New reconstructions suggest quasi-stable states and critical transitions in climate over the past 40 million years.
Sea level and deep-sea temperature variations are key indicators of global climate changes. For continuous records over millions of years, deep-sea carbonate microfossil–based δ 18O (δ c) records are indispensable because they reflect changes in both deep-sea temperature and seawater δ 18O (δ w); the latter are related to ice volume and, thus, to sea level changes. Deep-sea temperature is usually resolved using elemental ratios in the same benthic microfossil shells used for δ c, with linear scaling of residual δ w to sea level changes. Uncertainties are large and the linear-scaling assumption remains untested. Here, we present a new process-based approach to assess relationships between changes in sea level, mean ice sheet δ 18O, and both deep-sea δ w and temperature and find distinct nonlinearity between sea level and δ w changes. Application to δ c records over the past 40 million years suggests that Earth’s climate system has complex dynamical behavior, with threshold-like adjustments (critical transitions) that separate quasi-stable deep-sea temperature and ice-volume states.