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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