When you or I look at a mountain range running alongside an interstate highway, we might see a series of majestic peaks. When Kirk Johnson looks at a mountain range, he sees hundreds of millions of years of history. In the layers of rock and the detritus of once living things, he can trace the imperceptibly slow workings of evolution and geologic time and identify the short shocks of five mass extinctions that have periodically wiped out nearly all life on Earth in the last half billion years.
Johnson, a paleobotanist and geologist, runs the Smithsonian's National Museum of Natural History. He explained the long arc of the Earth's physical changes—and his lifelong interest in rocks—as we strolled passed the encased dioramas and dinosaur skeletons that make his museum one of the world's best.
"Museums are where we keep the evidence of life on our planet: fossils, rocks, specimens," he tells me. "They all end up in museums, and that's where our culture records all the oddities that have happened over the last four and a half, 4.6, or 4.7 billion years of Earth's history."
Since the mid 20th century, scientists have developed remarkably accurate methods for dating the emergence—and disappearance—of species and theories about how the world has taken the form it has: the continents scattered over the Earth's surface here and there, the particular way plants and animals are distributed across the globe. Carbon-dating technology and the theory of continental drift emerged only in the 1950s, Johnson said. So did methods for discerning how and why over the past 4.5 billion years most of life on Earth has disappeared. "All of these tools that allow us to tell the story of the planet have been growing up as I've been growing up," Johnson said. "And the last thing to enter the collective consciousness of the sciences," he added, "is that humans can impact the planet."
Johnson was talking about the Anthropocene—the Age of Man—the existence of which is perhaps the most significant current debate about humans' relationship to nature.
Human transformation of the environment has become so pronounced that Johnson, and researchers across dozens of scientific disciplines, now argue that we have entered a new phase in the history of the Earth. No longer are we in the Holocene epoch, the relatively warm period that began with the retreat of the glaciers about 12,000 years ago. Since the end of the last ice age and the proliferation of humans across the continents, our agricultural methods, our cities, our energy production, our transportation networks, our plastics, and our atomic tests are radically altering the biological and chemical composition of the air, soil, and water—and even leaving what some say will be a lasting impression on Earth's geology. The extraordinary rate at which these changes are occurring might even be bringing about the sixth great extinction in the history of the planet.
The notion that humans can scar the environment is hardly new. In 1854, the Welsh geologist and theologian Thomas Jenkyn coined a term for the likely impact of human activities on the geologic record: anthropozoic. American polymath George Perkins Marsh argued in his book Man and Nature, published in 1864, that by denuding the landscape of trees, humans bring about widespread ecological disruption, which in turn restricts the capacity of human societies to thrive and survive. Swedish chemist Svante Arrhenius found in 1895 that if atmospheric concentrations of carbon dioxide increased, so too would the Earth's surface temperatures.
In the early 20th century, Ukrainian geochemist Vladimir Vernadsky and two Frenchmen—Pierre Teilhard de Chardin and Édouard LeRoy—proposed the term noösphere to describe the growing influence of human technological innovation in shaping the future and the environment.
By the year 2000, Dutch Nobel Prize winner Paul Crutzen and his colleague Eugene Stoermer proposed in an issue of Global Change Newsletter that human influence over the physical world had reached such a tipping point that it required the designation of a new geologic age.
The human population, they said, had grown tenfold over the previous three centuries, and along with it the cattle population had exploded to nearly 1.4 billion. Urbanization had also ballooned tenfold during the 19th century, and that growth would exhaust fossil-fuel supplies that were several hundred million years in the making. Humans had introduced nitrogen-infused fertilizers, they wrote, and—echoing Marsh—transformed up to 50 percent of the Earth's land surface. The rate of species extinction had gone up by at least a thousandfold. Greenhouse gases had substantially increased in the atmosphere, and other pollutants had punched a hole in the Earth's ozone layer.
Their rundown reads like a technical account of a crime scene. But the terrain on which these transgressions were occurring wasn't the corner store—it was planetary in scale and reached into the fundamental biological, chemical, and physical properties of the world we all inhabit.
"Considering these and many other major and still growing impacts of human activities on earth and atmosphere, and at all, including global, scales," they wrote, "it seems to us more than appropriate to emphasize the central role of mankind in geology and ecology by proposing to use the term 'anthropocene' for the current geological epoch."
The pair went further than just describing the characteristics of the Age of Man. They also proposed a start date in the latter part of the 18th century, specifically the invention of James Watt's steam engine in 1784, which was integral to the Industrial Revolution.
The emergence of human influence on nature had come about swiftly, they concluded, and was likely to become a permanent fixture of the landscape.
"Without major catastrophes like an enormous volcanic eruption, an unexpected epidemic, a large-scale nuclear war, an asteroid impact, a new ice age, or continued plundering of Earth's resources by partially still primitive technology," they said, "mankind will remain a major geologic force for many millennia, maybe millions of years, to come."
For many scientists, though, including Kirk Johnson, the question is no longer whether there's something called the Anthropocene but when it began. Did it start, as Crutzen and Stoermer suggest, at the dawn of the Industrial Revolution, or on some other date?
Indeed, there is a scientific institution called the International Commission on Stratigraphy that is specifically tasked with making this decision. It's the kind of professional organization that one would normally never hear about, an obscure group with subcommittees on Precambrian stratigraphy, Ordovician stratigraphy, and Jurassic stratigraphy, among most of the other major geologic periods.
It's likely that sometime next year a committee of the ICS will decide whether or not the commission should formally adopt the Anthropocene and, if so, suggest when it began.
Jan Zalasiewicz is a lecturer in paleobiology at the University of Leicester and chairs the ICS committee that is weighing the merits of the Age of Man.
"You know the joke about geologists," he told me. "Put three geologists in a room and you get five different approaches to a question.
"With the Anthropocene, we're dealing with the sum of human action," he said, "and geologists are not really good at judging human action."
They are, however, very good at looking at rocks.
And in those rocks they've identified the traces of human activity, specifically radiation stemming from atomic weapons tests. It was a historic turning point, according to Zalasiewicz, who along with members of the working group proposed in a journal article that the first nuclear test, on July 14, 1945, was the beginning of the Anthropocene. The atomic age brought about a new form of energy—and a novel source of waste, one that can linger for thousands of years. And the July date coincided with many of the phenomena outlined in Crutzen and Stoermer's original paper: an explosion in the human population, concentrations of atmospheric greenhouse gases, species extinction, and production of concrete, plastics, and metals—often referred to as the "Great Acceleration."
Simon Lewis and Mark Maslin of University College London, writing in the March issue of Nature, offered either 1610 or 1964 as the dawn of the Anthropocene. For them, the great mixing of the Old and New worlds that began in 1492 was the largest reorganization of people in the past 13,000 years and an unprecedented global biological exchange of plants and animal species. The "Columbian Exchange" marked a turning point. Old World crops like sugarcane and wheat were sown in newly settled lands of the Americas, while New World crops like corn, potatoes, and manioc were grown across Europe, Asia, and Africa. Pollen from New World corn first appears in marine sediment cores dating to 1600. In other words, it was a global reorganization of life without precedent.
It also brought about a massive decline in the human population. The number of people in the Americas declined from an estimated 61 million people in 1492 to about 6 million in 1650 due to disease, famine, enslavement, and war. Fewer people meant less agricultural production and less slashing and burning of forests to make way for settlements and crops. That led, according to Lewis and Maslin, to a great expansion of the biomass of the Americas. With more trees and shrubs blanketing the landscape, more carbon dioxide was pulled from the atmosphere and, indeed, atmospheric carbon dioxide levels dipped slightly between 1570 and 1620.
Those two events—the appearance of New World pollen in Europe and the carbon dioxide dip—provide geologic markers that point to the onset of the Anthropocene, they say.
But like Zalasiewicz, Lewis and Maslin see the increase in radionuclides from atomic tests as a plausible boundary between one geologic period and the next. Yet instead of the date of the first test—July 14, 1945—they see 1964, when levels of radioactive carbon spike in tree-ring samples, as a sound marker for the start of the Anthropocene.
This might all seem academic. What difference does it make whether human influence on the environment began 12,000, 500, or 50 years ago, you might be asking. For Lewis and Maslin, the designation could impact our interpretation of what is driving all this unprecedented, global environmental change.
"The [dip in atmospheric carbon dioxide] implies that colonialism, globalized trade, and coal brought about the Anthropocene," they say. "Choosing the bomb spike tells a story of an elite-driven technological development that threatens planet-wide destruction."
Back at the Smithsonian, Johnson is also concerned about the stories people tell—after all, millions of visitors pass through his museum each year. How Johnson and his staff choose to curate the 4.5-billion-year history of the Earth impacts the perceptions of young and old, who participate daily, whether actively or passively, in broader social and political debates about dinosaurs, extinctions, climate change, and the future of civilization.
"The present rate of extinction is extraordinary—it's on par with the five major extinctions—and what's incredible is that we're causing it. There's no question," Johnson said, bringing us to a halt before a 1970s-era mural.
The dimly lit scene is set 15,000 years ago in what is modern-day Alaska, he said. Mammoths and mastodons, a stag-moose, American lions, short-faced bears, and muskoxen roam through a green and brown tundra spotted with snow. It's a representation, Johnson said, of the end of the last ice age, when humans began to spread out over the continents, also known as the Holocene.
Johnson points to the upper right corner of the mural, where four shaggy men with spears surround a large ground sloth. "In North America, people arrived around thirteen thousand years ago," he said, "and soon after, you start finding carcasses of mammoths with spear points in them. And shortly after that you don't find any more mammoths.
"We're well on our way to killing the elephant, the northern white rhino, the tigers. We're well on our way to killing major apex species," he said. "But we're also on our way to killing lots of smaller things. Even in the last hundred years we've lost Tasmanian tigers, certain types of antelopes. You start adding it up, and one hundred years is not very much time in the broad arc of geologic history, and the actual rate of extinction is well on its way to being on track with the other extinctions."
And those changes, he said, are ones brought about by the direct impacts of humans: deforestation, poaching, depriving predators of their prey. There are also the indirect impacts: levels of carbon dioxide in the atmosphere, pollution in the oceans.
"There are twice as many people on the Earth than when I was born, and whether you're poaching animals, eating a hamburger, or sitting in traffic, it all translates into the Anthropocene," he said.
Is it an innate human urge to kill, the outcome of capitalist production, or the sheer explosion of the human—and cow—population? He didn't offer an answer.
"We as humans can actually nudge this history toward a more pleasant outcome, rather than toward the less pleasant outcome," he said. "The past fifty years have demonstrated that humans can change the planet. Now the choice is whether we change it for better or for worse."