Transitions between persistent climate-carbon regimes coincide with elevated Phanerozoic biosphere vulnerability.

Linking abiotic forcing to coherent biotic responses over Phanerozoic time remains difficult to quantify because both environmental drivers and biological dynamics are heterogeneous, irregularly sampled, and affected by proxy and geochronological uncertainty. Here we introduce a regime-based description of the climate-carbon system in which Phanerozoic evolution is organized into a small number of recurring, long-lived mega-climate states separated by relatively sharp transitions. Using recurrence diagnostics on paired carbon and oxygen stable isotope records, together with complementary early-warning indicators, we identify five persistent regimes and their boundaries. We interpret the resulting structure with a conceptual climate-carbon cycle model that admits multiple coexisting equilibria, providing a mechanistic basis for state changes that switch between stable attractors under changing environmental forcings and perturbations. We connect these results to a biosphere vulnerability metric that incorporates biotic turnover with sample-standardized global diversity to show that vulnerability is systematically elevated near major regime transitions. These results suggest that transitions between persistent climate-carbon attractors provide a dynamical context for why some perturbation intervals coincide with heightened biospheric stress, beyond what is expected from more gradual background environmental changes alone.