How and why did genuine global changes happen? What were their local and global consequences on the physical environment, the ecosystems and the societies? What thresholds are involved?

Without paleoclimatic information, we would not know that atmospheric CO2 can vary naturally by up to 100 ppm on glacial-interglacial times, that abrupt climatic changes did occur on annual or decadal scales, that ice sheets may disrupt very rapidly, and basically that climate can change at all. These testimonies of how our Earth system is functioning are invaluable, yet still quite sparse and often not so well understood. They will likely deliver numerous further surprises. A great variety of climatic changes occured in the past, with many different amplitudes or consequences, and on many different time scales. When exceeding some thresholds, they were able to induce changes on the environment of past ecosystems and societies. These events should be traced back and quantified, before we can claim that we are in a position to predict future changes and their impacts.

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