Earth's Radiadion Budget

• Models illustrate the strong links among the radiation budget, the climate, and circulation of the atmosphere and the oceans.

• Simple model diagram

• Heat (heat energy) is the total kenetic energy of all the atoms in a substance.

• The Earth's climate system constantly tries to maintain a balance between the energy that reaches the Earth from the sun and the energy that goes from Earth back out to space.

• Scientists refer to this process as Earth's "radiation budget."

• Radiation budget diagram

• Earth's Radiation Budget Experiment image
  • The heat budget for the Earth is similar to balancing a checkbook.
    • Deposits are Sources

    • Withdraws are Sinks

• Insolation is the energy that reaches the surface.

• The flux of energy coming in from the Sun is 1370 W/m^2

• Heat is not evenly distributed on Earth's surface
  • Tropics receive large amounts of heat.

  • High latitudes receive small amounts of heat.

• As the sun's electromagnetic radiation penetrates the Earth's atmosphere it is selectivly adsorbed and scattered by molecules of gases, liquids, and solids.
  • The high energy X-rays and gamma rays are absorbed by molecules nitrogen and oxygen. At an altitude of 88 km (55 mi), the absorption is almost complete.

  • In the absorption of ultraviolet radiation, the atom or molecule gives up an electron to become a positvely charged ion.

• The components of the Earth system that are important to the radiation budget are the planet's surface (geosphere and oceans), atmosphere, and clouds.

• The energy coming from the sun to the Earth's surface is called solar or "shortwave" energy
  • Both the amount of energy and the wavelengths at which energy is emitted by any system are controlled by the average temperature of the system's radiating surfaces.

  • The temperature of the sun's radiating surface, or photosphere, is more than 5500 degree C (9900 degree F). However, not all of the sun's energy comes to Earth. The sun's energy is emitted in all directions, with only a small fraction being in the direction of the Earth.

  • The Earth's and Sun's electromagnetic energy diagram.

• Energy goes back to space from the Earth system in two ways: reflection and emission.

• Part of the solar energy that comes to Earth is reflected back out to space in the same, short wavelengths in which it came to Earth.

• The fraction of solar energy that is reflected back to space is called the albedo.

• Different parts of the Earth have different albedos.

• Table - Albedo of various surfaces
  • Ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the sun's energy.

  • Deserts and clouds, however, have high albedos; they reflect a large portion of the sun's energy. Over the whole surface of the Earth, about 30 percent of incoming solar energy is reflected back to space.

  • A cloud usually has a higher albedo than the surface beneath it, the cloud reflects more shortwave radiation back to space than the surface would in the absence of the cloud, thus leaving less solar energy available to heat the surface and atmosphere.

• Another part of the energy going back to space from the Earth is the electromagnetic radiation emitted by the Earth.

• The solar radiation absorbed by the Earth causes the planet to heat up until it is emitting as much energy back into space as it absorbs from the sun.

• Because the Earth is absorbing only a tiny fraction of the sun's energy, it remains cooler than the sun, and therefore emits much less radiation.

• Most of this radiation is at longer wavelengths than solar radiation. Unlike solar radiation, which is mostly at wavelengths visible to the human eye, the Earth's longwave radiation is mostly at infrared wavelengths, which are invisible to the human eye.

• When a cloud absorbs longwave radiation emitted by the Earth's surface, the cloud reemits a portion of the energy to outer space and a portion back toward the surface.

• If the Earth had no atmosphere, a surface temperature far below freezing would produce enough emitted radiation to balance the absorbed solar energy.

• Clear air is largely transparent to incoming shortwave solar radiation and, hence, transmits it to the Earth's surface. However, a significant fraction of the longwave radiation emitted by the surface is absorbed by the air.

• This heats the air and causes it to radiate energy both out to space and back toward the Earth's surface. The energy emitted back to the surface causes it to heat up more in order to emit enough radiation to balance the added amount it receives from the air.

• This heating effect of air on the surface, called the atmospheric greenhouse effect, is due mainly to water vapor in the air, but also is enhanced by carbon dioxide, methane, and other infrared-absorbing gases.

• In addition to the warming effect of clear air, clouds in the atmosphere help to moderate the Earth's temperature. The balance of the opposing cloud albedo and cloud greenhouse forcings determines whether a certain cloud type will add to the air's natural warming of the Earth's surface or produce a cooling effect.

Energy Loses in the Lower Atmosphere

• Rayleigh Scattering
  • Lord Rayleigh (1842-1919), English physicist

  • If the diameters of molecules are less than one-tenth the wavelength of the incoming radiation, as they are in gas molecules, scattering is governed by the Rayleigh Law.

  • Rayleigh's Law states that the amount of scattering is proportional to 1/(wavelength)^4.

  • Greater scattering for short-wavelengths

  • What we see when we look at the sky is scattered blue radiation

• Heat can be absorbed by:
  • Water vapor, dust, and gases (16%)

  • Clouds (3%)

  • Oceans and land (51%)