Transporting mass on the planet

With the radiative forcing being described in thermodynamic terms, in this chapter, we link this forcing to motion as the next step in the cascade of energy conversions of the Earth system that was shown in Fig. 1.5. Motion transports energy, mass, and momentum between different places of the Earth system. It is through large-scale energy and mass transports associated with motion that the processes in one region affect processes elsewhere. Motion thus plays the role of the global connector, allowing regions to interact. It is this global connection that makes the Earth system a highly interactive, planetary system with large-scale material cycling of geochemical elements.

The purpose of this chapter is to describe motion, the limits that it is exposed to as well as its planetary consequences from a thermodynamic perspective. This description is quite different from the common approach in which the natural starting point is the momentum balance in the form of the Navier–Stokes equation of fluid dynamics. The view formulated here does not contradict this common approach, but rather supplements it by placing motion explicitly into the context of the thermodynamic, planetary setting. Furthermore, the consequences of motion are then evaluated to show that motion can be interpreted as the result of a system advancing to its state of thermodynamic equilibrium at an accelerated rate.

Our starting point for the description of motion is kinetic energy and the processes that generate and dissipate this form of energy. Kinetic energy is then directly related to the velocity associated with motion and the magnitudes by which thermal energy and mass is transported. The generation of kinetic energy originates from differential heating. This differential heating causes density differences in fluids, which affect the potential energy of the system. The tendency of the system to deplete its potential energy to a lower value is associated with buoyancy in the system, which then drives the onset of motion. This mechanism to generate motion is very general and applies to most forms of motion found in the atmosphere, oceans, and the solid Earth. There are, however, also other forms of motion that result from different conversions that are either indirectly related to this generation mechanism or to other drivers. An example of an indirect relationship is the wind-driven oceanic circulation, which relates to atmospheric motion and its generation.