Increasing concentrations of carbon dioxide and other trace gases in the atmosphere, largely as a result of human activities, are expected to cause a significant increase in the earth's temperature over the next several decades. The increase would entail major ecological, economic, and social consequences. The likelihood of such global warming is high, and its causes seem to be fairly well understood. The emission of these gases into the atmosphere, therefore, should be monitored carefully and probably should be reduced considerably in order to prevent serious harm to human welfare. While the harmful effects would not be great in the short run, it would be extremely unwise to postpone serious efforts to limit the build-up of most of these gases. While "it may be too soon to start implementing drastic measures for either limitation or adaptation, . . . there is an immediate need for adoption of measures that ensure that the world is better prepared to respond to climatic change."

The concentration of carbon dioxide in the atmosphere by 1985 had increased by about 25 per cent since pre-industrial times. It is likely to increase by a further 40 per cent by the year 2050 if present trends in energy use continue; by 80 per cent if energy use accelerates. The additional warming effect of other trace gases, especially methane, nitrous oxide, and the so-called chlorofluorocarbons, is expected to be about equal to that caused by carbon dioxide. (The chlorofluorocarbons are also a principal cause of the depletion of stratospheric ozone, discussed below.) The expected climatic change would exacerbate the problems of drought, desertification, deforestation, and soil erosion and worsen the prospects for sustainable agriculture. Another consequence of global warming would be a higher sea level, which would profoundly influence habitation patterns, agriculture, and industry, particularly in river deltas, flood plains, and other low- lying coastal areas.

While most of these effects are expected in later decades, already in by the 1990s the increased climatic variability required a greater buffer capacity in food supplies than has been considered necessary in the past, at both national and international levels. In later decades adjustments of rainfed cropping patterns, extension or modification of irrigation systems, or changes between major land uses (e.g., arable use versus grazing or forestry) may be necessary.

Although the forces driving global warming are now broadly understood, its precise regional distribution and environmental impacts are not. Concerted efforts by the international scientific community will be necessary to clarify the prospects. If things are allowed to go on as at present, the world might in a few decades have to adapt very rapidly in the face of catastrophic change. But immediate measures to slow down the buildup of greenhouse gases would reduce the warming and its undesirable consequences. The 1987 Montreal Protocol on Substances That Deplete the Ozone Layer and the 1989 Helsinki Declaration on the Protection of the Ozone Layer⁸ are steps in that direction. They call for drastic cuts in production of fully halogenated chlo-rofluorocarbons (CFCs), which are greenhouse gases, by the end of the first decade of the 21st century.

Carbon dioxide is responsible for about 50 per cent of the greenhouse effect, so major attention should be given to strategies that would limit or even reduce its emission. The strategies include increased energy efficiency and modification of technologies in ways that lead to reduction in the use of fossil fuels. The preservation of forests, especially the tropical forests, is also essential.

About two thirds of the global carbon dioxide released by human activities arises from the combustion of fossil fuels. They released about 5.3 billion tons of carbon in 1986. In addition, deforestation, land exploitation, and burning of other components of the world's biomass currently release 2 to 3 billion tons of carbon annually, nearly 80 per cent due to deforestation. Annual emissions of carbon dioxide from fossil fuel are projected to contain about 7 billion tons of carbon in the year 2000, between 10 and 14 billion tons in 2030, and between 13 and 23 billion tons in 2050, depending on the rate of growth in the demand and supply of fossil fuel energy.

The developed market economies of North America, Western Europe, and Japan produce 49 per cent of global carbon dioxide emissions, the economies of Eastern Europe 25 per cent, and the developing countries 26 per cent. In addition to the enormous use of fossil fuels in the industrialized countries, virtually all production of chlorofluorocarbons takes place in these countries.They have the major responsibility, therefore, for devising ways to limit the output of the greenhouse gases. Although they have reached initial agreements to reduce the production of chlorofluorocarbons in order to preserve stratospheric ozone, very little has been done to reduce or even to slow the growth of CO₂ emissions.

In view of the opposition to nuclear power, adjusting the world energy mix to environmental concerns will be difficult unless technological breakthroughs can provide benign, economically competitive alternative energy sources and/or radically improve energy efficiency. Such developments can be expected in the long term at best, and adjustments in the medium term will by necessity be limited to greater reliance on the less malignant energy sources.

Vigorous application of the following policies to reduce carbon dioxide emissions could limit global warming to a rate that would make the necessary adaptation of global economic activity relatively manageable:

(a)     Reduce fossil fuel use through increases in end-use energy efficiency;

(b)   Shift the fossil fuel mix toward natural gas. For the same energy output the combustion of natural gas produces about 57 per cent of the amount of carbon dioxide produced from coal, and the combustion of oil produces about 83 per cent. Available technologies and reserves favour such an adjustment, particularly in the electric power sector;

(c)     Replace fossil fuel combustion with alternative energy technologies;

(d)   Eliminate net forest loss by more careful management of development in forest areas and by large scale reforestation;

(e)     Remove carbon dioxide from the flue gas of thermal power plants for disposal in the deep ocean. They account for 15 per cent of carbon dioxide emissions.

The first four of these measures can serve more than one environmental and economic purpose. Even if the worst scenario does not come about, the world would benefit in other respects by taking these measures now. Other greenhouse gases that could be controlled to varying degrees, based on present technical knowledge, are methane, nitrous oxides, and tropospheric ozone. Priority should be given to detailed studies of the sources of these gases, their interactions in the atmosphere, and technologies that would reduce their emissions.