Dr. Michael E. Mann recounts a story in his recent book (the Hockey Stick and the Climate Wars) about an encounter with Hillary Clinton during a hostile congressional hearing. While he was fending off hostile questions from senators like Senator James Inhofe (who described climate change as "the single greatest hoax ever perpetuated on the American public") Hillary Clinton asked a (friendly) question along the lines of "what was the world like the last time carbon dioxide levels were this high?" (the story is on pages 119-120 of the hardback version of Mann's book).
Mann responded thus:
We have to go back to the time of the dinosaurs probably to find CO2 levels that we know were significantly higher than the CO2 levels today.
This struck me as being something of a spur of the moment exaggeration (I still remember some of my training in geology), a view at least supported by the Wikipedia summary. Mann neither gives a source nor a comment on his response.
So my question is: has current thinking about historic CO2 levels changed? Do we have more CO2 than at any time since the Cretaceous when dinosaurs still roamed the earth? Or was Mann just using hyperbole for effect?
NB: This isn't a question about the effects of CO2 or about the broader issues of climate change. It is just about the specific topic of the history of the atmospheric level of CO2.