Earth's Radiation Balance

Earth's radiation balance is the equation of the incoming and outgoing thermal radiation.

The incoming solar radiation is short wave, therefore the equation below is called the short wave radiation balance Qs:

Qs = G - R = D + H - R or depending on the albedo (back-reflection to space): = (D+H)(1 - a)

• G = global radiation
• D = direct shortwave radiation
• H = diffuse shortwave radiation
• R = reflected portion of global radiation (ca. 4%)
• a = albedo

The Earth's surface and atmosphere emits heat radiation in the infrared spectrum, called long wave radiation. There is little overlap between the long wave radiation spectrum and the solar radiation spectrum. The equation below expresses the long wave radiation balance Ql:

Ql = AE = AO - AG

• AE = effective radiation
• AO = radiation of the Earth's surface
• AG = trapped radiation (radiation forcing, also known as the so called greenhouse effect)

The two equations on incoming and outgoing radiation can be combined to show the net total amount of radiation energy, total radiation balance Qt:

Qt = Qs - Ql = G - R - AE

The difficulty is to precisely quantify the various internal and external factors influencing the radiation balance. Internal factors include all mechanisms affecting atmospheric composition (volcanism, biological activity, land use change, human activities etc.). The main external factor is solar radiation. The sun's average luminosity changes little over time.

External and internal factors are also closely interconnected. Increased solar radiation for example results in higher average temperatures and higher water vapour content of the atmosphere. Water vapour, a heat trapping gas absorbing infrared radiation emitted by the Earth's surface, can lead to either higher temperatures through radiation forces or lower temperatures as a result of increased cloud formation and hence increased albedo.