One more excerpt from a book that simply has to go back to the library:
What the Vostok record shows is that the planet is already nearly as warm as it has been at any point in the last 420,000 years. A possible consequence of even a four- or five-degree temperature rise--on the low end of projections for the end of this century--is that the world will enter a completely new climate regime, one with which modern humans have no prior experience. When it comes to carbon dioxide, meanwhile, the evidence is even more striking. The Vostok record demonstrates that, at 378 parts per million, current CO2 levels are unprecedented in recent geological history. (The previous high, of 299 parts per million, was reached around 325,000 years ago). It is believed that the last time carbon dioxide levels were comparable to today's was three and a half million years ago, during what is known as the mid-Pliocene warm period, and it is likely that they have not been much higher since the Eocene, some fifty million years ago. In the Eocene, crocodiles roamed the Colorado and sea levels were nearly three hundred feet higher than they are today. A scientist with the National Oceanic and Atmospheric Administration (NOAA) put it to me--only half-jokingly--this way: "It's true that we've had higher CO2 levels before. But, then, of course, we also had dinosaurs."
--Elizabeth Kolbert, Field Notes from a Catastrophe