During the Cretaceous epoch, which ended 65 Myr BP, surface air temperatures were higher than they are at present. This is especially true for higher latitudes. An extended interval of cooling followed the Cretacous epoch, which went along with declining CO2 concentrations. The cooling culminated in the Pleistocene glaciation, which began about 2.5 Myr ago. Plate tectonics regulating the carbonate-silicate cycle along with the glaciation of the Arctic continent around 30-15 Myr ago and its drift toward higher latitudes are believed to have been responsible for this gradual trend in cooling. Since about 65 million years ago, Earth's climate has undergone a significant and complex evolution. Gradual shifts driven by tectonic processes and periodic cycles caused by orbital rhythms are responsible for rapid shifts and extreme transitions. An example is the Late Paleocene Thermal Maximum at about 55 Myr ago, which dominates the Cenozoic era (Zachos et al. 2001). This event could be the best ancient climate analogue for future increases in atmospheric CO2, and although there has been much progress in recent years in revealing the "carbon mysteries" (Pagani et al. 2006) there are still many aspects which remain unknown. The sudden global warming around 55 Myr ago provides evidence for high climate sensitivity to atmospheric CO2. The fast changes recorded in deep-sea sediment isotope cores have especially helped to improve our perspective on the mechanisms involved in rapid alterations in the climate system. A detailed treatment of the geological evidence and paleogeography can be found in Pierrehumbert (2009). The Cretaceous hothouse climate and the Pleistocene icehouse climate represent opposite extremes of the Earth's typical climate state over the past 500 million years.
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