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

Executive Summary

The amounts of some trace gases in the atmosphere, notably carbon dioxide (CO₂), nitrous oxide (N₂O), methane (CH₄), chlorofluorocarbons and tropospheric ozone, have been increasing. All of these gases are transparent to incoming short-wave radiation, but they absorb and emit long-wave radiation and are thus able to influence the Earth's climate. They are referred to in this report as greenhouse gases.

Increased concentrations of CO₂ and other greenhouse gases lead to a warming of the Earth's surface and the lower atmosphere. The resulting changes in climate and their impacts (e.g. on sea level, agriculture and forestry) can be estimated without associating the origin of the warming to anyone of these gases specifically. It is, however, necessary to study the effects of these greenhouse gases separately in order to estimate their relative contributions to the warming at any given time and, consequently, to develop strategies for reducing their possible harmful effects.

A review of previous assessments of the CO₂ problem shows that there are agreements on some basic issues. The net emissions of CO₂ from the biota (due to deforestation and land use changes) in themselves will be insufficient to cause a significant change of climate, while fossil fuel reserves are large enough for climatic changes to occur if these reserves continue to be exploited at a high rate in the future.

Generally it has also been agreed that regional patterns of climatic change cannot yet be predicted. Thus, the ways in which higher CO₂ concentrations and given changes in climate would affect ecosystems and human activities cannot be predicted either. This is presumably one of the main reasons why there has been substantial disagreement among previous studies regarding recommendations for future action.

Emission of CO₂

• The observations that began in 1958 have clearly shown that the atmospheric CO₂ concentration has increased from about 315 ppmv in 1958 to about 343 ppmv in 1984. We know the amount of CO₂ that has been emitted into the atmosphere by fossil fuel combustion and changing land use and can relate the observed increase of atmospheric CO₂ to these human activities.

• An evaluation of the 'pre-industrial level' of atmospheric CO₂ (concentrations occurring in the middle of the last century), based on direct measurements and analyses of air trapped in glacier ice, yields a value of 275 ± 10 ppmv.

• Combustion of fossil fuels-primarily oil, gas and coal-currently meets about 80% of the global energy demand. Future emissions of CO₂ will depend on how this global demand changes and what role fossil fuels play in the future supply. However, even short-term (a few decades) projections of energy use are very uncertain.

• The present assessment places an upper bound on possible CO₂ emissions of about 20 Gt C/year in the year 2050, i.e. about a fourfold increase of the present emission ( 5 Gt C/year). Higher values seem unlikely in view of environmental, social and logistic constraints.

• The lower bound on CO₂ emissions is placed at 2 Gt C/year in 2050. This value ,could possibly be achieved by sustained global efforts to limit the future use of fossil-fuel energy by decreasing energy demands and by increasing the use of non-fossil energy sources.

• Deforestation and land-exploitation cannot be major future sources of atmospheric CO₂. Much less carbon is stored as wood and in soils compared with fossil fuel deposits.

• Policy decisions concerning fossil-fuel use should take into account the negative effects of CO₂ emissions with regard to changes in climate while simultaneously considering their other environmental effects (e.g. air pollution, increasing ozone concentrations, acid rain).

Increases of atmospheric CO₂

• Understanding of the global carbon cycle has improved in recent years. Despite these advances, it is still not possible to balance the global carbon budget completely. However, the remaining uncertainties do not seriously influence the conclusions regarding the future levels of atmospheric CO₂ concentrations.

• Constant or very slowly increasing (0.5% per year) emissions of CO₂ during the next four decades, with slowly increasing emissions thereafter , would give an atmospheric CO₂ concentration of less than 440 ppmv at the end of the next century (i.e. less than 60% above the pre-industrial level).

• If the present increase of CO₂ emission (an average of 12% per year since 1973) continues over the next four decades with a slackening of the rate of increase thereafter, a doubling of the pre-industrial CO₂ concentration would be reached towards the end of the next century.

• The upper bound scenario implies that the CO₂ concentration might double by the middle of the next century, while the lower bound scenario implies that doubling of CO₂ concentration will not be reached until after 2100.

Other greenhouse gases and aerosols

• The equilibrium temperature change due to the increasing concentrations of other greenhouse gases (in particular, nitrous oxide, methane, tropospheric ozone and chlorofluorocarbons) up to the present is estimated to be about half of the temperature change attributed to the increase of atmospheric CO₂ alone. The concentrations of some of these gases, and hence their relative importance in changing the climate, are increasing more rapidly than that of CO₂.

• If present trends continue, the role of non-CO₂ greenhouse gases in changing the climate will be about as important as that of CO₂ during coming decades. The combined concentrations of atmospheric CO₂ and other greenhouse gases would be equivalent to a doubling of CO₂ possibly as early as the third decade of the next century.

• Chlorofluorocarbons may, within decades, become the greenhouse gas that next to CO₂, is increasing its importance for changing the radiative properties of the atmosphere most rapidly 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.

• Our knowledge of the global biogeochemical cycles that determine atmospheric concentrations of methane, nitrous oxide and ozone is still inadequate as a basis for policy decisions on how to reduce or limit the future growth of their concentrations.

• Although changes in global climate due to increasing concentrations of aerosols in the atmosphere have probably not been significant in the past, the possibility that they may become of importance in the future, particularly regionally, cannot be excluded. Future changes in aerosol concentrations cannot be projected with any certainty.

Changes in climate

• An evaluation of results from climate models leads to the conclusion that the increase in global mean equilibrium surface temperature due to increases of CO₂ and other greenhouse gases equivalent to a doubling of the atmospheric CO₂ concentration is likely to be in the range of 1.55.5 °C. Although no value within this wide range of uncertainty can be excluded, it is plausible that the increase may be found in the lower half of this range.

• The observed increase in mean temperature during the last 100 years (0.30.7 °C) cannot be ascribed in a statistically rigorous manner to the increasing concentration of CO₂ and other greenhouse gases, although the magnitude is within the range of predictions (0.31.1 °C).

• The expected change of the global mean temperature due to a doubling of CO₂ is of about the same magnitude as the change of global temperature from the last glacial period to the present interglacial.

• Continental or regional scale changes in climate cannot yet be modelled confidently, except that there are indications that warming will be enhanced in high latitudes and that summer dryness may become more frequent over the continents at middle latitudes in the Northern Hemisphere.

Changes in sea level

• The global average sea level has risen 12 ± 5 cm during the twentieth century.

• It is estimated empirically, on the basis of observed changes since the beginning of this century, that a global warming of 1.5 °C to 5.5 °C would lead to a sea-level rise of 20 to 165 cm. The major contributing factor to such a rise would be the thermal expansion of ocean water.

• A disintegration of the West Antarctic ice sheet is not judged to be imminent and would take a century or more if it were to occur. Many glaciologists consider, however, that further research is necessary before a reliable assessment of this possibility can be made.

Assessing the impact on ecosystems

• Based on evidence from climatic changes of the distant past there is little doubt that a future change in climate of the order of magnitude obtained from climate models for a doubling of the atmospheric CO₂ concentration could have profound effects on global ecosystems.

• Prediction of the future impacts on ecosystems is precluded by the lack of reliable estimates of climatic changes at regional scales, and by the lack of knowledge concerning the interactive effects of CO₂ and climate variables on vegetation. 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.

Consequences for agriculture

• In general, the direct effects of enhanced CO₂ concentrations on crop yields are beneficial. It is estimated from laboratory experiments on individual plants that, in the absence of climatic change, a doubling of the CO₂ concentration would cause a 010% increase in growth and yield of C₄ crops (e.g. maize, sorghum, sugar cane) and a 1050% increase for C₃ crops (e.g. wheat, soybean, rice), depending on the specific crop and growing conditions.

• In analysing the sensitivity of crop yields to possible changes in climate without including the direct effects due to higher CO₂ concentrations, most research has focused on average yields of cereal grains in core crop regions of the temperate latitudes. Less attention has been paid to the tropics and subtropics, to the climate-sensitive margins of production and to possible changes in year-to-year climatic extremes.

• Crop-impact analyses show consistently that warmer average temperatures are detrimental to both wheat and maize yields in the mid-latitude core-crop regions of North America and Western Europe. Given current technology and crop varieties, a warming of 2 °C with no change in precipitation might reduce average yields by 10 ± 7%. Increases in precipitation could partially offset these effects, while drier conditions could exacerbate them. Changes in the length of the growing season or in the frequencies of extreme climatic events could also have important effects.

• At the margins of crop areas, spatial shifts in cropping patterns might occur as a result of changes in climate. A limited number of marginalspatial analyses suggest that, in the mid- to high-latitude cereal growing regions, horizontal shifts of several hundred kilometres per °C change are possible, assuming unchanged technology and economic constraints. In North America, these are comparable in magnitude with shifts in crop patterns that have taken place over this century. At the cool, high-altitude limits of production, altitudinal shifts of more than l00 metres per °C may be possible.

• Models of agricultural production and trade suggest that numerous feedback mechanisms exist in many regions through which agriculture can adjust and adapt to environmental change. Over the long term, food production in such areas appears more sensitive to technology, price or policy changes than to climatic changes, and these factors are largely controllable, whereas climate is not.

• However, for some regions, particularly the lands marginal for food production in the developing world, agriculture may be acutely sensitive to climatic change, as evidenced by the tolls taken by year-to-year variations in climate. If these regions can adopt measures to reduce further the ill-effects of current, short-term climatic variability it is likely that they will be better prepared to adapt to some adverse effects of future changes in climate, should they occur.

Consequences for forests

• At present firm conclusions cannot be reached regarding the direct effects of elevated CO₂ concentrations on the productivity, species competition or size and areal extent of the world's forests. This is because of the paucity of experimental evidence for relevant tree species, particularly over one or more growing cycles, and the large uncertainties involved in 'scaling up' from the short-term responses of individual leaves or plants to complex forest systems.

• If, indeed, elevated CO₂ concentrations do result in increased growth of individual trees over the long term, increases in productivity would be most likely to occur in commercial forest plantations, and would be less likely to occur in mature natural forests, although biomass turnover rates would increase.

• The sensitivity of forests to climatic change has been analysed using forest simulation models. These studies suggest that temperature-increases of the size indicated by current climate models for a doubling of atmospheric CO₂ are potentially sufficient to produce substantial intermediate and long-term responses in the composition, size and location of forest ecosystems.

• These climate models predict the largest warming to occur at high latitudes as a result of increased concentrations of greenhouse gases, with smaller rises in temperature in the lower latitudes. The natural forests of the high latitudes in general, and the boreal forests in particular, may be most sensitive to temperature changes. Warmer conditions could thus 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 sub-tropical zones probably would be more sensitive to changes in precipitation than in temperature. However, because of the high uncertainty regarding future changes in precipitation in the tropics, and because of the lack of models that can be used to simulate the effects on tropical ecosystems caused by changes in climate variables, it is virtually impossible at present to make informed prediction of the responses of tropical forests to future climatic changes.

The possible problem of a change in climate due to the emissions of greenhouse gases should be considered as one of today's most important long-term environmental problems. It should be considered in the context of other ongoing changes of our environment also caused by human activities, such as air pollution, acid rain and deforestation. Only in this way can we achieve a realistic integrated view of the interplay between the environment as a whole and the global society that is required for thoughtful consideration of options and policies for avoiding long-term adverse consequences.