The central roles of energy and entropy

Different forms of energy and conversions among these are central to the dynamics of the Earth system and to the application of thermodynamics. Energy is defined as a property of matter and radiation that is linked to the capacity to perform work. Performing work relates to the conversion of one form of energy to another. The actual capacity to perform work is described by free energy, which is linked to the dispersal of energy across the microscopic scale of atoms and molecules and is described by its entropy. This chapter focuses on the description of different forms of energy and entropy relevant to the Earth system; but we should keep in mind that the dynamics of the Earth system are not shaped by the magnitudes of energy or entropy in the system, but rather by the conversion rates that are related to differences in energy and entropy. These conversion rates are subject to the laws of thermodynamics and are dealt with in the following chapters.

Earth system processes involve different forms of energy. Solar and terrestrial radiation involve radiative energy. Atmospheric motion is associated with kinetic energy, while cloud droplets are associated with gravitational, or simply potential, energy. Soil moisture on land is linked to the energy associated with the binding energy of water to the soil matrix and with potential energy. The concentration of constituents in air, water, and solids as well as biomass is linked to forms of chemical energy. Likewise, any other process within the Earth system is associated with some form of energy. In this chapter, the major forms of energy are described and broad estimates of their magnitude are given to illustrate how these are quantified. A more hidden aspect in these forms of energy is its spread at the scale of atoms and molecules that is described by entropy. At the microscopic scale, energy is stored in discrete, countable units. These units involve, for instance, photons, quanta of radiative energy, discrete energy levels of electrons in molecular bonds, and the distribution of kinetic energy over discrete number of molecules in a gas. When we describe Earth system processes at the planetary scale, such microscopic details are avoided to the extent possible, and aggregated, macroscopic variables, such as temperature, pressure, and density, are used instead that relate to macroscopic forms of energy.