The Earth is heated by radiation from the Sun. In turn, an equal amount of radiation must leave the top of the atmosphere for the Earth to maintain a balance so that it does not either heat up or cool down. The interaction of radiation with the atmosphere and the Earth's surface is very complicated. However, we can get a general picture of it by summarizing the Earth-atmosphere energy balance on average for the whole globe (i.e., global average; Figure 3.1).1 Suppose that 342 W/m2 of radiant energy impinge upon the top of the atmosphere, about the power of three and a half 100-W light bulbs. Not all of it reaches the surface of the Earth. About 67 W/m2 is absorbed by the atmosphere, mostly by ozone but some by water vapor and dust or other particulate pollution. Another 77 W/m2 is reflected by clouds and aerosols (small particles in the atmosphere). The Earth's surface reflects 30 W/m2 back to space. The remaining 168 W/m2 is absorbed by the surfaces of the land and the oceans. The surface of the Earth (land and oceans) directly heats the atmosphere (sensible heat), losing about 24 W/m2, whereas evaporation from the surface (latent heat) accounts for about 78 W/m2. The Earth's surface and the atmosphere emit radiation that has different characteristics from those of solar radiation. I will say more about that shortly. About 390 W/m2 of this "long-wave" radiation is emitted by the surface. Almost all of this is absorbed by clouds, water vapor, CO2, and several other gases. Forty watts per square meter escapes through the atmosphere. About 324 W/m2 is reemitted downward to the surface. The atmosphere, together with clouds, emits another 195 W/m2 into space.
Notice that the sum of the energy units must balance both at the surface and at the top of the atmosphere. The sum of the energy passing into the atmosphere from the Sun must be equal to the amount of radiant energy leaving the top of the atmosphere, which is the solar radiation reflected back plus the long-wave radiation leaving from the top of the atmosphere. Otherwise, the Earth-atmosphere would heat up. The same is true at the surface. The 168 W/m2 of solar energy absorbed plus the 324 W/m2 of long-wave heating from the atmosphere must equal 390 W/m2 of long-wave radiation emitted by the surface plus the sensible heat (24 W/m2) plus the latent heat leaving through evaporation (78 W/m2). Interrupting any of the processes can cause the system to be "out of balance." For example, if the radiation from the Sun were to increase to, say, 350 W/m2, the radiation escaping from the top of the atmosphere would have to increase by 8 W/m2 to bring the system back into equilibrium. Many things could happen to cause the radiation loss from the top of the atmosphere to increase. For example, the surface and the atmosphere could become warmer, and because the radiation increases with temperature, the long-wave radiation leaving the top of the atmosphere would increase. If that happened, there would probably be less ice and snow on the warmer Earth surface. Ice and snow are excellent reflectors of solar radiation, and so, less radiation would be reflected from the surface, reducing the 30 W/m2 of solar radiation leaving the atmosphere because of reflection from the surface, and again putting the system out of balance.
FIGuRE 3.1: The Earth-atmosphere global energy balance. If neither the atmosphere nor the surface of the Earth (including land and sea areas) is changing in temperature, then the net amount of heat entering and leaving both the atmosphere and the land and sea areas must be zero. This figure shows the pathways of 342 W/m2 of solar radiation bearing down on the top of the atmosphere. About 168 W/m2 of this very high temperature (or shortwave) solar radiation is absorbed by the Earth. The rest is either reflected back out of the atmosphere or absorbed by the atmosphere. In addition, about 324 W/m2 of long-wave radiation from the atmosphere and clouds is absorbed by the surface. The total (168 + 324 = 492 W/m2) must now leave the surface through sensible heat (caused by the temperature difference between the Earth and the atmosphere), latent heat (caused by evaporation), and long-wave radiation emitted by the surface. Most of the long-wave radiation is captured by the atmosphere, which emits about 235 W/m2 out of the top of the atmosphere. If the amounts of water vapor and CO2 in the atmosphere increase, more of the infrared radiation will be absorbed within the atmosphere, and the atmosphere and Earth must compensate by emitting more radiation. To do this, they must become warmer.
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