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Carbon Dioxide and Climate: A Scientific Assessment
be in agreement with observation when applied to the deduction of atmospheric temperature profiles from satellite infrared measurements. There is thus relatively high confidence that the direct net heating value ΔQ has been estimated correctly to within ±25 percent. However, it should be emphasized that the accurate calculation of this term has required a careful treatment of the thermal radiative fluxes with techniques that have been developed over the past two decades or more. Crude estimates may easily be in error by a large factor. Thus, in an interim report, MacDonald et al. (1979) obtain a ΔQ of 6 to 8 W m−2, a value about 1.5 to 2 times too large.
Greater uncertainties arise in estimates of the resulting change in global mean surface temperature, ΔT, for this quantity is influenced by various feedback processes that will increase or decrease the heating rate from its direct value. These processes will influence the feedback parameter λ in the expression ΔT=ΔQ/λ. For the simplest case in which only the temperature change is considered, and the earth is assumed to be effectively a blackbody, the value of λ=4σT3 is readily computed to be about 4 W m−2 K−1. For such a case, doubled CO2 produces a temperature increase of 1°C.
3.1.2 Feedback Effects
The most important and obvious of the feedback effects arises from the fact that a higher surface temperature produces a much higher value of the surface equilibrium water-vapor pressure through the highly nonlinear Clapeyron-Clausius relation. This, in turn, leads to increased water vapor in the atmosphere. A plausible assumption, borne out qualitatively by model studies, is that the relative humidity remains unchanged. The associated increase of absolute humidity increases the infrared absorptivity of the atmosphere over that of CO2 alone and provides a positive feedback. There is also increased absorption of solar radiation by the increased water vapor, which further increases the infrared feedback by about 10 percent. As with CO2, the radiative transfer calculation of water-vapor effects is relatively reliable, and the consequence is that λ is decreased and ΔT increased by about a factor of 2. For doubled CO2, the temperature increase would be 2°C.
One-dimensional radiative-convective models that assume fixed relative humidity, a fixed tropospheric lapse rate of 6.5 K km−1, and fixed cloud cover and height give λ=2.0 W m−2 K−1 (Ramanathan and Coakley, 1978). This value is uncertain by at least ±0.5 W m−2 K−1 because of uncertainties in the possible changes of relative humidity, temperature lapse rate, and cloud cover and cloud height.
Snow and ice albedo provide another widely discussed positive feedback mechanism (see, for example, Lian and Cess, 1977, and additional references therein). As the surface temperature increases, the area covered by snow or