Oklahoma Climatological Survey


Earth's Energy Budget

Part 2




Absorption and re-emission of radiation at the earth's surface is only one part of an intricate web of heat transfer in the earth's planetary domain. Equally important are selective absorption and emission of radiation from molecules in the atmosphere. If the earth did not have an atmosphere, surface temperatures would be too cold to sustain life. If too many gases which absorb and emit infrared radiation were present in the atmosphere, surface temperatures would be too hot to sustain life.



Figure 2 - Globally Averaged Energy Budget

Outgoing Terrestrial Radiation

  • The earth's surface, atmosphere, and clouds emit radiation in the infrared band and near-infrared band.


  • Outgoing infrared (IR) radiation from the earth's surface (also called terrestrial radiation) is selectively absorbed by certain molecules, particularly water vapor and carbon dioxide.

    Gases which absorb IR radiation are termed collectively as "greenhouse gases".

  • Water vapor and carbon dioxide emit infrared radiation.

    Infrared radiation from greenhouse gases in the atmosphere is emitted in all directions, including back to the earth's surface. It is this re-emission to the earth's surface that maintains a higher temperature on our planet than what would be possible without the atmosphere.

  • Condensed water is also an efficient absorber and emitter of IR radiation. Thus, clouds act in a manner similar to greenhouse gases.


  • Satellite infrared imagery detects infrared emission from clouds and the earth's surface and can be used during both day and night.



Global Energy Balance


  • When averaged over a year, the incoming energy in both the earth and its atmosphere equals the outgoing energy.

    If we consider the entire Earth-atmosphere system, then the amount of radiation entering the system must equal to the amount leaving, or the system would continually heat or cool. Not all of this energy is radiative energy; some is sensible and latent heat.

  • If we consider the atmosphere alone, we find that the atmosphere experiences radiative cooling.

    The atmosphere is kept from a net cooling by the addition of energy by latent and sensible heating.

  • The atmosphere has a warming effect on Earth's surface -- the "atmospheric greenhouse effect".

    If Earth had no atmosphere, the globally averaged surface temperature would be -18 degrees Celsius. Because Earth does have an atmosphere, the average surface temperature actually is 15 degrees Celsius.

    The atmosphere acts as a greenhouse because of gases that selectively allow solar radiation to pass through but absorb and then re-emit terrestrial radiation. These gases are collectively called "greenhouse gases" and include water vapor, carbon dioxide, ozone, molecular oxygen, methane and nitrous oxide. These gases are selective as to which wavelengths they will absorb. For example, ozone absorbs shortwave ultraviolet radiation whereas water vapor absorbs infrared radiation more readily.

  • Most of the sun's radiation that passes through the atmosphere to hit the earth is in the visible part of the spectrum.


  • Most of the earth's radiation that escapes the atmosphere is in the infrared band between 8 microns and 11 microns.

    This region of the spectrum is called the "atmospheric window".



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