5deg.C that currently define the northern and southern limits of the boreal zone (600 and 1,300 degree-days, respectively).[42] Under the warming projected for a 2 x CO2 climate (in this instance based on experiments with the GISS GCM) these limits are re-located about 500-1,000 km further north than at present. If taken as a proxy limit of northern agriculture, this indicates a substantial extension of agricultural potential, although much of this may be severely limited by inappropriate soils and terrain, particularly in northern America and Europe.

Similarly substantial shifts can be expected to occur for vegetation zones throughout the world. An illustration of the possible scale of these shifts, in this instance estimated for Europe for a 5deg.C increase in mean annual temperature and a 10 per cent increase in precipitation is given in Figure 4.9. On average, the major vegetation zones shift northwards by 1,000 km, the largest changes being in the boreal and Mediterranean regions.[43]


CO2-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 IPCC estimate of sea-level rise above present levels under the Business-As-Usual scenario is 9 cm - 29 cm by the year 2030 with a best estimate of 18 cm, and 28 cm - 96 cm by 2090, with a best estimate of 58 cm.[1, 44]

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 of taking protective measures.[45] It should be emphasized, however, that these surveys assume a much larger rise in sea levels (1.5 m) 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 effects on agriculture 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 per cent of all land (and about one-fifth of all farmland) in Bangladesh would be inundated and a further 6 per cent would become more prone to frequent flooding.[45] Altogether 21 per cent of agricultural production could be lost.

In Egypt, it is estimated that 17 per cent of national agricultural production and 20 per cent 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.[45]

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 re-locate 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 0.5m by 2090 under the Business-As-Usual scenario).


The combination of impacts on agriculture that could stem from the direct effects of increased atmospheric CO2, from effects of changes in climatic and, in coastal regions, from sea-level rise is likely to be extremely complex. It will certainly vary greatly from region to region and from one type of farming to another. The implications for agricultural potential are considered in the next chapter.

Chapter 4

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2. Acock, B., and Allen, L.H. Jr, "Crop responses to elevated carbon dioxide concentrations", in Strain, B.R., and Cure, J.D. (eds), Direct Effects of Increasing Carbon Dioxide on Vegeta~ion, DOE/ER-()238 (Washington, DC: US Dept. of Energy, 1985).

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8. Morison, J.I.L., "Intercellular CO2 concentration and stomatal response to CO2", in Zeiger, E., Cowan, I.R. and Farquhar, G.D. (eds), S~omatal Function (Stanford: Stanford University Press, 1987), pp. 229-251.

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16. Pitovranov, S.E., Iakimets, V., Kiselev, V. I. and Sirotenko, O.D., "The effects of climatic variations on agriculture in the subarctic zone of the USSR", in Parry, M.L., Carter, T.R., and Konijn, N.T. (eds), The Impact of Climatic Variations on Agriculture, Volume 1, Assessments in Cool Temperate and Cold Regions (Dordrecht, The Netherlands: Kluwer, 1988), pp.617-722.

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19. See, for example, Smith, J.B., and Tirpak, D., The Potential Effects of Global Climate Change on the United States, Report to Congress (Washington, DC: US Environmental Protection Agency, 1989). A summary is given in Adams, R.M. (and 9 others), "Global climate change and US Agriculture" Nature, vol. 345: 219-224 (1990).

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35. I am grateful to Julia Porter for permission to reproduce this section, originally drafted by her for the IPCC review of impacts on agriculture. See, IPCC, The potential impacts of climate on agriculture (Geneva and Nairobi: WMO and UNEP, 1990).

36. Porter, J.H., personal communication, 1990.

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