Spreading the power: comment on "Life, hierarchy, and the thermodynamic machinery of planet Earth" by A. Kleidon.

Kleidon’s ambitious review [1] of the thermodynamic machinery of Earth starts out as an accessible primer in non-equilibrium thermodynamics (this is necessary because entropy is probably the least understood and most central concept to the physics of life). He then builds a comprehensive model of how solar power spreads around the Earth through a hierarchy of levels – a trophic pyramid of power. Such a comprehensive analysis integrating various components into one big picture of our planet is refreshingly holistic and overdue. Usually ecologically-minded scientists analyse either ocean currents, or solar radiation, or erosion, or the hydrological cycle, or wind speeds, or cloud cover, or transpiration. Kleidon considers them all. The Earth is in a state of disequilibrium. Winds blow, the mantle convects, continents drift, rivers flow and life grows. To keep all of these processes going requires an input of power from the Sun and from the interior heat of the Earth. Kleidon quantitatively follows the input power as it spreads through the thermodynamic machinery of the Earth, maintaining a steady state of disequilibrium which powers all these processes. Solar radiation on the spherical Earth maintains an equator to pole temperature gradient and evaporates (desalinates) the ocean. This temperature gradient drives winds (“momentum gradients”), which transport humid ocean air up and over the continents (“potential gradients”). The resulting rain erodes mountains, carves river beds and dumps everything into the sea (“geochemical gradient”). All this takes a lot of power. And Kleidon uses a lot of accounting and math to keep track of the spreading power, just as one might keep track of a trail of laundered money, or the branching ratios of decaying particles. Ever since Lovelock introduced a Gaian world view, climate scientists have been wrestling with teleology, homeostasis, optimality (e.g. [3]) and the maximum entropy production principle (MEPP). Kleidon has been at the leading scientifically-respectable edge of the Gaian apologists and the MEPP advocates [2]. Kleidon’s new review contributes constructively to the Gaian and MEPP debate: “Using thermodynamics, we should be able to quantify the extent to