Contrasting dynamics of past climate states and critical transitions via dimensional analysis

While past major climate transitions can be unequivocally identified, understanding of underlying mechanisms and timescales remains limited. We employ a dimensional analysis of benthic stable isotope records across different timescales to uncover how Cenozoic climatic fluctuations are associated with changes in the number of feedbacks and mechanisms involved. Our analysis indicates that warmer and colder climates respond substantially differently to orbital forcing. Notably, large numbers of feedbacks dominated during the Icehouse (3.3 Ma to present) state at obliquity and eccentricity timescales, and during the Warmhouse (66–56 Ma and 47–34 Ma) and Hothouse (56–47 Ma) states at precession timescales. During the Coolhouse (34–3.3 Ma) state the number of active feedbacks was low and had no dominant timescale. Coupling between climate signals that affect oxygen and carbon isotope records appears high only in the Icehouse state, and low to absent in all other states. We also find that anomalously high active feedback numbers and very high coupling occurred across all timescales during the Paleocene-Eocene Thermal Maximum (PETM, 56 Ma), which suggests a complete system perturbation. In conclusion, our findings challenge the notion of a simple and unique conceptual model of interconnected feedbacks in reproducing Cenozoic paleoclimate variability, given that different numbers of active feedbacks with different levels of coupling governed different timescales between climate states, which then affected the inherent (in-)stability of each climate state.