What can be learned from large past climate variations?

This article introduces the natural factors acting on the climate system, on different time scales, and the characteristics of internal climate variability and feedbacks. The methods used to reconstruct and understand past large climate changes from the dating and analysis of natural archives and past climate numerical simulations are briefly described. The relationships between changes in atmospheric greenhouse gas concentrations and the mean temperature at the Earth's surface are investigated for very contrasted past periods, the warm phases of the Eocene and Pliocene, and the glacial climate. Past climates have also unveiled the repeated occurrence of abrupt events during glacial periods, linked with major reorganizations of the interhemispheric heat transport by the Atlantic Ocean leading to contrasted surface temperature changes in the two hemispheres. We finally depict the large-scale patterns of surface temperature changes during the present and last interglacial periods, in response to gradual changes of the Earth's orbit. All these past climates provide benchmarks to test climate models, and a framework against which recent and projected changes can be compared. (authors)