When did man dominate changes on Earth? A rock-hard question for scientists

By Gayathri Vaidyanathan | 03/12/2015 08:33 AM EDT

Evidence that humans have surpassed nature as the shapers of change on the planet is omnipresent, scientists say, and some say that shift marks the beginnings of a new epoch for the planet called the Anthropocene. But determining just when that epoch begins is a difficult task, and the ultimate arbiters of this shift would be geologists, who have yet to agree on assigning a date to the start of the Anthropocene Epoch, if it is formalized.

In 1607, a ship set sail from London carrying 40 soldiers, 35 "gentlemen," and 30 laborers and artisans to the Americas. They set up Jamestown, the first permanent British colony in the New World.

A century before their arrival, an estimated 54 million people were living in the Americas, both North and South. By 1650, the population crashed to 6 million.

"[The Europeans] brought with them so many diseases that the indigenous people had no immunity to," said William Ruddiman, a paleoclimatologist and emeritus professor at the University of Virginia. "[About] 85 to 90 percent of the people who had lived in the Americas died of those diseases."


These events are etched into the geological record of Earth and mark the beginnings of a new epoch for the planet called the Anthropocene, Simon Lewis, an ecologist at the University of Leeds, and Mark Maslin, a geologist at the same university, proposed in a study published yesterday in the journal Nature.

The Anthropocene, the "age of humans," implies a new world order where humans have surpassed nature as the shapers of change on the planet. The evidence is omnipresent, scientists say. Human activity has increased carbon dioxide in the atmosphere to levels not seen for at least 800,000 years. Some of the carbon has entered the oceans and changed its chemistry on a scale not seen in 300 million years. Humans have overfished the oceans, destroyed habitats and triggered a mass extinction event. Such changes happened in the past, but with nature at the helm.

Now, humans are the driving force, Lewis said in a phone interview.

"We really are seeing a shift to a new state in the Earth that would fit a new geological epoch," he said.

Consulting a ponderous ‘rulebook’

The ultimate arbiters of this shift would be geologists — scientists who bury themselves in deep time, examining things like the fossils of fungi that lived 460 million years ago. To better communicate such findings, geologists divide up Earth’s 4.6-billion-year history into eras, periods, epochs and stages.

Measured using their ruler, the planet is now in the Quaternary Period, encompassing the past 2.6 million years, and within that, the Holocene Epoch of the past 11,700 years.

Anthropocene — if it is formalized — is the period that would follow. The geological community is divided, and the matter will be decided in 2016.

The decision will be keenly watched beyond the geological community because the epoch, by emphasizing humans’ central role in the natural world, marks a seismic shift in communicating people’s responsibility toward the planet. The term has already gripped the public’s consciousness. "Enter the Anthropocene — the Age of Man," National Geographic declared in 2011. The Economist‘s 2011 cover welcomed readers to the Anthropocene.

Geologists are not so hasty. To start a new epoch, they have to consult a hefty rulebook created in the mid-1800s to assign Earth its time periods. The rules were crucial back then. To understand why, the next time you drive by an outcrop of sedimentary rock cleaved by the road, pause and examine its striations. Each layer of sediment, solidified into rock by compaction, has a time stamp.

Modern geochemists can dig out sediment from each layer and run it through machines that date carbon particles to reveal the original age of deposition more or less precisely. But in the 1800s, before the advent of radiocarbon dating, scientists relied on their rulebook to date layers and understand the origins of the objects of interest.

The geologists’ rulebook states that each period or epoch should correspond to a massive planetary upheaval that leaves a physical mark on the Earth. Consider, for example, the end of the Cretaceous Period and the beginning of the Paleogene, when most dinosaurs went extinct, the result, possibly, of an asteroid or comet strike 66 million years ago.

The crash spread a layer of iridium, a rare element, around the world, and it can be detected as a signal in the rock record. Geologists use this layer as the boundary between the Cretaceous and the Paleogene, and they marked the physical manifestation of this boundary with a golden spike hammered into a hillside in El Kef, Tunisia.

A similar global boundary has to be found for the Anthropocene. And that, in a nutshell, is the problem.

Human progress has been so incremental since the last ice age ended 11,700 years ago that geologists find it difficult to pinpoint in the rock record one year when the Holocene ended and the Anthropocene began.

Transition to a new world

In the Nature study, Lewis suggests 1610 as the beginning of the Anthropocene, a time when the first European settlers arrived in the New World. Indigenous people were already dying from disease, famine and war. Farming nearly stopped, and people reduced controlled forest burns, which allowed vegetation to reclaim savanna and grassland. The plant growth trapped carbon, and carbon dioxide levels in the air dropped by 7 to 10 parts per million.

Lewis and Maslin, the authors, call this drop in CO2 the "Orbis spike" and find evidence for it in ice cores drilled out of Antarctica. The marker in ice could be appropriate for a metaphorical golden spike, Lewis said — metaphorical, because it would be impractical to hammer a spike into an ice core.

To justify the choice of golden spike, the authors point to world history that followed the collision of the Old World with the new. It led to global movement as African slaves moved to the Americas and produced commodities for further trade to Europe. Silver was mined in Bolivia and shipped to China for use as the national currency. Ships crossed the oceans carrying crops and animals and mixing up biota. Today’s hybrid species are evolving on an entirely new stage, Lewis said.

"There were all sorts of accidental shifts of species," he said, and cited an example: "European earthworms are now all over North America."

Epoch of human dominance

Lewis is not the first to enter this debate, which began in 2000 when Nobel laureate Paul Crutzen offhandedly suggested during a geology conference that the world is in the Human Age.

Phil Gibbard, a geologist at the University of Cambridge, was leading the International Commission on Stratigraphy’s Quaternary subcommission at the time. The subcommission is responsible for the past 2.6 million years, including the present day, so it set up the Anthropocene Working Group to ponder this new epoch.

Gibbard dropped out of the working group when he became convinced that the Anthropocene is not needed, he said. He is not ready to end the Holocene Epoch of the past 11,700 years, which is already predicated on the rise of humans. What is the need, then, for the Anthropocene? Gibbard asked.

The Holocene is littered with evidence of human dominance. Humans who colonized North American 10,000 years ago killed off mastodons and mammoths; they lost their wandering tendencies and settled into agriculture beginning 11,000 years ago; about 6,500 years ago, farmers began cultivating rice using irrigation, which allowed greenhouse gas levels in the atmosphere to spike and keep global temperatures temporarily warm.

And by 1610, when the English set up colonies in the New World, indigenous people were already changing landscapes around the world, Ruddiman of the University of Virginia said. Emerging research suggests massive deforestation was ongoing in much of Europe, northern and central China, and central India, he said. These massive changes that fundamentally altered the planet would get excluded by a 1610 start date, he said.

"I’m in basic disagreement in defining such a thing as the Anthropocene," he said. "With all this long history that goes back more than 10,000 years, why compress all that into one single level that is formally defined?"

Was it 1610 or 1945 or 1964?

Naysayers aside, scientists continue to probe the idea. Some have suggested the Anthropocene started with the Industrial Revolution of the 1800s, when coal use heralded technological progress. But they have not found a global marker in the rock record that could satisfy the geologists’ rulebook.

The Anthropocene Working Group, which now has 38 members, has suggested that the epoch began on July 16, 1945. In the deserts of Alamogordo in New Mexico, the United States detonated the first nuclear bomb that day, and the fallout traveled to remote corners of the world. The radioactive fallout would be an ideal start date, the working group said.

The date would match a time of "Great Acceleration" for humanity, when populations burgeoned at an unprecedented rate, the working group said. Petroleum products accumulated in the environment, and CO2 levels in the atmosphere rose steeply from 311 parts per million in 1950 to 400 ppm today.

Lewis and Maslin also explore the Great Acceleration era in the Nature study and suggest 1964 as an alternate, though less favored, possibility for a start date. The golden spike could be placed on a tree ring of a pine in King Castle, Poland, that contains a carbon signal from the nuclear era, they suggest.

"Purely personally, I’m delighted by [the suggestion]," said Jan Zalasiewicz, a stratigrapher at the University of Leicester and chairman of the working group. "Niepolomice Castle is a lovely place which I know well."

Zalasiewicz still has reservations about the pine tree proposal and, particularly, about 1610 as the start of the Anthropocene, but welcomed suggestions from outside the working group.

Arbiters of planetary time

All proposals — from Lewis, the Anthropocene Working Group and various others working on the topic — would be submitted to the International Commission on Stratigraphy’s (ICS) Quaternary subcommission in 2016. If more than half the members vote in favor, the proposals would be forwarded to the main body of the ICS.

Its members would vote on the proposals, and if a majority agrees, it would pass the baton to the International Union of Geological Sciences, a UNESCO body composed of geologists that arbitrates on matters of geologic time.

Whether the proposals will survive the process remains uncertain because most geologists remain ambivalent about matters of such small consequence against the 4.6-billion-year backdrop of Earth history.

But everyone else cares deeply. The Anthropocene idea is so sticky because of the urgent need to convey to people that the planet "was a pleasure garden that existed before humans started messing around with everything, and we are now living in a dustbin of our own making," Gibbard said.

He agrees with that core message, and the idea could survive even if the epoch is not formalized.

"I think that the discussions we have been having and the concepts, I think, [it] has been a very valuable one," he said.