When Did the Anthropocene Start? Scientists Closer to Saying When.

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The official timeline of Earth’s history — from the oldest rocks to the‌ dinosaurs to the rise of primates, from the Paleozoic to the Jurassic and all points before and since — could soon include the age of nuclear weapons, human-caused climate change and the proliferation of plastics, garbage and concrete across the planet.

In short, the present.

Ten thousand years after our species began forming primitive agrarian societies, a panel of scientists on Saturday took a big step toward declaring a new interval of geologic time: the Anthropocene, the age of humans.

Our current geologic epoch, the Holocene, began 11,700 years ago with the end of the last big ice age. The panel’s roughly three dozen scholars appear close to recommending that, actually, we have spent the past few decades in a brand-new time unit, one characterized by human-induced, planetary-scale changes that are unfinished but very much underway.

“If you were around in 1920, your attitude would have been, ‘Nature’s too big for humans to influence,’” said Colin N. Waters, a geologist and chair of the Anthropocene Working Group, the panel that has been deliberating on the issue since 2009. The past century has upended that thinking, Dr. Waters said. “It’s been a shock event, a bit like an asteroid hitting the planet.”

The working group’s members on Saturday completed the first in a series of internal votes on details including when exactly they believe the Anthropocene began. Once these votes are finished, which could be by spring, the panel will submit its final proposal to three other committees of geologists whose votes will either make the Anthropocene official or reject it.

Sixty percent of each committee will need to approve the group’s proposal for it to advance to the next. If it fails in any of them, the Anthropocene might not have another chance to be ratified for years.

If it makes it all the way, though, geology’s amended timeline would officially recognize that humankind’s effects on the planet had been so consequential as to bring the previous chapter of Earth’s history to a close. It would acknowledge that these effects will be discernible in the rocks for millenniums.

“I teach the history of science — you know, Copernicus, Kepler, Galileo,” said Francine McCarthy, an earth scientist at Brock University in Canada and member of the working group. “We’re actually doing it,” she said. “We’re living the history of science.”

Still, the knives are out for the Anthropocene, even though, or maybe because, we all have such firsthand familiarity with it.

Stanley C. Finney, the secretary general of the International Union of Geological Sciences, fears the Anthropocene has become a way for geologists to make a “political statement.”

Within the vast expanse of geologic time, he notes, the Anthropocene would be a blip of a blip of a blip. Other geologic time units are useful because they orient scientists in stretches of deep time that left no written records and sparse scientific observations. The Anthropocene, by contrast, would be a time in Earth’s history that humans have already been documenting extensively.

“For the human transformation, we don’t need those terminologies — we have exact years,” said Dr. Finney, whose committee would be the last to vote on the working group’s proposal if it gets that far.

Martin J. Head, a working group member and earth scientist at Brock University, argues declining to recognize the Anthropocene would have political reverberations, too.

“People would say, ‘Well, does that then mean the geological community is denying that we have changed the planet drastically?’” he said. “We would have to justify our decision either way.”

Philip L. Gibbard, a geologist at the University of Cambridge, is secretary general of another of the committees that will vote on the working group’s proposal. He has serious concerns about how the proposal is shaping up, concerns he believes the wider geological community shares.

“It won’t get an easy ride,” he said.

Like the zoologists who regulate the names of animal species or the astronomers who decide what counts as a planet, geology’s timekeepers work conservatively, by design. They set classifications that will be reflected in academic studies, museums and textbooks for generations to come.

“Everybody picks on the Anthropocene Working Group because they’ve taken so long,” said Lucy E. Edwards, a retired scientist with the United States Geological Survey. “In geologic time, this isn’t long.”

The geologic time scale divides Earth’s 4.6 billion-year story into grandly named chapters. Like nesting dolls, the chapters contain sub-chapters, which themselves contain sub-sub-chapters. From largest to smallest, the chapters are called eons, eras, periods, epochs and ages.

Right now, according to the current timeline, we are in — deep breath — the Meghalayan Age of the Holocene Epoch of the Quaternary Period of the Cenozoic Era of the Phanerozoic Eon, and have been for 4,200 years.

Drawing lines in Earth time has never been easy. The rock record is full of gaps, “a jigsaw puzzle with many of the parts missing,” as Dr. Gibbard puts it. And most global-scale changes happen gradually, making it tricky to pinpoint when one chapter ended and the next one began. There haven’t been many moments when the entire planet changed at once.

“If a meteor hits the Yucatán Peninsula, that’s a pretty good marker,” Dr. Edwards said. “But other than that, there’s practically nothing out there in the geologic world that’s the best line.”

The early Cambrian Period, around 540 million years ago, saw Earth explode with an astonishing diversity of animal life, but its precise starting point has been contested for decades. A long controversy led to the redrawing of our current geologic period, the Quaternary, in 2009.

“It’s a messy and disputatious business,” said Jan A. Zalasiewicz, a geologist at the University of Leicester. “And of course, the Anthropocene brings a whole new range of dimensions to the messiness and disputatiousness.”

It took a decade of debate — in emails, academic articles and meetings in London, Berlin, Oslo and beyond — for the Anthropocene Working Group to nail down a key aspect of its proposal.

In a 29-to-4 vote in 2019, the group agreed to recommend that the Anthropocene began in the mid-20th century. That’s when human populations, economic activity and greenhouse gas emissions began skyrocketing worldwide, leaving indelible traces: plutonium isotopes from nuclear explosions, nitrogen from fertilizers, ash from power plants.

The Anthropocene, like nearly all other geologic time intervals, needs to be defined by a specific physical site, known as a “golden spike,” where the rock record clearly sets it off from the interval before it.

After a yearslong hunt, the working group on Saturday finished voting on nine candidate sites for the Anthropocene. They represent the range of environments into which human effects are etched: a peat bog in Poland, the ice of the Antarctic Peninsula, a bay in Japan, a coral reef off the Louisiana coast.

One site — Crawford Lake in Ontario, Canada — is small enough to walk around in 10 minutes. But it is so deep that the bottom layer of water rarely mixes with the upper layers. Whatever sinks to the floor remains undisturbed, gradually accumulating into a tree-ring-like record of geochemical change.

The working group’s members also voted this month on what rank the Anthropocene should have in the timeline: an epoch, an age of the Holocene, or something else.

The group isn’t disclosing the results of these or the other votes to be held in the coming months until they are all complete and it has finalized its proposal for the next level of timekeepers to ponder. It is then that a far more contentious debate about the Anthropocene could begin.

Many scholars still aren’t sure the mid-20th century cutoff makes sense. It is awkwardly recent, especially for archaeologists and anthropologists who would have to start referring to World War II artifacts as “pre-Anthropocene.”

And using nuclear bombs to mark a geologic interval strikes some scientists as abhorrent, or at least beside the point. Radionuclides are a convenient global marker, but they say nothing about climate change or other human effects, said Erle C. Ellis, an ecologist at the University of Maryland, Baltimore County.

Using the Industrial Revolution might help. But that definition would still leave out millenniums of planet-warping changes from farming and deforestation.

Canonizing the Anthropocene is a call to attention, said Naomi Oreskes, a member of the working group. For geology, but also the wider world.

“I was raised in a generation where we were taught that geology ended when people showed up,” said Dr. Oreskes, a historian of science at Harvard. The Anthropocene announces that “actually, the human impact is part of geology as a science,” she said. It demands we recognize that our influence on the planet is more than surface level.

But Dr. Gibbard of Cambridge fears that, by trying to add the Anthropocene to the geologic time scale, the working group might actually be diminishing the concept’s significance. The timeline’s strict rules force the group to impose a single starting point on a sprawling story, one that has unspooled over different times in different places.

He and others argue the Anthropocene deserves a looser geologic label: an event. Events don’t appear on the timeline; no bureaucracy of scientists regulates them. But they have been transformative for the planet.

The filling of Earth’s skies with oxygen, roughly 2.1 to 2.4 billion years ago — geologists call that the Great Oxidation Event. Mass extinctions are events, as is the burst of diversity in marine life 460 to 485 million years ago.

The term Anthropocene is already in such wide use by researchers across scientific disciplines that geologists shouldn’t force it into too narrow a definition, said Emlyn Koster, a geologist and former director of the North Carolina Museum of Natural Sciences.

“I always saw it not as an internal geological undertaking,” he said of the Anthropocene panel’s work, “but rather one that could be greatly beneficial to the world at large.”

Sumber: www.nytimes.com

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