Adjusted Radiative Forcing

Estimates of radiative forcing due to tropospheric ozone are listed in Table 2. CTM calculations of changes in tropospheric ozone and the radiative forcing are based on a variety of assumptions. Most models, but not all, include temperature adjustment in the stratosphere and effects of clouds. Neglect of either of these effects has been estimated to lead to an overestimate of radiative forcing by approximately 10-25% (Berntsen et al., 1997). 

Radiative forcing defined as a change in the energy flux at the tropopause is referred to as instantaneous forcing (Figure 23.10a). The adjusted radiative forcing, which is the measure now commonly used, is the radiative forcing at the top of the atmosphere (TOA) in which the stratospheric temperature profile is allowed to adjust to the perturbation and... 

FIGURE 23.10 Assumptions used in calculating (a) AF , the instantaneous radiative forcing (b) AFa, the adjusted radiative forcing (c) AT0, the surface temperature response with no feedbacks and (d) AT, the surface temperature response with feedbacks (Hansen et al. 1997). (Reprinted by permission of American Geophysical Union.)... 

Following this immediate shock the stratosphere cools. The increased CO2 in the stratosphere enhances the thermal emission. Because the stratospheric temperature increases with altitude, this has the effect that the cooling into space is larger than the absorption from layers below. This is in fact the fundamental reason for the COi-induced cooling in the stratosphere. After stratospheric cooling a new radiative equilibrium develops with the new doubled CO, concentration. This reduces the increased downward emission at the tropopause by about 0.2 W in - and the tropopause forcing is adjusted accordingly. 

Figure 23.10 illustrates the differences between the no-feedback surface temperature response ATo and the ultimate equilibrium response AT,. AT0 tends to be directly proportional to the adjusted forcing this proportionality provides the rationale for using the adjusted forcing as a measure of the expected climate response to the perturbation. This proportionality assumes that the lapse rate in the troposphere is fixed (and that in the stratosphere is determined by radiative equilibrium). Ultimately, the question is whether AT, is proportional to the adjusted forcing, when the tropospheric lapse rate is allowed to change in response to climate feedbacks such feedbacks include changes in clouds and precipitation. 

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