Energy conversions, maximization, and evolution

So far, we formulated Earth system processes in terms of the different forms of energy that these involve and identified fundamental limits to the conversion of one form of energy into another. We have not yet discussed why processes should evolve to thermodynamic limits and how this evolution should take place. The goal of this chapter is to explore exactly these questions of why, how, and to what extent systems should be expected to evolve towards their thermodynamic limits.

At the core of these questions are the dynamics that take place within a system. The term dynamics is used here in a general sense on a thermodynamic system in disequilibrium that maintains fluxes that are directed towards depleting this thermodynamic disequilibrium. The extent of dynamics can thus be measured by how much free energy is being generated and dissipated. The motivation of this definition of dynamics is that a system in thermodynamic equilibrium would not show macroscopic dynamics. In contrast, a system with free energy generation can sustain fluxes and macroscopic changes taking place inside the system. As these fluxes and changes involve energy, mass, momentum, and other variables that are represented in the conjugated pairs of variables described in the earlier chapters, these dynamics relate to conversions of different forms of energy. We can thus view dynamics as the consequence of how a certain form of energy is generated and dissipated. The term evolutionary dynamics is then used here to characterize dynamics that go beyond energy, mass, and momentum balances and are specifically characterized by a change in free energy generation, dissipation, and the thermodynamic state of the system.

A general template for the conversions involved in a particular form of energy is shown in Fig. 5.1. At the top of the diagram is a driving gradient, for instance, a temperature difference, from which a certain form of free energy is generated. This form of free energy is then represented by a gradient in another variable. For kinetic energy, this gradient is represented by velocity differences within the fluid and its surroundings. Ultimately, this form of free energy is dissipated into thermal energy, with a greater amount of free energy being typically associated with a greater dissipation rate. For simplicity, conversions into other forms of energy during dissipation are not included here.