Incoming & Outgoing of Energy of the Earth

The Earth’s Energy Balance The Earth's average temperature remains fairly constant from year to year. Therefore, the Earth must be releasing into space the same amount of energy that it receives from the sun. If this did not occur, the atmosphere would measurably warm or cool, depending on the amount of heat lost to space.

Incoming Solar Radiation The sun radiates mostly in the visible band, but also in the ultraviolet (shorter wavelength). When averaged globally and annually, only 50% of the solar radiation striking the earth and its atmosphere is absorbed at the surface. The atmosphere absorbs 20% of incoming solar radiation and the remaining 30% is reflected back into space.

Incoming Solar Radiation The shortest wavelengths of solar radiation (those most dangerous to life) are absorbed by molecules in the upper and middle atmosphere. In particular, ozone selectively absorbs ultraviolet radiation while allowing visible radiation to pass through relatively unhindered. Reflection significantly affects the solar radiation that reaches the ground, as the sun's rays could be reflected off of air molecules (termed scattering), clouds or the ground itself.

Incoming Solar Radiation Light-colored or shiny objects reflect more radiation than dark objects. Energy that is reflected cannot be absorbed or transmitted through an object. Different surfaces have different albedos. (Refer to T.B. for details, p ) (Refer to T.B. for details, p )

Average Reflectivity of Surfaces to Solar Radiation Surface type % Reflected Albedo Fresh snow ~ 90 % ~ 0.9 Thick clouds ~ 80 % ~ 0.8 Thin clouds ~ 40 % ~ 0.4 Ice ~ 35 % ~ 0.35 Soil ~ 20 % ~ 0.2 Forest ~ 5 % ~ 0.05 Ocean (high sun angle) ~ 5 % ~ 0.05 Ocean (low sun angle) ~ 95 % ~ 0.95

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”.

Outgoing Terrestrial Radiation Water vapor and carbon dioxide emit infrared (IR) 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.

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

Global Energy Balance Since the earth-atmosphere system is neither warming up nor cooling down, there must be a balance between the incoming insolation and outgoing terrestrial radiation. In the entire Earth-atmosphere system, the amount of radiation entering the system must equal to the amount leaving, or the system would continually heat or cool. However, not all of this energy is radiative energy; some is sensible and latent heat.

Global Energy Balance 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. latentsensible heatinglatentsensible heating The atmosphere has a warming effect on Earth's surface -- the "atmospheric greenhouse effect".

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

Global Energy Balance 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 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.

Global Energy Balance 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".

Global Energy Budget 1. Atmospheric Subsystem - radiation budget: deficit - heat budget: surplus (from ground) 2. The Earth / Ground Subsystem 2. The Earth / Ground Subsystem - radiation budget: surplus - heat budget: deficit (to atmosphere) * Radiation budget and heat budget result in a balanced energy budget in both the ground / earth system. (For details, please refer to the WS p.3-5) (For details, please refer to the WS p.3-5) * Work out also the radiation budget involved.

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