Back to the Miocene

It’s getting harder and harder to find something optimistic about. Seeing science knee-capped and obnoxious snots under Musk’s employ rifling through the IRS files and plotting to destroy Social Security (Hey! That’s my money! It’s not for billionaires to steal) is incredibly discouraging. I found something that looks on the bright side of climate change, though. The Miocene might be a good model for our future.

The Miocene, roughly 5-20 million years ago, had CO₂ levels similar to where we’re going as we blast past recommended limit. It was generally warmer and wetter! That has some appeal as I sit here in a region at -30°C. It wasn’t a terrible world at all — primates were diverse and thriving, we had all these interesting mammals, “From Dryopithecus, a lineage of extinct primates that included forerunners of humans, to the toxodonts, large-hoofed mammals with long, curved incisors, to mammals similar to sloths, armadillos and anteaters, to marsupial carnivores”…it was great!

Significantly, the Miocene was a nearly 18 million year epoch full of change, albeit far slower change than ours. It started with a period of glaciation that must have been a chilly change from the greenhouse-like Oligocene, and ended with a prolonged period of glaciation, too. But through much of the Miocene, it was a warm world compared to today’s, a high CO2 planet that gradually cooled over millions of years until ice sheets developed in the Northern Hemisphere and Antarctica.

Around the middle of the epoch, we reached what is called the Miocene Climate Optimum (MCO), a roughly two million year-long greenhouse period when the world experienced its last period of sustained warmth, and the CO2 level was at least 500 ppm. This is the period we’re talking about, most specifically, when we talk about the Miocene as a proxy for our future, although changes throughout the Miocene are relevant: basically, from the middle Miocene Earth went through a process roughly opposite the one we are experiencing (and causing) today.

See? How can you dislike something called a “Climate Optimum”? It looks like paradise! Sign me up — these Minnesota prairies would be so exciting with little horses and hippos and Thylacosmilusand chalicotheres gamboling about in the lush vegetation.

The plants are going to love it.

Carbon dioxide levels affect plants by allowing for greater photosynthesis rates, and by increasing water use efficiency, in that plants can achieve the same amount of photosynthesis with less loss of water through the pores in their leaves, because higher availability of CO2 absorbed through open pores means they can keep them closed more of the time. Thanks to all this, it was also “a globally greener Miocene world,” as Reichgelt and West write in the 2025 paper. Various forms of evidence suggest that the biosphere was more productive during the Miocene compared to now, and that at higher latitudes, this effect was more pronounced.

Except for one major problem: evolution does not run backwards. No chalicotheres await us, especially since we’d be entering a neo-Miocene with a depauperate fauna.

Sadly, the taxodonts will not grace our future world. The long-armed, horsey Chalicotheriidae, reminiscent of Bojack Horseman, won’t be joining us at the bar. Smilodon, the catty predator whose ancestors emerged in the early Miocene, will not smile on us again. Nor the “bizarrely specialized” family of carnivorous marsupials, Malleodectidae, which used their massive ball peen-like third premolars to crush snails. Not the dog bears, Hemicyoninae, who emerged before and lived through the Miocene, nor the bear dogs, Amphicyonidae, which died out by the late Miocene. Evolution doesn’t work like that. Barring the odd de-extinction attempt, what’s lost is gone forever (that includes, thank goodness, the terror birds.)

Expect wild pigs and deer, already doing well, and novel species exploring new environments: I expect the descendants of raccoons and rats to thrive. Humans, not so much. We don’t do so well in the face of widespread environmental disruption, we like nice stable tame-able places where we can rely on crops to come in dependably. We’ll be starting with ecological wreckage and then amplifying the swings of climate and weather, which is a recipe for radical destabilization.

It’s also possible that we’re being seduced by the idea that the Miocene might represent a “happy medium.” As Steinthorsdottir and colleagues write, “More pessimistic scenarios of unmitigated greenhouse gas emissions quickly move us beyond the Pliocene state, pushing Earth’s systems into a potentially vulnerable position where many of its ‘tippable’ subsystems such as glaciers, sea ice, forest biomes, deserts and coral reefs will be permanently destabilized […] an ‘intermediate’ deep-time climate analog, where boundary conditions are close to modern but extreme climate changes occurred, is therefore of great interest.”

As humans we have a notorious tendency to believe that whatever’s in the middle of two given extremes is moderate, cozy, all around OK. (In politics, this results in the Overton Window.) But Miocene-style hydrological or water cycles favor high altitude wind events, like cyclones and hurricanes, that transport heat and moisture evaporating from the tropics to higher latitudes, or California’s intense seasonal rainstorms. The future may be lush, sure, but it’ll also be erratic and dangerous for us. And the “tippable” subsystems Steinthorsdottir mentions may have tipping points that occur well within a Miocene-like context, as scientists have warned.

Whenever a paleoclimatologist tells you a scenario is “of great interest”, it’s time to run.

Sorry. I told you it’s hard to find anything to be optimistic about.