What is the Anthropocene? by Pecier Decierdo

There was a time when dinosaurs ruled the Earth. Then there was a time, far more ancient, when life moved from sea to land. Deeper yet in time, there was an age when the activities of single-celled microorganisms left lasting marks on the planet. It is then that our atmosphere got filled with oxygen. We are still breathing the effects of that long-gone but momentous era. 

Right now, many scientists argue, we are in the age when human activities have become a dominant force in the planet. We are in the "age of humans". We have entered the Anthropocene. 

A shale oil plant emitting smoke. Experts think that human activities such as agriculture and energy production
are leaving lasting marks on the planet.
(Photo credit: Wikimedia Commons)

In a recent meeting, scientists in the Working Group on the Anthropocene (WGA) voted to officially declare a new age in the history of the Earth. Called the 'Anthropocene epoch', this is the stage in Earth's history when human activities have made lasting marks in the geologic record. Previous to this decision, geologists defined the present as part of the Holocene epoch. Scientists in the working group propose the mid-20th century as the end of the Holocene and the beginning of the Anthropocene. 

This decision, however, is not yet written in stone. Not all scientists are as excited as the members of the WGA about changing the textbooks to include the Anthropocene epoch. It has something to do with the very unique way the idea of the Anthropocene came about. 

Reading the rocks

Geologists look at the layers of rocks and ice for clues about what happened in the Earth's past. By piecing together these clues, geologists have come up with a scenario about what happened in different stages of the Earth's 4.6 billion year history.

The clues in the rock and ice are collectively called the geologic record. The geologic record allows geologists to answer questions about the Earth's past. How old is this layer of rock? Was the Earth relatively warm or cool during the time the rock formed? What was the composition of the atmosphere then? What organisms were plentiful at the time? Geologists answer these questions by looking at changes in the chemical composition and kinds of fossils found in the geologic record.

Using the geologic record, scientists have organized the Earth's 4.6-billion year history in what is known as the geologic time scale. 

Each part of the geologic time scale represents a stage in the Earth's history that has left a permanent mark in the geologic record.

The geologic time scale.
(Photo credit: Ray Troll)

One can think of it this way. The geologic record is the evidence. The geologic time scale is the scenario. The geologists are the investigators who gather the evidence and try to come up with the scenario that best fits the evidence.

Often, like good investigators, geologists start with the geologic record. Using the record, they come up with stages of the geologic time scale.

For example, by analyzing layers of ice, geologists discovered that the last major ice age ended 11,700 years ago. Before the end of that ice age the Earth was much cooler; the glaciers covered a larger part of the planet, and sea levels were lower. When that major ice age ended, the Earth warmed, the glaciers receded, and the 'land bridges' were flooded and cut. Geologists use this as evidence for the end of the Pleistocene epoch, an epoch ruled by ice, and the beginning of the Holocene. The relatively stable and warm climate of the Holocene is what made human civilization possible. 

A unique epoch

The Anthropocene was proposed by Paul Crutzen and Eugene Stormer in 2000. The idea behind it was to illustrate how far-reaching the effects of human activities were. Humans, Crutzen and Stormer argued, are shaping the planet in ways that will leave permanent marks in the geologic record. We are disrupting the balance of the Earth to the point that we are out of the stable, balmy Holocene and have entered a new era.

This makes the Anthropocene unique among other stages in the geologic time scale. Instead of being suggested by the geologic record, the Anthropocene was suggested by observations of the present day world.

Ever since Crutzen and Stormer proposed the Anthropocene, scientists have added to the list of human-caused events that they think might leave marks in the geologic record. 

One example of the effects of human activities is the extinction of a lot of species.

Species go extinct all the time. The average number of species going extinct every one million years is known and is called the 'background extinction rate'. Ever since humans have come into the picture, the rate of extinction has dramatically risen above this background extinction rate. This has led many scientists to propose that a mass extinction is currently underway. 

There have been five other mass extinctions in the history of the planet. The most popular among these is the extinction of the non-avian dinosaurs that happened about 65.5 million years ago. It was probably caused by a meteor strike. In her book, The Sixth Extinction: An Unnatural History, Elizabeth Kolbert argues that humans are responsible for the current sixth wave of mass extinction.

The Sixth Extinction by Elizabeth Kolbert, where Kolbert argues that humans are causing the
sixth mass extinction even in the Earth's history.
(Photo credit: Henry Holt & Company)

Many have argued that this mass extinction can be an indicator for the Anthropocene. However, since most of the species humans have made extinct had limited ranges (the dodo only lived on the island of Mauritius), such extinctions are not likely to leave marks on the geologic record.

In order to justify the addition of the Anthropocene in the geologic time scale, an identifiable geologic marker must be found. Scientists in the WGA have suggested several possible markers.

One contender is the increase in the amount of artificial radioactive isotopes in the rocks. These isotopes mark the beginning of the nuclear era, when humans started detonating nuclear bombs that spread these artificial isotopes around the globe. 

Another contender is the increase in the amount of plastic found in the sediments that will eventually become rock. Most of these are "microplastics" which have made their way from human settlements to areas as far as the deep sea floor.

Another popular candidate is the spike in global temperature brought about by the rise in greenhouse gases. This is closely linked to two other candidates, the concentration of carbon dioxide in the atmosphere (which can be recorded by bubbles trapped in ice) and the spread of unburned carbon spheres emitted by factories.

Other candidates include aluminum and concrete particles in the soil, the doubling of nitrogen in phosphorus in the ground due to fertilizer use, and even the widespread fossil remains of farmed chickens. 

Some way to go before going official

The WGA does not have the power to officially recognize a new epoch in the geologic time scale. That responsibility goes to the International Commission on Stratigraphy (ICS). The working group's decision only serves as a proposal to the ICS. Upon approval by the ICS, an even higher body, the International Union of Geological Sciences (IUGS), must ratify the addition of a new epoch.

Workflow for approval of a revision to the geologic time scale. 
(Photo credit: The Geological Society of America)

This can be an issue, as some members of the ICS are not very sure about the validity of the new epoch. Stanley Finney and Lucy Edwards, both members of the ICS, point out that the geologica time scale is about the past while the Anthropocene is about the present and future. They also pointed out that, unlike previous epochs, which have been suggested by the geologic record, the Anthropocene was proposed before geologic evidence can be gathered and analyzed. 

Finney and other scientists have another objection to the Anthropocene - it is very short. "Its duration is that of an average human lifespan," Finney wrote in a commentary published by the Geologic Society of America. Critics of the Anthropocene say there has not been enough time since the mid-20th century to leave a record in the rocks and ice. 

This has led Finney, who chairs the ICS, to ask whether scientists are being asked to make a political rather than a scientific statement in ratifying the Anthropocene epoch.

Jan Zalasiewicz, chair of the WGA, understands Finney's skepticism. "Our stratigraphic colleagues are very protective of the geologic time scale, " Zalasiewicz told The Guardian. "They see it very rightly as the backbone of geology and they do not amend it lightly." Still, he is confident he can convince his fellow scientists about the Anthropocene. "I think we can prepare a pretty good case," he said.

And many other scientists are on his side.

Chris Rapley, a climate scientist at University College London, agrees with the recommendation of the WGA. "The Anthropocene marks a new period in which our collective activities dominate the planetary machinery," he told The Guardian. "It is highly appropriate that geologists should pay formal attention to the change in the signal within sedimentary rock layers that will be clearly apparent to future generations of geologists for as long as they exist."

Rapley also said that the 'great acceleration' in human activity in the past century constitutes a "strong, detectable, and incontrovertible signal" for the Anthropocene. Rapley is not part of the WGA. 

Not all experts who believe in the Anthropocene are excited, however. 

Clive Hamilton, an ethicist at Charles Sturt University in Australia, warned against scientists who declared, "Welcome to the Anthropocene".

"At first I thought they were being ironic, but now I see they are not. And that's scary," Hamilton wrote in the journal Nature. "The idea of the Anthropocene is not welcoming. It should frighten us. And scientists should present it as such."

Even if approved, it might take years for the new Anthropocene epoch to be added to the textbooks. But if it happens, most of us woulud have lived our lives in an age dominated by the effects of human activities. And there might be no coming back.


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