The main driver of the Earth system

Radiation – obviously – plays a major role in the Earth system. It is by far the most important driver for energy conversions on Earth, both in terms of the sheer magnitude of the energy flux of solar radiation, and also in terms of its quality, as solar radiation represents radiation with a very low entropy. In the thermodynamic view of the Earth system shown in Fig. 1.5, it is the start point and endpoint for most energy conversions taking place within the Earth system. Spatial and temporal variations in the absorption of solar radiation are the causes for various heat fluxes and associated dynamics that distribute imbalances in radiative heating and cooling rates. Variations in radiative forcing in combination with dynamics shape most of the observed climatic variations, from the seasons in mid-latitudes to the large-scale variation of surface temperature from the tropics to the poles. The focus of this chapter is to describe the thermodynamic nature of radiation which is then used to understand the dissipative nature of radiative transfer processes, to derive the limits of energy conversions from radiation to other forms, and to describe radiative transfer as a dominant process shaping the environmental conditions that affect other energy conversion processes.

In thermodynamic terms, the radiative exchange between the Earth and space generates the most important driving gradients, and it exports the entropy that is being produced by Earth system processes to space. The entropy of radiation is represented mostly by the spectral composition of the radiative flux. The spectral compositions of the incoming solar radiation and the outgoing radiation from the Earth system are shown in Fig. 6.1. Solar radiation is composed mostly of visible light which is characterized by relatively short wavelengths. This spectral composition essentially corresponds to the composition of the radiation when it was emitted from the Sun at a high temperature of about 5760 K. When solar radiation is absorbed by the Earth system, it is subsequently reemitted to space at a much lower temperature of about 255 K, which is approximately the Earth's radiative temperature. The associated spectral composition of the emitted radiation is centered at much greater wavelengths in the infrared, so that this radiative flux has a markedly different spectral composition.