The changing greenhouse effect

Palaeoenvironmental evidence suggests that the greenhouse effect fluctuated quite considerably in the past. In the Quaternary era, for example, it was less intense during glacial periods than during the interglacials (Bach 1976; Pisias and Imbrie 1986). Present concern is with its increasing intensity and the associated global warming. The rising concentration of atmospheric CO2 is usually identified as the main culprit, although it is not the most powerful of the greenhouse gases. It is the most abundant, however, and its concentration is increasing rapidly. As a result, it is considered likely to give a good indication of the trend of the climatic impact of the greenhouse effect, if not its exact magnitude.

Svante Arrhenius, a Swedish chemist, is usually credited with being the first to recognize that an increase in CO2 would lead to global warming (Bolin 1972; Bach 1976; Crane and Liss 1985). Other scientists, including John Tyndall in Britain and T.C.Chamberlin in America (Jones and Henderson-Sellers 1990), also investigated the link, but Arrhenius provided the first quantitative predictions of the rise in temperature (Idso 1981; Crane and Liss 1985). He published his findings at the beginning of this century, at a time when the environmental implications of the Industrial Revolution were just beginning to be appreciated. Little attention was paid to the potential impact of increased levels of CO2 on the earth's radiation climate for some time after that, however, and the estimates of CO2-induced temperature increases calculated by Arrhenius in 1903 were not bettered until the early 1960s (Bolin 1972). Occasional papers on the topic appeared (e.g. Callendar 1938; Revelle and Seuss 1957; Bolin 1960), but interest only began to increase significantly in the early 1970s, as part of a growing appreciation of the potentially dire consequences of human interference in the environment. Increased CO2 production and rising atmospheric turbidity were recognized as two important elements capable of causing changes in climate. The former had the potential to cause greater warming, whereas the latter was considered more likely to cause cooling (Schneider and Mesirow 1976). For a time it seemed that the cooling would dominate (Calder 1974, Ponte 1976), but results from a growing number of investigations into greenhouse warming, published in the early 1980s, changed that (e.g. Idso 1980; Manabe et al. 1981; Schneider and Thompson 1981; Pittock and Salinger 1982; Mitchell 1983; NRC 1982 and 1983). They revealed that scientists had generally underestimated the speed with which the greenhouse effect was intensifying, and had failed to appreciate the impact of the subsequent global warming on the environment or on human activities.

Worldwide concern, coupled with a sense of urgency uncommon in the scientific community, led to a conference on the 'International Assessment of the Role of Carbon Dioxide and other Greenhouse Gases in Climate Variations and Associated Impact', held at Villach, Austria in October 1985. To ensure the follow-up of the recommendations of that conference, an Advisory Group on Greenhouse Gases (AGGG) was established under the auspices of the International Council of Scientific Unions (ICSU), the United Nations Environment Program (UNEP) and the World Meteorological Organization (WMO) (Environment Canada 1986). The main tasks of the AGGG were to carry out biennial reviews of international and regional studies related to the greenhouse gases, to conduct aperiodic assessments of the rates of increases in the concentrations of greenhouse gases, and to estimate the effects of such increases. Beyond this, they also supported further studies of the socio-economic impacts of climatic change produced by the greenhouse gases, and identified areas such as the monsoon region of south-east Asia, the Great Lakes region of North America and the circumpolar Arctic as likely candidates for increased investigation. The AGGG suggested that the dissemination of information on recent developments to a wide audience was also important, and in keeping with that viewpoint Environment Canada began the production of a regular newsletter to highlight current events in CO2/climate research. Its annual reports Understanding CO2 and Climate, published in 1986 and 1987, were also devoted to that theme. Throughout the 1980s, Environment Canada funded research on the environmental and socioeconomic impacts of global warming in such areas as agriculture, natural resource development and recreation and tourism. The Department of Energy in the United States has also been active in the field with more broadly based reports on the effects of increasing CO2 levels on vegetation (Strain and Cure 1985) and on climate (MacCracken and Luther 1985a and 1985b) as well as the effects of future energy use and technology on the emission of CO2 (Edmonds et al. 1986; Cheng et al. 1986).

In Europe, Flohn's (1980) study of the climatic consequences of global warming caused by human activities, for the International Institute for Applied Systems Analysis (IIASA), included consideration of CO2. More recently, the Commission of European Communities (CEC) funded research into the socio-economic impacts of climate changes which might be caused by a doubling of atmospheric CO2 (Meinl et al. 1984; Santer 1985). Most of these investigations involved the use of GCMs. The UK Meteorological Office five-layer GCM, for example, provided information on CO2-induced climatic change over western Europe (Wilson and Mitchell 1987). Several other European countries, including Germany and the Netherlands, also launched research programmes.

Table 7.2 A condensation of the IPCC Executive Summary on Climate Change: 1990

(a) There is a natural greenhouse effect which keeps the earth warmer than it would otherwise be.

(b) The natural greenhouse effect is being enhanced by increasing atmospheric concentrations of greenhouse

(a) Some greenhouse gases are more effective than others in causing climate change. For example, C02 has been responsible for more than half the enhanced greenhouse effect in the past, and that situation is

(b) Atmospheric concentrations of long-lived gases adjust only slowly to changing emissions. Thus, the longer emissions continue at current rates, the greater will be the reductions required to stabilize

(c) Immediate reductions of over 60 per cent in the emissions of long-lived gases would be required to

(a) Under the IPCC 'business-as-usual' scenario, mean global temperatures will rise by about 0.3°C per decade. The rise will not be steady, but the prediction is an increase in mean global temperatures of about

(b) Increasingly stringent emission controls could reduce the temperature increase to between 0.1°C and 0.2°C

(c) Land surfaces will warm more than the oceans and high northern latitudes will warm more than the global

(d) Regional temperature changes will differ significantly from the global mean, although confidence in the predictions is low because of the inaccuracy of GCMs at regional scales.

(e) Global mean sea level will rise about 20 cm by 2030 and 65 cm by 2100, but with significant regional

Predictions of the timing^ magnitude and regional patterns of climate change are hampered by incomplete

To improve current predictive capability, it will be necessary to:

• understand better the climate related processes associated with clouds, oceans and the carbon cycle.

• improve systematic global observation of climate-related variables and further investigate past changes.

• increase support for climate research activities, especially in developing countries.

Source: Houghton et al. (1990)

With the worldwide increase in the number of government agencies involved in the study of global warming, it became clear that an attempt had to be made to assess the overall state of the research. This was made possible by the WMO and UNEP, which together established the Intergovernmental Panel on Climate Change

(IPCC) in 1988. The IPCC was charged with assessing the status of the scientific information on climate change so that potential environmental and socio-economic impacts could be evaluated. It was also asked to formulate appropriate response strategies. With the cooperation of several hundred scientists from around the world, the IPCC produced the first part of its report—the scientific assessment—in 1990 (Houghton et al. 1990). The reports on impact assessment (Tegart et al. 1990) and response strategies (IPCC 1991) followed shortly thereafter. The scientific assessment included a summary of current knowledge of global warming as well as predictions for future developments (see Table 7.2). A supplementary report was issued in 1992, generally confirming the results of the earlier assessment, but also paying greater attention to the effects of sulphate emissions and ozone depletion on global warming trends (Houghton et al. 1992).

Concern over global warming was also an integral part of the Framework Convention on Climate Change signed at the Earth Summit in Rio de Janeiro in 1992. The convention was intended to deal with the human contribution to global warming, and create a vehicle for international action comparable to the Montreal Protocol on ozone depletion. For most observers, however, it was no more than a symbolic statement with few specific targets and no proper enforcement mechanisms (Clery 1992; Hulme 1993). The refusal of the United States to agree to even modest greenhouse gas emission targets, and the unwillingness of some Third World countries to support the energy efficiency standards necessary for reduced emissions, further weakened the convention (Pearce 1992c).

The perceived weakness of the convention is in part a reflection of the different views of the issue held by policymakers and scientists (Leggett 1992). Despite the immense volume of research on global warming, many key elements—such as the magnitude and timing of the warming— remain imperfectly understood, and therefore difficult to predict with any accuracy. Uncertainty of this type is not uncommon in scientific research, and scientists have come to accept it, but among planners and politicians it is often seen as undermining the arguments of those calling for immediate attention to the issue. Since policymakers have the ultimate say in where and when policy will be implemented to deal with the warming, it appears that they must shoulder much of the blame for the delays. However, some environmentalists also claim that scientists have failed, by giving too much attention to the uncertainties and too little to the dire consequences of full-scale warming (Leggett 1992).

No matter how the blame is apportioned, one of the results of the uncertainty has been to allow those who remain unconvinced by the scientific assessment, or perhaps have a vested interest in retaining the status quo, to argue successfully that no steps be taken to deal with the issue until the real impact of global warming is revealed by additional research. That may yet take several decades, and some of the researchers investigating the problem warn that by then it may be too late (Roberts 1989; Henderson-Sellers 1990).

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