| The study of high-resolution oxygen isotope (18O) records from the Cenozoic, the last 65 million years (Ma) of Earth?s history, has revolutionized ideas about the evolution of the oceans and the quasi-periodic response of the climate system to astronomical (Milankovitch) forcing. Among the most intriguing questions that emerged is why the response of global climate to astronomical forcing varied with different atmospheric pCO2 concentrations, while variations in solar insolation remained unchanged. Current explanations call for a nonlinear response of the climate system to insolation forcing or a stochastic process of any kind within ice sheet, oceans or carbon reservoirs. Here, I propose to constrain these mechanisms, using an integrated data-modeling comparison approach applied to four key intervals, which experienced different background pCO2 concentrations. These intervals comprise: (1) the onset of major Northern Hemisphere glaciations (~3 Ma) when pCO2 values declined below pre-industrial (~280 p.p.m.v.) values, (2) the mid-Pleistocene transition when dominant obliquity-paced glacial cycles culminated into a ~100-kyr rhythm, (3) the evolution of the Antarctic ice sheets from the Oligocene to the mid-Miocene (~25-12 Ma) when background pCO2 values were still well above pre-industrial values, and (4) the warmest period of the Cenozoic (~55-50 Ma), when pCO2 reached values of more than four times the present-day (~385 p.p.m.v.) value. Emphasis of the proposed research is on the amplitude, timing and rates of deep-sea temperature and ice volume changes and the long-term modulations of the carbon cycle in relation to the occurrence of punctuated glacial periods and extreme global warming events (hyperthermals). The proposed integration of climate modeling experiments with geological observations will not only lead to a much better understanding of astronomical forcing during past states of the climate system, but may serve as a template for improving current projections directed at the rates of sea level change and (deep-sea) temperatures for the coming centuries. |
