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the oceans and the continental land masses. Heat is returned from the oceans and the land by infrared radiation from the surface (390 W/m2). Ninety W/m2 are returned by nonradiative processes that lead to the upward transport of water vapor (with its latent heat) and of sensible heat. All of the 90 W/m2 flux and a substantial part (call it B) of the 390 W/m2 radiative flux are deposited in the atmosphere. The total energy flux to the atmosphere (80 + 90 + B) is necessarily in balance with the infrared emission from the atmosphere, 320 W/m2 of which is directed downward into the ocean-land mass interface and the rest of which emerges from the top of the atmosphere (along with that fraction of the interface radiation (390 - B) that is not absorbed in the atmosphere). One can see that these numbers are mutually consistent by noting that the total nonreflected solar input is equal to the total (infrared) output of the system, that the total energy flux downward through the interface balances the upward flux from that interface, and that the energy received by the atmosphere (80 + 90 + B) plus the nonabsorbed upward flux from the interface (390 - B) is in balance with the radiation from the atmosphere that supplies the output of the system (at the top) and the radiative flux downward into the interface.

The interplay among the transmission of infrared radiation through the atmosphere, the absorption of infrared in the atmosphere, and the emission of infrared from the atmosphere is distributed throughout the height of the gaseous envelope. The foregoing, highly oversimplified splitting of these aspects of the infrared interchange into individual macroscopic items is needed only for the schematic presentation within this chapter. One need not choose a particular, necessarily artificial, value for B, because here a chosen value for B would not affect the conclusion drawn.

Note that, since each flux shown in Figure 12.1 is a globally averaged quantity, there is no depiction of the horizontal heat fluxes within the atmosphere and ocean that redistribute heat from one part of the planet to another. Nevertheless, such internal transfers play vital roles in the physical processes that determine the climatic state.

There are two important points to note from Figure 12.1. The first is that the 240 W/m2 emission at the top of the atmosphere (TOA) is 150 W/m2 less than the 390 W/m2 emission from the surface. This radiative flux difference is the greenhouse effect of the earth's present atmosphere, and it is caused by the absorption of infrared radiation by greenhouse gases and clouds. 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/m2). By itself, the additional 320 W/m2 provided by infrared surface heating produces substantial warming of the surface above the temperature that would otherwise prevail; thus it is the greenhouse effect that makes our planet habitable.