An elementary form of agency at planetary-scale

Abstract The claim that climate homeorhesis—non-equilibrium steady-state dynamics that underlie Earth’s long-term habitability—depends on an integrated biosphere–climate system capable of acting on its own is central to debates surrounding the Gaia hypothesis. This paper presents a proof of concept suggesting a minimal form of planetary agency based on a simple biosphere–climate system model operating through active inference. This means that the internal states of the system ( biosphere ) will appear to infer shifts in net incoming solar radiation ( external states ) through its boundary states , i.e. by integrating past and current surface temperatures ( sensory states) and actively changing greenhouse forcing ( active states ) to maintain climate trajectories within geophysiological bounds. When surface temperature is altered by solar fluctuation changes, the system adjusts greenhouse and albedo parameters to restore expected climate trajectories, analogous to correcting sensory prediction errors in biological systems. Perturbing active states (e.g., imposed changes in albedo) does not reduce inference precision. Inference robustness decreases when the system’s degrees of freedom increase. Active inference falls, and thus climate homeorhesis fails when there is no biosphere activity (lifeless planet) or when it is decoupled from the climate system. Predictive behaviour operates within the range of solar constants consistent with life’s presence on Earth (~ 4 Ga to ~ 1.5 Ga expected in the future); outside this range, inference fails, undermining a putative form of planetary agency.