Thermodynamic foundations of the whole Earth system

We have now dealt with the basics of thermodynamics and its application to a range of dominant Earth system processes, from the radiative forcing of the planet to human activity. At the end of this book, the goal is to synthesize these thermodynamic components to a comprehensive picture of how the Earth system functions as a whole and how its dynamics is a reflection of the second law, and to illustrate what insights this picture may provide for the questions raised in the motivation for the book. This chapter closes with a perspective of the possible future directions.

Themainmotivation for formulating the Earth system in thermodynamic terms is that thermodynamics is so general that it is applicable to all Earth system processes. It thus provides a unifying basis for describing the directions, connections, and interactions of processes so that we can get an understanding of how the whole system functions and evolves. While for thermal energy and heat this thermodynamic formulation is common and straightforward, other processes such as radiation or motion are less commonly formulated in thermodynamic terms. Yet, when this is done with the use of conjugate variables, Earth system processes can be dealt with in the same units of energy, thus making them comparable, and the dynamics of these processes can be formulated in terms of energy conversions. Here, thermodynamics provides another critical component for a unifying basis. The second law of thermodynamics formulates the overall direction of these conversions and imposes a condition that results in thermodynamic conversion limits. These thermodynamic limits act as relevant constraints to the dynamics of Earth system processes. The first and second law of thermodynamics thus provide the bare minimum of essential physics to consistently formulate Earth system processes and their interactions.

Equally important in establishing a thermodynamic foundation of the Earth system is to place thermodynamics in the context of the Earth system. This was illustrated in the introduction by Fig. 1.5, which shows how the planetary forcing creates gradients that are further converted to motion, hydrologic and geochemical cycling, biotic and human activity and that result in the dynamics of the Earth system. This hierarchical view of Earth system processes allows us to separate the drivers from the driven processes and apply thermodynamic limits to these conversions.