RadTech 1895: How Earth Budgets Its Energy Through Radiation

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 is emitted to space. Scientists refer to this process as Earth's "radiation budget".

On the Moon where there is no atmosphere, a surface temperature far below freezing emits enough radiation to balance the absorbed solar energy.

Because of the tilt of the Earth's axis, incoming solar radiation is not evenly distributed on the Earth's surface and seasonal changes occur.

The Sun is not in the exact center of the Earth's orbit. During the Southern hemisphere summer the Earth is closer to the Sun than during the Northern hemisphere summer. The Earth is farthest from the Sun during the Southern hemisphere winter.

As the Sun's electromagnetic radiation penetrates the Earth's atmosphere it is selectively absorbed and scattered by molecules of gases, liquids, and solids.

The energy coming from the Sun to the Earth's surface is called solar insolation 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).

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

Reflection:

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 percentage of solar energy that is reflected back to space is called the albedo.
Different surfaces have different albedos. Over the whole surface of the Earth, about 30 percent of incoming solar energy is reflected back to space.
Ocean surfaces (26% albedo) and rain forests (15% albedo) reflect only a small portion of the Sun's energy.
Deserts however, have high albedos (40%); they reflect a large portion of the Sun's energy.
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.

Emission:

Another part of the energy going back to space from the Earth is the longwave radiation emitted by the Earth. The solar radiation absorbed by the Earth increases the planet's temperature. Heat energy is emitted into space, creating a balance.

A cloud can absorb radiation emitted by the Earth's surface and radiates in all directions.
The longwave energy emitted from the surface of the Earth and absorbed by the atmosphere results in an increase in the ambient temperature (i.e., the greenhouse effect). This absorbed energy is then emitted both to space and back towards the Earth's surface.

The greenhouse effect or the global warming is due mainly to water vapor in the atmosphere. This effect is enhanced by carbon dioxide, methane and other infrared-absorbing gases.

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