About 50 million years ago, when mammals had just started taking over the planet, atmospheric CO2 levels were about 1000 ppm. And wow, was it hot here:
[A]round 50 million years ago there were crocodiles, palm trees, and sand tiger sharks in the Arctic Circle. On the other side of the blue-green orb, in waters that today would surround Antarctica, sea-surface temperatures might have topped an unthinkable 86degrees Fahrenheit, with near-tropical forests on Antarctica itself. There were perhaps even sprawling, febrile dead zones spanning the tropics, too hot even for animal or plant life of any sort.
[U]nder this past regime of high CO2, in the ancient U.K., Germany, and New Zealand, life endured mean annual temperatures of 23–29°C or 10–15°C warmer than modern times.
And yet, there is a seeming disconnect, between traditional projections for future warming—like those made by the International Panel on Climate Change (IPCC), which predicts around 4°C of warming by the end of the century under a business-as-usual emissions scenario (still frightening) and sea-level rise measured in mere inches (still frightening)—and the scarcely recognizable Earths buried in the rocks and created under similar CO2 regimes, like those that Eberle unearths.
One obvious way to reconcile this disparity is by noticing that the changes to the ancient earth took place over hundreds-of-thousands to millions of years and (IPCC graphs notwithstanding) that time won’t stop at the end of the 21st century. The changes that we’ve already set in motion, unless we act rapidly to countervail them, will similarly take millennia to fully unfold. The last time CO2 was at 400 ppm (as it is today) was 3 million years ago during the Pliocene epoch, when sea levels were perhaps 24 m higher than today. Clearly the climate is not yet at equilibrium for a 400-ppm world.
Good thing we've stopped pumping CO2 into the atmosphere! Oh, wait...