The human layer

By Helen Gordon

Concrete is a new kind of rock…We’ve made about 500 billion tonnes of it, which is enough for one kilo for every square metre of the Earth’s surface, land and sea.”

In the summer of 2015 Jan Zalasiewicz, professor of palaeobiology at the University of Leicester, was speaking at the Tate Modern as part of the Anthropocene Project conference. Concrete, he explained, can be considered a characteristic deposit of the Anthropocene Epoch just as coal is of the Carboniferous Period.

Leaving the stage Zalasiewicz quickly found himself at the centre of a crowd of artists and curators who wanted to talk about the Anthropocene. Men in black-framed glasses and women with Jean Seberg haircuts discussed tectonic forces, millennial time and the infrastructural unconscious of contemporary urbanism. The Otolith Group, Turner prize-nominated artists, showed a film about a Californian woman who claims to be able to predict earthquakes and Dipesh Chakrabarty, a historian, suggested that “the term…invites you to place human beings in the context of deep time.”

The idea of the Anthropocene carries in its tail familiar narratives of planetary disaster and extinction

Anthropos: “human being”. Kainos: “new”. In 2000 Paul Crutzen, an atmospheric chemist, and Eugene Stoermer, a biologist, crystallised a radical idea that had been floating around: humankind’s impact on the Earth’s ecology has been so profound that it is fashioning an entirely new geological epoch. The change is being wrought not just by concrete and plastic but also by soaring extinction rates and changes to the carbon cycle, the nitrogen cycle and temperature. The Holocene Epoch, according to this theory, is finished: we have entered the Anthropocene.

The term’s power derives from the intellectual heft it borrows from an academic discipline, geology. By measuring humanity’s planetary influence on a geological timescale, it implies an impact vaster than empires, though not more slow. It carries in its tail familiar narratives of planetary disaster and the looming threat of extinction.

The term has spread rapidly from the sciences through the arts and out into popular culture: there is a graphic novel, several death-metal albums, a Brian Eno track and an “Anthropocene Fashion” Pinterest site board. Yet its air of authority is misleading, for the Anthropocene has no formal scientific status. Among geologists there is no consensus as to when, or indeed whether, it began; it is a source of disagreement and, to some, embarrassment.

The Earth has existed for 4.6 billion years. For convenience, geologists divide this span into units of time: ages, epochs, periods, eras and eons. Some of the periods are familiar: Jurassic, Carboniferous. But a list of ages – Wordian, Roadian, Kungurian – sounds more like the roll call for some intergalactic council meeting in the Alpha Centauri system. Divisions between units are based on the occurrence of “significant” events that leave a changed world and a record of this change in the strata (layers of rocks or occasionally ice). The Pleistocene/Holocene division is a global-warming event that can be seen as a sudden change in the composition of trapped air in the Greenland ice cores. The Cretaceous/Paleogene division is the mass extinction of some three-quarters of plant and animal species, including all non-avian dinosaurs. Evidence from the rocks might include a radically altered fossil record. “Geology is often a forensic science,” said Zalasiewicz. “It’s Sherlock Holmes stuff basically.”

Right now we are living in the Holocene Epoch, in the Quaternary Period, in the Cenozoic Era, in the Phanerzoic Eon. Geological units are hierarchical and nest inside one another like matryoshka dolls. An “epoch” is defined by relatively minor geological change and typically covers between 2m and 30m years. Eons, by contrast, are massive. All of geological time is divided into only four eons. The most recent, the Phanerozoic, covers the last 540m years and the significant event that led to its creation was the beginning of multicellular life.

The branch of geology that concerns itself with the classification of layers of rock (or ice) is known as stratigraphy. By building up a picture of the layers of strata you build up a history of the Earth and, eventually, a geological timescale. Today the official version of that timescale is known as the International Chronostratigraphic Chart, produced by the International Committee on Stratigraphy (ICS). The result of centuries of painstaking work by thousands of geologists, it provides an organising structure that supports all studies of the Earth’s past. “It’s the one thing you’ll find in every professional geologist’s office,” a friend who lectures in Earth sciences told me. “It’s the equivalent of the periodic table.” This is the chart that would need to be amended should the Anthropocene be formally accepted.

Used to working with timescales of thousands, millions and billions of years, stratigraphers are not famously impulsive. In their discipline, “BP” stands for “years Before the Present”, and the “Present” is defined as 1950. According to ICS guidelines any new boundary definition must be left in place for a minimum of ten years before amendments are allowed, and proposals relating to changes to geological units involve a lengthy, hierarchical procedure. Zalasiewicz chairs the Anthropocene Working Group, the official body that must debate whether the term “Anthropocene” is (a) scientifically justified and (b) useful to the scientific community as a formal unit. If the answers are yes, then a summary paper will be handed to the Subcommission on Quaternary Stratigraphy (SQS). If the SQS are in agreement, the proposal will be passed up the chain to the ICS. If accepted by the ICS it will be sent finally to the International Union of Geological Sciences for formal ratification.

The Working Group was set up in 2009 and now has more than 30 members drawn from around the world. In 2014 they published a collection of papers: “A Stratigraphical Basis for the Anthropocene”. In 2016 they hope to present one or two summary papers to the ICS. Plenty of jokes have been made about the work proceeding on a “geological time scale” but for the traditional stratigraphers this feels more like break-neck speed: the haggling over older divisions of the timescale often lasts for decades.

Earlier this year I travelled to Leicester to speak with Zalasiewicz. In addition to his day job he writes popular science books and is translating the work of 18th-century French naturalist the Comte de Buffon. “I think about the 18th- and 19th-century naturalists and how they came at everything fresh. We come at it with clutter, with all this bloody education,” he said. For Zalasiewicz the Anthropocene is, among other things, an opportunity to see anew what has always been in front of us. “We can look at buildings, for example, and say that they are also part of the rock cycle. They are made of rocks and will go back to being rocks. They will leave a distinctive record.”

We drove out through the suburbs of Leicester to an old railway cutting. Tall nettles and purple-flowering willowherb grew in the shade of a steep tree-lined bank. One hundred and eighty-five million years ago, during the Early Jurassic, we would have been standing on the floor of the shallow sea that covered the whole of Britain. Today that sea floor has been translated into a sandy-coloured limestone rock filled with fossils: brachiopods, ammonites and belemnites. Also preserved are the tracks and burrows of floor-dwelling organisms. These are what are known as “trace fossils” or “ichnofossils” – not the organism itself but a mark it left behind. Considered one way the London Underground system (or the New York Subway, or the Tokyo Metro) is a giant trace fossil. Another type of trace fossil is what Zalasiewicz calls a “technofossil” – the fossilised trace of a manufactured product. A paleolithic flint axe might become a technofossil, so might a double-decker bus or a toothbrush. Zalasiewicz was excited: “One thing we want to pursue is, what is the true diversity of techno-fossils? Has anybody ever counted how many different types of toothbrush have been made?”

One hundred and eighty-five million years from now, what else might be preserved from our own time? What evidence discoverable in the rocks? Suggestions include changes to the fossil record (this organism stopped existing; this one suddenly appeared on the other side of the world); anthropogenic deposits (sediments containing manmade materials such as metals, plastics, glass); evidence of anthroturbation (underground holes and tunnels dug by humans); the altered composition of air trapped inside ice cores. From a stratigrapher’s point of view what matters is not that humans are leaving an imprint but that the Earth system is changing. Humans just happen to be important geological drivers of that change.

We walked farther along the cutting. Here, shelly limestone stopped about a metre from the floor; above it was a shale, very dark blue-grey and composed of thin, crumbly layers like sheets of paper. There were no fossils. This was something called the Toarcian Extinction Event, which marked the end of the Pliensbachian Age. It took place around 183m years ago when an estimated 2 trillion-5 trillion tonnes of carbon were released into the atmosphere and the world warmed up by some five degrees Celsius. The oceans became anoxic (depleted of oxygen), and stagnant. A lot of things died out, which was why there were no fossils in the layers. The presence of so much carbon was responsible for the dark, almost black colour. “It’s one of the events used to compare with modern global warming,” Zalasiewicz said. “We’re beginning to create a modern Toarcian event. We’ve not gone as far yet but we’ve done about half a trillion tonnes of carbon, mostly in one century.”

For an epoch to be formalised it must have a starting-point – either a Global Stratigraphic Section and Point (GSSP) or a Global Standard Stratigraphic Age (GSSA). A GSSP is one physical location where the transition between units can be seen. Typically a GSSP is located in a section of rock and marked with a “golden spike” (in actuality a bronze disc). The GSSP for the base of the Silurian Period can be viewed at Dob’s Linn in Moffat, Scotland; the base of the Jurassic at the Kuhjoch section, Tyrol, Austria. Where no GSSP can be identified – for the 4-billion-year-old Eoarchean, say, where there are few physical traces – then a GSSA is used. This gives a chronological point but not a physical reference.

The Anthropocene’s proponents have been debating when that point might be. Crutzen argued for 1784 – to coincide with the beginnings of the Industrial Revolution. Other suggestions include the expansion of agriculture and livestock cultivation during the Neolithic, or the growth of mining around 1,400BC; though, inconveniently, these developments happened at very different times in different places around the world. A paper published earlier this year proposed 1610: there was a noticeable drop in global levels of CO₂, which the authors linked to the settlement of the Americas, the deaths of some 50m indigenous people, the abandonment of agricultural lands, forest regrowth and thus the fall in CO₂ levels.

Zalasiewicz and others have argued for a mid-20th-century date. This would coincide with the so-called “Great Acceleration” in global economic activity following the first world war, and the extraordinary growth of cities and major infrastructure projects that may be considered a distinctive feature of the Anthropocene. Geological evidence might include the spread of nuclear waste, nitrous gases from exhaust fumes and industrial pollution and the upturn in levels of carbonaceous fly-ash particles, which are a by-product of the burning of fossil coals.

Zalasiewicz is cautious when asked about his chances with the ICS: “It’s a very conservative body. I think it’s unlikely that they will formalise the Anthropocene next year, but the right questions are now being asked…In my personal opinion there’s no question that we’ve entered the Anthropocene but not all the effects have been felt yet. We haven’t yet had climate change, for instance. We’ve had a fraction of a degree, almost certainly human driven, but we’re still not yet higher than peak temperature in the last interglacial, 125,000 years ago.”

Not everybody in the geological world agrees. Last year Michael Walker, emeritus professor of quaternary science at the University of Wales, left the Anthropocene Working Group. Unconvinced of the need for a formal Anthropocene Epoch and with views increasingly at odds with the majority of the group, he felt that his presence there no longer served a useful purpose. “In the geological community there is a growing sense of disquiet about the whole ‘Anthropocene’ business,” he wrote in an e-mail.

A close colleague of Walker’s is Philip Gibbard, professor of quaternary geology at Cambridge University and a former chair of the SQS: “I never thought I’d be cast in the role of the man with his foot on the brake but I do think we need to say slow down, let’s look at what’s really going on here,” he told me. “Certainly there have been huge amounts of anthropogenic change – and when the green lobby say that we’re a bit of a plague on this planet I tend to agree with them – but does it warrant a change to the [ICS] chart?”

Gibbard’s laboratory lies within a jumble of buildings behind the stately lines of Downing College. His office is upstairs. “It’s fine to say it’s a tip,” he said. Shelves are crammed with files and books, piles of paper covered the work surfaces and a toy woolly mammoth looks down from a precarious perch above the sink. Gibbard is dressed in black hiking shorts and sandals, and his grey hair sticks out to form a sort of halo around his head. At university in Sheffield he was a few years ahead of Zalasiewicz though they didn’t know each other then, meeting later and becoming friends while working on East Anglian ice sheets. Both speak warmly of the other and it was Gibbard who, as SQS chair, asked Zalasiewicz to form the Anthropocene Working Group.

The media spotlight rarely falls on stratigraphers. Society tends to leave them to work quietly on events that took place hundreds of millions of years ago. The debate over the Anthropocene has focused unusual attention on them. Gibbard calls it “extraordinary” and “welcome” but worries that the science is being simplified and misrepresented: “In many ways we’ve already lost control and this is exactly like the dinosaur business all over again – the whole thing grossly oversimplified by rocks falling from the sky etcetera, etcetera.”

Crutzen, not himself a geologist, sees the Anthropocene concept as fundamentally interdisciplinary, and while the Working Group is made up predominantly of Earth scientists, it also includes archaeologists, a philosopher, a lawyer and a historian. This makes Gibbard anxious. “However they want to use the term ‘Anthropocene’ in their own disciplines is fine but do they understand the geology? How is the term useful for geologists?” he asked. “That’s the real test of the term: is it useful?”

Stratigraphers worry that the search for evidence of the Anthropocene turns the discipline on its head. Normal practice is for observations of rocks to lead to conclusions about what must have been happening on Earth when those rocks were laid down. With the Anthropocene we know what has been happening, and the stratigraphers are trying to find the geological evidence for it. But a lot of things that may be fundamental in defining the proposed epoch haven’t happened yet. It’s far too soon, for example, for a fossil record to have formed. Indeed, some have argued that a period of 50 years is simply not long enough for any “anthropogenic detrital material” to have formed a geologically useful stratum. Gibbard said: “In many ways, we are just too close to the events to be able to talk usefully about them from a geological perspective. Even the Holocene is very short. It begins 11,700 years ago and that’s nothing, just a tiny period, a blink of the eye. Geologically, even 45 years ago is still ‘now’.”

Nor is it obvious that circumstances warrant a new epoch. “The reason the Holocene is defined as a separate epoch is the presence and activity of humans. Now from my perspective as a geologist I would say you can’t play the card twice, and so that means that what’s been going on to establish the Holocene can’t then be used again to define the Anthropocene.”

Gibbard says that for now he is staying in the Working Group but plans to take a back seat. Despite the problems a lot of good work has come out of the project, he says, but for now he favours the Anthropocene as an informal term – something like Bronze Age or Iron Age – making it part of human rather than Earth history. “Those are cultural terms and they don’t have fixed global boundaries because they are dependent on the migration of people or ideas with time. That might be a better way of using it because obviously in China 5,000 years ago there was industry well before we got on the scene.”

Stan Finney of California State University and chairman of the ICS has published a paper setting out a series of issues to be addressed by the Working Group. He points out, for instance, that other organisms have had far greater, longer-term impacts on the planet that have not been recognised with a formal time unit. Consider the development of photosynthesising vascular land plants that produced great volumes of oxygen for the first time in the atmosphere. Finney writes: “Might the desire to establish the ‘Anthropocene’ as a formal unit be Anthropocentric?”■

Images: ALAMY