…sea levels were 25 meters higher, the planet was a full four degrees warmer (which is much more catastrophic than it sounds), and the arctic was 11-16 degrees hotter.
Because of the lag time between CO2 and planetary warming, we're not quite at Pliocene-level conditions right now, but it's a pretty persuasive argument for getting back to 350 as soon as possible.
(infographic courtesy of our friends at Grist)