Agriculture: Climatic change and its implications

Agriculture: Climatic and its Implications

Change TREE vol. 5, no. 9, September 7990

M.L. Parry, J.H, Porter and T.R. Carter

In spite of many uncertainties surrounding the nature of future changes in climate, a

, num6er of indications are emerging of the h%ely imphcations for agriculture. At high mid-latitudes, agriculture is at present con- strained 6y low temperatures; proiections of preferential warming at these latitudes suggest that their productive potential may 6e enhanced, although the traditional bal- ante between agriculture and forestry may 6e disrupted 6y a poleward retreat of the boreal zone, and by the problem of over- production of grain crops. Mid-latitude con- tinental areas, containing the world’s ‘6readbaskeP regions, may suffer declines in productivity due to increased moisture stress during the growing season. The most inherently vulnera6le regions, generally located at lower middle and low latitudes, appear to be particularly at risk as any changes in climate may further stress the already limited production capacity.

changes in climate (see Henderson- Sellers, this issue). Although the broad-scale prediction is for a smooth increase in global temperature, there may be rapid warming in some regions and possibly even periods of cooling in others2. One important question is whether agricultural systems can adapt to such rapid rates of change.

There is much concern over the possible impacts on agriculture (and the implications for global food security) of climatic changes due to increased emissions of ‘greenhouse’ gases. Three main issues need to be addressed in con- sidering the likely effects of climatic change on agriculture: first, the nature of the expected changes in climate; second, the estimated impacts of these changes on crop and animal production; third, the range of appropriate responses required to adapt to climatic change, through adjustments both at the farm level and in regional and national policy.

Current best estimates suggest that if greenhouse gas concen- trations in the atmosphere continue to increase at present rates, then an increase in global mean annual sur- face air temperatures of approxi- mately 0.5’C by 1995-2005, I .5”C by 2015-2050and3”Cby205&2100can be expected, with greater increases at high than at low latitudes’.

The changes in precipitation that are likely to accompany any increases in temperature are of particular importance to agriculture. At a global level, warming will lead to an intensified hydrological cycle and thus higher precipitation. How- ever, there are great uncertainties about the extent of precipitation changes at the regional level, where increases or decreases could occur. One further effect of increases in temperature is a likely increase in potential evapotranspiration. Marked drought stress may occur in regions where there is no accompanying increase in precipitation.

The present year-to-year fluctu- ations in crop yields give some indi- cation of the wide-ranging effects that weather can have on agriculture. For example, high temperatures during ear growth, low temperatures during flowering, drought during grain filling and wet weather at harvest are some of the critical climatic factors that may adversely affect grain yields. Thus, any change in climate that alters the frequency, intensity or timing of such events could have a profound effect on agriculture.

To date, general circulation models (GCMs) simulating climatic change have been unable to pro- vide useful projections of regional

There are three broad types of impact on agriculture that are likely to stem from climatic change. Changes in climate will directly influence both crops and livestock; in addition, there will be indirect effects due to changes in other important environmental factors, such as insect pests, diseases, weeds, soils and water supplies.

Effects of climatic changes on crop potential

M.L. Parry, 1.H Porter and T.R. Carter are in the Crop potential is likely to be af- Atmospheric Impacts Research Group, School of fected by climatic change in two Geography, University of Birmingham, PO Box 363, ways. The first is through the ‘direct’ Birmingham 815 2lT, UK. physiological effects on plants re-

318 0 1990, Elsevier Science Publ,sher:s L!d iLKI 0169 5347!90:$02 00

sulting from the increase in levels of ambient CO, in the atmosphere. The direct effects on plants are con- siderecl in more detail by Wood- ward (this issue). It is sufficient to note here that, for many crop species, elevated levels of CO, will lead to increases in yields, provided that all other factors remain con- stanPA.

Secondly, changes in climate will have indirect effects on crop potential, in a number of ways; for example, there may be changes in the length of the growing season, changes in crop yields and spatial shifts in agricultural potential. How- ever, each of these changes is likely to be spatially heterogeneous: the absolute magnitude of climatic change will vary between different locations, and the local effect will be a function of the change in climate relative to the existing conditions. Thus, since agricultural potential at mid-latitudes generally decreases poleward, due to reduced thermal resources, the same increases of temperature will have relatively greater effects on crop potential at higher latitudes than at lower latitudes. In addition, because the magnitude of future temperature increases is expected to be greater at higher latitudes, substantial effects on crop potential can be anticipated in these regions.

Changes in length of the growing season For crops grown in cool-

temperate and cold regions, an increase in temperature will lead to a lengthening of the potential growing season and an increase in plant growth rates, and thus a shortening of the required growing period. At lower latitudes, particularly where the growing season is determined by rainfall, it is much less clear what the consequences may be.

Figure I shows the potential growing period in Europe under present climatic conditions, and under the UK Meteorological Office GCM projection of the climate resulting from an increase in greenhouse gases that is equivalent in effect to a doubling of CO, from pre-industrial levels (the UKMO 2 X CO, scenario). The growing period is taken to be the number of months with average temperatures above 5°C and rainfall exceeding half of potential evapotranspi-

TREE vol. 5, no. 9, September 7990

nJion7. It can be seen from Fig. lb that in northern Europe there would be an increase in the potential grow- ipg period, whereas around the Mediterranean the growing season could shorten significantly due to warmer and drier conditions in spring and autumn. Seen in these simple terms, there is a shift of cropping potential from southern Euro- pean countries to northern Europe.

C llanges in mean crop yield Yields of most crops in cool-

temperate and cold regions can be expected to increase with increasing temperature, except in regions where moisture is a limiting factor. Under the Goddard Institute for Space Studies (GISS) 2 x CO, scenario, yields of barley and oats in Finland are increased by 9-18%, depending on the region*. It is possible that agriculture in Fenno- scandia could gain from global w,arming more than that in any other part of the world.

This increase of biomass poten- ti,sl in northern Europe is in contrast to the decreases in southern Europe that are implied by those GCM projections indicating reduced soil moisture in this region9. As already noted, this combination of factors could lead to a significant northward shift of the balance of agricultural resources in Europe’O. However, such a shift is likely to be impeded bv the poor quality of soils in many regions of northern Europe.

Reductions in yields of some crops are likely to occur in many al’eas where climatic change results in reduced soil moisture availability. There are indications that global warming could lead to decreases in cereal production in North America, and this would be mainly due to the accompanying reduction in soil moisture”-‘3. Even assuming an irrigated crop, under a number of 2 x CO, scenarios potential maize yields in North Amer- ica are estimated to decrease by I& 2’i%“.

Increases in precipitation leading tc excess soil moisture can be equally serious for agriculture. In the Leningrad region of the USSR, for example, the temperature and precipitation increases anticipated en route to a 2 x CO, climate could initially produce increased yields of rye. However, the higher rainfall is

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Fig. I. (al Length of growing period (months) under the present-day ( 1931-1960) climate. lb) Length of erowine ueriod lmonthsl under a 2 x CO, climate simulated by the UK Meteorological Office general &la& model. Redrawn from Ref 7. h

likely to cause more leaching and’ erosion, eventually decreasing soil fertility and therefore rye yieldsi4. Elsewhere, in contrast, increased rainfall could be beneficial to crop yields. In China there are indications that global warming could lead to increased rainfall during the summer monsoon. Under a l°C warming and with precipitation increases of 100 mm, national yields of rice, maize and wheat are estimated to increase by about 10%15. Increased yields of rice are also expected in northern japani6, yet yields may well decrease in other regions of Southeast Asia due to an accelerated rate of crop growth.

Many other regions face the prospect of reduced agricultural productivity as a consequence of decreases in rainfall. In north- eastern and East Africa any change in precipitation could substantially affect maize yields and grass growth, and thus the carrying capacity of rangelands 17. Any change in the frequency of dry years would significantly influence the average output of agriculture in these regions.

Spatial shifts of agricultural potential The combination of different

effects of climatic change on agricultural crops is likely to bring about a spatial shift of crop potential. Areas that are, under present climatic conditions, judged to be most suited to a given crop or combination of crops, or to a specified level of management, will change location.

Several studies have examined changes in the climatic limits for a range of crops in mid-latitude regions, under a variety of climatic

scenarios7,‘o,‘6,i8. They suggest that a 1°C increase in mean annual temperature would tend to advance the thermal limit of cereal cropping in mid-latitude regions by about l50- 200 km, and to raise the altitudinal limit of arable agriculture by about 150-200 m. In the UK, for example, the temperature limit for the successful ripening of grain maize - which at present lies in the extreme south of England - would gradually shift northwards as temperatures began to rise (Fig. 2).

One potentially useful method of interpreting future climatic scenarios and their likely impact is to identify ‘analogue regions’, which have a present-day climate that is analogous to the future climate estimated for a study area. Analogue regions of this kind have been identified for a number of areas (Fig. 3). For example, under the GISS 2 x CO, scenario the climate of Ice- land is similar to the climate of northern Britain today. The present- day farming types in analogue regions are a useful indicator of the adaptive strategies that are likely to be required to retune agriculture to altered climatic resources in the study regions19.

Implications of climatic change for livestock

Changes in climate will affect livestock in two main ways. Firstly, through direct effects on the ani- mals themselves; and secondly, through effects on the production of feedstuffs.

Any change in external temperature will affect the heat balance of an animal. Within a certain range of temperatures, known as the ‘comfort

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Fig. 2. Hypothetical limits for the successful ripening of grain maize based on temperature. The present limit and future limits on lowland areas under warmer temperatures are shown. 1976 was a particularly warm summer in the UK. Reproduced from Ref 10.

zone’, livestock can maintain a relatively constant body temperature. If temperatures rise above the upper level of this comfort zone, the appetite of the animal will be reduced, feed-conversion efficiency will be lower and reproductive capacity, particularly of the male, will be reduced20. Therefore a rise in temperature that is near to this critical tolerable threshold could have an adverse effect on livestock production.

Changes in the growing area of particular crops, in the yield of these crops and in crop quality will affect

the cost of feed productionZo and, ultimately, the viability of retaining livestock farming in some regions. Another important factor relates to the timing of crop production with respect to livestock requirements. In Australia, winter rainfall is important for the growth of herbaceous C, plants, which greatly contribute to animal production2’. A combination of a decrease in winter rainfall and an increase in summer rainfall would lead to a decline in these important plant species, resulting in greater seasonality of animal production. Elsewhere, more favourable conditions may develop. For example, under the GlSS 2 x CO, climate scenario for Iceland, the carrying capacity of improved grass- land for sheep is estimated to increase by about two and a half times, and on unimproved range- land by more than a half22.

Effects of climatic changes on pests and diseases

Global warming has a number of important implications for agricultural insect pests. Increased temperatures will generally lead to increases in the rate of development of insects, with earlier establishment of pest populations in the growing season. Thus, for multi- voltine (multi-generational) insects, such as aphids or the European corn borer (Ostrinia nubilalis), warming may increase the number of gen- erations produced per year”. The geographical range of some species is likely to extend beyond their present distributions, and there will be an increased risk of invasion by migrant and ‘exotic’ species as climatic conditions become more favourable for their establishment and development23.

- Cherdyn Region

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Fig. 3. Present-day regional analogues of the conditions predicted under a 2 x CO, climate for Saskatchewan, Iceland, Finland, and the Leningrad and Cherdyn regions of the USSR. Reproduced from Ref 19.

Many of these factors could lead to increases in pest density and damage. In the US Grain Belt, an increase in the overwintering range and population density of the corn ear-worm (Heliothis zeal under a warmer climate will increase damage to soybeans and result in significant economic losses”. Cli- matic changes will also affect livestock pests. For example, the Old World screw-worm fly (Chysomya bezziana) - an obligate parasite - represents a major threat to much of the world’s livestock, and it is known to pose an immediate threat to livestock in Australia and North and Central America. With temperatures 3°C warmer than at present and more summer precipitation, the potential for C. bezziana infestation would be significantly increased24.

Most agricultural diseases have greater potential to reach severe levels under warmer conditions. For example, under present climatic conditions potato blight (Phytoph- thora infestans) causes relatively little damage to potato crops in Ice- land, because it is restricted by low summer temperatures. However, historical evidence suggests that the blight is present in Iceland at sub-outbreak levels, and that climatic warming (whilst potentially increasing crop yields) may lead to more severe blight attacks22s25. Fungal and bacterial pathogens are also likely to increase in areas where precipitation increases, as free water is of primary importance in their germination and dispersa12’.

Likely farm-level adjustments in agriculture

As climate changes, it is likely that agriculture will adapt to meet some of these changes and that these adjustments may significantly influence future impacts. The nature of the adjustments will depend on future developments in technology, demand, prices and national policy, and changes in these factors cannot be predicted. However, there are a number of farm-level adaptations that could be implemented now, under present levels of technology, and that do not require a change in government policy. These adaptations include changes in crop variety, crop type and management practices.

In regions where substantial in-

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creases in the warmth of the growing season are expected to occur, a change from the existing, often q Jick-maturing, crop cultivars to later-maturing varieties, with higher tlrlermal requirements, may be necessary to take full advantage of tte longer and more intense growing season. It has been estimated ttat yields of present-day quick- maturing rice varieties in northern Japan would probably increase by about 4% with the increased temperature predicted under the GISS 2 X CO, scenario. However, the adoption of late-maturing rice (at pa-esent grown in central Japan) might increase yields by 26%16.

Conversely, in areas where growing-season length is reduced b4, high temperatures, there may be a shift to shorter-season, more heat- tclerant varieties’j. Similarly, in aleas where the availability of soil moisture declines, earlier maturing, drought-tolerant cultivars may be needed. In SaskatchewanI and the Moscow region of the USSRi4, winter N neat would give higher yields than spring cereals under a warmer climate.

A shift in agricultural zones as a re,sult of the switching of crops to take advantage of changing environmental conditions is likely to be most pronounced in middle and high latitudes, where latitudinal zoning is most evident as the result 01 differences in available warmth for crop maturation. In this way, the potential for the production of early-maturing sunflowers in the UK could shift northwards by about 300 km for each 1°C rise in mean annual temperaturelo. Similarly, northward shifts of citrus, olives and vines have been projected for southern Eu- rope2$ and, in the southern hemi- sphere, southward shifts of land use have been suggested for crops such a:; white wine grapes and kiwifruit in New ZealandIs.

Many changes in management will occur as the effects of a changing climate are perceived. As well as adjustments to the timing of sowing and harvesting, there will be changes in the quantity and timing o!: fertilizer and pesticide appli- cations. In addition, the projected changes in temperature and precipitation under a 2 x CO, climate are likely to favour the use of irri- gation for crop production in some

countries, for example in regions of the USA27. However, this is likely to lead to significantly higher costs of production and possible shifts to- wards less water-demanding uses”. Conversely, increases in the amount or intensity of rainfall, particularly in regions characterized by monsoon rainfall, will require changes in management to prevent soil erosion.

Adiustments in agricultural policy The implications of the changes

in agricultural potential described above are likely to be felt through- out regional and national econom- ies. As a result, policy adjustments may be required in a number of areas.

As the climate changes, there may be a need for government assistance in substituting new ac- tivities for old, in order to match the actual pattern of production to the changed patterns of agricultural potential. For example, incentives could be offered to farmers to switch to different crops in order either to maximize total production, or to stabilize year-to-year variations in production. In many drought-prone countries in the developing world, the latter strategy is probably worth pursuing even in the absence of global climatic change28. in some cases, however, these adjustments might prove very costly, involving large- scale replacement of existing agricultural infrastructure.

Several countries in high mid- latitude regions stand to gain from increased productivity under a warmer climate. However, many of these countries (e.g. Finland and Japan) are already struggling to maintain a precarious balance between ensuring self-sufficiency on the one hand and avoiding costly over-supply on the other. Price- support policies encourage home production, often at prices well above those of the world market, and increased production would lead to major national surpluses of foodgrains, creating a problem of disposal. Radical changes of policy, including the promotion of ‘set- aside’ land and reduction of price support, would be required to take cropland out of production8J6.

Elsewhere, existing policies that encourage inputs such as fertilizers, and support for land, water and pest management schemes, may need to

be retuned to account for altered levels of agricultural potential. Fi- nally, since there are likely to be regional disparities in the effects of climatic changes on agriculture, the present regional pattern of farm in- comes is likely to alter; as a result, government policies designed to reduce regional discrepancies may need substantial revision in order to maintain a level of equitable support’9.

Conclusions Present indications are that, with

a changing climate, overall global food production could be sustained at current levels29, by a combination of spatial shifts of agricultural zones and adjustments in agricultural technology and management. How- ever, there are likely to be critical reductions in production in some regions. Indeed, there are a number of inherently vulnerable regions - particularly in Africa and South America, and elsewhere in the lower middle and lower latitudes -where changes in temperature and precipitation may further stress the already limited productive capacity. These are the regions most at risk from the effects of climatic changes.

There is still great uncertainty about some aspects of climatic change that are important for agriculture. These include the regional and local (rather than large-area) variations in climate, changes in the annual and seasonal variability of rainfall, and the rate at which climate is likely to change. It is important to increase substantially the range of current research into how agriculture can best adapt to, or mitigate, such changes. Some of the research priorities include: ?? Improved estimates of the possible changes in climate, particularly at the regional level. ?? Improved knowledge of the effects of climatic change on crop yields and livestock production. ??Greater understanding of the relationship between the effects of climatic change and changes in other systems (for example, changes in the incidence of pests and diseases). ?? Further examination of the range of potentially effective technical adjustments and policy responses to prevent, adapt to or mitigate climatic changes.

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There is a high probability that the earth is already committed to a certain degree of climatic change resulting from past emissions of greenhouse gases into the atmosphere. Thus, it is clear that the need for further research into the impacts of climatic change, to underpin political decision making, is both vital and urgent.

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Agriculture: Climatic change and its implications