Humanity left an irreversible imprint upon Earth's rocks. Here's how.

BerlinOn a stage in central Berlin one night last month, Jens Zinke slid a white slab out of a clear plastic sleeve. At first it looked like a piece of Styrofoam, with a pencil-sized groove cut along its surface. But on closer inspection the slab proved as hard as rock: It was a piece of coral cut from Flinders Reef, a towering undersea formation some 150 miles off the east coast of Australia.

Flinders is the kind of secluded place you’d think would preserve pristine nature. The immediate area “is devoid of the usual human influences—tourism, agricultural runoff, and industrial pollution,” says Zinke, a paleoclimatologist at the University of Leicester in the United Kingdom.

That may make the reef an ideal spot to illustrate the concept that humans are changing the Earth not just locally but globally—and in geologically lasting ways. Since World War II, rapid increases in the human population and in our industrial and agricultural activity have created what’s been called a “Great Acceleration” in human impact.

The shift in Earth’s systems is so profound, some researchers argue, that we’ve entered a new geological epoch. After the Pleistocene ice ages and the warm and stable Holocene epoch that, over the last 12,000 years or so, gave rise to human civilization, we’ve now created the “Anthropocene.”

If so, then geologists need a way to pinpoint its beginning in a tangible way. Zinke was one of dozens of scientists who gathered at a conference in the German capital to talk about sites around the world that could mark the onset of the Anthropocene.

“Is the Anthropocene a real thing in the geological record? The answer to that is yes,” says Anthony Barnosky, a biologist who manages Stanford University’s Jasper Ridge Biological Preserve and who also attended the Berlin meeting. “The next step is: Find us a site that clearly shows the transition, and a time it starts, and a signal you can look for … a globally synchronous marker that will last in the rocks forever.”

Flinders Reef is one of a dozen candidates still under consideration. The corals there grow about a centimeter a year, or four tenths of an inch. In capturing chemicals from seawater they create a precise record of changes in its chemistry. X rays of the pencil-thick samples Zinke removed from the coral reveal annual growth lines—like tree rings, but invisible to the naked eye—that can date the coral precisely. His samples from Flinders Reef go back more than 300 years, to 1710.

For most of that time, graphs measuring the chemical content of the coral don’t change much. But beginning in 1957, the Flinders Reef cores capture a sharp spike in radioactive isotopes like plutonium and radiocarbon, the legacy of above-ground atomic testing carried out before a global ban went into effect in 1963. The coral also records higher amounts of salt and nitrogen.

“It all shows the impact of people on the planet,” Zinke says. 

Golden spikes

To define the beginning of a new stage on the geological time scale, geologists use markers called “global stratotype section and point” (GSSPs) or “golden spikes." They’re conceptual as well as physical: Researchers look for the earliest feature that sites from that period around the world have in common. Then they may attach a physical marker—more likely a brass plaque than an actual golden spike—to the base of that layer at a site where the feature is readily recognizable. (Ice, too, can qualify for a golden spike–in which case samples stay untouched in a freezer.)

Often the key feature is a fossil, but not necessarily the most famous one. The Jurassic Period is best known for Diplodocus, Stegosaurus, and other dinosaurs, but its onset is defined by the rapid spread of a particular marine mollusk species, a kind of ammonite called Psiloceras spelae. “Most studies focus on one primary marker—one fossil and its appearance, or one biochemical marker,” says geologist Colin Waters, one of the Berlin conference’s organizers.

Qualifying for a “golden spike” is no easy task. Each geological boundary gets just one, and potential sites go through a years-long vetting process. A committee of experts called the Anthropocene Working Group (AWG) has been at work for more than a decade.

After settling on the 1950s as the most likely starting point for the Anthropocene, AWG members began looking for a site that would capture physical evidence. A dozen contenders soon emerged, from Flinders Reef to Beppu Bay in Japan and an Antarctic ice sheet. At the meeting in Berlin, each site’s results were presented in detail, giving researchers a chance to compare evidence. “This is a coming-out party for all these sites,” Barnosky says.

Over the next few months, the researchers will pore over the assembled data. By the end of the year, they will choose a single site. Their choice will then have to be ratified by a larger group of geologists, the International Commission on Stratigraphy, which is itself part of the International Union of Geological Sciences. According to the rules, the golden spike of the Anthropocene has to be in a place that other scientists can go and see, sample repeatedly and get the same results–although that can include stored coral or ice cores.

Bombs and plows

At the Berlin meeting, presentation after presentation told a similar story: Whether in Antarctic ice cores, California mud or Australian coral, something dramatic changed in the 1950s—and has continued changing in the decades since. “It’s not just one single piece of evidence,” says University College London geographer Simon Turner. “We have an abundance of data that shows an acceleration of human activity in the environment.”

Jerome Kaiser, a researcher at the Leibniz Institute for Baltic Sea Research in Germany, extracted a tiger-striped, 18-inch-long sediment core from the bottom of the Baltic Sea, a few hundred kilometers off the coast of Germany. Unlike the Antarctic, the Baltic region is heavily populated—85 million people live in the sea’s catchment area. Its depths are mostly oxygen-free and still, so sediment settles gently to the bottom, forming a compact record of everything that flows into the sea. Kaiser’s core captures 150 years of sea-floor mud.

Pointing to a spot a little more than halfway up the core, Kaiser says things really begin to change around 1956. That’s when the thin layers of mud begin to include the invisible residue of changes happening around the world: radioactive plutonium and americium from bomb tests in the far-off Pacific, the appearance of the toxic pesticide DDT, and higher levels of soot particles from coal-fired electrical plants that proliferated after WWII.

Though many such indicators can only be detected under a microscope or by testing for specific chemical residues, one change is more obvious. Kaiser says that in the post-war period, as European farmers adopted artificial fertilizers on a massive scale, dozens of rivers began flushing enriched soil straight into the Baltic, nutrients that boosted algae and other marine plant populations. The shift is starkly visible: The color of the sediment changes abruptly as more organic matter appears, from gray to dark brown.

“There is a clear mid-1950s transition that’s visible to the naked eye,” Kaiser says. “You can really say, ‘this is the Anthropocene starting, right here.’”

Some candidates for a golden spike show even clearer signs of human influence. In the hills overlooking San Francisco Bay, a mostly silted-up lake preserves 130 years of annually deposited mud, in sharply defined layers an inch or so thick. Called the Searsville Reservoir, it’s an artificial lake created by a dam project in 1892, now part of Stanford’s Jasper Ridge Biological Preserve.

“It’s a geologic record created by human activity,” says Stanford paleobiologist Allison Stegner. 

Each year, the mud at Searsville traps substances typical of the accelerating Anthropocene. Beginning in the 1930s, for example, there are measurably more spheroidal carbonaceous particles—a technical term for the fine-grained soot emitted by power plants and factory smokestacks—along with other pollutants like mercury. There is also lead from increased use of leaded gasoline around the world. “We see global signals of the great acceleration,” Stegner says. (After leaded gas began to be phased out in the 1970s, lead levels declined.)

An especially clear indicator at Searsville, as at Flinders Reef, is the spike in radiation caused by nuclear bomb explosions, which started in the 1940s and peaked in 1963.

“Radionuclides are coincident with important things changing on the planet, have a specific onset and peak, and are evenly distributed around the planet,” Stegner says. “It doesn’t matter what the cause is, we’re just looking for something that’s simultaneous.”

What’s reversible, what’s not

Over coffee and vegetarian lunches in the warm spring sunshine, members of the AWG agreed that searching for the start of the Anthropocene can be depressing. Human activity has set in motion an enduring shift—and it’s not clear what comes next. “It’s not going back, it’s not reversible,” says Stanford biologist Elizabeth Hadly. “It’s going to be different, and it’s going to be a hard transition.”

But there was a current of optimism at the meeting nonetheless. Hope came from unlikely places–like the Antarctica Peninsula Ice Sheet, where British researchers have collected glacial ice going back centuries. The site, more than 400 miles from the nearest research station, can only be reached using a Twin Otter airplane equipped with skis.

In 2012, a team of researchers drilled down 133 meters (436 feet), removing ice in meter-long sections and packing them into dozens of cardboard boxes for the flight back to the research station. They made the long sea voyage to labs in the U.K. in a container chilled to -25ºC, about the same temperature as their source on the Antarctic Peninsula. The cores preserved four centuries of annual snowfall, starting in 1621—along with bubbles of air trapped in the snow.

On the conference’s second day, British Antarctic Survey scientist Liz Thomas took the stage to present data from the ice cores. The air bubbles, which pop and crackle in the lab when the ice is melted for analysis, reveal something remarkable: Methane, a greenhouse gas 80 times more powerful than carbon dioxide, began ticking up in 1800, but skyrocketed in the mid-1900s, mirroring the global expansion of industry and agriculture. It’s emitted by everything from oil wells to rice fields to belching cows.

“Between the 1950s and the 1970s, the acceleration in methane is 100 times what we saw in the previous 1,000 years,” Thomas says.

For Thomas, the methane measurements are actually a hopeful sign. Unlike CO2, methane in the atmosphere dissipates after a decade or so. “If you start to make changes in land use and agriculture, you will see a drop-off in methane quickly, whereas CO2 lasts much longer,” Thomas says. “Which is why we could change it if we stopped eating cow.”

In other words, golden spikes, whatever changes they mark in the geologic record—the proliferation of ancient mollusks, of radioactive isotopes from bomb testing, or soot from coal-fired power plants—are forever. But it’s not too late to hit the brakes on the Great Acceleration.

“I worry if we say we’ve reached a new epoch, people will say ‘the damage is done, let’s just give up,’” Thomas says. “But we can still make a positive difference.”