3.1 Earth radiation budget

The global average equilibrium temperature of the Earth is determined by a balance between the energy acquired by the absorption of incoming solar radiation and the energy lost to space by the emission of thermal infrared radiation. The amount of solar energy absorbed depends both on the incoming irradiance and on the Earth’s reflective properties. If either of these changes then the temperature structure of the atmosphere-surface system tends to adjust to restore the equilibrium. Figure 18View Image shows the components of the global annual average radiation budget and how much radiation is absorbed, scattered and emitted within the atmosphere and at the surface. The value for the incoming radiation, 342 Wm–2, is equivalent to a total solar irradiance at the Earth of 1368 Wm–2 averaged over the globe. Of this 31% (107 Wm–2) is reflected back to space by clouds, aerosols, atmospheric molecules and the surface, with the clouds playing the most important role, so that only 235 Wm–2 is absorbed by the Earth system. 20% (67 Wm–2) of the incident radiation is absorbed within the atmosphere leaving 49% (168 Wm–2) to reach and heat the surface.

The temperature and emissivity of the surface are such that 390 Wm–2 of infrared energy are emitted into the atmosphere. Only 40 Wm–2 of this, however, escapes to space with the remainder being absorbed by atmospheric gases and cloud. The atmosphere returns 324 Wm–2 to the surface. The energy balance at the surface is achieved by non-radiative processes such as evaporation and convection. The radiation balance at the top of the atmosphere is achieved by the 195 Wm–2 of heat energy emitted to space by the atmosphere and clouds.

View Image

Figure 18: Globally averaged energy budget of the atmosphere. Figure from External Linkhttp://asd-www.larc.nasa.gov/ceres/brochure/ based on data from Kiehl and Trenberth (1997).

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