Beyond Gaia: Thermodynamics of Life and Earth System Functioning

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 nonequilibrium 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.