A Time of Our Own: Defining the Anthropocene

In one of the most memorable sequences of his TV show Cosmos, Carl Sagan mapped the entire history of the universe onto a “cosmic calendar.” On this 12-month scale, it took until September for the Earth to be formed, and humanity did not enter the picture until 10:30 p.m. on December 31 – a sobering reminder of our minor existence in time.

Despite our relatively brief residence on this planet, humans have severely altered Earth in ways no other species has come close to – we cultivate vast amounts of land for our food, emit billions of tons of chemicals into the atmosphere annually and move more sediment with our mining activities than all the world’s rivers combined. A Public Library of Science One paper found evidence of discarded stone tools from early hominids littering an area in the Sahara Desert – evidence that we’ve affected the planet’s landscapes for millions of years.

Even more worryingly, our activities are killing off many of Earth’s species – a paper in Science last year estimated that between 11,000 and 58,000 of Earth’s approximately 5-9 million animal species are going extinct every year. This is an extinction rate equaling those observed in past mass extinctions.

In the face of these massive effects, Nobel Prize-winning atmospheric chemist Paul Crutzen and  biologist Eugene Stoermer argued in 2000 for emphasizing “the central role of mankind in geology and ecology by proposing to use the term ‘anthropocene’ for the current geological epoch.” Since then, the proposal has gained support from a large segment of the scientific community (there are now 3 extant journals focusing on the concept), and in 2009 the International Commission on Stratigraphy, the scientific body in charge of defining the geologic time scale (Earth’s “calendar”), formed “the Anthropocene Working Group” to figure out the details of adding a new period of geologic time.

The March of Time

Because of the overwhelming size of Earth’s timescale (4.54 billion years), scientists use the geologic time scale to discuss earth’s various ages. Scientists have divided Earth’s history into five scales in descending size: eons, eras, periods, epochs and ages. As you can see in the diagram below, these divisions are not uniform in duration because they are based on the Earth’s strata, or layers of rock and fossils (hence the “geologic” part). This chart shows the current standard for the GTS, with the Earth’s formation at the bottom and the present day at the top (Ma=millions of years ago).

Geologists mark new divisions based on where significant changes in the strata occur, usually because of some Earth-altering event. In the picture below, the black band running through this rock formation marks the boundary between the Cretaceous and Paleogene periods (the larger “Tertiary” period represented in the chart above is increasingly being discarded by scientists in favor of the two smaller Paleogene and Neogene periods). This band was caused by the Cretaceous-Paleogene extinction event, more commonly known as the asteroid impact that killed off the dinosaurs. In the Paleogene period that followed, increasing numbers of mammalian fossils show how mammals went from being Earth’s losers to one of its dominant forms of life.

Scientists have traditionally recognized five major extinctions events in Earth’s history. Each one caused massive shifts in the planet’s number and diversity of species, and represented the start of a new geologic period. The current geologic period, the Holocene, began about 11,700 years ago at the end of the last glacial period (or “ice age”).


A feature article by science journalist Richard Monastersky in last week’s issue of Nature analyzes the support and dissension among scientists used to studying the distant past on how to define the present day.

An ally of the push is botanist John Kress, interim undersecretary for science at the Smithsonian, which has hosted two symposiums on the Anthropocene. The Smithsonian has so fully embraced the epoch that it plans to include a section on it in its renovated fossil hall at the National Museum of Natural History. “Never in its 4.6 billion-year-old history has the Earth been so affected by one species as it is being affected now by humans,” Kress told CBS News at the time of last fall’s symposium.

Another advocate is Australian climate scientist Will Steffen, who even gave a TEDx talk on the Anthropocene in 2010. Referring to graphs of metrics including global biodiversity, greenhouse gas levels, and hemisphere temperatures, Steffen said “In each case […] we have left that envelope of environmental stability which typifies the Holocene.”

On the other side of the debate are scientists who question whether it’s really appropriate to create a new division of geologic time based on no rock or fossil records. A 2012 review paper by geologists Whitney Autin and John Holbrook made its intentions clear in its title: “Is the Anthropocene an issue of stratigraphy or pop culture?”

“If there is an underlying desire to make social comment about the implications of human-induced environmental change,” the authors wrote in GSA Today, “Anthropocene clearly is effective. However, being provocative may have greater importance in pop culture than to serious scientific research.”

Autin and Holbrook’s assertion strikes at the increasingly controversial nature of this debate, with some scientists feeling that the push is being driven more by the media and climate change scientists like Steffens than by geological facts. “What you see here is, it’s become a political statement,” said Stan Finney, chairman of the ICS in Nature. “That’s what so many people want.”

Alternatively, some other scientists suggest that it’s too early into the proposed Anthropocene epoch to define it, and in the words of paleoclimatologist Eric Wolff, “it might be wise to let future generations decide, with hindsight, when the Anthropocene started, acknowledging only that we are in the transition towards it.”

In the middle of the debate is Jan Zalasiewicz, the chairman of the Anthropocene Working Group. In a 2008 review paper advocating the new epoch, Zalasiewicz wrote that the changes humans have made to the Earth since the Industrial Revolution are feasibly large enough to leave a unique impression in the rock record for this era. “These changes,” he and his colleagues wrote cautiously, “although likely only in their initial phases, are sufficiently distinct and robustly established for suggestions of a Holocene–Anthropocene boundary in the recent historical past to be geologically reasonable.” That paper got him the job of leading the working group, according to Nature, and the unenviable task of impartially sorting through many opinions.


In a review article, British scientists Simon Lewis and Mark Maslin discuss one of the most pressing questions about the Anthropocene besides whether it should exist: When does it start? Several dates have been put forward over the past decade and a half: Paul Crutzen initially suggested the beginning of the Industrial Revolution in the 1800s, while others have suggested the Agricultural Revolution about 11,000 years ago.

In January, the Anthropocene Working Group announced tentative support for dating the epoch’s start to the beginning of the Nuclear Age – specifically the July 16, 1945, Trinity nuclear test in New Mexico.

Lewis and Maslin, however, argued that two different dates make the most sense for the start of this new epoch: 1610 or 1964. While these years may seem a bit arbitrary compared to the other events put forth, the authors sought to find a time when definite changes occurred on the Earth that would be preserved in the strata – a reference point called a Global Boundary Stratotype Section and Point.

Following Christopher Columbus’ initial voyage to the Americas in 1492, a wave of European settlement on the continents changed them drastically. In particular, the deaths of more than 50 million Native Americans from disease and warfare led to the regrowth of over 50 million hectacres of formerly developed land. This caused massive uptake of carbon dioxide from the atmosphere in a very short span, something which would definitely show up in ice cores and rock samples. The Columbian Exchange that occurred during this time also caused many plant and animal species to appear outside their natural habitats suddenly, which will appear unusual in the fossil record.

The other candidate date is 1964, when radioactive carbon levels peaked in the atmosphere. This year is close to the beginning of the “Great Acceleration” – a rapid growth of human population during the second half of the 20th century, and also a year after the Partial Test Ban Treaty ended above-ground nuclear testing.

Epoch TBD

The Anthropocene Working Group will present its initial recommendations to the ICS next year, but the voting and revision process could take years. Of course, that’s just the blink of an eye in the geologic time scale.


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