Ivan Alvarado / Reuters

Here is the hypothesis: Not so long ago, the very nature of planet Earth suffered a devastating rupture. The break was sudden, global, and irreversible. It happened on a Sunday within living memory. Mick Jagger, Meryl Streep, and Caitlyn Jenner were all born before this crack in time. Vladimir Putin, Liam Neeson, and Mr. T were all born after it.

That idea might soon carry the weight of scientific fact. Later this month, a committee of researchers from around the world will decide whether the Earth sprang into the Anthropocene, a new chapter of its history, in the year 1950. If accepted, this delineation will signal a new reality, that human activities, not natural processes, are now the dominant driver of change on Earth’s surface—that carbon pollution, climate change, deforestation, factory farms, mass die-offs, and enormous road networks have made a greater imprint on the planet than any other force in the past 12,000 years.

Starting next week, the committee’s 37 members will vote on two questions. First, should the Anthropocene be added as a new epoch to the Geological Time Scale, the standard scientific timeline of Earth’s 4.5-billion-year history? Second, should the Anthropocene, if it does exist, commence in the middle of the 20th century?

William Ruddiman, a professor of environmental sciences at the University of Virginia, is extremely worried about climate change, but he nonetheless hopes the committee votes against both questions. For the past two years, he has lobbied its members to think of the Anthropocene not as a sudden upheaval, but as a gradual change, a slow transformation of the planet that began 5,000 years ago. “Where could you possibly pick a single start date in this ever-evolving story?” he once asked me in an email.

Last week, he and 23 other researchers argued the topic at length in the scientific journal Progress in Physical Geography. At stake is a seemingly simple question: When did human influence over the environment reach a tipping point?

For Jan Zalasiewicz, a professor of geography at the University of Leicester, the answer is clear. Zalasiewicz chairs the Anthropocene Working Group, the committee that will soon vote on the existence of the epoch.

“If you look at the main parameters of the Earth-system metabolism, then … things only began to change sharply and dramatically with industrialization,” he told me. He believes that the most significant event in humanity’s life on the planet is the Great Acceleration, the period of rapid global industrialization that followed the Second World War. As factories and cars spread across the planet, as the United States and U.S.S.R. prepared for the Cold War, carbon pollution soared. So too did methane pollution, the number of extinctions and invasive species, the degree of surface-level radiation, the quantity of plastic in the ocean, and the amount of rock and soil moved around the planet.

It was “the Big Zoom,” he said, borrowing a phrase from the journalist Andrew Revkin. There is “nothing really comparable” to that shift in any other period of Earth history. Even setting carbon pollution aside, he said, the spike in fertilizer use led to the largest jump in surface nitrogen levels in 2.5 billion years. Zalasiewicz hopes the committee will start the Anthropocene in the middle of the 20th century.

Ruddiman isn’t so sure. He believes that humanity’s effect on the planet is spread throughout time and is driven primarily by agriculture. Before the year 1750, he argues, humans had already cleared so much forest as to produce 300 billion tons of carbon emissions. Since 1950, deforestation has only led to 75 billion tons of emissions.

Humans remade the planet in other ways, too. About 12,000 years ago, we drove a huge swath of American mammals, including the giant ground sloth, into extinction. About 11,000 years ago, we entered into unprecedented relationships with crops and some livestock, domesticating them and taming their genome. Between 6,000 and 7,000 years ago, humans began clear-cutting forests to create new agricultural land; they may have transformed much of Europe by doing so. And by about 1,000 years ago, as humans embraced tilling and made rice paddies, they began moving more dirt and rock around the surface of the planet than is moved naturally.

“I don’t think it’s possible to put an exact date” on the Anthropocene, Ruddiman told me last week. “It goes on continuously for 12,000 years. There’s no obvious break point. Even just the invention of tilling—it’s huge.” For that reason, he believes that the committee shouldn’t add a capital-A Anthropocene to the geological timeline. Instead, scientists should talk about the “lower-a anthropocene”—a set of profound changes wrought to Earth over the course of millennia, across many different places. They culminate in the biggest anthropocene of all: modern, human-caused climate change.

It is important to say modern, for Ruddiman believes that humans have already shifted the climate once before. About a decade ago, he proposed what’s called the “early anthropocene hypothesis”—a theory that ancient agricultural clear-cutting added so much carbon to the atmosphere that it effectively stopped Arctic glaciers from expanding more than 3,000 years ago. If not for that deforestation, then there would be an additional Greenland’s worth of ice in the Canadian Arctic today, he said.

While Ruddiman’s hypothesis is not widely accepted, it is taken seriously by the community. And his broader skepticism of codifying a late Anthropocene is shared by several members of the working group. In a separate paper published last week, five members of the committee rejected the idea of the 1950s Anthropocene. Today’s scientists are simply too close to the events at hand to place a division in geological time, they argue. We don’t yet know how significantly the planet’s climate will change in the centuries to come: Will the shift be of the same magnitude as what occurred at the end of the last Ice Age, 12,000 years ago? Will it be equal to the first time that ice seized the surface of Earth, 2.1 million years ago? Or does it signal something far larger, a cataclysm on par with the asteroid impact that ended the dinosaur-dominated Mesozoic Era, 66 million years ago? “There is no testable way of knowing at present,” they wrote.

The five authors also point out that the last 12,000 years would be understood as a single geological instant if they had happened millions of years ago. (Indeed, it would be one of the most shocking geological moments in the whole rock record.) And they worry about the sudden divisions that a great split in 1950 would impose on geology. If the Anthropocene is adopted as a formal time division, it will mean that any process that began in 1947 and ended in 1953 would straddle two epochs.   

So far, the committee at large has not seemed to accept these criticisms. In another paper published last week, Zalasiewicz and 16 of his colleagues wrote that any human-induced changes prior to 1950 paled in comparison with those that came after.

“The difference between before and what’s happening now … it’s geologically quite dramatic,” Zalasiewicz told me. “We hadn’t realized that at the beginning. In 2009, I didn’t know that the Anthropocene would be as clear and sharp as it has been. I thought it might fade away into a fuzzy gradational change.” Instead, the committee has accumulated more and more evidence that a new epoch lurched into existence during the mid-20th century, he said.

Carbon pollution, methane pollution, and world population all spiked after 1950 as they never had before, he argues. Ruddiman told me he doubted some of the committee’s reconstructions of human population, but appreciated their “good-faith effort to respond.”

Population, Carbon, and Methane All Spiked After 1950

Average values of relative change to (a) global human population, (b) atmospheric CO2 concentration, and (c) CH4 concentration over the past 20,000 years. (Zalasiewicz et al. / Science)

The idea of the Anthropocene was first proposed by the Nobel-winning chemist Paul Crutzen in 2000. Since then, it has caught on more broadly in culture, even though it is not a formal term in geology. (The musician Grimes is releasing an Anthropocene-themed album later this year.) But it could soon have its day: If the working group accepts its existence, that will clear the way for the International Commission on Stratigraphy and the International Union of Geological Sciences to accept it in full.

Of the working group’s 37 members, 17 members signed their name to Zalasiewicz’s paper, and only five signed their name to the more skeptical review. That leaves 15 committee members unaligned in advance of the upcoming vote. “You’d think people who served on a committee for years would be more willing to put their name on paper,” Ruddiman said. The vote will take place electronically and continue through May. If it succeeds, then the committee will busy itself with the next task: finding evidence in the rock record of the precise moment that humanity pushed Earth into a bewildering new era.

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