T

SOLAR 8 TERRESTRIAL

FIGURE 2.9. An "opaque" greenhouse made up of two layers of atmosphere. Each layer completely absorbs the IR radiation impinging on it.

FIGURE 2.9. An "opaque" greenhouse made up of two layers of atmosphere. Each layer completely absorbs the IR radiation impinging on it.

layer-by-layer—which depends on the vertical distribution of absorbers, particularly H2O, CO2, and O3 (see section 3.1.2)—and do the required budgets for each layer and at the surface (we are not going to do this). An incomplete schematic of how this might look for a rather opaque atmosphere is shown in Fig. 2.10.

The resulting profile, which would be the actual mean atmospheric temperature profile if heat transport in the atmosphere occurred only through radiative transfer, is known as the radiative equilibrium temperature profile. It is shown in Fig. 2.11. In particular, note the presence of a large temperature discontinuity at the surface in the radiative

FIGURE 2.10. Schematic of a radiative transfer model with many layers.

FIGURE 2.11. The radiative equilibrium profile of the atmosphere obtained by carrying out the calculation schematized in Fig. 2.10. The absorbers are H2O, O3, and CO2. The effects of both terrestrial radiation and solar radiation are included. Note the discontinuity at the surface. Modified from Wells (1997).

FIGURE 2.11. The radiative equilibrium profile of the atmosphere obtained by carrying out the calculation schematized in Fig. 2.10. The absorbers are H2O, O3, and CO2. The effects of both terrestrial radiation and solar radiation are included. Note the discontinuity at the surface. Modified from Wells (1997).

equilibrium profile, which is not observed in practice. (Recall from our analysis of Fig. 2.8 that we found that the atmosphere in our slab model is always colder than the surface.) The reason this discontinuity is produced in radiative equilibrium is that, although there is some absorption within the troposphere, both of solar and terrestrial radiation, most solar radiation is absorbed at the surface. The reason such a discontinuity is not observed in nature is that it would (and does) lead to convection in the atmosphere, which introduces an additional mode of dynamical heat transport. Because of the presence of convection in the lower atmosphere, the observed profile differs substantially from that obtained by the radiative calculation described above. This is discussed at some length in Chapter 4.

Before going on in Chapter 3 to a discussion of the observed vertical profile of temperature in the atmosphere, we briefly discuss what our simple greenhouse models tell us about climate feedbacks and sensitivity to changes in radiative forcing.

2.3.4. Climate feedbacks

The greenhouse models described previously illustrate several important radiative feedbacks that play a central role in regulating the climate of the planet. Following Hartmann (1994) we suppose that a perturbation to the climate system can be represented as an additional energy input dQ (units Wm-2) and study the resultant change in global-mean surface temperature, dTs. Thus we define dTs/dQ to be a measure of climate sensitivity.

The most important negative feedback regulating the temperature of the planet is the dependence of the outgoing longwave radiation on temperature. If the planet warms up, then it radiates more heat back out to space. Thus using Eq. 2-2 and setting SQ = s(aT4) = 4T3e8Ts, where it has been assumed that Te and Ts differ by a constant, implies a climate sensitivity associated with blackbody radiation of

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