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Explore the balance between incoming solar radiation and outgoing terrestrial radiation in the Earth's energy system. Learn about the greenhouse effect and its impact on maintaining Earth's temperature.
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The Earth’s Energy Balance • The Earth's average temperatureremains 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, ozoneselectively absorbs ultraviolet radiation while allowing visible radiation to pass through relatively unhindered. • Reflectionsignificantly 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.207-208)
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 issensible 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. • 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. • Because Earth does have an atmosphere, the average surface temperatureactually is 15 degrees Celsius.
Global Energy Balance • The atmosphere acts as a greenhouse because of gases that selectivelyallow 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 selectiveas 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 - 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) * Work out also the radiation budget involved.
