Late Pleistocene climatic trajectory in the phase space of global ice, ocean state, and CO₂: Observations and theory

We consider the evolution of three main slow-response variables describing the state of the climate system during the late Pleistocene (global ice mass, atmospheric carbon dioxide, and the ocean state) as measured, respectively, by the SPECMAP δ¹⁸O record, the Vostok CO₂ record, and the North Atlantic sea surface temperature record at 50°N (core K708-1), which is coherent with other ocean state properties. Their coevolution is portrayed in the form of a trajectory in the phase space of the three variables and its projections on its three phase planes. The oscillatory nature and phase lags of the variables are clearly illustrated, suggesting a “physical process” scenario that can account for the observations. The basic element of this scenario is a free, near-100 kyr period, oscillation driven by internal instability, involving feedbacks between all three variables under the influence of Earth-orbital (Milankovitch) forcing and long-term tectonic CO₂ forcing. It is shown that a phenomenological theory advanced by the authors, emphasizing the role of CO₂, provides a credible account of the phase-space trajectory.