In a high-profile vote in March 2024, an international scientific committee chose not to support designating a new geological epoch, the Anthropocene [
1]. Prior to this somewhat unexpected decision, scientists spent 15 years gathering abundant evidence for a shift in the geological timescale that would acknowledge humankind’s accelerating impact on the Earth, which they argued has already left a distinctive mark in the planet’s geological strata. While some will continue to work for formal recognition of the Anthropocene Epoch, others say the concept remains useful whether it is integrated into the official geological time scale or not [
2].
The idea was first articulated by atmospheric chemist Paul Crutzen, a Dutch 1995 Nobel Prize in Chemistry winner, and Eugene Stroemer, a professor of biology at the University of Michigan (Ann Arbor, MI, USA), both now deceased. In 2000, in the newsletter of the International Geosphere-Biosphere Programme, Crutzen and Stroemer wrote that the Earth is no longer in the Holocene Epoch but has, thanks to the tremendous impact of human activity on the planet, advanced into the “Anthropocene” [
3]. Crutzen expanded on the idea in a
Nature article in 2002 [
4], and the idea has since become deeply rooted in the scientific community’s lexicon.
A group of stratigraphers, geologists who specialize in studying and dating the planet’s rock layers, adopted the idea and started to study whether the Anthropocene’s impacts were visible in the strata. There are no explicit, objective criteria that must be met to designate new units of geological time, such as new epochs. Scientists first form a working group to develop the argument and gather evidence. That group then submits its proposal to the relevant subcommission, in this case the Subcommission on Quaternary Stratigraphy (SQS) of the International Commission on Stratigraphy (ICS), which is a body within the International Union of Geological Sciences (IUGS). If the proposal is approved by the wider ICS, it must be ratified by the executive committee of the broader IUGS.
The Anthropocene Working Group (AWG) was officially convened by the SQS in 2009 and began gathering evidence [
5], submitting its formal proposal on 31 October 2023. A core part of the AWG’s argument was that, though humans have affected the Earth throughout their existence, the explosion in population and economic activity that started around 1950—which is called The Great Acceleration—has already and will continue to mark the Earth for tens of millennia to come [
6].
Throughout most of the Holocene, which followed the Pleistocene beginning approximately 11 700 years ago with the retreat of the great glaciers of the ice age, human impacts on the environment were relatively small and localized. Since 1950, however, there has been an exponential growth of atmospheric greenhouse gas concentrations, increasing ocean acidification, and global temperature increases [
2]. These changes were also detailed on webpages summarizing the AWG’s work that have now been deleted from the ICS website [
7]. But the Anthropocene is not just about climate change, according to Jaia Syvitski, professor emeritus of geological sciences at the University of Colorado in Boulder, CO, USA. Syvitski, who served on the AWG, said that human energy consumption and productivity, irrespective of greenhouse gas emissions from burning fossil fuels, has driven planetary changes.
Those changes include the disruption of the planet’s nitrogen cycle due to the use of fertilizer, the transformation of the planet’s rivers by dams and reservoirs, the compaction and erosion of the world’s soils, the distribution of contaminants including pesticides and pharmaceuticals to remote areas including the Arctic, and a major shift in the makeup of the planet’s animal life [
8]. The magnitude of these changes since 1950, across many of Earth’s systems, is vast. For example, humans and their livestock account for about 96% of the mammalian biomass on the planet; poultry raised by people now make up 70% of avian biomass [
8].
And many geologists point to how these changes have marked the Earth. “They are recorded in geological sediments and other natural archives,” said Martin Head, professor of earth sciences at Brock University (St. Catharines, ON, Canada), who served as vice chair of the AWG. “Technically we are still living in the Holocene, but from a functional standpoint, the relatively stable conditions of the Holocene ended in the mid-20th century.”
As part of their proposal, the working group had to select a “golden spike,” a place in the Earth’s strata where there is a clear mark of the beginning of a new epoch. The group chose a sediment core from Crawford Lake (
Fig. 1) near the city of Toronto, ON, Canada, a small but deep meromictic lake in which the top layers of water do not mix with and disturb its bottom layers. The lake’s sediments contain contaminants including plutonium from nuclear bomb testing that was deposited in 1952, the year the AWG proposed as the start date of the Anthropocene [
9].
Head said it is unusual for the SQS to reject proposals once they have been fully ironed out by a working group. But that is what happened in March. Head and Syvitski both point to the human, political side of science as the reason for the rejection. The work of geologists and stratigraphers typically focuses on very long periods of time, Head said, with some voting members of the SQS objecting to the brevity of the proposed Anthropocene, reaching back just to 1952. “There was an ideological split,” he said. Head and the SQS’s chair, Jan Zalasiewicz, emeritus professor of geology at the University of Leicester (Leicester, UK), protested the vote because they believed too little time had been allowed to discuss the proposal, contrary to ICS regulations [
10].
Other scientists have protested that the voting members rejected clear scientific evidence and called the rejection of the Anthropocene Epoch the result of “savage academic infighting” [
11].
Some members of the AWG became uncomfortable with its methods, said field archaeologist Matthew Edgeworth, a research fellow affiliated with the University of Leicester. Edgeworth said he resigned from the AWG because he felt the project was, in a sense, working backwards. Instead of starting with physical evidence, the process took a top-down approach, wherein the team had decided the Anthropocene began around 1950, and then looked for physical evidence to support that. “We should go by the evidence, rather than imposing it on the stratigraphy,” he said.
Edgeworth said it did not make sense to him to focus so much on a specific date, when a marker could have been chosen from any number of years around 1950 or at another time, depending on the criteria. “We can all agree we are in this amazing planetary transformation,” he said. “There is no disagreement about climate change or The Great Acceleration—it is just the idea of imposing a date.”
Edgeworth argues that the Anthropocene should be viewed not as an epoch but as an unfolding event that does not have a specific start date. He and others contend that it is like The Great Oxidation Event, a period from 2.4 billion to 2.1 billion years ago when oxygen concentrations rose. This event transformed the planet, making life as it is now known possible, but is not formally recognized as an interval in the geological time scale [
12].
Regardless of whether it is deemed an epoch or an event or gains no official recognition at all, the Anthropocene “... will remain an invaluable descriptor of human impact on the Earth system,” according to the joint statement by the IUGS and ICS announcing the rejection of the AGS proposal [
13]. The statement acknowledges that almost all areas of science have adopted the concept, and it has additionally entered the popular culture. None of the scientific journals with “Anthropocene” in their titles are likely to change their names or their mission statements, said Syvitski. Added Edgeworth: “It does not cease to exist just because it has been rejected.”
Head said he expects the AWG will be invited to resubmit its proposal at some point and that the new epoch will be adopted by the IUGS. He said there has already been some turnover in the voting group that may turn the tide in the proposal’s favor. Head was involved with previous proposals to refine the geological time scale, including the definition of the Quaternary Period, which he said took more than ten years to be formally adopted. In geology, it seems, things just take time.
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