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.
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