SCOPE 29 - The Greenhouse Effect, Climatic Change, and Ecosystems

1.1 INTRODUCTION

Man's expanding activities have reached a level at which their effects are global in nature. The natural systems, i.e. the atmosphere, land and sea as well as life on this planet, are clearly being disturbed. We know that some natural trace gases in the atmosphere, such as carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄) and tropospheric ozone (O₃), have been increasing during the last century. In addition, other gases are being emitted that are not naturally part of the global ecosystem, notably chlorofluoro-carbons. These trace gases absorb and emit radiation and are thus able to influence the Earth's climate. They will be referred to collectively as green- house gases.

It is also clear that the major natural terrestrial biomes are changing and it is generally accepted that the area covered by tropical forests is decreasing, although there is still considerable debate about the rate of ongoing changes. Significant changes of the marine system on a global scale are less well documented, but it has been shown clearly that man-made pollutants are invading even the deep sea. Air and water pollution are generally increasing. It is, of course, hardly surprising that the presence of almost 5000 million people on Earth is altering the natural systems significantly. Some such changes must be accepted in order to permit the exploitation of the natural resources on which man is dependent. We must ask, however, if and to what extent the speed of recent development represents a threat to the renewable resources on Earth. What do the changes of the terrestrial and marine ecosystems and other ongoing changes mean to man in a long-term perspective? It is important to keep in mind throughout the following discussion that the CO₂ problem, or rather the problem of a possibly changing climate due to emissions of greenhouse gases into the atmosphere, cannot be considered in isolation. It is one of many important environmental problems that must be addressed but in a long-term perspective probably the most important one.

The realization that the climate might change as a result of emissions of carbon dioxide into the atmosphere is not new. Arrhenius (1896) pointed out that the burning of fossil fuels might cause an increase of atmospheric CO₂ and thereby change the radiation balance of the Earth. During the 1930s Callendar (1938) for the first time convincingly showed that the atmospheric CO₂ concentration was increasing. Earlier attempts had not been successful primarily due to non-representative sampling. The problem was revived again by C. G. Rossby in the 1950s, who was the driving force behind the initiation of CO₂ measurements in Scandinavia, and by R. Revelle, who was instrumental in getting C. D. Keeling engaged in the observational programmes on Mauna Loa, Hawaii, and at the South Pole in 1957-58. At about the same time Revelle and Suess (1957) presented the first more careful assessment of the likely future CO₂ increases due to fossil fuel combustion. This was followed by a more elaborate analysis by Bolin and Eriksson (1959).

The observations begun in 1958 have clearly shown that the concentration of carbon dioxide (CO₂) in the atmosphere has increased from about 315 ppmv then to about 343 ppmv in 1984. We know today approximately the amounts of CO₂ that have been emitted into the atmosphere by fossil fuel combustion and changing land use (deforestation and expanding agriculture) and can relate the observed increase of atmospheric CO₂ to these human activities. Since a continued increase of the atmospheric CO₂ concentration might lead to changes of the global climate, it is essential to be able to project the likely future concentrations that may occur due to various possible rates of CO₂ emission.

The reason for concern about climatic effects is the so-called 'greenhouse effect' of CO₂. While CO₂ is transparent to incoming short wave radiation from the Sun, it absorbs outgoing long wave radiation and re-emits this energy in all directions. Therefore, an increase of the atmospheric CO₂ concentration leads to a warming of the Earth's surface and lower atmosphere. In addition, it is becoming increasingly clear that a number of other green-house gases in the atmosphere similarly affect the radiation budget. Their concentrations are also changing as a result of natural and human causes. Since increased concentrations of CO₂ as well as of these other greenhouse gases all lead to a warming of the Earth's surface and lower atmosphere, the estimated climatic effects and further impacts (e.g. on sea level and agriculture) must be considered as a result of a combined effect of these potential origins of the warming. However, in order to be able to make estimates of their relative contributions to the warming and associated climatic changes at any given time, their effects are studied separately.

A major characteristic of the CO₂ problem is the uncertainty that is encountered when considering each of the aspects identified above. This uncertainty has often been ignored or underplayed in the past. Here it will be considered more specifically in the final sections of this chapter, together with other principal questions that confront us. However, the results from earlier assessments and the most important findings of the present one will be summarized first.

1.2 PREVIOUS ASSESSMENTS OF THE CO₂ PROBLEM

Of the numerous assessments that have been made of the CO₂ problem we shall limit ourselves here to considering those that have been conducted by national bodies and similar groups and those that have dealt most comprehensively with the problem. Table 1.1 compares some of the results of these previous assessments with those of the present study.

The first international assessment of the CO₂ issue organized by UNEP, WMO and ICSU resulted from an expert meeting held in Villach, Austria, in November 1980 (World Climate Programme, 1981). The projection of future fossil fuel use made at that meeting was largely based on a scenario developed at the Institute for Energy Analysis, Oak Ridge (USA) (Rotty and Marland, 1980). The net emissions of carbon dioxide from the biota were estimated on the basis of a review of available studies. The projection of the atmospheric carbon dioxide concentration in 2025 was made by assuming that 40-55% of the total emissions would remain in the atmosphere (the so-called airborne fraction). The globally averaged surface temperature response to a doubling of the atmospheric CO₂ concentration was estimated, as in all the studies referred to in Table 1.1, by examining the results of numerical models of the climate system. The WCP (1981) report concludes that CO₂-induced climatic change is a major environmental issue but that, because of existing uncertainties, the development of a management plan for control of CO₂ levels in the atmosphere and for preventing detrimental impacts on society would be premature. It was felt that research to place decision-making with respect to CO₂ on a firm scientific basis merits high priority. The meeting further emphasized that the CO₂ problem affects both developing and developed nations and calls for a special partnership of effort.

The report of the Carbon Dioxide Assessment Committee (CDAC,1983) of the U.S. National Research Council gives a detailed assessment of the various aspects of the CO₂ problem. To estimate future emissions of CO₂ from fossil fuels, Nordhaus and his co-authors conducted a review of previous global long-range energy studies and developed a model of the global economy and carbon dioxide emissions. The model used a range of paths and uncertainties for major economic, energy and carbon dioxide variables, which allowed a 'best guess' of the future path of carbon dioxide emissions and a reasonable range of possible outcomes given present knowledge. The estimate of the net emissions of CO₂ from the biota was made on the basis of information presented in the report and available in the scientific literature. The possible future atmospheric CO₂ concentrations were calculated using an estimate of 0.60 ± 0.10 as the likely future airborne fraction of the projected emissions due to fossil fuel combustion (the value referred to here is higher than given in the CDAC report, but is a correction communicated by Nordhaus; see further Chapter 3 of the present report). The effects of CO₂-induced climatic changes on agricultural, social and economic system were also assessed with emphasis on the United States. It was concluded that the longer-term agricultural effects are uncertain and depend strongly on the outcome of future research, development, and new technology in agriculture. The CDAC report reached a general conclusion similar to that of the WCP (1981) report, i.e. that the evidence at hand about CO₂-induced climatic change does not support steps to change current fuel-use patterns away from fossil fuels, although such steps may be necessary or desirable at some time in the future. The report pointed out that steps to control climatic change should start possibly with reductions of the emissions of other greenhouse gases, since their control may be more easily achieved. Further, the CDAC report suggested that the CO₂ problem might serve as a stimulus for increasingly effective cooperative treatment of world issues.

Table 1.1 A comparison of results of recent assessments of the CO₂ problem

The study of the US Environmental Protection Agency, EPA (Seidel and Keyes, 1983) took a different approach to the problem by examining whether specific policies aimed at limiting the use of fossil fuels would prove effective in delaying temperature increases over the next 120 years. In contrast to the other assessments discussed here, the EPA study based its conclusions on the results of a set of three particular models, as opposed to a review of earlier model results. The projections of future energy demand and supply were made using the world energy model of the Institute for Energy Analysis (Edmonds and Reilly, 1983). A global carbon model developed at Oak Ridge National Laboratory (ORNL) (Emanuel et al., 1981) was used to estimate the atmospheric CO₂ concentration. The changes of the atmospheric temperature were evaluated using a simplification of a one-dimensional radiative-convective model developed at the Goddard Institute for Space Studies (Hansen et al., 1981). The EPA analysis concluded that only a ban on coal use instituted before 2000 would effectively slow down the rate of a global temperature change and delay a 2 °C increase until 2055. It was concluded further that major uncertainties include the increasing concentrations of other greenhouse gases and the atmospheric temperature response and that alternative energy futures produced only minor shifts in the calculated date of a 2 °C warming. Although the results suggested that bans on coal and shale oil are most effective in reducing temperature increases in 2100, the EPA study concluded that a ban on coal is probably economically and politically infeasible. It was suggested that action is required in the following three areas: accelerating and expanding research on improving our ability to adapt to a warmer climate; narrowing uncertainties about the future effects of greenhouse gases other than CO₂; and reducing uncertainty in our knowledge about the temperature response of the atmosphere.

In a study by the Nuclear Centre in Jülich, F.R. Germany, completed for the Federal German parliament (Volz, 1983) the future emissions of CO₂ due to fossil fuel use were estimated on the basis of scenarios produced by other groups (IIASA, 1981; Colombo and Bernadini, 1979; Lovins et al., 1981). The atmospheric CO₂ concentration in 2030 was calculated using a model of the global carbon cycle. The study concluded that, although our knowledge is presently not good enough to be able to describe quantitatively and unambiguously the relationship between CO₂ emissions and specific climatic changes in the individual world regions, the world-wide and long- ranging potential threat demands fast and specific action. In fact, the Jülich study concluded that if a 'threatening climatic catastrophe' is to be avoided with certainty, the necessary steps must be taken immediately.

In contrast to the latter view, Clark et al. (1982) concluded, on the basis of a detailed review of each of the aspects of the CO₂ problem, that there is no justification for immediate advocacy of a zero-growth policy for fossil fuel use, although it is possible that increased understanding, coupled with a return to high economic growth rates, could justify serious efforts to constrain future growth of fossil fuel use beginning as early as 1990. They suggested that during this 'grace period' two courses of action are rational: develop, maintain, and evaluate options for satisfying energy demand with less reliance on CO₂-emitting fossil fuels and promote further monitoring and research on the effects of increased atmospheric CO₂.

A Committee of the Health Council of the Netherlands made an assessment of the CO₂ problem in 1983 (CHCN, 1983). The energy scenarios upon which the assessment was based were taken from the IIASA (1981) study, with CO₂ emissions from fossil fuels in 2030 ranging between 8.9 GtC/year and 16.2 GtC/year. The changes in the atmospheric carbon dioxide concentration were calculated using a model of the carbon cycle. The concentration in 2030 was estimated to be 431 ppmv and 482 ppmv for the IIASA 'low' and 'high' scenarios respectively. Considerable emissions from the further exploitation of the terrestrial ecosystems were assumed but also more effective uptake by undisturbed forest systems and by soils due to charcoal formation in the process of burning during deforestation. The likely future temperature-change corresponding to the atmospheric CO₂ increase was calculated using a simple climate model. The global average surface temperature increase from 1980 values was calculated to be between 0.86 and 1.15 °C in 2030 for the two IIASA scenarios. The Netherlands study concluded by pointing out that CO₂ is an international problem and that measures taken by individual countries will hardly be effective. Further, the Netherlands assessment found that since the CO₂ issue is so complex, the only recommendation it could make was the creation of an authority to deal with the CO₂ issue in all its aspects and to keep the Government informed.

A working group at an international conference on energy/climate interactions (Bach et al., 1980) in 1980 concluded that in view of present uncertainties prudence dictates a cautious and flexible energy strategy. The group recommended a 'low climate-risk energy policy', which would promote the more efficient end use of energy, secure the expeditious development of energy sources that add little or no CO₂ to the atmosphere, and keep global fossil fuel use, and hence CO₂ emission, at the present level. A further working group at the same conference noted that decisions that will have to be made in the decades ahead to prepare for or avert a carbon-dioxide climatic change will have to be made before all the answers have been obtained. It was concluded that the assessment of the impacts of climatic changes will have to be made now despite the uncertainties.

It is clear from this review that there is agreement on some basic issues. The net emissions from the biota (due to deforestation and land use changes) in themselves will be insufficient to bring about a significant change of climate, while fossil fuel reserves are sufficiently large that environmental disturbance would occur if the reserves are exploited at an increasing rate in the future. Although there are differences in the estimated globally averaged surface temperature response to a doubling of the atmospheric CO₂ concentration, these differences are not large, but the uncertainties of these estimates are considerable. It is also generally agreed that regional differences of climatic change cannot be predicted at present, and similarly the general way a given change of climate would influence people and nations around the world cannot be predicted. This is presumably the reason why there is substantial disagreement regarding the recommendations for future action. While some assessments conclude that there is not sufficient evidence to support changes of current fossil fuel use patterns, other assessments conclude that immediate action is necessary. The implications of the existing uncertainties have been evaluated differently in the various studies. We shall return to these questions later in this introductory chapter.

1.3 MAJOR FINDINGS OF THE PRESENT STUDY

The analysis of the 'CO₂ Problem' presented in this report has been pursued along the lines usually adopted by scientists. In the individual chapters, the different aspects of the problem are treated in the logical sequence: emissions- projections of atmospheric CO₂ and other greenhouse gas concentrations-climatic change-environmental and ecosystem effects. However, when an assessment is to be made of the problem as a whole, it is not sufficient to proceed step by step through this sequence. There will always be uncertainties in the successive analyses and it is difficult to assess what can be said with some degree of reliability with regard to the problem as a whole. It is, therefore, necessary to single out findings that are significant and upon which there is general agreement. At the same time, the nature of the uncertainties must be examined. It should be borne in mind that policy will ultimately be based on a judgment of the problem as a whole in relation to other societal problems, some caused by other environmental problems, some of a totally different nature. In this process the urgency of the problem is of decisive importance in the establishment of a policy for action. Long-term problems, such as the CO₂ issue, are therefore often deferred for later consideration. Both scientific progress and the adoption of a policy for action depend very much on the availability of factual information about ongoing changes. In the former case the information is essential for verification of model projections of future changes, in the latter case the evidence of observations usually is necessary to convince the general public and make action politically possible. We shall return to a discussion of the overall problem after having summarized the most important findings that are now available. The following sub-sections bring together the conclusions of the detailed analyses of the different problem areas given in Chapters 2-10. For a c1oser study of the major issues reference is made to these chapters and the extensive scientific literature.

1.3.1 Likely Emissions of Carbon Dioxide into the Atmosphere Due to Future Energy Demands (cf Chapter 2)

Combustion of fossil fuels-primarily oil, gas and coal-today meets about 80% of the total global energy demand. Future emissions of carbon dioxide will depend on how this global energy demand will change and what role fossil fuels will play in the future global energy supply system. The characteristics of the energy system ('the energy mix') change only slowly because of the large capital investments that are required to establish the units that supply the energy needed, and because the lifetime of existing installations is long and accordingly the time required to develop new supply systems is long. Even so, projections of future energy demands made during the last decade or two have not been very successful, a principal reason being the rather dramatic events that have taken place in the oil market since the early 1970s. A considerable decrease of the energy use per production unit has occurred and a further decrease is technically possible. Whether this will take place or not depends primarily on the relative costs for investments in new energy supply systems on one hand and those for conservation and energy savings on the other. Many other factors, however, also play an important role in this context.

It has been pointed out in a few recent analyses of the problem and has become increasingly clear in the present analysis that the projections of energy use beyond the early decades of the next century are very uncertain. This also implies that the options for choices increase in the long-term perspective. It is therefore not very meaningful to attempt precise projections beyond 30-40 years, but instead likely upper and lower bounds for future energy demands are estimated. The degree to which these may be met by fossil fuels or other energy supply systems is assessed in Chapter 2.

The estimated upper bound of possible scenarios implies an emission of about 20 GtC/year in the year 2050, i.e. about a fourfold increase of present emissions during the next 65 years. It is interesting to note that the last four-fold increase of emissions has occurred since the late 1930s, i.e. during the last 45-50 years. Although it is recognized that some studies have projected an even more rapid future increase, it is argued that this seems quite unlikely because of a number of environmental, social and logistic constraints. On the other hand it is not likely that the CO₂ emissions can be reduced to a value below about 2 GtC/year in 2050, and this value could only be achieved by sustained global efforts to limit the future energy demand and, particularly, the use of fossil fuels. Even lower values have been projected but are judged to be unrealistic on economic grounds. Low CO₂ emissions would only be achieved if there were some global acceptance of a much more considerate attitude towards the natural environment, implying decreasing use of fossil fuels. Although some people already consider this as fundamental for the continued well-being of human society, a general acceptance will take a long time. The range of possible future CO₂ emissions into the atmosphere as a function of time is shown in Figure 1.1 and discussed in Chapter 2.

At present about 5 GtC/year are emitted into the atmosphere through the combustion of fossil fuels. Although biotic emissions of carbon dioxide as a result of deforestation and land use changes have also contributed to the rise of the atmospheric carbon dioxide concentration in the past, it is clear that in the future the emissions from the biota will be comparatively small, since there is a physical limit to the extent of deforestation. If there is going to be a significant increase of the atmospheric carbon dioxide concentration during the next hundred years, it will come from the emissions of CO₂ by fossil fuel use, and, in a longer term perspective, particularly from the use of coal.

The range of likely carbon emissions in 2050 is not an uncertainty of the kind that natural scientists assign when they add an uncertainty range to an observation or to a prediction by a model. The future CO₂ emissions due to the burning of fossil fuels as well as changing land use will depend on human decisions. In any evaluation of the problem, economic considerations will play an important role, but other environmental concerns are also being expressed, as shown by the increasing use of concepts such as 'quality of life'. Therefore, policy with regard to CO₂ emissions will depend on how well we understand the negative effects of continued and possibly increased use of fossil fuels for energy supply, not only with regard to CO₂ emissions and possible climatic changes, but also with regard to other environmental effects such as air pollution, acid rain, etc.

 Figure 1.1 Projections of carbon emissions in 2050. Sources: EPA, Environmental Protection Agency (1983); ER, Edmunds and Reilly et al. (1984); Goldemberg et al. (1984); IIASA, International Institute for Applied Systems Analysis (1981, 1983); Legasov et al. (1984); Lovins et al. (1981); MIT, Massachusetts Institute of Technology (1983); NY, Nordhaus and Yohe (1983); Reister (1984); and WEC, World Energy Conference (1983)

1.3.2 Projections of Future Atmospheric CO₂ Concentrations (cf Chapter 3)

Future atmospheric CO₂ concentrations resulting from a given scenario of CO₂ emissions depend on the transfer processes, whereby CO₂ is partitioned between the major carbon reservoirs in nature. It is well established that the crucial questions are:

• How rapidly is the huge storage capacity of the world ocean (including the role of marine biota and sediments) becoming available as a sink for the CO₂ emitted into the atmosphere?

• In which way do the terrestrial ecosystems modulate the increase of atmospheric CO₂ and what are the implications of man's exploitation of the terrestrial biosphere, particularly by deforestation and changing land use?

The assessment shows an improved understanding of the global carbon cycle in recent years. This is particularly due to the development of more realistic models of the carbon cycle, the simultaneous consideration of the observed changes in the global distribution of all three carbon isotopes, ¹²C, ¹³C and ¹⁴C, as well as other global data for validation of carbon cycle models. Progress in documenting past changes of the carbon cycle, particularly by analysis of the air trapped in glacial ice, has also been of great significance. It is still not clear how to balance the global carbon budget, partly because of the poor knowledge of land use changes and partly because of the uncertainty about pre-industrial atmospheric CO₂ concentrations. However , the remaining uncertainties about the global carbon cycle do not seriously influence the conclusions about future levels of atmospheric CO₂. The main source of uncertainty is, rather, the projections of future CO₂ emissions. The general conclusions illustrated in Figure 1.2 and discussed in Chapter 3 can be summarized as follows:

• A low CO₂ emission scenario, i.e. constant or only very slowly increasing emissions during the next 4 decades and slowly decreasing emissions thereafter, would give atmospheric CO₂ concentrations below about 440 ppmv, i.e. less than about 60% above the pre-industrial level. It is accordingly quite possible that a doubling of CO₂ will be reached only after year 2100.

• A modest increase (1-2%/year) of CO₂ emissions during the next 4 decades (and a decline thereafter) will lead to about a doubling of the CO₂ concentration (i.e. 550 ppmv) towards the end of the next century.

• The upper bound scenario implies CO₂ doubling by about 2050 and it is accordingly rather unlikely that the CO₂ concentration in reality will double before the middle of the next century.

These conclusions suggest a somewhat slower development than that discussed in the first WMO/ICSU/UNEP assessment (World Climate Programme, 1981) and in the U.S. National Academy of Sciences study (CDAC, 1983). It depends on the conclusion reached in Chapter 3 that the airborne fraction of the CO₂ emissions is probably 45% ± 10% rather than about 50%, as has usually been assumed previously. It is also important to note that the more slowly the emissions increase, the smaller is the fraction that remains in the atmosphere. A warmer climate induced by increasing CO₂ concentrations could, however, diminish the transfer of CO₂ to the deep sea and thereby increase the future airborne fraction.

 Figure 1.2 The lower set of curves indicates scenarios for future atmospheric CO₂ concentrations. The upper and lower bounds (UB and LB*) values are given in Figure 1.1. The value 550 ppmv corresponds to a doubling of the pre-industrial concentration of CO₂ (275 ppmv). The range, 410 to 490 ppmv for the year 2025, is the projection presented in the previous UNEP/WMO/ICSU CO₂ assessment (1981). The upper set of curves shows the corresponding scenarios if account also is taken of the other greenhouse gases (see Chapter 4)

1.3.3 Expected Increases of Other Greenhouse Gases that May Affect the Earth's Radiation Budget (cf Chapter 4)

Carbon dioxide is not the only atmospheric constituent that is of importance for the heat budget of the atmosphere and, thus, the global temperature. Moreover, the concentrations of some of these other constituents are also observed to be changing. Water vapour is the major radiatively active constituent of the atmosphere, but systematic and significant changes of atmospheric water vapour will primarily occur in association with changes of the climate and have been implicitly included in numerical models simulating climatic change (see sub-section 1.3.4 and Chapter 5).

It has been recognized for a long time that ozone is important in the Earth's radiation budget and, since changes of atmospheric ozone concentrations can be induced by human activity, attention should be paid to its importance in comparison with CO₂. Some slight decrease of stratospheric ozone (less than 1%) seems to have been detected and this decrease might continue in the future. Ozone in the Northern Hemisphere troposphere has probably on average increased by more than 10% presumably because of human activities. An additional increase by 10% during the remainder of this century and the first decades of the next may well occur .

Methane (CH₄, present average global concentration 1.65 ppmv) is increasing at a rate of about 1.2% per year, presumably due to more extensive use of paddy fields in cultivating rice, increasing numbers of domestic ruminants, biomass burning and to leakage of natural gas when exploiting gas fields. Analyses of air trapped in glacier ice indicate pre-industrial concentrations probably only about 40% of those observed today. Until we know better the reasons for this increase it is difficult to project likely future increases with any degree of certainty, but a 20-50% increase during the next 50 years seems plausible.

Nitrous oxide (N₂O, present average concentration 0.30 ppmv) is increasing by about 0.3% per year. Pre-industrial concentrations may have been 5-10% below the values observed today. The increase is probably due to the use of nitrogen fertilizers in agriculture and forestry and also to combustion processes and is expected to reach 0.35-0.40 ppmv towards the middle of the next century.

Man is also producing a large number of gases that were not present naturally in the atmosphere or only in small and insignificant amounts. The analysis presented in Chapter 4 shows that we may expect that also the two most common chlorofluorocarbons (CFCs), i.e. F11 and F12, will become of increasing significance for the radiation budget of the Earth, while a series of other compounds probably will be of less importance combined than any of the gases considered separately above. An annual increase of F11 and F12 emissions by 4% does not seem unlikely, if no preventive measures are taken.

It is fairly straightforward to compute the direct radiative effects of these greenhouse gases. Such computations can be made with adequate accuracy. They can also be included in General Circulation Models (GCMs). The uncertainties about the climatic changes that these greenhouse gases might cause are similar to those for CO₂. Therefore, their role can be expressed approximately in terms of an equivalent amount of CO₂.

It is concluded that the temperature change due to the changing concentrations of these greenhouse gases upto the present is about one half of the change calculated for the increase of the atmospheric CO₂ alone (about 70 ppmv). The effect of these other greenhouse gases is equivalent to an additional increase of CO₂ by 40-50 ppmv.

The concentrations of several of these gases are increasing more rapidly than that of CO₂. If the rates of increase as given above are applied during the next 50 years, we find that such a scenario would be equivalent to a doubling of atmospheric CO₂ concentrations well before the middle of the next century (see Figure 1.3). Chlorofluorocarbons would then become the most important gases in addition to CO₂, if no preventive measures are taken. On the other hand, their regulation would be easier to achieve than the limitation or reduction of CO₂ emissions.

 Figure 1.3 Cumulative equilibrium surface temperature warming due to increase in carbon dioxide and other trace gases from AD 1980 to 2030 as computed by a one-dimentional model. (After Ramanathan et al., 1985.) Due to feedback mechanisms as revealed by general circulation models (cf. sub-section 4.4.3 and Chapter 5) expected changes are 0.8-2.6 times the values given in this figure

1.3.4 Modelling of Future Climate (cf Chapter 5)

There are many factors that are known to cause changes of global climate but our understanding of the cause-and-effect linkages is limited. Global climate can be affected by changes of solar energy output, changes of the Earth's orbit around the Sun, volcanic eruptions, changes of atmospheric composition, changes of cloudiness, the Earth's albedo and atmosphere-land-ocean interactions. These factors can act individually or together. In spite of recent theoretical advances and quite detailed understanding of many processes, it has not been possible to establish unequivocally the causes of documented past climatic fluctuations. It seems likely, however, that changes of the incident solar radiation caused by the slow variation of the characteristics of the Earth's orbit around the Sun have played a significant role on the glacial/interglacial time scale.

Estimates of the effects of changes in atmospheric composition on climate are made using models of the climate system. Since the climate system is very complex, various approximations and simplifications have been made in the development of such models. Different modelling approaches have been adopted, giving rise to a range of climate models from simple zero-order models, simulating global mean temperature changes, to comprehensive three-dimensional general circulation models of the atmosphere-land-ocean system. Only the latter are capable of providing adequate information for evaluating the characteristic features of future climatic change, that is, not merely average changes for the Earth as a whole, but also some details regarding regional climatic change and changes in surface hydrological processes. Even though significant advances have been made in modelling the climate system, present models are not yet able to simulate reliably the many processes that govern the regional climate. However, comparison of model computations with observed features of the general circulation of the atmosphere, particularly the model capability to reproduce seasonal variations of weather and climate, has given us some confidence in available results. Figure 1.4 shows a comparison of the estimated global equilibrium temperature change due to a CO₂ doubling as deduced in Chapter 5 with estimates made in previous assessments.

An evaluation of the large number of results from climate models leads to the conclusion that the global equilibrium temperature change expected from increases of CO₂ and other greenhouse gases equivalent to a doubling of the atmospheric CO₂ concentration is likely to be in the range 1.5-5.5 °C. The largest sources of uncertainty in modelling global average temperature change appear to be the levels of feedback from clouds, ice-albedo and, possibly, lapse-rate and water-vapour changes. Moreover, storage of heat in the oceans delays the warming expected for an equilibrium response to carbon-dioxide-induced warming and may also significantly modify the geographical distribution of climatic change. The role of the oceans is important but also uncertain. Model results suggest that the global warming resulting from increases in greenhouse gases to date is in the range 0.3 °C to 1.1 °C. Continental-scale or regional-scale climatic change cannot yet be modelled realistically but a few tentative conclusions can be drawn. All three- dimensional model results suggest that the largest temperature increases would occur in the high latitudes in the fall and winter seasons, and there is unanimous agreement that the stratosphere will cool. There is also some evidence for mid-latitude, mid-continental drying during the summer.

 Figure 1.4 The estimated global equilibrium annual temperature change for doubling of the CO₂ concentration compared with estimates from previous assessments, and values obtained using climate models (see Chapter 5)

1.3.5 The Detection of a CO₂-induced Climatic Change (cf Chapters 5 and 6)

An analysis of surface temperature data since the middle of the last century shows that both hemispheres experienced a general warming from the late 19th century until about 1940 and a cooling until the mid-1960s. Since then the globe as a whole appears to have warmed, with a delay in this warming trend in the Northern Hemisphere. The observed global temperature record from 1850 is shown in Figure 1.5 and discussed further in Chapter 6.

Figure 1.5 Variation with time of the global mean annual surface temperature obtained from land and marine temperature records. The filtered curve has been obtained by suppressing variations on time scales less than 10 years (see chapter 6)

Detection of a CO₂-induced warming in the observational record has become a high priority issue. The detection problem can be viewed in terms of the concept of the signal-to-noise ratio, i.e. one can claim to have detected a change in climate once the signal (e.g. the increasing temperature) has risen appreciably above the background noise level (e.g. natural variability of temperature). Past records show that climate has varied naturally on timescales from a few years to centuries, millennia and longer. Chapter 6 concludes that the observed global mean temperature increase (about 0.5 °C) during the last 100 years cannot be ascribed in a statistically rigorous manner to the changing CO₂ concentration, although the magnitude is within the range of predictions. Considering also the warming due to the increase of other greenhouse gases, it seems that the observed temperature change during the last 100 years is in the lower part of the projected range. It is not possible to conclude whether this might be due to a more modest role of the positive feedback mechanisms as described in the climate models or to a delay of a warming due to the inertia of the oceans. A major problem in detecting the climatic effects of CO₂ increases is to explain the medium to longer time-scale (decadal or greater) fluctuations in the observed temperature record. Until the comparatively rapid global warming of 1920 to 1940 and the cooling between 1940 and the mid-1960s, in particular, have been adequately explained, claims regarding the detection of CO₂ effects can be easily criticized.

1.3.6 Projecting the Rise in Sea Level Caused by a Warming of the Atmosphere (cf Chapter 7)

Changes of global temperature affect the various components of the hydrological cycle in different ways and with different response times. For example, changes in precipitation patterns over land affect runoff from rivers and glaciers into the sea. Ocean waters expand when warmed. Catastrophic collapse of ice sheets has also been proposed as a potential risk causing a comparatively rapid rise of sea level, i.e. 1-2 cm/year.

Measurements of sea level changes since early this century show an average rate of rise for global sea level of 14 ± 5 cm per century. The changes of global sea level are illustrated in Figure 1.6. If the observed (modest but significant) correlation between sea level and air temperature during this time is assumed to be valid in the future, it is estimated empirically that a global warming of 1.5 °C to 5.5 °C would lead to a sea level rise of 25 to 165 cm. Probably the major contributing factor to such a sea level rise would be the thermal expansion of the ocean water.

The small glaciers would probably decrease in extent and might contribute to a rise of sea level by 20 ± 10 cm in the course of a century for a warming of 3.5 ± 1.0 °C. It seems likely that the Greenland ice sheet would decrease, but probably enhanced precipitation (snow) in the Antarctic would increase the accumulation and balance approximately the net flux of ice and water from Greenland to the sea.

Better oceanographic knowledge is required to assess the influence of any climate warming on the stability of the West Antarctic ice sheet. One possibility is that global warming could warm ocean waters and change ocean circulation sufficiently to cause a catastrophic collapse of the West Antarctic ice sheet following a global warming of 3-4 °C. Another possibility is that increased precipitation over the ice sheet may outweigh any effect of increased ocean temperatures, which may be confined mainly to the top 100 m, in which case the volume of West Antarctic ice may slowly increase in a stable manner. Even if a collapse started, it is likely to take several hundred years to raise sea level by, say, 5 m, corresponding to the possible discharge of the West Antarctic ice volume into the ocean.

Figure 1.6 Comparison of the global mean sea level (Gornitz et al., 1982) with the global mean surface temperature given in figure 1.5

1.3.7 Global Issues in Assessing the Effects on Terrestrial Ecosystems (cf Chapter 8)

Global vegetation can be affected in two general ways: by the direct effects of higher CO₂ concentrations on plant growth, and by changes in climate. At the scale of individual plants, higher CO₂ concentrations have been shown consistently in short-term laboratory experiments to stimulate growth and yield of both C₃ and C₄ plants. This results principally from enhanced net photosynthesis, and from increased water use efficiency through a reduction of the stomatal aperture in the leaf and, hence, transpiration. However, there are large uncertainties involved in extrapolating these experimental results to longer time scales and larger area scales.

A comparison of global vegetation and climatic changes of the distant past leaves little doubt that a climatic change of the order of magnitude indicated by climate models for a doubling of atmospheric CO₂ could, potentially have profound effects on global ecosystems. However, the prediction of the direction, magnitude and rate of changes in ecosystems requires reliable estimates of climatic change at the regional scale, and such estimates are currently unavailable.

Despite the inability to make predictions, sensitivity analyses can produce useful information for judging the possible direction and magnitude of effects for given changes in CO₂ levels or climate variables, and thus for identifying specific regions and environmental changes which may warrant policy attention in the future. There is ample opportunity to test the sensitivity of global ecosystems to changes in climate variables by using scenarios of regional climatic change derived from GCM results or the instrumental record, or simply arbitrary climatic changes. In this volume, the emphasis is placed on agriculture and forests.

From a global perspective, geographic differences in agricultural regions have important implications for assessing the effects of increased CO₂ and climatic change. For example, rainfall is the principal constraint on agriculture in the tropics and sub-tropics, whereas in the temperate and higher latitudes, temperature has a relatively greater influence. In many developing countries of the lower latitudes, advances in food production have been achieved in large part through the expanded use of marginal lands, a trend which may be increasing the sensitivity of agriculture in these regions to climatic change or variability. In other areas, notably most of the major grain- producing countries of temperate latitudes, food production has risen principally through intensification and, hence, increases in average yields within the core crop regions. An unresolved issue here is whether the technological applications, which have made these long-term yield gains possible, have increased or decreased the sensitivity of crops to short-term climatic variations.

Another important trend has been the expanding volume of global grain trade during the past several decades, a trend which has increased interregional reliance for food production and distribution. Any adverse climatic change in the core crop regions of the temperate zones, where both major centres of supply and demand are located, could have large socio-economic impacts on the developing countries of the lower latitudes, whose purchasing power cannot always compete during times of scarcity. In this respect, the world has become increasingly interconnected in the face of increasing CO₂ and climatic change which, itself, is global in nature.

For the forest ecosystems of the world, three major issues need to be addressed, according to scale. First, at the microscale, the issue is how changes in processes such as photosynthesis, stomatal conductance or development may modify plant growth and, ultimately, forest productivity. The effects of changes in CO₂ concentrations or climate variables (like temperature) have to be analysed with respect to factors that currently limit productivity (like nutrients or moisture supply). Second, at the mesoscale, the issue is how the dynamic interaction and competition between ecosystem species would be affected, with consequences for local forest composition. Third, at the macroscale, the issue is how the size and areal extent of the world's forests may be altered. The largest spatial response could be expected from changes in temperature and precipitation at the cold or semi-arid margins of forest extent. At this scale, the lag time between climatic change and the response of forests could range from decades to hundreds, or even thousands, of years, based on evidence from the last glaciation.

In short, for both agriculture and forests the basic questions regarding increased CO₂ and climatic change are similar. How would crop yields or forest productivity be affected? How would crop types or forest composition change, particularly at the margins of production or ecological transition zones? How would the global patterns of forest ecosystems or food production be altered? These issues are addressed in Chapters 9 and 10.

1.3.8 The Response of Global Agriculture to Increasing CO₂ and Climatic Change (cf Chapter 9)

In the context of agriculture, four broad approaches to assessing the impacts of increasing CO₂ and climatic change address the major issues noted above: (a) crop impact analysis; (b) marginal-spatial analysis; (c) agricultural sector analysis; and (d) historical case studies.

Crop impact analyses are concerned specifically with the effects on plant growth and crop yields. Considering the direct effects of higher CO₂ concentrations in the absence of climatic change, it is estimated from laboratory experiments on individual plants that a doubling of CO₂ from 340 to 680 ppmv could result in a 0-10% increase in the growth and yield of C₄ crops (e.g. maize, sorghum, sugarcane) and a 10-50% increase for C₃ crops (e.g. wheat, soybean, rice), depending on the specific crop and growing conditions. These positive effects are obtained under most environmentally stressful  well as non-stressful conditions (see Table 1.2), and would therefore benefit both the environmental margins and the core of crop regions. Greater yield benefits would be expected to accrue to those regions of the world where C₃, rather than C₄, crops predominate.

Considering the sensitivity of crop yields to climatic change without including the direct CO₂ effect, crop impact analyses have focused largely on grain yields in temperate and higher latitudes, to the neglect of the tropics and sub-tropics. From studies using various types of crop-climate model and climatic change scenarios, it is estimated that, with no precipitation change, a warming of 2 °C might reduce average yields of wheat and maize in the mid-latitudes of North America and Western Europe by 10 ± 7% assuming instantaneous warming and no change in cultivars, technology, or management. These yield reductions would be offset by wetter conditions and exacerbated by drier conditions. These estimates pertain to core region; in contrast, average yields at the cool margins of cereal production, for example, might well benefit from a lengthening of the growing season and a reduction of damaging frosts.

Marginal-spatial analyses examine the margins of production where conversion to other crops (or genotypes) or land uses is most likely to take place. A very limited number of studies in the mid- and high-latitudes suggest potential shifts in the boundaries of cereal regions of the order of magnitude of several hundred kilometres per °C change (assuming that existing crop regions are largely climatically determined and optimally located). Other studies of high altitude locations with steep environmental gradients suggest potential altitudinal shifts of more than a hundred metres per °C change. While these estimates are highly uncertain, any such shifts at the margins would certainly modulate the effects of climatic change on regional crop yields and production.

Table 1.2 Relative effects of increased CO₂ on growth and yield: a tentative compilation¹

Agricultural sector analyses and historical case studies examine the range of environmental, agricultural and socio-economic impacts and explore the ways in which agricultural systems adjust to climatic change and variability. There are many feedback mechanisms that can enhance or diminish the potential impacts of environmental changes on crop yields and food production, as illustrated in Figure 1.7. A limited number of studies, using linked regional models or global agriculture and trade models, suggest that, in many regions of the world, agriculture would readjust crop yields and food production to changing climate. Over the long term, yields and production in such regions may be more sensitive to technology, price or policy changes than to climatic changes, and these factors are largely manipulatable whereas climate is not.

 Figure 1.7 A generalized diagram of some feedbacks influencing crop-yields and production over time. The agricultural systems approach to impact assessment examines the dynamics of agriculture and the mechanisms which can diminish or accentuate the primary yield effects of increased CO₂ and climatic change

In past studies the pragmatic approach has been to consider the impacts of climatic change separately from primary responses of plants to increased CO₂. But it is clear that the effects are interactive and non-linear, not simply additive, and should be studied accordingly. In any case, there exist large uncertainties in extrapolating laboratory results to field conditions.

Furthermore, most studies of agricultural impacts have focused on average climatic changes. However, it has been demonstrated in several instances that small changes in average climate can result in relatively large shifts in the frequencies of climatic extremes like droughts. These shifts may be equally, if not more, important to farmers than long-term changes in mean climate, especially in the marginal lands of many developing countries where climatic extremes take heavy economic and human tolls. The degree to which agriculture in such regions can be assisted in developing adjustments to buffer the ill-effects of present climatic variability, the better prepared they will be to adapt to any adverse effects of future climatic change, should they occur.

In general, given the uncertainties in regional scale estimates of climatic change and the numerous deficiencies in methodologies of impact assessment, there is presently no firm evidence for believing that the net effects of higher CO₂ and climatic change on agriculture in any specific region of the world will be adverse rather than beneficial. But it is certain that some will gain and others will lose, although we know neither where they will be found nor the magnitude of the impacts.

1.3.9 The Response of Global Forests to Increasing CO₂ and Climatic Change (cf Chapter 10)

The forests of the Earth constitute a complex system with many possible responses, both to the direct effects of an increase in atmospheric CO₂ concentration and to the possible changes in climate. These responses may originate from phenomena that operate on very different scales of time and space. In general, formidable difficulties are encountered in the 'scaling-up' of the short-term physiological and biochemical response of leaves and individual plants to estimate the intermediate and long-term responses of forests. The difficulties arise from the large uncertainties involved in the methods of extrapolation and from the complex interactions that occur at larger scales. The two uncertainties are presently large enough to preclude meaningful estimates of the effects on forests of higher CO₂ concentrations and climatic change except in the most general way.

With respect to the direct effects of CO₂, these problems of scaling-up are compounded by the lack of experimental evidence for relevant forest species, particularly for plants that have been allowed to acclimate to enhanced CO₂ concentrations over one or more growing cycles. Although higher concentrations of CO₂ have been shown to increase the growth rates of individual trees in controlled conditions over the short term, it is highly uncertain whether such effects would be sustained and would lead to increased productivity in actual forest environments over the long term. In uncontrolled environments, the direct CO₂ effects are complicated by micrometeorological differences in the degree of coupling between forests and atmosphere (within as well as between forest systems), and by species competition and interaction. If, indeed, elevated CO₂ concentrations do result in long-term growth enhancement, increases in productivity would be more likely to occur in commercial forests than in mature forests in which the capacities for increased carbon storage are more limited. Direct experimentation at this scale, however, is largely impracticable. In order, therefore, to assess the responses of forest systems to both higher CO₂ concentrations and changes in climate, experimental studies must be augmented by empirical observation and simulation modelling.

Figure 1.8 Estimation of the change in life zone extents for a CO₂-induced warming according to Emanuel et al. (1985) from present climate (hatched columns) to that deduced by Manabe and Stouffer (1980) in their climate simulation experiment for a doubling of the atmospheric concentration of CO₂ (open columns). It should be stressed that in calculating these changes in potential vegetation the changes in precipitation were not taken into account. Note also that potential vegetation corresponds to extension of ecosystems unaffected by man's direct impact

With respect to the effects of climatic change, empirical climate-vegetation models and forest simulation models have been used to assess the responses of forests at scales ranging from a single point in a forest to an entire continental system. In general, the results of a limited number of such studies suggest that climatic changes of the order of magnitude predicted by climate models for a doubling of atmospheric CO₂ are potentially sufficient to produce substantial intermediate and long-term changes in the composition, size, and location of the forests of the world. The natural forests of the high latitudes in general and the boreal forests in particular, appear sensitive to predicted temperature changes, and it is at these latitudes that climate models predict the largest warming to occur as a result of increased concentrations of greenhouse gases (see Figure 1.8). Warmer conditions could possibly lead to large reductions in the areal extent of boreal forests and a poleward shift in their boundaries. The forests of the tropical and subtropical zones, on the other hand, would probably be more sensitive to changes in precipitation than temperature. Because of the high uncertainty regarding future changes in precipitation in the tropics, and because of the present lack of models that can be used to simulate the effects of tropical ecosystems to changes in climate variables, our knowledge of the responses of tropical forests to future changes in climate is meagre.

1.3.10 Concluding Remarks

The later chapters analyse the individual aspects of the 'greenhouse gas' problem in detail.

Clear priorities can be set for future research in carbon-cycle modelling, better estimation of the effects of other trace substances, sea-level projections and climate modelling. In the modelling of agricultural impacts, model validation and cross-model comparisons are required. The assessment of impacts on forest ecosystems indicates an essential lack of direct observations of whole system performance of forests under altered environmental conditions. Finally, it is clear that there needs to be a better integration of available methods and approaches in impact assessment.

1.4 WHERE DO WE GO FROM HERE?

It is clear from the above review of the following chapters and of previous assessments that there are uncertainties with regard to each aspect of the 'greenhouse gas problem'. Some are due to incomplete knowledge about the natural systems with which we are concerned, including uncertainties about the way these systems can be described in models. Others arise because it is not certain how global society will develop and respond to changes. We are not able to predict factors, such as future energy demand, use and management of renewable and non-renewable resources, very far into the future. It has been pointed out in previous studies and must be emphasized here that some of the uncertainties will still exist in the future, despite intensive research on the individual topics. However,  prevailing uncertainty does not mean that the problem can or should be dismissed. Instead, it is necessary to examine the characteristics of these uncertainties and assess what can be said about future changes and to consider if and when some actions are needed in view of such possible changes.

1.4.1 Climatic Change

As far as the expected climatic change is concerned, this assessment concludes that a doubling of the CO₂ concentration would lead to an increase of the globally averaged surface temperature by 1.5-5.5 °C. The uncertainty is considerable, but there is almost unanimous agreement that a substantial warming would occur.

Some global mean surface warming most likely has occurred but may well be partly obscured by natural climatic fluctuations. Model estimates as well as observed changes are subject to considerable uncertainty. Further, the observed warming could be attributable to other causal factors. It is not possible to state unequivocally that a CO₂ or greenhouse gas signal has been detected. The observed general increase of mean global surface temperature during the last hundred years is, however, in general accord with model results.

Our knowledge about past changes of climate as well as model computations of future changes indicate that marked regional differences can be expected. This implies, obviously, that some parts of the globe will experience significantly larger changes than indicated by the average global change, in other regions they will be less. The spatial patterns of future changes are, however, not known. Some of the regional climatic anomalies observed today and usually attributed to the natural variability of the climate, might be due to some extent to an ongoing man-induced change.

1.4.2 Future Changes of Global Society, Emission Scenarios and Atmospheric Concentrations of Greenhouse Gases

Although there are gaps in our understanding of the response of the climate system to emissions of greenhouse gases into the atmosphere, it seems clear that the major uncertainty with regard to future environmental changes is due to our inability to predict man's future behaviour. We cannot predict the future of society in the way that natural phenomena can be predicted using the fundamental laws of nature. Some limited success in foreseeing changes of global society has been achieved by recognizing its inertia and by using econometric models. These methods are not applicable for long-term changes. In the present case, in particular, we do not know to what extent a wider awareness about general environmental problems might influence the behaviour of individuals, groups or even nations during the next 100 years. It is interesting in this context to note the shift of opinion about the impact of air pollution and acid deposition that has taken place in Europe, since an impact on forests has been detected.

The uncertainties of projections of future atmospheric CO₂ concentrations from a given CO₂ emission scenario are considerably less than those of the emission scenarios themselves. The major uncertainty is thus related to the difficulty of projecting the future use of fossil fuels. It is also difficult to foresee how mankind will respond if there is conclusive evidence that the increasing emission of other greenhouse gases into the atmosphere is an equally important factor in causing climatic change.

We conclude that the main aim of studies of future changes of atmospheric greenhouse gases is not only to predict their concentrations but also to analyse to what extent and how future increases can be limited and/or the impact on our environment can best be managed.

1.4.3 The Holistic View of Man and His Environment

Different kinds of emissions contribute to the same environmental problem (i.e. the greenhouse effect), while, on the other hand, one single human activity may cause different environmental problems (e.g. fossil fuel combustion emits CO₂, sulphur dioxide and nitrogen oxides into the atmosphere leading to air pollution, acid deposition, etc.).

It is becoming increasingly clear that many of these key problems are closely related both because physical, chemical and biological processes interact and also because one single human activity can contribute to several processes. It is obvious that environmental policy must be developed with such a holistic view of man and his environment.

It is emphasized in Chapter 2, that even if physical, economic and environmental variables were fixed and immutable, future energy and greenhouse gas emission policies would still be subject to considerable uncertainties. The same is true for other environmental problems. On the other hand the full spectrum of policy choices has usually not been emphasized. Such choices are for example the improvement of energy use efficiency, the introduction of conservation measures, accelerated development of specific technologies, the introduction of stricter environmental regulations and development of strategies of how to adapt to a changing climate. There is a wide range of possible energy futures because such choices can be made. It would be of great interest to know under which circumstances a fuller awareness of potential or real threats to our environment, in particular due to climatic change and impacts on agriculture, might lead to policy decisions to limit further emissions.

1.4.4 Man's Response to Environmental Change

With regard to the impact of climatic change on natural terrestrial ecosystems, agriculture, forestry, etc., the lack of projections of regional climatic change due to increasing concentrations of greenhouse gases in the atmosphere means that more precise assessments of likely future changes on this scale cannot be made. For quite some time, research will be restricted to examining the sensitivity of these ecosystems to given climatic change scenarios. This is a serious shortcoming, since it will be difficult to tell more specifically what the implications of a change might be for particular groups of people (e.g. farmers). Those engaged in national planning will also usually be hampered because of lack of regionally specific information.

In view of these difficulties it is important to adopt a strategy for action that would yield useful information in any case of future changes of climate. Rather than predicting likely future impacts we should study the sensitivity of the agricultural system and explore the question of what adaptation to climatic change would imply, e.g. the development of new varieties to safeguard crop diversity under a wide range of climatic conditions. Obviously such a measure would be of value, since it could increase efficiency in the agricultural system even if there is no change of climate.

Agricultural systems analysis looks at the readjustment that would be needed on the local, national and international scale in response to climatic changes. It is clear that these would be quite different depending on the spatial scale being considered. A shift of a climatic zone might not change the total production of a particular crop in a large and well developed country, because of possible shifts within the country of the regions where production takes place and the introduction of new varieties is possible. Such alternatives are not usually available to small countries, and considerable difficulties may be encountered at the national level. The individual farmer will also be affected very differently depending on whether or not cultivation of other crops than the traditional ones is feasible and on whether or not migration to regions with improved climate is possible. The seriousness of the problem further depends on the rate of the expected changes, which also warrants study.

Although a careful analysis of such impacts requires knowledge about the likely changes of all key variables and their variability in the region concerned, the assessment of the most sensitive parameters might still be possible. The agricultural impacts in the tropics and sub-tropics have not been studied in detail, although the impacts could be felt the most in these regions. Clearly, this imbalance should be redressed in future studies.

We obviously encounter considerable difficulties in trying to assess the more specific consequences of a possible change of the global climate. It does not seem likely that even intensified research will make it possible to foresee the geographical patterns of a climatic change in more detail, before clear signs of an ongoing change are available. In view of the far-reaching consequences that might be expected for some parts of the globe, it might be desirable to take some preventive steps before much more clear evidence is available than today. We note that the emissions of greenhouse gases other than CO₂, particularly chlorofluorocarbons, could be more easily controlled than CO₂ emissions. It is also clear that there are technically and economically viable strategies for long-term global energy development that are compatible with a high level of concern about the effects of a CO₂ increase. Actions that simultaneously contribute to solving other environmental and societal problems should then of course be given high priority. However , because of the uncertainties and the difficulty of arriving at and implementing a global policy the problem of adaptation of society to a climatic change will also become important.

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