All this is not to say that if we sustain 400 ppm in the atmosphere we are destined to have a world like the Oligocene, as other factors may have contributed to global warmth then. Other greenhouse gases especially methane may have contributed to global warmth, as may the very different pattern of ocean and atmospheric circulation. We cannot use the past to generate simplistic links and predictions or try to prove or disprove the greenhouse effect which is the realm of modern atmospheric physics.
But if there is one simple message it is this: The geological record gives us no reassurance that 400 ppm of CO 2 in the atmosphere is a safe level to maintain in the long term( safe, in terms of preserving our continental ice caps). Only climate models with high sensitivity to CO 2 seem to be able to capture the main features of past geological warm climates like the Oligocene and Miocene. And of course halting pCO 2 at 400 ppm is not going to happen.
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