Beyond Gaia: Thermodynamics of life and Earth system functioning

Authors:

Axel Kleidon

Abstract:

Are there any general principles that govern the way in which life affects Earth system functioning? Most prominently, the Gaia hypothesis addresses this question by proposing that near-homeostatic conditions on Earth have been maintained “by and for the biosphere”. Here the role of the biota in the Earth system is described from a viewpoint of non-equilibrium thermodynamics, particularly with respect to the hypothesis of Maximum Entropy Production (MEP). It is argued that the biota introduce additional degrees of freedom to Earth system processes. Therefore we should expect biotic activity, and Earth system processes affected by the biota, to evolve to states of MEP. The consistent effects of the biota on entropy production are demonstrated with a conceptual model of biogeochemical cycling, by using extreme climate model simulations of a “Desert World” and a “Green Planet”, and by a simple coupled climate-carbon cycle model. It is shown that homeostatic behavior can emerge from a state of MEP associated with the planetary albedo. This thermodynamic perspective is then discussed in the context of the original Gaia hypothesis and in light of a recent discussion in Climatic Change. Potential implications of the MEP hypothesis for global change research are also discussed. It is concluded that the resulting behavior of a biotic Earth at a state of MEP may well lead to near-homeostatic behavior of the Earth system on long time scales, as stated by the Gaia hypothesis. However, here homeostasis is a result of the application of the MEP hypothesis to biotically-influenced processes rather than a postulate. Besides providing a fundamental perspective on homeostasis, the MEP hypothesis also provides a framework to understand why photosynthetic life would be a highly probable emergent characteristic of the Earth system and why the diversity of life is an important characteristic of Earth system functioning.

See also commentaries by:

  • Tyler Volk: The properties of organisms are not tunable parameters selected because they create maximum entropy production on the biosphere scale: A by-product framework in response to Kleidon. >> more
  • Axel Kleidon: Thermodynamics and environmental constraints make the biosphere predictable – a response to Volk. >> more and here
  • Ken Caldeira: The maximum entropy principle: A critical discussion. >> more

Reference:

  • Climatic Change, 66 (3), 271-319.
  • Weblink to publisher's web page.
  • Postprint of this manuscript (accepted version of the paper formatted by author).
  • BibTex entry.

Figure 1: An estimate of the global entropy production budget of the Earth after Peixoto et al. (1991). The entropy production of the latent heat flux differs from Peixoto et al’s value and has been calculated as 79 W m-2 (1/266K - 1/288K).

Figure 2a: Polar heat transport as an example for a process close to a state of maximum entropy production (MEP). (a) Entropy production associated with polar heat transport σHT (dotted line) and total entropy production σTOT (solid line) as a function of heat conductivity k as simulated by the two box model described in the text.

Figure 2b: Polar heat transport as an example for a process close to a state of maximum entropy production (MEP). (b) Polar heat transport QHT (solid line) and equator-pole temperature difference TT - TP (dotted line) as a function of heat conductivity k. The plots were obtained by using QIN,T = 288 W/m2 and QIN,P = 192 W/m2.

Figure 3a: Turbulent heat fluxes (i.e. sensible and latent heat flux) between the Earth’s surface and the atmosphere as an example for a process close to a state of MEP. (a) Entropy production associated with turbulent heat fluxes σTF (dotted line) and total entropy production σTOT (solid line) as a function of heat conductivity k as simulated by the two box model described in the text.

Figure 3b: Turbulent heat fluxes (i.e. sensible and latent heat flux) between the Earth’s surface and the atmosphere as an example for a process close to a state of MEP. (b) Turbulent heat transport QTF (solid line) and surface-atmosphere temperature difference TS - TA (dotted line) as a function of heat conductivity k.

Figure 3c: Turbulent heat fluxes (i.e. sensible and latent heat flux) between the Earth’s surface and the atmosphere as an example for a process close to a state of MEP. (c) Entropy production σTF a function of the ratio of turbulent heat fluxes to the net longwave radiative flux for three different values of atmospheric absorptivity ε (dotted: ε = 0.25; dashed: ε = 0.5; solid: ε = 0.75). The plots were obtained by using QIN = 240 W/m2 and ε = 0.5 for plots (a) and (b).

Figure 4a: Simulated climate of the simple climate model described in section 3.4. (a) Planetary albedo αP as a function of atmospheric carbon dioxide concentration pCO2 for current solar luminosity (solid line) and 70% of its value (dotted line).

Figure 4b: Simulated climate of the simple climate model described in section 3.4. (b) same as (a), but surface temperature TS.

Figure 4c: Simulated climate of the simple climate model described in section 3.4. (c) same as (a), but total entropy production sTOT. Present-day luminosity is taken as L = 1370 W m-2.

Figure 5a: Homeostatic behavior emerging from a state of MEP associated with a biotically mediated carbon cycle. (a) Value of the respiration coefficient ρ (solid line) at which biotic entropy production is maximized and the associated atmospheric carbon dioxide concentration pCO2 (dotted line) as a function of solar luminosity L (expressed as a fraction of the present-day value).

Figure 5b: Homeostatic behavior emerging from a state of MEP associated with a biotically mediated carbon cycle. (b) total entropy production σTOT associated with an optimally chosen value of ρ (solid line) and for the extreme cases of high pCO2 (pCO2 = 1, dotted line) and low pCO2 (pCO2 = 10-6, dashed line).