FAKHRI A. BAZZAZ
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, USA
WIM G. SOMBROEK
Land and Water Development Division, and Interdepartmental Working Group on Climate Change, FAO, Rome, Italy
Present knowledge, problems and uncertainties
Implications and needs
The predicted changes in climate, especially increased atmospheric CO2, temperature and precipitation, associated with changes in nitrogen deposition, tropo- and stratospheric ozone levels, UV-B radiation, etc. can have great impacts on world agricultural production and supply patterns. In order for agricultural production to be sufficient to meet the demands of the ever-growing human population, the impact of the climate must be understood and integrated in any future planning. The Food and Agriculture Organization of the United Nations is much concerned with this issue. The Organization formed an Interdepartmental Working Group on Climate Change and charged it with coordinating FAO activities in the critical area.
A Consultation 'Global climatic change and agricultural production: direct effects of hydrological and plant physiological processes' was held from 7 to 10 December 1993 at FAO Headquarters in Rome, with the support of the United Nations Environment Programme in Nairobi.
The objectives of the Consultation were: (1) to analyse and assess the effects of higher atmospheric carbon dioxide (CO2) levels, higher ultraviolet (UV-B) radiation, higher near-surface ozone concentrations, higher temperatures and changing precipitation/evapotranspiration ratios on plant growth and food production; (2) to provide an overview of the state of knowledge on individual and combined effects, including a description of the processes, and availability of data for specific crops; (3) to identify gaps in our present knowledge and to focus on critical research needs.
The meeting was attended by 40 external participants, the 12 members of FAO's Climate Change Group and other interested FAO staff. The discussions concentrated on crops, with limited consideration of natural ecosystems. Because the majority of the presentations and discussions emphasized agricultural systems, it was decided to focus the book on agriculture alone. However, we believe that there are strong interactions between agricultural and natural ecosystems and these natural ecosystems play a significant role in the global carbon cycle which can greatly impact agriculture. The two are separated for convenience only. No discussions took place on the effects of climate change on oceanic biotic production because of the absence of specialists in that field among the assembled groups.
The chapters contained in this book are recent revisions of the presentations given in the meeting and therefore reflect the fast-growing research on the impact of global climatic change on agro-ecosystems. The views expressed in various chapters of the book reflect those of authors who wrote the individual chapter and therefore do not reflect the positions of their respective institutions, nor do they necessarily represent the views of FAO.
Due to increasing consumption of fossil fuels such as oil, gas and coal, in order to satisfy human needs, large quantities of carbon dioxide are being emitted into the atmosphere (Boden et al., 1994). In addition, agricultural and industrial activities add considerable amounts of methane (CH4), nitrous oxides (N2O) and chlorofluorocarbons (CFCs) to the atmosphere as well. Collectively, all of these gases lead to what is called the enlarged greenhouse effect because all of them absorb infrared radiation (Houghton et al., 1990). It is estimated that about 50% of CO2 emitted to the atmosphere remains in it and the other 50% is taken up by the ocean and terrestrial ecosystems.
General circulation models based on the equivalent of doubling CO2 concentration have predicted a global increase in mean global temperature ranging from 1.5 to 4.5°C. Furthermore, all current models show that the increase will be unequally distributed globally. For example, it is predicted that temperature rise in higher latitudes will be much more than in equatorial regions. These models also predict a change in precipitation with some regions receiving more rain than the present and others receiving much less rain.
Agriculture is totally dependent on weather and climate. Despite much effort by climatologists, there is considerable uncertainty about the potential impact of climate change on this sector. Little is known as to how, when, where and to what extent climate change will occur; one incontestable fact is the rising concentration of carbon dioxide (CO2) in the earth's atmosphere. An additional certainty is the soundness of the basic greenhouse theory: the composition of the gas mix in the atmosphere strongly affects the planet's temperature. If the model scenarios are realistic, correctly reflecting future realities, such an increase may have serious consequences for agriculture and, in particular, for the regional food security in some regions (Ruttan, 1994). This security is already strained by increased demand and intensification of resource use, by the fast-growing human population and by an increase in per caput consumption of agricultural products (Rosenzweig and Parry, 1994). There is little question that agriculture must keep pace with the burgeoning human population which is expected to reach 10 thousand million in the 21st century. Since the timing, spatial pattern and magnitude of climate change are uncertain, policy-makers face a dilemma as to what measures, if any, should be taken to face the predictions of climate change. At present, some of the proposed preventive and mitigation measures would have enormous economic and social costs, particularly in relation to energy use. But these costs could even be larger in the future, if uncertainties are not resolved and we would learn that changes in the climate would be larger and more devastating than initially thought.
Thus, agriculture faces a particularly difficult dilemma: should it begin adapting at high cost to uncertain climatic changes while seeking the resolution of the scientific issues concerning the magnitude of climate change and its impact on agriculture, or choose the 'business-as-usual' principle and run the risk of leaving future generations unprepared when changes materialize? Although clear-cut answers might not be available for at least the next decade, improving scientific knowledge on the agronomic and ecological effects of any climate change, both adverse and positive, and on the ability of humans and ecosystems to adapt, might reduce the uncertainty and help formulate better policy (Fajer and Bazzaz, 1992). Therefore, whatever policies are adopted should be subject to frequent reassessment and must be flexible enough to accommodate change dictated by the gaining of new knowledge.
The panel of experts assembled by FAO, made up of persons with a wide range of backgrounds in various aspects of agriculture and natural ecosystem science, reached consensus on the following issues.
A. EFFECTS ON PLANT PHYSIOLOGY AND SOIL PROCESSES
CO2 fertilization and anti-transpiration effects
As plant type and plant productivity are major determinants of food production, it is critical to understand and quantify the response of the most important crops to changing environmental conditions. Different crop species have different responses to increased atmospheric CO2 concentrations and to the combined changes in other factors, such as temperature, precipitation, pollutants, ultraviolet radiation (UV-B), etc. For example, free-air CO2 enrichment experiments (FACE) in Arizona, USA, show that cotton is highly responsive to elevated CO2 whereas wheat is much less responsive (see chapter by Allen et al, in this volume).
CO2 is a key factor in photosynthesis and in plant growth. After diffusion into the plant through stomata, it is transformed by photosynthesis into carbohydrates. A large number of water molecules are lost by transpiration through the stomata for every CO2 molecule entering the leaf. In a CO2-rich environment, the larger concentration gradient forces more CO2 into the plant, while partial closure of the stomata will reduce water losses from the leaf. As stomatal opening decreases in a high CO2 environment, water loss from the plant is also reduced, increasing water-use efficiency. For example, there are indications that doubling present CO2 concentrations reduces stomatal conductance (opening) by 30-60% depending on the species. The reduction in water consumption is called the CO2 anti-transpiration effect. The water-use efficiency (WUE) of the plant improves, since less water is used for equal or more CO2 transformed into dry matter. At the same time, net photosynthesis might increase, because photorespiration, which reduces carbon gain, is less at high CO2 concentrations. In optimal conditions of light, moisture and availability of nutrients, this fertilization effect could increase above- and below-ground biomass production by 10 to 40% depending on crop type and even to higher levels such as the case in cotton. Earlier and more rapid leaf production is expected, and the incremental increase in biomass can benefit the root system even more. The earlier establishment of ground cover, because of the early canopy development, may also limit water loss by direct soil evaporation. This response is even greater at high temperatures, since the optimum temperature for photosynthesis increases with high atmospheric CO2. In contrast, combined low temperatures and elevated CO2 concentration could reduce plant growth.
The importance of the anti-transpiration and fertilization effects varies with crop type. For example, at double atmospheric CO2 the biomass production of C3, plants, including major crops such as rice, wheat, potatoes, beans, soybean, sunflower, groundnut and cotton, can be expected to increase, on average, by some 30%, provided other factors are not limiting.
On the other hand, and independently of CO2, the physiology of C4 plants permits a generally higher photosynthetic capacity than in C3 plants. This efficiency, however, is quickly saturated at increased CO2 concentration. Therefore, in a CO2-rich environment, the net improvement in photosynthesis of C4 plants is proportionally small (about 10%, mostly in stem), although the WUE might significantly improve (by about 40%). This category includes crops of major importance for food production, such as maize, sorghum, sugar cane and millet, but also tropical grasses, pasture, forage and some weed species that are critical for agricultural production. CAM plants seem to be less sensitive to CO2 enrichment as well (Poorter, 1993).
Effects on soil fertility
A sudden doubling of the atmospheric CO2 concentration and associated higher temperatures - as in most experimental set-ups, both enclosed and under free-air enrichment - may result in soil degradation including nutrient depletion. A potential increase in initial soil fertility under elevated atmospheric CO2 can be expected if the increase in CO2 occurs gradually, as in practice and as is the case in crop models that take into account the gradual transient increases. Additional litter is likely to raise soil organic matter content unless litter chemistry drastically changes which would cause a decline in litter decomposition rate. However, the higher soil temperature can stimulate microbial respiration and the decomposition of the organic matter (i.e., mineralization) and cause the release of nutrients which become available for plant uptake through the root system unless microbial competition with plants for the available nutrients is intensified. Furthermore, since both the nutrient uptake efficiency and the structure (length and density) of root systems improve under elevated CO2 concentrations, overall plant nutrient uptake can also increase. Furthermore, there are indications that the expanded root system can penetrate more deeply into the soil and reach extra sources of moisture and nutrients.
An additional driving force is an enhancement of the symbiotic association between root systems and the fungi and bacteria of rhizosphere under high CO2 concentrations. If these concentrations do not greatly alter the existing gas composition in the rhizosphere, the colonization of the root system by mycorrhizae (fungi in symbiosis with roots, facilitating plant uptake of occluded phosphate) and nitrogen-fixing bacteria (bacteria with the ability to incorporate atmospheric nitrogen into nitrogenous compounds which can be utilized by living organisms) is likely to improve the nutrient uptake by the host plants. However, it is possible that this process will ultimately slow down if changes in litter chemistry, e.g., an increase in C/N ratios and an increase in tannins, occur. In such a case there will be a greater demand for the use of additional fertilizers.
This promising picture of improved food production under higher atmospheric CO2 is modified by other factors that can limit growth. First, most of our understanding of the positive effects on crops relies on short-term and controlled studies usually at the individual plant level. Despite some evidence from field experiments such as FACE, extrapolation and generalization to large-scale field conditions or to long-term global food production are still uncertain. Also, since crop responses to climate change are site-specific and species-dependent, the knowledge on one kind of grouping or plants (e.g., annuals) may have little relevance to other species or groupings (e.g., perennials). Second, under conditions of limited soil nutrients or solar radiation (e.g., through enhanced cloud cover), higher CO2 may not improve overall yields; in much of the world such stress conditions are the rule rather than the exception. In fact, nutrients and other present-day limitations might be more critical to agricultural production than the potential impact of climate change especially if urbanization and increased demand for agricultural products push agriculture toward marginal land.
Experimentation and scientific explanation provide additional arguments for a balanced assessment of the CO2 fertilization and anti-transpiration effects. There are feedback mechanisms that might lower the direct effects of higher CO2 and temperatures. For example, because transpiration rate per unit leaf area goes down when stomatal conductance decreases, leaf temperature can rise, triggering a general increase in canopy temperature and potentially higher water use. There are indications that the foliage ages more rapidly and the critical period of seed filling is shortened at high canopy temperature. Such feedbacks can undercut the decrease in crop seasonal water use that might result from the improvement in water-use efficiency of plants.
There are physiological and biochemical mechanisms that can further limit the long-term benefits of CO2. For example, after an initial improvement, the growth rate of many perennial species exposed to elevated CO2 concentrations tends to fall to near that of unexposed plants. One reason is a possible biochemical feedback, such as the reduction in the enzyme rubisco due to the accumulation of carbohydrates in leaves, that slows down photosynthesis and reduces phosphorus availability to carry carbohydrates into the growing parts of plants. There is also insufficient knowledge of the exact causes and consequences of this downregulation in various crops. It appears that sink strength plays a major role in this process as downregulation seems to occur more strongly in crops which are sink limited than in crops which are sink unlimited. The direct physiological responses of plants (e.g., changed stomatal resistance, leaf area index) may be different at different growth stages to varying CO2 concentrations above and beyond the finding that early stages of plant development are more responsive to elevated CO2 relative to later stages.
Interactions with pollutants in the atmosphere are also important for agriculture. Tropospheric ozone (O3) levels, sulphur dioxide (SO2) concentrations and UV-B radiation are likely to increase in parallel with the rise in CO2 Depending on the species, it is estimated that crop damage from peak O3 exposure may result in significant reduction in yield in the proximity of the source (e.g., downwind of major urban areas). Because most studies are performed in closed systems, there is considerable uncertainty on the applicability of such results in open-field environments where significant spatial and temporal variability of O3 concentration are to be expected. Moreover, the damage attributed to O3 alone is confounded by the interaction among elevated CO2, increase in temperature and changes in water availability. For example, under some conditions, yield losses attributable to water stress can outweigh those resulting from 0. exposure. Yet, all indications suggest that the increase in tropospheric O3 will have a negative effect on crops; what is unknown is the magnitude of this effect at the local scale or in combination with other factors.
Losses of stratospheric ozone are contributing to higher levels of UV-B radiation but, although some experimental work suggests reduced growth, there is little field evidence that this would significantly affect plant growth. Furthermore, it seems uncertain that the interaction between UV-B and ozone will be of importance to crop productivity.
B. EFFECTS ON HYDROLOGICAL PROCESSES
Many countries allocate a large percentage of their freshwater resource to agriculture. Since water is a critical factor for crop yields, it is obvious that the advantages of elevated atmospheric CO2 cannot be realized if water is limiting. There is strong evidence, however, that responsiveness to CO2 increases in dry years in some grasslands. Although some recent models show increased drought in some regions and increased rainfall in other regions, available information on the potential effect of climate change on the global water availability is conflicting and remains largely fragmented, except for the nearly gratuitous statement that the hydrological cycle and ultimately the overall water supply are likely to be affected.
Current models predict that there may be an increase in the Indian monsoon and a decrease in moisture availability in the Amazon region and parts of tropical Africa. Numerous Global Circulation Models (GCMs) have attempted to predict these changes in rainfall patterns based on increased global warming potential. In most cases the predictions compare poorly with the observed data. There is no solid evidence that recorded changes in precipitation in the last three decades are related to the increasing atmospheric CO2. Three reasons are cited for the mismatch between real and simulated data obtained from GCMs. Firstly, precipitation is the least reliable model output, because of the temporal and spatial variability in rainfall and more importantly because physical processes governing short-term variations are not well understood. Secondly, runoff resulting from precipitation depends on a large number of parameters many of which are specific to each river basin. GCMs rarely account for these parameters; their predictions are therefore inaccurate as long as the horizontal processes (i.e., overland flow, channel routing, groundwater movement, etc.) governing local water balance are largely ignored. Finally, many GCMs generally do not take into account specific cloud effects on regional heat balance, the influence of oceans, or extreme events (e.g., drought, flooding, etc.), the frequency of which is predicted to increase with climatic change.
However, in spite of their inherent deficiencies, models are valuable analytical tools for the assessment of the potential impacts of climate change on the water resources systems. Often these models use hydrological time series (e.g., measurement of river flow) to associate past meteorological events (e.g., precipitation) with recorded changes in lake levels and river flow regimes. This is not justified in view of the non-linear relationship between rainfall and runoff; small changes in precipitation have a magnified effect on river flow. For example, when regional precipitation decreased ~20% around Lake Chad, flow in the local river system was reduced ~50%. Similarly, in the Volga basin where regional precipitation is already increasing, the discharges will substantially increase if GCM predictions are correct. In regions with significant changes in precipitation, monitoring of river flow might therefore be a good indicator of any 'climate change effect'. However, even when based on this indicator, the analysis of hydrological time series of major lakes and river systems (Lake Chad, River Nile, Caspian Sea, Great Lakes, etc.) fail so far to detect any trend likely to be caused by global warming. Factors other than climate change (e.g., population growth, industrial and agricultural development, new methods of irrigation) seem to have a far greater impact on water resources.
C. EFFECTS ON GLOBAL FOOD SECURITY
The important question remains: how do the above processes influence the global food production system? Models have attempted to simulate climate change impacts on the world food situation by pulling together population data, components of the earth-atmosphere-ocean system and scenarios of climate change as well as estimates of potential changes in yields of important crops. The purpose of these models is to predict changes in land productivity and the geographical shift in agricultural land use as a function of changes in climate and food demand. The main outputs of these models reveal some new predictions, and confirm previous ones, that a doubling of atmospheric CO2 is likely to cause the following:
· agro-ecological zones would shift because of temperature increase and improved water-use efficiency, with significant regional differences;
· crop yields and winter grazing in mid- and high-latitude regions (i.e., mostly developed countries) would improve because of increased photosynthesis, longer growing periods and extension of frost-free growing regions, provided optimum growth conditions are maintained, e.g., by judicious fertilizer and biocide use on agricultural land;
· in most developing countries, crop productivity would diminish (some 10% reduction in cereals), which could raise agricultural prices on local and world markets and increase the need for cereal imports, although the global food supply/demand ratio might change only little;
· there can be much risk in tropical and subtropical regions, and the greatest risk to food security would be in Sub-Saharan Africa. The magnitude of the threat will also depend on the behaviour of non-agricultural sectors of the economy in the future.
There remains much uncertainty around these predictions. For example, soil conditions in part of the new lands becoming available through shifting climatic zones may be unsuitable for sustainable crop production. New crop varieties may have to be developed. Also, the predictions are confounded by the uncertainties about the role of volcanoes, oceans and terrestrial ecosystems in global carbon fluxes. For example, recent evidence from the Arctic tundra (Oechel et al., 1994) suggests that due to soil warming the system is already a net source for atmospheric carbon. Present and future alterations of land use (deforestation, extensification or intensification of land use, etc.) are extremely important determinants of the terrestrial carbon fluxes into the atmosphere. At the same time, since elevated CO2 concentrations can increase net primary biomass production, some enhancement of the terrestrial sink (i.e., more carbon storage in vegetation and soils), is to be expected, at least in the short term.
The inherent weaknesses of the current GCMs further exacerbate these uncertainties. In spite of recent advances, only few models, the so-called Coupled General Circulation Models (CGCMs) consider the role of oceans and the more realistic transient scenario (i.e., progressive build-up of CO2). Spatial and temporal resolutions are still poor at the regional level and of little use at the local level where farmers must act. Important parameters, such as cloudiness, atmospheric aerosols, ocean-atmosphere-terrestrial linkages, cost of technological and social adaptations, extreme climatic events, lateral water transfers, availability of water resources and soil nutrients, and multiple and inter-cropping systems must be included in the models.
Our current scientific knowledge provides a good understanding of plant physiology, morphology and growth. For example, plant response to varying temperatures, changing precipitation, soil moisture and humidity are well documented. Likewise, there are some vegetation models that adequately correlate the present distribution pattern of vegetation and climate. Nevertheless, a high degree of uncertainty still clouds the potential impacts of climate change on both managed and natural ecosystems. The reasons for uncertainty have been discussed in the previous sections. They relate mainly to the paucity of relevant climatic data, the incomplete understanding of the processes underlying the global circulation models, the coarseness of their spatial resolutions, and the insufficient knowledge on the long-term, direct and indirect biological and physical responses of crops and other ecosystems to elevated CO2 and other correlated factors. The working groups at the Expert Consultation identified several research areas which would reduce uncertainty, improve knowledge, increase preparedness in the face of climate change and provide better grounds for policies related to climate change:
· Firm quantitative assessment is needed of site-specific crop responses as a function of time and growth stage, in particular for perennial crops and crops of greatest importance to food production. Such assessment needs to be properly structured, and its parameters should be identified and tested for priority crops and regions. Open-field multi-variable experiments should consider important plant biological stresses along with changing atmospheric composition (CO2, O3, SO2, N2O and other non-CO2, greenhouse gases), precipitation, and UV-B radiation.
· As some of the soils of the new lands made available through shifting of climatic zones may be unfavourable for crop production, it is important that they are categorized and mapped to avoid or lessen the chances of inappropriate land-use choices.
· The long-term effects of elevated CO2 on the uptake of nutrients (e.g., nitrogen, phosphorus, etc.) are unknown and studies on this subject should start soon. Nitrogen balances should be linked to plant-soil-water balances, and the impact of elevated CO2 on biological nitrogen fixation and phosphorus uptake need to be quantified for various crops under field conditions.
· Agronomists need to work closely with climatologists at a regional level to provide a sound basis for optimizing crop, soil and water management under the changing conditions. Special attention should be given to evaluating probabilities of extreme events (droughts, floods) and their effects on plant growth and yield. This should be accompanied by concerted projects on plant breeding by conventional techniques and by using genetic engineering and selection for stress-resistant genotypes.
· The known physical mechanisms regulating plant water use should form the basis for assessing water balances for drought- or flood-prone regions, in specific soil conditions, and for selected crops and changing climatic scenarios. Water management assumes even a greater importance under a changing climate. Water resources harnessing criteria (e.g., for irrigation structures) should achieve the necessary flexibility to accommodate future changes, particularly in vulnerable areas of the world. Long-term, continuous climate and hydrographic observations are essential not only for detecting climate change signals but also for judicious water resources development.
· A concerted effort should be directed to increase the quality of global modelling projections. High-resolution databases are needed on land use and land cover, soil carbon, nutrients and mineralization rates, crop cultivars and their climatic requirements, N2O emission rates from fertilizer applications related to local climate, soil and vegetation conditions. Remote sensing, field-level evaluation of soil and terrain conditions, monitoring of nutrient inputs and use are important tools for these tasks.
· Improving high-resolution databases will not only help generate better GCM projections, it will also contribute to the planning of overall development process in the developing countries. Better information improves knowledge and helps devise good policies and sound agricultural management practices. These in turn would increase the resilience of production systems to inter- and intra-seasonal climatic variations and to global climate change. Forums for communicating progress on data collection and availability should be strengthened.
· Better socio-economic data on household income and expenditure are also critically important. Much of this information does not exist or is not at a suitable resolution and consistency, and therefore is not directly comparable between or within regions.
· There is a lack of basic biological knowledge about how tree species and forest ecosystems are affected by climate change. The rate at which a particular species is expected to move into a newly available area needs to be studied, as well as the effects of elevated CO2, alone and in conjunction with other global change parameters on successional, mature, degraded and recovering forests in different world biomes.
· Readily available assessments (e.g., FAO's Agriculture Towards 2010) should extend their perspectives by enhancing their treatment of indicators of sustainability, with implications of climate change and consequences for greenhouse gas emissions.
Several qualitative judgements can be drawn from the Expert Consultation and the various preceding (updated) chapters that resulted from it:
· The scientific uncertainty surrounding the issue of climate change will not be resolved soon. The time scales of climate change are usually so long that observational studies are usually too short to provide adequate answers. The uncertainty is exacerbated by limitations in modelling techniques, especially at the local scale, and by the lack of knowledge about the complex biophysical responses in field conditions to global change.
· Although the rising CO2 concentration in the atmosphere is currently the primary cause of climate change, the correlated changes in environmental conditions (temperature, precipitation, O3 UV-B, humidity, etc.) are likely to be as important as CO2 in determining the responses of managed ecosystems. To determine any needed changes in management practices of graziers or ranchers, farmers and foresters, both positive and negative responses need to be fully understood and tested in field conditions.
· In spite of many uncertainties, global warming, if it happens, can be a serious problem that could have great implications on agriculture and on natural ecosystems.
· Feedbacks among biophysical, economic, social and technological mechanisms are likely to accentuate the uneven distribution of climate change impacts between developed and developing countries. Although global food security might not be affected, the developing countries are presently the least able to make the necessary adjustments. The most vulnerable regions should therefore receive priority for determining the impact on food security, even though the effects of social, economic and technological constraints on food security are likely to match or exceed those associated with climate change.
· Past and present activities of the industrial countries are currently the major sources of CO2: it is their responsibility to reduce emissions first and prepare for the likely consequences; imposing reduction targets on agriculture in developing countries is impractical and non-equitable.
· Whether or not climate change takes place, improving the resilience of food production and minimizing risks against weather variability are essential if agriculture is to meet the challenges of ensuring food security, raising rural employment in developing countries and protecting natural resources and the environment. This can serve at the same time ('no-regrets policies') to prevent or mitigate the negative impacts of climate change. For example, good land husbandry and better agronomic management adapted to variable conditions are appropriate both to face higher inter-annual weather variability and the more modest and gradual global climate change. Similarly, crop and livestock breeding for heat and drought stresses is a pressing need on its own, because of growing human populations, but can also serve as a potential response to global climatic changes.
Whether climate change impact scenarios will ultimately materialize depends on how precipitation patterns change and on the magnitude of temperature increase and its spatial and temporal distribution. Unfortunately, there is no scientific consensus as yet on the answers to these questions, nor is there certainty that the slight global temperature increase observed in the 20th century was caused by the greenhouse effect. Nevertheless, past climate fluctuations have provided circumstantial evidence that temperature variations are linked to greenhouse gases. It would be perilous for agriculture to ignore the potential impacts of any global warming on the basis that the likelihood of warming is uncertain or because scientific consensus is not yet achieved. The Expert Consultation and the present consolidated texts may provide a contribution in identifying uncertainties and pointing to the emerging issues and needs of agriculture in the face of a potential global climate change.
A good part of the needs for further research is being addressed within the International Geosphere-Biosphere Programme (IGBP) of the International Council of Scientific Unions. In particular its research programme on Global Change and Terrestrial Ecosystems (GCTE) is relevant, and therefore a description of it is added to this publication as a free-standing chapter (No. 12).
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