SCIENCE

Geologists drive golden spike toward Anthropocene's base

MAINZ, Germany -- The man who named the Anthropocene has had a change of heart.

Twelve years ago, Paul Crutzen, a Nobel laureate and atmospheric chemist, coined the term "Anthropocene" as shorthand, an argument wrapped in a word. Geology had long relegated humanity to the sidelines, but in recent history, the human fingerprint on the Earth had grown too deep to be ignored, he said. We had created our own geological time. The world had left the Holocene behind and entered an epoch of humanity.

While foreign to stratigraphy, the arcane and sometimes internecine discipline that judges geological time, Crutzen even hazarded a guess at when this transition occurred: the early 19th century, when the Industrial Revolution unleashed the energy found in fossil fuels. Man could move mountains, and the steady buildup of carbon in the air began.

Clearly, he was onto something. Over the past few years, the Anthropocene has become a defining idea of environmentalism. It does much in little space. It ends the separation of humanity from nature. It changes the discussion for a politicized electorate weary of global warming. It broadens the tent, encircling a host of realities: biodiversity loss, resource scarcity, population growth. And best of all, geologically speaking, it might even be true.

On further thought, Crutzen's start date may have been off, he said during a visit to the gleaming new building of the Max Planck Institute for Chemistry, nestled just west of downtown Mainz, the city famous as Gutenberg's home. The institute's elder statesman, Crutzen speaks with a frail voice and resembles a wise gnome.

"I'm starting to think the strongest signal, one of them, is just nuclear explosions -- the test cases of atomic material," Crutzen said. "There were the first two nuclear explosions in Japan, but then [much more] testing took place, and anytime that radioactive material came into the world, into the sediments, we had an example of a good marker. Now I'm more in favor of declaring the nuclear tests as the real start of the Anthropocene."

Crutzen's shift is a reminder that, while his neologism is all-conquering, it is far from settled as a scientific fact. Indeed, it's only recently come to the attention of geologists. Humans are altering the face of the world, no doubt. But the world has had many faces over its eons. And so the question is set: Is humanity's touch mere makeup, or does it cut to the bone of deep time, in a pattern consistent with geology's demands?

It's not a problem to be resolved in an Internet minute, said Jan Zalasiewicz, a University of Leicester geologist who is leading a small band of academics considering whether to formally propose the Anthropocene as an epoch.

"All of these discussions about any unit in geology take an age, almost literally," he said.

By nature, stratigraphy is a conservative profession, resisting proposals even from its own eminences, let alone offhand remarks from climate scientists. Notoriously, the field wrangled for 60 years over the Quaternary, the geological period that enwraps the Holocene. Debates about the boundaries often turn political and bitter, and last forever, said Michael Ellis, head of climate change science at the British Geological Survey.

"Geologists have gone to their death beds arguing about these boundaries," Ellis said.

The world is not waiting for the geologists to decide, of course. Run a Web search for the Anthropocene and 520,000 results pop up. This year, more than 200 academic papers have used the term, many of them outside geology -- or the hard sciences. It made the cover of The Economist and National Geographic. The publishing giant Elsevier will soon launch a journal on the Anthropocene. It is a phenomenon.

But while the Anthropocene fad may wane, the judgment of stratigraphers will bear the test of time. If the International Commission on Stratigraphy, the bureaucracy that rules on issues of geological time, decides that the world has entered an epoch dominated by man, the verdict will reverberate with a clamor matched by few academic findings.

Getting to that decision won't be easy. Zalasiewicz's working group has embarked on an exhaustive search to find the uniform signals that will remain embedded in the Earth for millions of years, after much of our world erodes. At their most optimistic, they hope to find a "golden spike," a distinct global chemical or fossil marker between the Holocene and Anthropocene. It is that hunt that helped spark Crutzen's recent shift to the postwar atomic age, when humanity's touch seemed to accelerate and go global.

Many stratigraphers are dubious any proposal will meet their code. The debate over the Anthropocene promises to be an academic tussle that could outstrip all past episodes for its intellectual jousting. More attention will be paid to the discipline than ever before. Its reputation will be on the line. And so the evidence demanded will be steep, said Will Steffen, an Australian climate scientist who has popularized the term with Crutzen.

"The geologists have set the bar pretty high for us," Steffen said.

It's not that stratigraphers want to fight a rear-guard action against the realities of human influence on the planet, added Whitney Autin, a geologist at the State University of New York, Brockport, who has criticized the Anthropocene's stratigraphic credentials. It's great that it's raising environmental awareness, he said.

"But you're talking about our code," Autin said. That's different. "When you bring it down to the nuances of how we practice our science, the requirements go up."

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'We like boundaries, don't we?'

Before there was the Anthropocene, there was the "Coca-Cola layer."

Geologists have long joked about the traces left behind a million years in the future if humanity wiped itself out. There would certainly be local records. The late Derek Ager once said that to study the stratigraphy of Alaska in the future, beer bottle caps could be an excellent guide. But beyond a few intellectual misfits of the 19th and 20th centuries, most geologists demurred from formalizing their after-dinner chatter.

"Geologists had often made jokes about the Coca-Cola layer that we would leave behind," Zalasiewicz said. "But in general, it was thought that human effects looked dramatic in somewhat [local situations], but they were not geologically significant."

It took a fed-up atmospheric chemist to change all that.

Known for his Nobel-winning work on the ozone hole, Crutzen famously blurted out his new geological epoch at a meeting of the International Geosphere-Biosphere Programme (IGBP) near Mexico City in 2000, after a paleoclimate scientist invoked the Holocene 10 times or so. Anthropocene, described that year in a newsletter by Crutzen and his colleague, the late Eugene Stoermer, soon became the unifying theme of the IGBP, a global network of environmental scientists that began in the 1980s.

"Whenever you go somewhere in the world and make measurements, you cannot avoid having to deal with humankind," Crutzen said. "That is the idea of the Anthropocene."

Crutzen was not alone in tapping the zeitgeist. Various near-misses sprouted up in the 1990s. Chinese geoscientists called the current age the "Anthroposphere," while Andrew Revkin, a New York Times reporter, dubbed it the "Anthrocene" in one book. (Revkin is actually a member of the Anthropocene working group.) Several high-profile scientists, including Jane Lubchenco, the future National Oceanic and Atmospheric Administration chief, laid out the evidence for human domination in 1997 -- just without a catchphrase. And Stoermer had been using Anthropocene in his correspondence since the 1980s.

Still, it wasn't until Crutzen's outburst that the phrase went global.

The head of the Anthropocene working group, Zalasiewicz, remembers reading the 2002 Nature essay in which Crutzen introduced his word to the wider world. Zalasiewicz was then writing a book exploring what from humanity would linger in the rock record. The term seemed destined for his use. But he waited, and watched, until he saw the idea catch on, used without irony or scare quotes, in scientific work.

As time went on, Zalasiewicz persuaded his friend Phil Gibbard at the University of Cambridge to give the term a look. Gibbard was then head of the international panel overseeing the Quaternary, our current stratigraphic period. (Geological bureaucracy is nested like a Russian doll, much like its subject; a period encompasses an epoch or age but falls under an era or eon.) Gibbard had just ended a battle over the Quaternary's existence, only to find an invented unit of geological time quaking around the globe.

"It was then that I realized we need to have a long, cold look at this thing, rather than trying to waffle about it," Gibbard said. "We needed an international group to weigh the case for and against."

He asked Zalasiewicz to look into it.

Zalasiewicz pulled together a diverse group of geologists and academics to reflect the wide interest in the subject. The group subsists on shoestring funding, its members gathering on the fringes of large events. Its mandate is to decide whether there will be a decision: to vote, eventually, on whether to submit the Anthropocene to the grinding gears of the geological hierarchy.

It's a process that will be wracked with scientific, and social, uncertainty.

So far, the great value of the Anthropocene is that it has made humanity open its eyes and look at the legacy it's leaving, Gibbard said. But once talk turns to boundaries, our species tends to go haywire. Look at the fuss over country borders, he said.

"Humans, we like boundaries, don't we?" he said. "I guess it's territorial."

Time and 'time-rock'

In many ways, imagining the Anthropocene requires a grim creativity.

Wipe every human from the planet, and it may be surprising how much of the world would return to its pre-civilization state. Rivers would break their bounds, bending to gravity's demands. Temperatures would fall, allowing displaced species to return. Farm crops would fail and cows would die. Even cities would erode, ground back into the mineral grist from which they're made.

The planet's cycles, whirring in an absence of humanity, could render moot even some of our most obvious changes. For example, humans have increased atmospheric carbon dioxide by more than 100 parts per million over the past two centuries, causing a global temperature rise of about 0.9 degrees Celsius. But end human emissions and CO2 will, over geological time, retreat from the air, dragged into the ocean's depths through the chemical weathering of eroded minerals, borne down from the mountains.

Will our temporary CO2 imbalance, and the traces it leaves in the shells of minute sea creatures, leave a lasting mark? How high will CO2 go? And those shells falling into the ocean's sedimentary murk are mixing with dirt that could be considered part of the Holocene; the same is happening with soils. Many cities, which some see as hallmarks of the Anthropocene, began 1,000 years ago or more. The Earth is tangled.

"What we'll have to do here is not simply think about the Anthropocene history, but the nature of the Anthropocene strata even as they're forming today," Zalasiewicz said.

There are technical issues of stratigraphy to consider, as well. For example, biologists warn that the Earth has begun a great wave of human-induced extinction. Because much of the geological record is divided by such extinctions, these losses seem an obvious indicator. There's one catch, though. Despite popular notions, geological divides are defined not by the disappearance of creatures, but by when evolution has filled their void.

"In geological terms, we define on the appearance of something new," Gibbard said.

The ultimate technicality in stratigraphy, however, comes down to its idea of time. Turns out, in geology, there's more than one type of time. There are two.

Like everyone else, geologists first count time in years. It's the abstract, astronomic system that neatly divides Earth history into, say, the Jurassic period. But then there's another kind of time, the one geologists call time-rock, or chronostratigraphy. Time-rock is the physical record left in the Earth, and it is a mongrel. Its divisions can be difficult to see. Rocks rise and rocks fall, in a melange the defies the layered drawings of youth.

"Nothing in the record of our past is ever permanent," said Ellis of the geological survey.

Stratigraphy is dominated by time-rock. The field is constantly refining its ability to test samples, prompting an emphasis on distinct magnetic, chemical, paleontological or climatic signals -- golden spikes -- that can be detected worldwide. The most well-known signal is the global iridium layer that spread when a meteor detonated into the Yucatan Peninsula, causing mass extinction -- dinosaurs survived only as birds -- and a new era, the Cenozoic. Anthropocene or no, Earth remains in the Cenozoic to this day.

The oldest eons, their evidence buried deep, are counted in years. But for the modern periods, starting 600 million years ago or so, geologists are now engaged in a great hunt to nail down all the golden spikes they can find. While Zalasiewicz thinks it would be fine to define the Anthropocene according to regular time, some stratigraphers have insisted that beginning the Anthropocene will require precise time-rock data.

That was a point of two geologists from the University of the South Pacific, Stephen Gale and Peter Hoare, writing in a recent issue of The Holocene. The earliest evidence of human environmental influence could go back for 40,000 years, since fire became one of our first terraforming tools. How do you untangle when this shifted to dominance?

"The short time scales of the Holocene and the high resolution of the techniques available for dating this episode make it impossible to hide behind dating uncertainties in defining a single instant for the initiation of human impact," they wrote. "Under these circumstances it is questionable whether efforts to establish a single date for the start of the Anthropocene can have any meaning or value."

Their concerns speak to a fundamental problem: Geologists don't yet have an exact definition for the Anthropocene, let alone firm evidence. Is it the start of human-induced environmental change, or a tipping point when that change went global? If humans, through agriculture, began to inch global carbon emissions up millennia ago -- as the University of Virginia's William Ruddiman argues -- does that work as a start date?

One thing is certain. The mere presence of human activity is not enough. That chip has been spent. Without the rise of humans, our current epoch, the Holocene -- which started 12,000 years ago, as the glaciers began to retreat -- would be an unremarkable time within the Pleistocene's rhythm of ice ages, Cambridge's Gibbard said.

"If you wish to hive off the last couple of hundred years, you've got to have more than just the activities of humans," he said. "We've used that card. We've played that card."

The Great Acceleration

On July 16, 1945, in New Mexico, the United States ushered in the atomic age.

Over the next decade and a half, the Soviet Union and the United States competed to build, and explode, ever-larger atmospheric bombs, until the countries agreed to halt airborne tests in the early 1960s. Those tests released a patina of radioactive particles -- fallout -- that is easily detected in the world's soil. Some common isotopes, like cesium-137, are often used to track environmental change.

Many of these particles are irrelevant geologically, though. Their half-lives last for years or days, their radioactivity consigning them to a brief stay on the planet. Even plutonium has a half-life of only 24,000 years. Most intriguing for the Anthropocene is iodine-129, which has a half-life of 15.7 million years. In other words, 200 million years from now, 0.01 percent of the iodine-129 could remain.

The global distribution of this detritus has made atmospheric testing a strong candidate for the Anthropocene's start. It helps that the testing coincides with what IGBP scientists call the Great Acceleration: the postwar period when oil- and coal-fired growth took off like a rocket throughout the world. By 1950, CO2 emissions sat at 315 parts per million, barely outside the Holocene's normal variation; that soon changed. Synthetic fertilizer become common in farming. Dam construction boomed.

"Each one of these bits of evidence ... may have a different beginning," said James Syvitski, a sedimentary geologist at the University of Colorado, Boulder, and the current chairman of IGBP. "But if you were to look at the ensemble of them, after World War II, you started to see the beginnings of the exponential curve."

Simply getting at all those fossil fuels was bound to leave a mark. For example, all the world's rivers move about 13 gigatons of sediment in a year, Syvitski said. Meanwhile, each year, humanity mines 9 gigatons of coal. ("Just coal," he added.) There's another statistic everyone was just talking about at lunch, he said.

"There are 568,000 abandoned mines in the [United States] alone. Millions throughout the world," Syvitski said. It just seems improbable to think this isn't a global signal. "The more I check," he added, "the more I'm convinced."

If there is a global signal, it will ultimately be recorded where the continents meet the sea. Geological boundaries need a stratotype, a model rock, its Platonic ideal. It's quite possible the Anthropocene's stratotype is forming downstream of, say, New Orleans.

"Most likely to be preserved are places that are subsiding and falling rapidly in sea level," Ellis said. "In most instances, that means the big deltas of the world and the coasts. ... We know we are sending all sorts of things down to those sediments."

First, the amount of silt has changed, as each year dams trap about 2.3 gigatons of sediment in reservoirs. The sediment that does reach the delta is rich in particulate forms of carbon and agricultural byproducts, getting buried alongside that 1950s fallout. And then there's one of the community's favorite potential signals: the shift in isotope ratios that stems from the wholesale extraction of atmospheric nitrogen for fertilizers.

The change in the nitrogen cycle is truly global, scientists have recently found. A study published in Science late last year found a coherent signature of human-induced changes to nitrogen in the murk of a host of remote lakes and watersheds in the Northern Hemisphere. It's a model of the research prompted by the Anthropocene.

Expect much more work like this, Syvitski said. Erle Ellis, a geographer at the University of Maryland, Baltimore County, has won grants to look at earth evolution on a global scale. More methods will be developed to connect the tools of biology and stratigraphy. Earlier this year, Michael Ellis began discussing a "pre-Anthropocene" biome project that would go back thousands of years to document what buried fossils and pollen say about the distribution of life before humanity's dominance.

"All sorts of spinoffs are happening faster than I can keep up with," he said.

'Overtaken by events'

Still, it's possible that the Anthropocene is an idea before its geological time.

There's a degree of uncertainty in the concept that could make even nongeologists a bit squeamish. Say the world takes a turn -- solar power becomes too cheap to meter -- and the beginning of the end of global warming appears. Industrial and organic farming mate and turn into high-yielding, sustainable agriculture. Population levels off.

"If humanity does change course, one could view [the Anthropocene] not even as an epoch," said Steffen, the Australian climate scientist. "One could view it as a minor excursion from the Holocene."

Alternatively, CO2 will increase and the ice sheets could melt: Hello, Anthropocene.

That uncertainty goes to the heart of stratigraphers' concerns about the Anthropocene. Their code is not about prediction, about what humanity will do next. It's about the rock.

"Is there a demonstrable record in the rocks today that I can put my finger on?" asked Autin, the geologist at SUNY Brockport. "No."

There's also a question of usefulness -- one of the main hurdles for any proposed geological age. At their heart, geological divisions are simply a tool, a means of communication. If the Anthropocene proves itself useful for the scientific world -- many would say it has -- that's a point in its favor. But if it causes the field of stratigraphy to devolve into acrimony and animus? Not as helpful.

"There's no point in having a spanner that doesn't work," Gibbard said.

Gibbard knows how this can go. He was a main player in the drama to restore the Quaternary above the Pleistocene. Both names hark back to the field's early years, in the 1800s, but some saw the Quaternary as redundant, because it encompasses only the Pleistocene and Holocene, and the latter is a fairly unremarkable interglacial. The debate turned on nationalism and politics as much as it did on facts, Gibbard said.

The feud over the Quaternary lasted 60 years. Partially, this is by design. After the International Commission on Stratigraphy rules on a geological time, it imposes a 10-year moratorium before any changes can be considered. The losing side stews, waits and researches -- and returns for another fusillade in a decade. And then another.

"It can get very political," Gibbard said. "And the lesson is not to let it, if you can."

It will be up to Zalasiewicz, the working group chairman, to navigate those politics. The Geological Society of London is preparing a volume with work supporting and criticizing the Anthropocene from a stratigraphic view. And it wants to pull in ecologists and biologists documenting human-induced changes to plants and wildlife. By 2016, expect a chunky book from the group on the pros and cons of the Anthropocene.

A vote on whether to propose the period would follow soon after.

"That's my personal goal," Zalasiewicz said.

Autin is looking forward to seeing it.

"I'd like to slow the boat down," he said. "I'm not trying to stop it. If the proponents offer a cogent idea that's workable -- I'm good. That's what we're supposed to be doing."

Four years from now, there's no telling where the popularization of the Anthropocene will go. It is already serving as the basis for a historic joining of the four major global scientific organizations devoted to environmental change into a group called Future Earth, IGBP's Syvitski said. Lawyers have begun to use it. Even clergy.

"Nobody's waiting for that [stratigraphic] process," Syvitski said.

There's no guarantee the working group will even decide to propose the Anthropocene. Gibbard is serving on the group, and he remains a "bit negative on the concept," as he put it. And should the momentum behind the Anthropocene continue to grow outside geology, there's a chance the field could feel bullied into the decision and push back.

"They're being overtaken by events beyond their control," Ellis said.

Back at the Max Planck Institute, Crutzen had time only for a brief talk before his dinner plans. Now 78 years old, he's cut down on what has been a lifetime of scientific travel. Although he's changed his mind on when the Anthropocene started, he's not a part of the formal investigations. His presence would only slow the process, he said. No need for that.

"That's not my business," Crutzen said. "That's the business of the geologists."

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