Earth's radiation balance

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

An instrument for measuring the net radiation balance and albedo. Model shown CNR 1. Courtesy of Kipp & Zonen

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): = G (1 - a)

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

The Earth's surface and atmosphere emits heat radiation (in the infrared spectrum). There is little overlap between this and the solar spectrum. Since this is long wave radiation, this formula also is known as 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)

From those two equations for incoming and outgoing radiation, the total amount of energy now can be calculated (total radiation balance (Qt), net radiation):

Qt = Qs - Ql = G - R - AE

The difficulty is to precisely quantify the various internal and external factors influencing the radiation balance. Internal factors are all mechanisms affecting atmospheric composition (volcanism, biological activity, land use change, human activities etc.). The main external factor is solar radiation. It is interesting to note in this context that over its lifetime the sun's average luminosity to date has increased by ca. 25%.

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 vapor, 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. The situation is complicated since countless factors are involved, and therefore the concept of radiation balances is - albeit physically correct - of limited value.