Thermodynamic roles of water

So far we have dealt with radiative forcing and how differences in radiative heating result in motion that depletes differential heating, thereby following the second law. Our next step in the sequence of energy conversions from radiation to Earth system processes shown in Fig. 1.5 is to relate the differential heating and the resulting motion to hydrologic cycling and associated thermodynamic directions and limits.

There are two aspects closely related to water that greatly shape the thermodynamic setting of the Earth system. First, hydrologic cycling involves energy conversions of substantial magnitude that are associated with the different phases of water, so that the latent heat involved in the different phases contributes a considerable share of the heat fluxes in the atmosphere. Second, the presence of water in its different phases is associated with the abundance of ice and clouds, two aspects that greatly affect the albedo, the magnitude of the solar radiative forcing, and thereby the radiative environment of the Earth system. Furthermore, water vapor in the atmosphere and clouds contributes substantially to the atmospheric greenhouse effect, so that hydrologic cycling affects both absorption and reflection of solar radiation as well as the radiative transfer of terrestrial radiation. These highly relevant aspects illustrate how water and the magnitude of its cycling affects the overall thermodynamic state of the Earth system.

The thermodynamic treatment of hydrologic cycling starts with the thermodynamics of phase transitions. The different phases of water – solid, liquid, and gas – correspond to different intensities by which water molecules are bound to each other. The water molecules are bound most strongly in their solid state, and are unbound when in the gaseous state. When we consider the entropy of a system in which water is present in two states, for instance, liquid water and vapor then the total energy of the system involves thermal energy, but also uncompensated heat related to the water vapor pressure as well as intermolecular binding energies. Binding energies are described in thermodynamics by the mass and the respective chemical potential, as introduced in Section 2.3. The total energy of the system is thus spread over the thermal energy, the pressure of water vapor, and across the intermolecular bonds in the liquid state.