Why did geologists reject the “Anthropocene” epoch? It’s not rock science.

Scientists have dealt the final blow in a long-running fight over one big question: Have humans messed up the Earth so badly that we’re now living in a new climate epoch?

For 15 years, an intrepid band of geologists has been trying to argue exactly that. They claimed that humanity has ushered in the Anthropocene, a new chapter in the Earth’s history borne of our impact on the planet. And they hunted all around the globe for proof.

But it’s not easy to make the case that we’re in a new epoch. That’s a technical term that describes a chunk of time typically lasting a few million years (sounds like a lot, but it’s nothing compared to a geological “period” like the 54-million-year Jurassic or “era” like the 186-million-year Mesozoic). Scientists have to vote on whether the term “epoch” applies.

In February, a top body of Earth’s professional timekeepers voted — against canonizing the Anthropocene. That vote was immediately contested, with some members arguing that the decision was premature and more time was needed to weigh all the evidence. But this week, an overseeing body called the International Union of Geological Sciences decided to uphold the vote, making it official.

And while this might just seem like a smackdown over semantics, the fight over “Anthropocene” is much more. It’s a deeply political fight over how to make meaning of what we humans are doing to the planet.

Earth has gone through distinct geological epochs, chunks of time defined by changes in rock layers. To prove that the Anthropocene represents a new chunk, a group of geologists had to find a “golden spike” — a physical site where the rock, sediment, or ice clearly records the change from a previous chapter in time to a new one.

In 2009, they started scouring the planet and found a range of strong candidates, from a peat bog in Poland to a coral reef in Australia to the ice of Antarctica.

But the Anthropocene Working Group (AWG), as the group was called, wanted to pick a site where the rock record indisputably shows that we’ve left behind the Holocene epoch, which started 11,700 years ago when the last ice age ended.

In 2023, the geologists said they’d found their holy grail: little Crawford Lake in Ontario, Canada.

There, the waters are so deep that whatever sinks to the floor usually remains without mixing with the upper layers of water, so it stays preserved, offering an amazingly good record of geological change.

Since the middle of the 20th century, the sediment there has been inundated by the byproducts of human activity: plutonium isotopes from the nuclear bombs we’ve detonated, ash from the fossil fuels we’ve burned, and nitrogen from the fertilizer we’ve used.

That was also when we started to see major changes in phenomena like global warming, sea-level rise, ocean acidification, and the explosive growth of domestic animal populations. So the AWG said the Anthropocene began around 1950.

That would prove controversial.

Some scientists argued that it doesn’t make sense to recognize our current interval as its own epoch, since it’s incredibly brief in geological time.

If the previous epoch, the Holocene, lasted 11,700 years, does it make sense to give the same designation to an interval that hasn’t yet spanned 75 years?

But even among those who agreed that human activity had ushered in a new epoch, there was disagreement over when the epoch started.

Paul Crutzen, the atmospheric chemist who originally coined the term “Anthropocene,” said it started in the late 18th century thanks to the greenhouse gas emissions that took off with the Industrial Revolution. Others looked further back to the colonial powers that ravaged the so-called New World. Still others said humans have been transforming the planet since the dawn of agriculture, so trying to pinpoint any later starting point would be arbitrary.

Erle Ellis, an ecologist who’d been part of the AWG for 14 years, objected so strongly to its idea of drawing a bright line between pre- and post-1950 that he ultimately resigned.

Carving up time that way “does real damage by denying the deeper history and the ultimate causes of Earth’s unfolding social-environmental crisis,” Ellis wrote in his resignation letter.

“Are the planetary changes wrought by industrial and colonial nations before 1950 not significant enough to transform the planet? The political ramifications of such a misleading and scientifically inaccurate portrayal are clearly profound and regressive.”

The thing is, carving up time is inherently political, because scientists are not the only ones who use geological labels. The public uses them too. They feature in our school textbooks, our museum exhibits, and even our music.

The term “Anthropocene” is already widely used and understood — in 2020, the musician Grimes even released an album dubbed Miss Anthropocene. The term has become a way to get people to take climate change more seriously.

While some scientists were uncomfortable with the idea of using the “Anthropocene” label to make a political statement about what humanity is doing to the planet, other scholars embraced that.

The geologist Emlyn Koster, for example, told the New York Times in 2022 that geologists shouldn’t think of defining the Anthropocene as solely the AWG’s business. “I always saw it not as an internal geological undertaking,” he said, “but rather one that could be greatly beneficial to the world at large.”

Now that the Subcommission on Quaternary Stratigraphy, the body in charge of recognizing geological time units, has rejected “Anthropocene” as a new epoch, and its parent body has upheld the vote, some scientists are at pains to emphasize that humans are still screwing up badly.

“We are in the Anthropocene, irrespective of a line on the time scale,” said Francine McCarthy, an earth scientist at Canada’s Brock University who participated in the AWG. “And behaving accordingly is our only path forward.”

This story appeared originally in Today, Explained, Vox’s flagship daily newsletter. Sign up here for future editions.

Update, March 20, 4 pm: This story, originally published March 7, has been updated with the official decision of the International Union of Geological Sciences.