earth’s albedo and hydrological budget. Although such possibilities deserve investigation, they are too remote to justify further discussion here.

4.2
CLIMATIC IMPACT OF CARBON DIOXIDE

Carbon dioxide, CO2, is an important natural factor, as are water vapor and ozone, in controlling the temperature of the atmosphere. This gas is nearly transparent to visible light but is a strong absorber of infrared radiation, especially at wavelengths between 12 and 18 µm, where a considerable proportion of the outgoing radiation from the earth’s surface is transmitted to outer space. An increase in atmospheric CO2 to levels appreciably above the preindustrial concentration of about 290 ppm (mole fraction in parts per million) might act, much like adding glass to a greenhouse, to increase the temperature of the lower atmosphere.

As discussed below, CO2 will be produced by man in large quantities relative to the amount now present in the atmosphere. Atmospheric CO2 concentrations five to ten times the preindustrial level may be attained during the twenty-second century. If high levels are once reached, they will probably decrease only slowly and thus remain well above the preindustrial level for at least a thousand years.

The magnitude of the temperature rise attending an increase in atmospheric CO2 has been estimated with mathematical models of increasing plausibility and complexity. Early studies, summarized by Manabe and Wetherald (1975), estimated the global average temperature at the earth’s surface without regard for changes in the turbulent energy exchanged with the overlying air column and need not be examined here. Manabe and Wetherald (1967) overcame this limitation with a one-dimensional convective adjustment, or “radiative convective,” model, which allowed for temperature adjustments in the air column to preserve a reasonable vertical gradient, or lapse rate. They also considered the influence on radiation of varying amounts of water vapor but assumed cloud cover to be unaltered by the change in CO2 concentration. Their model has been widely quoted and has been used to compare the temperature effect of other industrial gases to that of CO2, as discussed below.

Recently, Manabe and Wetherald (1975) have developed a three-dimensional model that takes into account both vertical and horizontal atmospheric motions. The model employs an idealized continent and ocean with snow cover, rainfall, and vertical lapse rate of temperature treated as dependent variables. Heat transport by ocean currents is neglected. The distributions of cloudiness at low, middle, and high levels are preset at average values that remain invariant because no general circulation model has yet been devised that predicts clouds reliably. On the basis of what the authors considered to be the best available radiative transfer scheme, the model predicts an average rise of 2.5°C in air temperature in the lower atmosphere for a doubling of atmospheric CO2 from 300 to 600 ppm. This rise is about 25 percent higher than predicted by their one-dimensional model. Also, precipitation, not predicted in the one-dimensional model, is increased by 7 percent. The temperature rise is most evident in polar latitudes (see Figure 9.2 of Chapter 9) as a result of decreased snow cover and suppressed vertical air motion. This positive feedback explains in large part why a higher temperature rise is predicted than by their one-dimensional model.

Although the climatic effects of CO2 levels higher than 600 ppm have not been investigated for the three-dimensional model, the one-dimensional convective adjustment model predicts nearly equal additional rises in average air temperature for each successive doubling of CO2 level (Augustsson and Ramanathan, 1977). For example, an eightfold increase in CO2 might raise average air temperatures by 7°C. Such a global average rise would be comparable with the extreme changes in global temperature believed to have occurred during geologic history.

The Manabe-Wetherald three-dimensional model, in spite of its complexity compared with one-dimensional models, still falls short of an accurate portrayal of atmospheric processes. The neglect of feedback mechanisms for clouds and ocean circulation leaves open the possibility that the temperature rise might be considerably larger or smaller than the model predicts, even possibly of opposite sign, as discussed by Smagorinsky in Chapter 9. Nevertheless, the prediction of a substantial temperature rise cannot be arbitrarily dismissed. As pointed out by Schneider (1975), there is no strong evidence that the present models are more likely to overestimate the rise than to underestimate it.

Our limited knowledge of cloud feedback illustrates this point. As Manabe and Wetherald (1967) and others have shown, increasing either low or middle stratoform clouds, per se, would produce lower surface temperatures. It is not clear, however, that these forms of cloudiness necessarily increase with increasing atmospheric CO2. The Manabe-Wetherald three-dimensional model predicts higher relative humidity in the low troposphere and, probably, as Smagorinsky suggests in Chapter 9, more low clouds. But in the middle troposphere, the model predicts lower relative humidity, associated with the stronger hydrological cycle that accompanies added heating from CO2. If this lower humidity leads to substantially less middle clouds, the cooling effect of more low clouds could be partially or wholly canceled.

With respect to the role of the oceans, progress in modeling is hampered by the long adjustment times involved in the intermediate water circulation. This water, which lies just below the ocean surface at high latitudes and circulates to depths as great as 1000 m at low latitudes, probably contributes significantly to the atmospheric heat budget and should be included in models of climatic change.

Also, all the present modeling attempts have involved so-called equilibrium models, which compare the climate for two or more distinct CO2 levels each maintained indefinitely. As discussed below, the CO2 concentration of the atmosphere is likely to vary considerably over the next several hundred years. This time is too short for the atmosphere and ocean to attain equilibrium either with respect to climate or chemical processes



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