The thermodynamic setting of land

The next step in describing the thermodynamics of the Earth system deals with the conditions on land. The land surface – in contrast to the oceanic surface – has a particular relevance in the Earth system. While it covers less than a third of the Earth's surface, it harbors a disproportionately large share of the biosphere, culminating in the lush and highly productive tropical rainforests that show among the highest levels of photosynthetic activity on the planet. These high levels of photosynthetic activity on land are achieved by large, complex, and highly organized vascular plants rather than by small and comparatively primitive microorganisms that are the primary producers of the oceans. On land, the high rates of photosynthetic activity are associated with a physical imprint on the characteristics of the surface. Note how different the land surface covered by rainforest is compared to an ocean surface, which is almost entirely described purely by its physical state, as exemplified by Fig. 10.1. On land, forest canopies provide dark and heterogeneous surfaces which absorb solar radiation while their root systems reach deep into the soil where they are able to extract water and transport it into the canopies to sustain an evaporative flux into the atmosphere. By dominating the absorption of solar radiation and evaporation, forests shape the partitioning of the surface energy balance. Biotic effects on the functioning of the Earth system are thus particularly strong at the land surface. In this chapter, we want to understand the conditions that allow for and favor these strong biotic effects and how these feed back to biotic activity from the insights gained so far from thermodynamics.

We have seen in the previous chapters that the partitioning of absorbed solar radiation into radiative and convective cooling in the surface energy balance is constrained by the maximum power limit as it results from a close interaction of the convective heat flux with its driving temperature difference between the surface and the atmosphere. Hence, the effects of forests on the surface energy balance extend further into the atmosphere and have the potential to alter the thermodynamic limit of the surface–atmosphere system. As convection drives the mass exchange between the surface and the atmosphere, this potentially feeds back on the gas exchange between vegetation and the atmosphere and thus on the level of biotic activity, as was hypothesized in the previous chapter.