Life as the major driver of planetary geochimical disequilibrium. Reply to comments on "Life, hierarchy, and the thermodynamic machinery of planet Earth"

Authors:

Abstract:

First of all, I thank the commentators for their willingness to work through a rather long paper, and to come up with insightful commentaries [1–5]. These are excellent points for discussion on what the critical issues are in understanding the thermodynamic role of life within the Earth system, which aspects need further clarifications, and what the future needs are for researching this topic further. The commentary by Bejan [1] focuses mostly on how the paper [6] fits into the context of his “constructal law of nature” [7]. While I agree that there are aspects in common, I disagree with the apparent claim that this work is mostly explainable by his theory and that new laws of physics are needed. In fact, the description in [6] contains no new physics or approaches, it merely represents a planetary “integration” that is based on non-equilibrium thermodynamics. On the other hand, we note that Bejan’s constructal “law”, as it has been defined so far, does not constitute a rigorously formulated, testable approach that allows us to quantify the extent to which life contributes to geochemical disequilibrium at the planetary scale. Bejan describes his law as “For a finite-size flow system to persist in time (to live) its configuration must evolve such that it provides easier access to its currents” [1]. With such a definition, we must ask which thermodynamic variable expresses “access” and which units does it have? If the constructal “law” is indeed a fundamental law, then it ought to be expressible in precise and well-defined thermodynamic terms and it should relate to some form of optimization as well. Such clear definitions are absent and as such it is hard to compare his constructal law to the work presented in [6]. I do not see this lack of a precise definition as a problem per se. The Gaia hypothesis, when first conceived, was loosely formulated as well, as pointed out by its critics [8]. Likewise, the proposed principle of Maximum Entropy Production (MEP) was proposed originally without a theoretical basis. Just because ideas are not rigorously formulated does not mean that they are necessarily wrong, but it should encourage the proposers (and others) to evolve these ideas further into testable hypotheses and fundamental theory. Where Bejan’s work, or similarly phrased approaches such as fractal river basins [9] or the metabolic theory of life [10] become relevant in the context of the optimized, thermodynamic hierarchy of power generation and transfer is that in [6] I do not address the question about the generalities in how systems achieve and maintain their state of maximum power in terms of their internal organization and dynamics.

Reference:

Physics of Life Review, 7, 473-476, 2010.
Weblink to publisher's web page.