Humans have had such a dramatic impact on the planet that we've changed geological history. Future scientists will be able to find humanity's distinctive chemical signature written into the rocky record of Earth's crust. That signature marks the beginning of the Anthropocene, or the new human age.

It sounds dramatic, but the Anthropocene has humble beginnings. It all started when Nobel-winning atmospheric chemist Paul Crutzen became flustered at a rather obscure scientific meeting back in 2000. Members of the International Geosphere-Biosphere Programme (IGBP) scientific committee were meeting in Mexico, to discuss research into environmental change. The meeting was long, and several researchers gave papers referring to the Earth's current geological era, the Holocene, which began about 11,700 years ago when the last ice age abated. Each time somebody said "Holocene," Crutzen became visibly agitated.


Will Steffen, director of the Australian National University's Climate Change Institute, said that Crutzen finally just lost it. In an essay about the meeting, Steffen recalled Crutzen spluttering, “Stop using the word Holocene. We’re not in the Holocene any more. We’re in the...the…the…the Anthropocene!” The word was a play on Holocene, combining "anthro," meaning human, and "cene," meaning new. Though ecologist Eugene Stoermer had coined the term back in the 1980s, it had never caught on. But after Crutzen's passionate outburst at that IGBP meeting, the word took off like wildfire.

Geologists and other Earth scientists began using the term in their papers to describe the unique properties of the human era on Earth. And slowly it began to trickle into everyday usage, too. Journalists like The New Yorker's Elizabeth Kolbert popularized the term in the late 2000s, and even artists began taking up the idea on their work. It was a word that seemed to make sense to everyone — not just experts.


Jamie Kruse and Elizabeth Ellsworth, the artists behind New York's Smudge Studio, have created several successful art projects — including the "geologic time viewer" and Friends of the Pleistocene — around the idea. "We were excited to have a word to put to the feeling that monumental events and dynamics capable of changing the Earth's geologic realities were unfolding under our feet," they told io9.

Still, the word never lost its scientific heft. UC Berkeley environmental biologist Anthony Barnosky summed up its relevance to his work:

Geologic epochs must be characterized by physical criteria in rocks, namely distinctive paleontological, chemical, or physical signatures (such as distinctive and unique types of sedimentary particles). It turns out the biologic (when fossilized, paleontologic), chemical, and physical signatures in deposits that are being formed today and that will be preserved into the geologic future are every bit as distinctive as those used to define the past geological epochs.


In other words, there is a distinctive signature that says "human" written into the layers of rock, sand, and other particles forming on the surface of the planet today. But when did those layers first start to form?

Putting a Start Date on the Anthropocene


Earth's deep history is divided up into geological periods, which scientists have identified using stratigraphy, the study of ancient layers of rock. As the planet ages, new layers of dust and stone settle on its surface. Over time, these layers are covered by more layers — and stratigraphers can look back through the ages of the Earth by studying these layers as if they were planetary tree rings. Many of the layers have distinctive chemical signatures that reveal everything from the molecular composition of the atmosphere to how many trees were growing at a given time in history.

The question is what it would take for humans to put their distinctive imprint on those geological layers. California State University Fresno ecologist Madhusudan Katti explained what that would mean via e-mail:

Geological periods (eras or epochs) are generally defined by our ability to see clear boundaries separating distinct layers of rocks. Different eras are also characterized by changes in the composition of flora and fauna in the fossil record (including plant and animal fossils as well as pollen records and other deposits), and sometimes by the composition of the rocks themselves. For our actions to result in a new geological period, they would have to be of sufficient magnitude of impact as to cause such distinct geological changes. Remember also that geological eras may not represent overnight changes, but a series of gradual changes compressed into relatively small time periods - but in geological time, [small time periods] could mean centuries or millennia.


For Crutzen, the answer was obvious. We'd created a new geological period during the Industrial Revolution.

Crutzen wrote a paper after that meeting in Mexico, where he identified what he thought might be the origins of the Anthropocene. He said it would likely be sometime in the early nineteenth century, during the Industrial Revolution, after people had been burning fossil fuels for long enough that atmospheric CO2 levels rose higher than anything we'd seen so far in the Holocene. CO2 levels are easy to track in rocks, and they're an obvious chemical signature to search for. Previous geological periods have also been identified by higher and lower levels of CO2 signatures.

Steffen, working with Crutzen, created a simple chart of the origins of the Anthropocene, mapped to CO2 levels.


In an email, Steffen told io9 that the nineteenth century isn't the only possible start date for the Anthropocene:

There are several proposals for the starting date for the Anthropocene. The three most common ones are (i) the advent of early agriculture (the so-called "Ruddiman hypothesis"), (ii) the beginning of the industrial revolution; and (iii) the beginning of the "Great Acceleration", the post-World War II period of exceptionally strong economic growth and technological development.


Steffen actually prefers to use the post-World War II period, because that era left behind an indisputable mark on the planet: a layer of radioactive materials deposited by the use of atomic bombs.

Many scientists refer to the past 60 years as the "great acceleration," a period when the human population exploded and our resource use left dramatic changes behind in the geological record. Here's another chart from Steffen, tracking all the transformations that mark the great acceleration.


Jumping off this idea, ecologist Katti noted that the Anthropocene isn't just quantified by CO2 and radiation. It's also a measurement of how much humanity has changed the Earth. That's why he suggests that the rise of cities thousands of years ago could also be a possible start date for the Anthropocene. If you want to take things even further back, the widespread migration of humans from Eurasia to other continents like Australia and the Americas started about 50 thousand years ago and forever altered the world's ecosystems.

Another feature of the Anthropocene could be humans doing large-scale geoengineering of the planet, mostly for agricultural purposes. Katti said that this includes "big dams which altered the flow of rivers, sometimes causing earthquakes; also our harvest of underground aquifers, which in some cases cause rock layers to shift." Fracking might be included in this list too.

The point is that the Anthropocene is a geological period whose uniqueness comes from a whole set of human practices. Perhaps the most obvious marker to a chemist or physicist would be the radiation layer of the post-World War II era. But CO2 levels are also crucial, as are all the ways we've moved around plants, animals, and even rivers to suit our needs.


Plus, geological layers usually encompass thousands if not millions of years. So future geologists may identify a pretty long period of time as the Anthropocene and still consider it a blink of an eye in deep time.

Is It Science or Politics?

Though it began purely as a stratigraphic term, Anthropocene has taken on a political meaning. Many of humanity's chemical signatures, such as elevated CO2 levels, are associated with climate change — and that means some people don't want to use the term because it's as good as admitting that humans are changing the nature of the planet.


Scientists like Barnosky and Steffen believe it's valid as a purely scientific term. Still, it has undeniable social implications. As Steffen put it:

If the Anthropocene has now attracted political interpretations, those have come from non-scientists who bring a political lens to a scientific concept. It isn't the first time science has challenged the world views of many people. Copernicus and Galileo did so, and of course Charles Darwin challenged the political and religious worldviews of many people.

Katti added:

I think the term has validity both scientifically and in the realm of human politics. Recognizing the magnitude of human impact on the planet, i.e., that we are leaving permanent marks on the planet's very surface, has clear political implications. And that is regardless of whether one considers those impacts good or bad.


Jamie Kruse and Elizabeth Ellsworth, the artists who focus on the Anthropocene in their work at Smudge Studio, say that the term in important precisely because it gives people a human perspective on the very difficult concept of deep time. It's hard to wrap our minds around the idea of a 4.5 billion year planetary history. But thinking in the context of an Anthropocene — whether it starts thousands of years ago with agriculture, or last century with the atomic bomb — allows us to understand our role as a species in the planet's long and complicated drama.

Kruse and Ellsworth told io9:

It makes science more accessible to the general public by providing an entryway into the entire geologic record and timescale that pivots on the realities of today's daily life. It also encourages scientists to acknowledge that their work can be increasingly relevant to the humanities . . . From our point of view, the more social, political — and aesthetic — scientific terminology can be, the better. The Anthropocene is a great term to use to stage conversations about the complexities of the material realities of our contemporary moment. It is useful for naming and locating the time we're experiencing as a time of great change that "breaks" with the Earth's material realities up to this point, and sets up deep geologic futures to come.


So if the term is political, it isn't just about the politics of climate. It's about the way humans interact with their environments, affecting other humans as well as other life forms. The Anthropocene describes the vast transformation the world has undergone because of our presence.

Photo by travelwayoflife

It might be useful to view the term Anthropocene in the context of another geological period, the Carboniferous. Beginning about 358 million years ago, it was a period that marked the widespread growth of trees across all the planet's land masses. These trees completely transformed the land, breaking it up with their root systems; and they transformed the atmosphere by converting CO2 into oxygen. And when those trees died, they left a distinctive layer of carbon in the geological record. Geologists named the period "Carboniferous" because of all those trees. In a sense, trees were as disruptive as humans during their first geological age, transforming the planet's systems from the crust all the way to the stratosphere.


Whether it's become a political term or not, the Anthropocene is likely destined for science textbooks. An Anthropocene working group led by geologist Jan Zalasiewicz has submitted a proposal to the International Commission on Stratigraphy to formally adopt the term in the geology community. The proposal will go before the ICS in 2016, and that will usher in a long process of deliberation.

How long? "Hard to say, quite," Zalasiewicz told io9. "It has to be considered by the Subcommission on Quaternary Stratigraphy and then by the ICS itself — and then, if it gets through that, it has to be ratified by the International Union on Geological Sciences." It sounds as if the Anthropocene may eventually wind up in exactly the kind of obscure meeting where it began. Despite its humble journey, this is an idea that seems destined to change the way we understand our impact on the world — and our future prospects on it.

Annalee Newitz is the editor-in-chief of io9. She's also the author of Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction. Follow her on Twitter.