Quantifying the Global Energy Balance

We can summarize and quantify these concepts by defining the energy balance for the earth as is shown in Figure 2.

If we assume that the earth receives 100 units of solar energy per unit time, then what happens to this energy? As shown in the accompanying sketch, about 25 units of this energy are reflected by the atmosphere, another 25 units are absorbed by particles and gaseous molecules in the atmosphere, and 5 units are reflected by the earth's surface. This leaves 45 units to be absorbed by the earth's surface.

Transfer of energy from the earth to the upper atmosphere in the amount of 29 units occurs by conduction, convection, and evaporation of surface water (absorption of latent heat) with subsequent condensation in clouds and fog (release of latent heat). Radiation from the surface accounts for 104 units of energy loss, with a concurrent gain of 88 units of radiation back to the surface from greenhouse gases, clouds and atmospheric particulates. The planet, as a whole, releases 70 units of infrared radiation to outer space, mostly as radiation from cloud tops and the upper atmosphere. The effective radiating temperature of the planet as seen from outer space is 255 K, or -18⁰C. Notice that the 70 units of outgoing longwave radiation, combined with 30 units of solar energy that is reflected, gives an outgoing energy total of 100 units, which balances the incoming amount.

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