Solar and terrestrial radiation

Whenever the solar radiation reaching the surface increases, the warmer surface will emit more terrestrial (heat) radiation and counteract further warming. In a very simple radiation balance model this can be expressed by the energy balance of a sphere with radius R

Emission = Absorption

The average temperature TB of the planet treated as a blackbody radiator, emitting a flux density F according to Stefan-Boltzmann's law with F = c T4, is only determined by solar flux density, So, reaching the Earth, often called solar constant or solar irradiance, diminished by the Earth's albedo a. Using So = 1367 Wm-2, a = 0.3 and c = 5.67 • 10-8 Wm-2 K-4, we get a temperature Tb of 255 K. Thus the radiation to space originating from the Earth is equivalent to a blackbody with a temperature of about -18°C. This temperature occurs in the atmosphere on average at about 5 km height, thus indicating that emission to space originates largely in the atmosphere and stems less from the surface, whose average temperature is 288 K. Hence the greenhouse effect of the atmosphere is about 33 K.

Increasing So by one percent leads to A TB ~ + 1 K. The interplay between solar and terrestrial radiation is a strong constraint for the planetary radiation budget. It also becomes clear from equation 2. 1 that the climate system could counteract solar radiation changes by changes in planetary albedo. As the latter is to a large degree determined by the clouds in the atmosphere a potentially strong negative feedback is less cloud reflection in glacials and more in interglacials or warmer climate periods without any continent-wide glaciation. Whether this is true we do not know yet.

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