Policy Implications of Greenhouse Warming Mitigation, Adaptation, and the Science Base (1992) / Chapter Skim
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12 Radiative Forcing and Feedback
12 Radiative Forcing and Feedback
Pages 100-110
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From page 100... ...
In this discussion of radiative forcing, the planet's heat balance, and these feedbacks and their consequences, emphasis is given to global mean quantities, since, conventionally, the concept of radiative feedback mechanisms is applied to global mean quantities associated with changes from one equilibrium climate to another. Many aspects of these feedback mechanisms are controversial.
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From page 101... ...
The second important point of Figure 12.1 is that the atmospheric greenhouse gases and the clouds emit infrared radiation downward to the surface, and this direct radiative heating of the surface by the atmosphere (320 W/m2) is twice the direct solar heating (160 W/m24.
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From page 102... ...
This scenario is adopted here, not because CO2 doubling has some special significance but because it is an atmospheric modification that may occur within the next century and particularly because the CO2 doubling scenario has become the baseline case most commonly used by atmospheric scientists to compare the performance of one model with that of another. Immediately after this augmentation of greenhouse gas content, no changes in interface temperature, atmospheric temperature distribution, atmospheric moisture content, or any other component can yet have occurred.
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From page 103... ...
RADIATIVE FEEDBACK MECHANISMS In order to demonstrate radiative feedback mechanisms, it is convenient to assume initially that climate change is manifested solely by temperature changes within the climatic system and that all other climate parameters remain fixed at their unperturbed values. In this framework, there is no change in the climatic system's 240 W/m2 solar absorption.
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From page 104... ...
, recent combined observational and model results strongly support the conventional interpretation that water vapor provides positive feedback (Ravel and Ramanathan, 1989; Rind et al., 19911. Notwithstanding the connection between water vapor and clouds in the climatic system, they are treated separately for analytic purposes (see also the "Cloud Feedback" section, below)
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From page 105... ...
The most detailed climate models for the purpose of projecting climate change are three-dimensional atmospheric general circulation models (GCMs) , and these models seem to depict properly the infrared component of water vapor feedback.
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From page 106... ...
These results suggest that water vapor feedback is not overestimated in models and should amplify the climatic response to increased concentrations of greenhouse gases. Snow-Ice Feedback An additional feedback mechanism is snow-ice feedback, by which a warmer earth has less snow and ice cover, resulting in a darker planet that in turn absorbs more solar radiation.
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From page 107... ...
For example, if global warming displaces a given cloud layer to a higher and colder region of the atmosphere, this will produce a positive feedback because the colder cloud will emit less radiation and will thus enhance the greenhouse effect. · Cloud water content: There has been considerable recent speculation that global warming could increase cloud water content, thereby resulting in brighter clouds and hence a negative component of cloud feedback.
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From page 108... ...
, and in some models the net effect may be positive (Schlesinger, 1988; Cess et al., 1990; Rind et al., 1991~. Recent analysis using both satellite and model results showed that highly reflective cirrus clouds are produced in tropical regions when sea surface temperature increases sufficiently (Ramanathan and Collins, 1991~.
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From page 109... ...
There is, however, a nearly threefold variation of the global (clear plus overcast) sensitivity parameter; clearly, given the clear-sky agreement, most of the variation in the global sensitivity parameters of current models can be attributed to cloud feedback.
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From page 110... ...
1991. Positive water vapor feedback in climate models confirmed by satellite data.
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