Modeling earth system complexity : theoretical biology approaches to the Gaia hypothesis

The Gaia hypothesis is best presented as “the hypothesis that supposed the Earth to be alive…as a living organism”; therefore a necessary and sufficient characterization of “what is a living organism?” is required to address this hypothesis. The cell is the minimal organism that has been formally characterised as self-fabricating causal organization and constructive dynamics capable of active inference of its environment by the theoretical biology approaches of autopoiesis, the (M,R)-system, and the Free Energy Principle. Modelling the Earth complexity while taking the Gaia hypothesis at face value –the Earth is complex because it instantiates life at the planetary scale– can be, thus, addressed by the same the theoretical biology approaches. A first proof-of-concept based on Chemical Organization Theory and the Zero Deficiency Theorem applied to Earth's biotic and abiotic reaction networks shows that at molecular level there are Earth’s reaction networks organized as an autopoietic system, i.e. able to self-fabricate. A second proof-of-concept based on the Free Energy Principle–here active inference, shows that a biosphere-climate dynamical system model can remain at non-equilibrium steady-state climate dynamics by active inference of net incoming solar radiation. Finally, considering that Earth system components and operational processes satisfies the formal entailments of the (M,R)-system would mean that its complexity has a formal equivalence to a self-referential (impredicative) system, which cannot, in principle, be completely surrogated by an algorithmic representation. Yet, it may indicate that potentially the Earth components could have multiple functions and that the Earth’s climate dynamics could be context-dependent. These theoretical biology approaches open up the plausibility that the 4-billon years of Earth’s habitability (climate homeorhesis) has occurred by autopoiesis and active inference at the planetary level and that the Earth system is likely to be akin to a living system, which must be borne in mind in future Earth's climate modelling.