Epoch or event? Defining the Anthropocene

The Anthropocene is one of those rare scientific terms that has entered the public consciousness, which is ironic, given that geologists have not yet fully defined it. As Ian Randall discovers, the proposed formal definition sits at odds with the public’s understanding

“It’s probably fair to say that no single geological phenomenon has resonated in so many communities, with so many people, and in so many different ways,” says the British Geological Survey on the topic of the Anthropocene – the current geological era, during which human activity is considered to be the dominant influence on the Earth’s climate, environment and ecology. “The discovery of plate tectonics was the last time geology made a significant change to our understanding of how the Earth works. The Anthropocene is no less significant.”

Significant though the concept may be, it is still rather nebulous. Indeed, the term “the Anthropocene” has permeated through to other disciplines; graced the May 2011 cover of the Economist magazine; and even inspired the work of numerous artists. But members of the earth sciences community (specifically stratigraphers, who study the way sedimentary rocks are layered) are still debating exactly when this new geological period dominated by human activity is supposed to have begun.

Many of these changes will persist for millennia or longer, and are altering the trajectory of the Earth system, some with permanent effect

International Commission on Stratigraphy’s Anthropocene Working Group

The Subcommission on Quaternary Stratigraphy (SQS) – which is a constituent body of the International Commission on Stratigraphy (ICS), the largest scientific organization in the International Union of Geological Sciences – has been developing a formal definition of this the Anthropocene. According to the SQS, there are various phenomena associated with this new epoch, including an increase in erosion and the movement of sediments associated with urbanization and agriculture. Unprecedented new materials, such as concrete, fly ash (a coal combustion product) and plastics, are now part of the sedimentary record. The impact also extends to ecosystem changes such as rampant deforestation and the biodiversity losses that have led some to assert that we are in the middle of a sixth great mass extinction.

“Many of these changes will persist for millennia or longer, and are altering the trajectory of the Earth system, some with permanent effect,” the SQS’ Anthropocene Working Group (AWG) wrote in its 2019 report of the definition of this epoch, published on the SQS website. “They are being reflected in a distinctive body of geological strata now accumulating, with potential to be preserved into the far future.”

The Anthropocene begins, etymologically speaking

The roots of the Anthropocene concept can be traced back to at least as early as 1873, and the writings of the Italian geologist Antonio Stoppani. A former Catholic priest, Stoppani regarded “the creation of man” as “the introduction of a new element into nature, of a force wholly unknown to earlier periods”. Given this, he argued, an “Anthropozoic era” had begun “with the first trace of man” and would end only when Earth had “escape[d] the hands of man…thoroughly and deeply carved by his prints”.

Four years later, American geologist Joseph Le Conte of the University of California Berkeley took this a step further, arguing that humankind had become the “chief agent of change”. Based on the notion that previous ages were “reigns of brute force and animal ferocity”, while the age of man was “characterized by the reign of mind”, he proposed the “Psychozoic era”. In a similar fashion, the Russian geochemist Vladimir Vernadsky suggested in 1926 that the biosphere had become the “Noosphere”, with nous being the Greek for “mind” and “reason”.

Around the same time, in 1922, fellow Russian Alexei Petrovich Pavlov proposed that the present geological era be dubbed the “Anthropogene”. The prefix “anthro”, meaning “human”, referred to how the era would be defined based on the emergence of the genus Homo – making it roughly equivalent to the “Quaternary”, the period covering the last 2.6 million years – while “gene” is the established suffix for a geological period. Despite being adopted in Soviet circles, and being formally approved by the Soviet Union’s Interdepartmental Stratigraphic Committee in 1963, the term never caught on elsewhere.

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The first use of “Anthropocene”, meanwhile, is credited to the American limnologist Eugene Stoermer, who used the term informally in the late 1980s to refer to the impact and evidence of human activity on the planet. It would not be until the year 2000, however, that the term truly achieved scientific popularity after being re-coined by the Nobel-prize-winning atmospheric chemist Paul Crutzen, during a meeting of the International Geosphere–BioSphere Programme (IGBP) in Cuernavaca, Mexico.

According to an account by meeting organizer and chemist Will Steffen – amid a series of presentations describing various significant and geologically recent changes in Earth’s atmosphere, oceans and sediments – Crutzen became agitated by repeated references to the Holocene, the geological epoch covering the last 11,700 years. Bringing proceedings to a screeching halt, Crutzen remonstrated that they were no longer in the Holocene and on the spot, independently of Stoermer, cooked up “Anthropocene” to describe the current epoch.

A few months later, Crutzen – now in collaboration with Stoermer – wrote a short article in the IGBP newsletter expanding on the concept (Global Change Newsletter May 2000 p17). Detailing many of the ways in which humans have affected the Earth from the transformation of natural landscapes to the hole in the ozone layer, the pair wrote that “considering these and many other major and still growing impacts of human activities on Earth and atmosphere, it seems to us more than appropriate to emphasize the central role of mankind in geology and ecology by proposing to use the term ‘Anthropocene‘ for the current geological epoch”.

The Anthropocene begins, geologically speaking

The key difference between Crutzen’s conception of the Anthropocene and its predecessors is that while the latter imagined humanity’s influence as an incremental one, the former saw something more abrupt.

As science historian Jacques Grinevald, formerly of the University of Geneva, and his colleague Clive Hamilton, a public ethics expert from Australia’s Charles Sturt University, put it in a 2015 paper (The Anthropocene Review 2 59), “The Anthropocene represents, according to those who initially put it forward, a dangerous shift, and a radical rupture in Earth history. This means that the Holocene can be no guide to the Anthropocene geologically or intellectually.” This is, of course, rather in keeping with the way in which previously established divisions of geological time are defined.

Ediacaran GSSP

Geologists mostly use two approaches to divide Earth’s history. The first is geochronological, based on a combination of absolute and relative dating. Absolute dating involves analysing radioactive isotopes. Relative dating uses tools such as records of palaeomagnetism and stable isotope ratios. Geologists then divide time into spans of varying size, with eons being the largest, then eras, periods, epochs and finally “ages” as the smallest. Each span has an equivalent unit in the stratigraphic record – ages of time, for example, match up to successions of rock strata called “stages”.

The lower boundary of each stage in the rock record across the globe is established with respect to a single example in the field. Established by the ICS,  these paragons are formally (and verbosely) known as the Global Boundary Stratotype Sections and Points (GSSP). However, geologists commonly refer to them as “golden spikes”, after the metal pins driven into the rocks to mark the precise location of the boundary. Most, but not all, of these boundaries are defined by changes in the fossil record.

So, for example, the start of the Sinemurian stage 199.3 million years ago is marked by the first appearance of two species of ammonite in the rock record, with the golden spike located in a rock outcrop near East Quantoxhead in Somerset, UK. The divide between the Maastrichtian and Danian stages 66 million years ago is marked by a section in El Kef, Tunisia, of the iridium layer created by the asteroid impact thought to have triggered the mass extinction that killed the dinosaurs.

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Spiking up debate

So where in the geological record should the Anthropocene begin? This is the thorny question that members of the AWG have been grappling with since 2008. Indeed, as AWG convener and palaeobiologist Jan Zalasiewicz once joked, “There have been suggestions that our own work is proceeding on a geological timescale!”

In Crutzen’s original conception, he proposed that the Anthropocene might start in 1784, during the Industrial Revolution. Writing in a 2002 paper (Nature 415 23), he justified this as “when analyses of air trapped in polar ice showed the beginning of growing global concentrations of carbon dioxide and methane” – a date, he noted, that coincides with the refinement of the steam engine by the Scottish engineer James Watt.

However, various alternative proposals have been made, the earliest being the advent of agriculture some 12,000 years ago. Other suggestions include 1492, when the “Columbian exchange” – referring to Italian explorer Christopher Columbus’ 1492 transatlantic voyage, and the related European colonization and global trade that followed – saw the widespread transfer of animals across the continents. Another is 1610, which saw a drop in carbon-dioxide levels associated with the arrival of imperial colonists in the Americas (European diseases having devastated local populations, leading to forest growth across abandoned settlements).

The AWG, meanwhile, has zeroed in on the “Great Acceleration” of the mid-20th century, when various socioeconomic and Earth system trends began to increase dramatically as a result of “population growth, industrialization and globalization”. According to Zalasiewicz and colleagues, the acceleration has left an array of proxy signals in the geological record, “the sharpest and most globally synchronous of [which] is made by the artificial radionuclides spread worldwide by the thermonuclear bomb tests from the early 1950s”.

Illustration of possible start dates for the Anthropocene

In 2016 the AWG presented the preliminary summary of its analysis at the 35th International Geological Congress, where a majority voted to consider the Anthropocene as “stratigraphically real”. It recommended a move to formalize the epoch with a suitable GSSP located in the mid-20th century.

At present, 12 candidate sites for the Anthropocene Golden Spike are being considered. They include coral reefs off the coast of Australia and the Gulf of Mexico; a peatland in Poland; an ice core on the Antarctic Peninsula; and a series of anthropogenic sediments in Vienna. Even once this list has been whittled down to one, the process of ratification is elaborate, and involves the proposal being approved at various stages before final evaluation by the IUGS.

A turn of events?

As Zalasiewicz himself has noted, there is no guarantee that the working group’s recommendation for an Anthropocene epoch will ultimately be accepted. In fact, there are some scientists who believe that there may be better ways to conceptualize it.

Most of humanity’s impact on the environment has been diachronous, occurring at different times in different places

The leader of this charge, ironically enough, is Quaternary geologist Philip Gibbard of the University of Cambridge, UK, who was responsible for creating the AWG in the first place, joking that “I’ve only myself to blame!” In a series of papers – the latest published in the Journal of Quaternary Science (JQS 37 1188) – Gibbard and his colleagues argue that while the definition of an epoch calls for a boundary that occurred at a fixed point in time across the globe, most of humanity’s impact on the environment has been diachronous, occurring at different times in different places.

Instead, they propose, it might be better to sidestep the problem of setting a single start date for the Anthropocene and instead classify it less formally as an “event”. It would run in parallel to the geological timescale and encapsulate all of humanity’s diverse effects and their complex spatial and temporal variations. In this way, they argue, the Anthropocene would be seen more like other so-called diachronous transformations in Earth’s history that occurred on different geological timescales, from place to place.

These include the Great Oxidation Event of 2.4–2.0 billion years ago, which saw oxygen accumulate in the atmosphere after the evolution of photosynthesis; or the explosion of marine life over some 25 million years during the Great Ordovician Biodiversification Event.

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Epoch, event or episode?

Listening to the comments of the proponents of the epoch approach, Gibbard and colleagues’ proposal is perhaps too radical a departure from the established protocol they are following. Some have also suggested that “episode” would be a more appropriate name than “event” for such a long-lasting phenomenon – although this does not seem to negate the concept.

AWG member and stratigrapher Martin Head of Canada’s Brock University, for example, said that the group “is defining the base of the Anthropocene as a formal epoch using geological event stratigraphy. This is standard procedure when defining units of the International Geological Time Scale”. Event stratigraphy is the study of the traces of (geologically) short-lived events – lasting up to thousands of years – in the sedimentary record.


In contrast, Head says, the Anthropocene “event” proposal is “neither strictly geological nor an event”, at least in normal Quaternary usage. “We consider it an interdisciplinary concept that combines elements of geology and the social sciences. It could coexist with a formal Anthropocene epoch if found useful, but conceptually cannot replace it and would need to be renamed to avoid confusion.”

Zalasiewicz agrees, adding “The ‘Anthropocene event’ concept is very different to that of the Anthropocene as a formal epoch of the Geological Time Scale…these are very different concepts, and it invites confusion to apply the same term of ‘Anthropocene’ to both of them. Under a different name, the ‘event’ idea could be potentially complementary to a precisely defined Anthropocene epoch, rather than in conflict with it or as an alternative to it.”

A challenge to this rationale, Gibbard and his colleagues note, is that the term Anthropocene has already taken on a life of its own. “Most of the world is not a geologist though, right? I think, in a lot of ways, the cat’s out of the bag,” says palaeoecologist Jacquelyn Gill of the University of Maine.

She adds, “What an event framework allows us to do is to characterize the Anthropocene in the way that it’s already being used by the broader public, by journalists, by historians, environmentalists, etc – all of whom sort of use the word as almost a metaphor for the suite of activities that humans do that leave a marked impact on the planet.” The issue with setting up the Anthropocene as a time period with a particular global start date, Gill argues, is that it risks trivializing the impacts that humanity had on the Earth before that point.

Not all scholars outside of geology, however, appear to share these concerns. Historian Julia Adeney Thomas of the University of Notre Dame, who has written extensively on the Anthropocene, tells Physics World that the idea of a 50,000-year event doesn’t resonate with the way the concept is seen in history, the social sciences or allied disciplines. In fact, she explains that “what works best for us is a definition of the Anthropocene shared by everyone, and rooted in biostratigraphy and Earth system science.” While us human beings have always had an impact on our environment, she adds, it is only in the last 70 years or so that we have truly transformed the way the Earth system functions.

Ultimately, defining the Anthropocene is less important than engaging with the issues it invokes. “It’s a great conceptual framework, and people are going to continue to use it, regardless of whether we say it’s 1950 or 1750, or whatever it ends up being,” notes Gill. “The reason we name and delineate things in nature is so that we have a common language to ask questions. We’re already doing that about the Anthropocene,” she says. “As it stands right now, the decision about when it starts has no bearing on the questioning that we’re doing. I just hope that this definition doesn’t limit that questioning in the future.”