The presence of these animals suggests a very warm world indeed. 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 degrees Celsius (7 degrees Fahrenheit) 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 CO 2 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 CO 2 was at 400 ppm (as it is today) was 3 million years ago during the Pliocene epoch, when sea levels were perhaps 80 feet higher than today. Clearly the climate is not yet at equilibrium for a 400-ppm world.

And it won’t be for quite some time. And anyway, we’re clearly not content to stop at just 400 ppm. If we do, in fact, push CO 2 up to around 1,000 ppm by the end of the century, the warming will persist and the earth will continue to change for what, to humans, is a practical eternity. And when the earth system finally does arrive at its equilibrium, it will most likely be in a climate state with no analog in the short evolutionary history of Homo sapiens. Most worryingly, the climate models that we depend on as a species to predict our future have largely failed to predict our sultry ancient past. And though the gulf is narrowing, and models are catching up, even those that come close to reproducing the hothouse of the early Eocene require injecting 16 times the modern level of CO 2 into the air to achieve it—far beyond the rather meager doubling or tripling of CO 2 indicated by the rock record.

Clearly we are missing something, and Naafs thinks that one of the missing ingredients in the models is methane, a powerful greenhouse gas which might help close the divide between model worlds and fossil worlds.

“We know nothing about the methane cycle during these greenhouse periods,” he says. “We know the hotter it gets the more methane comes out of these wetlands, but we know nothing about the methane cycle beyond the reach of ice cores which only goes back 800,000 years ... We know tropical wetlands pump much more methane into the atmosphere compared to [cooler] wetlands. And we know methane can actually amplify high-latitude warming, so maybe that’s some of the missing feedback.”

In many ways these ancient worlds are not analogs to our own. We have to be careful when making comparisons between the two. The early age of mammals was a different world. The continents were in slightly different positions, leading to a vastly different ocean circulation and boundary conditions quite unlike our own world, 50 million years on—with all the tectonic, oceanographic, and biological changes that come with such a yawning expanse of time. But artificially jam enough greenhouse gases into the atmosphere and Naafs thinks that many of the wildest features of the early age of mammals could be recreated.

“If we were to burn all the fossil fuels and wait a few centuries we might return to this,” he says. “Basically every type of paleoclimate research that’s being done shows that high CO 2 means that it’s very warm. And when it gets very warm, it can be really, really, really warm.”