The rate, at which the earth and its atmosphere receive energy from space, is equal to the rate at which energy is being returned to space and is known as ‘global energy balance’.
To calculate global energy, three separate regions namely the earth, its atmosphere and the outer space need to be considered.
If the global temperature does not change with time, then the rate at which energy is received by earth and its atmosphere, must be radiated back. Out of 342 W/m2, 107 W/m2, is reflected back into space, hence, the absorbed 235 W/m2 radiation must be radiated back into the space.
If the earth’s surface was at 254 K, it would radiate 235 W/m2 to balance the incoming energy. But it is known that greenhouse gases absorb most of the outgoing radiation, hence, required energy balance would not be realized.
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Therefore, to achieve necessary energy balance, enough energy should be forced out through the atmosphere, i.e., the temperature of the earth’s surface must be higher than 254 K.
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In an attempt to pass through the atmosphere, it is affected by various atmospheric gases as well as by aerosols. It is known that a molecule absorbs energy only when the frequency of its oscillation matches with the frequency of radiation.
Thus, most of the long wavelength energy radiated by the earth is absorbed by gases like H20 (water vapour), C02, CH4, N20 and 03 (greenhouse gases). However, radiant energy having wavelength between 7 to 12 pm, is not absorbed by any atmospheric gases due to frequency mismatch.
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As a result, the radiated energy (40 W/m2) in the frequency range of 7 to 12 pm easily escapes to the space. This relatively clear sky, for this outgoing energy is referred to as the ‘atmospheric radiation window’.
Out of 390 W/m2, only 40 W/m2 passes directly through the atmosphere, mostly through the atmospheric window between 7 and 12 pm.
The remaining 350 W/m2 is absorbed by the greenhouse gases in the atmosphere. The atmosphere then radiates 324 W/m2 back to the surface and (169 + 26 = 195) 195 W/m2 to the space.
Heat transfer also occurs from the surface to the atmosphere by convective heating as well by evaporation and condensation of water. Convection transfers 24W/m2 to the atmosphere and condensation of water vapour provides 78W/m2 of latent heat.
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If the model is internally self consistent, the rate of energy gain must be equal to the rate of energy loss in all of the three regions: the space, the atmosphere and the earth’s surface.