In the late 1980s, Elizabeth Hadly became the first researcher to excavate the caves of Yellowstone National Park. By studying fossils and other markers of the past, the evolutionary biologist and ecologist helped reconstruct how surviving species adapted to the global warming that ended an ice age nearly 12,000 years ago, and with it, the Pleistocene epoch. Retreating ice left behind rocks and water and not much else. Photosynthesizing organisms eventually developed into vegetation that helped create soil. Gradually, the terrain became more complex and hosted more species. Today, it is the iconic American landscape of willow, aspen, wolves, and grizzly bears.
While contemplating the deep past, Hadly also began to observe unprecedented changes in real time. Small mammal populations were abruptly shifting, and fire frequency increased. “Going out every day for years, I started to see changes in the landscape,” says the professor of biology and of Earth system science. Over the course of 17 years, Hadly documented disappearing ancient ponds and vanishing amphibians. In 1988, fires ravaged the park. “I was evacuated from the caves I was working in,” Hadly recalls. “This was a part of the park that didn’t normally burn. It was a tipping point for fires in the West.” Decades later, she would realize she was witnessing the bookends to an entire geologic epoch—evidence of the start, and signs of the finish, of the Holocene, a time period marked by the ever-increasing influence of modern Homo sapiens.
Since 1998, when Hadly joined Stanford, she and her lab members have focused mostly on analyzing ancient DNA and other markers to assemble a picture of how mammal populations have evolved into today’s ecosystems. In 2016, she also took on the role of faculty director of Stanford’s Jasper Ridge Biological Preserve, a 1,200-acre protected area in the Foothills where her husband, geologist and paleontologist Anthony Barnosky, served as executive director from 2016 to 2022, and where more than 70 scientists conduct fieldwork in any given year. Barnosky spent much of his career as an integrative biology professor at UC Berkeley, researching past mass extinctions. The couple have collaborated on projects and traveled together for years. But at Jasper Ridge, their work would dovetail in a new way.
‘The only way to understand nature now is in the context of the Anthropocene.’
As nature morphed before their very eyes, Hadly and Barnosky set about documenting the onset of the Anthropocene, a new epoch proposed for the geological time scale. A commission is expected to decide on it this year. “I’ve done work all around the world—witnessed ice caps melting on the Tibetan plateau, the impacts of poaching, and other increasing human footprints,” Hadly says. “How can we protect biodiversity now? What does it mean to support it under conditions we can’t predict?” The Anthropocene departs from the relatively stable climate that has characterized the Earth system for approximately 12,000 years. Its changes are moving targets and don’t resemble historical patterns. “The only way to understand nature now is in the context of the Anthropocene,” Hadly says. She hopes the new designation will help people better understand how fundamentally different Earth has become in recent decades. It will also give leaders and educators a common language for discussing what Hadly and Barnosky had been seeing on—and in—the ground. “We didn’t go looking for the Anthropocene,” Hadly says. “It found us.” Soon, they realized they could study it right in Stanford’s backyard.
The Importance of Telling Time
Officially, we’re in the Holocene epoch, a time span that began some 11,700 years ago, characterized by relatively predictable seasonality and a temperate climate. On the geological time scale, it sits within the Quaternary, the third period of the Cenozoic era (see graphic). If the 4.5-billion-year history of the Earth can be conceived of as a book, the time scale acts as an ordering system dividing the narrative into chapters and numbering the pages. Epochs, periods, and eras are subdivisions that help us understand how the present came to be. The transition from one major subdivision to another is frequently marked by profound ecological change, including mass extinctions.
Infographic timeline: U.S.G.S. (source); all illustrations via Getty images: Tanarch (Earth); Alinabel (10); Alexey Makarov (human)
In 2000, Earth system scientists began to question whether we have passed from the Holocene into a new epoch. The story goes that Nobel Prize–winning chemist Paul Crutzen lost his cool at a meeting of the International Geosphere-Biosphere Programme. One after another, researchers presented evidence of recent, profound changes to planet Earth due to the impacts of just one among millions of species. “Stop saying Holocene,” Crutzen burst out. “We’re not in the Holocene anymore. We’re in the Anthropocene.” Crutzen subsequently joined with biologist Eugene Stoermer to argue that we have entered a new category of history in which the activities of Homo sapiens have accumulated irreversible changes to the way the Earth system functions. Crutzen proposed the term Anthropocene to “emphasize the central role of mankind in geology and ecology.” In 2009, Earth system scientist Will Steffen published a seminal paper in Nature warning that these changes are destabilizing what, to this point, has been a “safe operating space for humanity.”
That safe operating space is maintained by interactions between the atmosphere, the hydrosphere, the lithosphere, and the biosphere. (That is, between air, water, earth, and life.) This unified system interacts partly through geochemical cycles, including the well-known beast of climate change. Burning fossil fuels puts too much CO2 into the atmosphere, which changes patterns in the hydrosphere. In turn, those weather changes affect biological life, which we are changing in other ways. Humans and our domesticated animals now make up more than 90 percent of the mass of all vertebrates. We are losing not only microbes, plants, and animals but also the interactions they contribute to the Earth system. By 2008, the Stratigraphy Commission of the Geological Society of London concluded that, yes, a new epoch had begun. But when?
The next year, a subgroup of the International Commission on Stratigraphy—the Anthropocene Working Group—focused on what it would mean to establish the Anthropocene. In 2012, Barnosky became a member of the group based on his research in biostratigraphy, the science of using fossils to tell time. Earth’s strata—its layers of rock and soil—show evidence of human influence in many places and times. Some researchers argued that the onset of agriculture marks the decisive turn in humanity’s power to shape-shift the Earth, but that evidence is not uniform around the globe. Likewise, the industrial revolution would seem to be an obvious beginning point, but its impacts are unevenly distributed. For formal designation, a geological epoch must be discernable like the title of the Oscar-winning film Everything Everywhere All at Once. “Global synchronicity,” Barnosky explains, “is the gold standard for marking time in the rock record—for marking time, period.” He compares it to specifying time zones so that everyone knows when to log into Zoom.
In 2015, the working group recognized the Great Acceleration of the mid-20th century as marking the beginning of the Anthropocene. “Humans have been gradually changing the planet since we first became a species,” Barnosky says, “but nothing approaches the changes we see mid-20th century.” Industrialization, population, pollution, nitrogen fertilizer use, and more ratcheted up significantly around 1950, and all these markers continue to rise. From steel to concrete to plastics, the residues of our activities are found in the fossil record. The accumulation of human detritus has grown so massive that it has its own name: the technosphere. The Earth system has a new driver, and it’s too late to revoke our license.
The working group identified radionuclides from atomic testing as key geological markers for the new epoch. Aboveground atomic testing from the 1940s to the 1960s released distinctive isotopes into the atmosphere. The group decided that plutonium, which is not detectible in sediment that predates atomic testing, is the most useful synchronous marker of the Anthropocene. It is followed by “bomb carbon,” which introduced new levels of carbon 14 molecules into the atmosphere that have gradually accumulated at the bottom of freshwater lakes, sealed in layers by subsequent atmospheric depositions. They have also settled into polar ice, coral reefs, and stalactites. Extracting a cross-section of sediment—a core—from these repositories and identifying the plutonium and bomb carbon layers in them makes it possible to mark the Great Acceleration: a point in geological time where we can see a before-and-after in Earth history.
Geologists mark exactly when a new time period in the geological record begins and ends with a Global Boundary Stratotype Section and Point—a Golden Spike, for short. Much as the railroad barons used the term to indicate the transition between railway lines, geologists use it to indicate where one period yields to another. In 2019, the working group initiated a competition among scientists around the world to establish a single site that exemplifies the transition from Holocene to Anthropocene. Once it is chosen, a metal spike will be hammered into the site to serve as a reference point. The area will then be made available for researchers studying global change. Hadly and Barnosky put Searsville Lake at Jasper Ridge up for consideration.
Brave New Nature
In her 25 years as a Stanford professor, Hadly has traveled all over the world, continuing to chronicle the changes she began to study in Yellowstone. In 2012, she and Barnosky co-authored a paper in Nature entitled “Approaching a State Shift in Earth’s Biosphere” that caught the attention of the then governor of California, Jerry Brown. “Why aren’t you guys shouting this from the rooftops?” he asked the couple. “Well, we are trying,” Hadly replied. Brown asked Hadly and Barnosky to summarize the paper in lay language. The result, “Scientific Consensus on Maintaining Humanity’s Life Support Systems in the 21st Century,” describes the many pressures we are putting on nature and why we must curtail them to safeguard future survival. The paper carries the signatures of hundreds of scientists from around the world and has been distributed globally, often by Brown, who hauled boxfuls on his international travels.
The Hadly Lab uses multiple data-based strategies to reveal the invisible past, including interactions among species and ecosystems. Her work shows that over many thousands of years, plants and animals have evolved in relationship with each other, and their interactions contribute to the functioning of the Earth system. She has also shown that as human impacts reduce other life forms, the evolutionary destinies of millions of species are being decided by humans, often without our knowing it. This can be viewed as a moral issue, but it is also a practical problem. We know humans need pollinators like bees and decomposers like beetles to carry out functions vital to our own well-being. We don’t know how many other species we need to support our “safe operating space.”
Hadly continues to study the genetic capacity of mammals to adapt as their habitats are altered or destroyed. Her research has helped show that tigers may need genetic intervention for their species to survive. Tigers are top predators and have an outsize role in regulating the food web. Hadly found that some Asian pikas in Tibet are moving to higher ground as the climate warms but may not survive the lower level of oxygen at higher elevations. Asian pikas are ecosystem engineers. Their activity modifies soil and helps host myriad plant and animal species. Life begets life, and we are unwittingly extinguishing parts of the process. “I couldn’t continue to simply publish findings about what is happening,” Hadly says. “Scientific papers can only take us so far. I wanted to tackle the challenge of managing a place with high biodiversity potential into the future.”
Just a short drive from campus, the Jasper Ridge Biological Preserve sits in what is sometimes called the urban-wildland interface, where city meets suburb meets relative wilderness. Seismic activity from the San Andreas Fault has mashed together a high diversity of geology, soils, and landscape features. Searsville Dam, erected across San Francisquito Creek in 1892 to create a drinking water supply, transformed a riparian valley into a lake and has been steadily filling up with sediment for more than 125 years. To quantify change at Jasper Ridge, Hadly hired paleoecologist Allison Stegner, ’10, as a postdoc to pull long cylinders of mud from Searsville Lake. “I had worked with cores for years,” Hadly says. Like tree rings, lake sediment cores retain evidence of temperature and precipitation patterns. They contain pollen, which can be analyzed to identify biological responses, such as changes in tree and wildflower communities. “I have cored many lakes,” Hadly says, “but I’ve never seen any [cores] so long as the ones from Searsville Lake, and so discrete”—so clearly marked by historical events.
The Core
SEARSVILLE SEDIMENT: A CT scan of the Hadly team’s core shows plutonium, as well as increased human influence around the time of the proposed Global Boundary Stratotype Section and Point (GSSP), aka the Golden Spike. (Chart: Modified from Stegner et al. 2023)
Hadly and Barnosky began to see Jasper Ridge as an exemplary illustration of the Anthropocene. “Tony and the [Anthropocene Working Group] were talking about using cores to identify historic markers like plutonium,” Hadly says. “I realized we had that information in our Searsville sediment.” She and Barnosky nominated the site for Golden Spike consideration. Eleven other sites are vying for the label, including lakes in Canada and China, a peat bog in Poland, layers buried under Vienna, ice in Antarctica, a cave in Italy, coral reefs off the coasts of Australia and Texas, bays in Japan and California, and the Baltic Sea. As of this writing, voting on the Golden Spike is underway. “It’s a tricky decision,” says Jan Zalasiewicz, chair of the Subcommission on Quaternary Stratigraphy, “because there are too many excellent choices.” There will be one winner but no losers in this competition. All the sites will become reference points for researchers working to elucidate the new epoch.
The View from Jasper Ridge
In winter 2019, migrating cormorants and mallards touched down on Searsville Lake, taking no notice of a Rube Goldberg–like contraption floating alongside them. Buoyed by pontoons and sporting a motorized coring drill, the Vibracore was operated by researchers from Stanford and the United States Geological Survey. Stegner, now a research scientist in the Hadly Lab, guided a tall metal cylinder as it plunged into the sediment at the bottom of the lake. She leaned her tall frame against the coring mechanism and, pushing with all her weight, levitated briefly as the coring device went down.
DELVING DEEP: At Searsville Lake in Jasper Ridge, researchers extract sediment cores to show more than 125 years of Earth history. (Photos from top: Nona Chiariello; Anthony Barnosky)
Nearby, Hadly and others watched from a rowboat. Winching the Vibracore back up out of the depths, Stegner gingerly extracted the muddy bounty, capped the aluminum tubes, and passed them to Hadly. In all, her team extracted 14 cores from Searsville and nearby Upper Lakes. Back at the lab, Stegner and colleagues analyzed the plutonium and bomb carbon in the cores, as well as mercury and other heavy metals, and correlated them with specific time intervals captured in the sediment. Evidence of the 1906 and 1989 earthquakes—marked by disturbances in otherwise continuous sediment—helped them establish dates for each layer. The team also compared their findings with archival material, such as newspaper accounts, oral histories, and old photographs. Species disappeared when the area was logged and plowed for agriculture; once Jasper Ridge was protected, oak populations increased. The presence—and disappearance—of microfossils correlated with the recorded dates of herbicide and pesticide applications at Searsville Lake. “This is a different kind of science,” Hadly says. “The history and its geographical markers are intertwined. Teasing them out creates a picture in which the past becomes the present and the Holocene becomes the Anthropocene.”
Rob Dunbar, a professor of Earth system science and of oceans, says Hadly is helping pioneer a necessary intellectual approach to our changing world. “There is a strong case to be made for defining reference sections for the Anthropocene wherein interdisciplinary, precise, and well-dated scientific knowledge tells us what happened and why, the extent to which humans contributed to change, and the outcome on biodiversity, hydrology, climate, and community resilience.”
What’s in a Name?
Not everyone is keen on the term Anthropocene. Some argue it redoubles our human-centric bias with respect to the rest of the living world, although some of the suggested alternatives, including Capitalocene, Plantationocene, and Homogocene, are not much better. Tadashi Fukami, a professor of Earth system science and of biology, counts himself among those resisting the word’s reference to humanity and “the very arrogance that has got ourselves into this environmental crisis in the first place.” Regardless of the terminology, Fukami says, Hadly’s research helps demonstrate “how intricately humans are embedded in complex interactions with other species.”
Anthropocene itself has become a cultural meme. Zalasiewicz calls it “a new way of understanding the human role in environmental transformation.” In a 2019 textbook, he and his co-authors, including Barnosky, reference international law and medicine as arenas in which an official designation will be useful. International treaties assume planetary stability, based on “current conditions for an objective and unchanging reality that has surrounded us since time immemorial.” But the very geological boundaries of sovereign nations are changing as sea levels rise, ice melts, and coastlines move, raising issues about the extent of treaties and, for example, fishing rights. A formal designation is not going to change the “underlying geological realities” of our new epoch, they wrote, but may help us anticipate international aggravations arising from it. In 2015, a Commission on Planetary Health reported that the current systems supporting human well-being are inadequate to address Anthropocene issues, including pollution-related mortality. Establishing the epoch will provide a common reference point to help redefine some of humanity’s most basic guidelines around how we live.
Last May, at the House of the World’s Cultures in Berlin, the 12 teams vying for the Golden Spike presented their evidence. Stegner spoke for Searsville Lake, explaining how the long tubes of mud bear witness to history. She explained the land’s original occupation by Muwekma Ohlone people and its subsequent colonization. She elucidated places in the core that reflect Mexican and American “chopping up” of the landscape for ranching. “These are global signals of the Anthropocene,” she said. The news wasn’t all bad. She showed where tree communities had recovered when cattle grazing was discontinued. “When you limit impacts,” she said, “things tend to recover.”
In the end, 11 of the sites had the same punch line: plutonium. That evidence of human activity was so clearly discernible in every core presented that even the staid members of the Anthropocene Working Group were taken aback. “The major moment coming out of the last few days is progressively clear,” Zalasiewicz commented. To find another such pattern in the Earth, discernible everywhere on the globe, would require reaching back more than 11,700 years to the Pleistocene.
The five-day meeting included workshops and discussions among the scientists and the general public. An exhibition called “Earth Indices” by European artists Giulia Bruno and Armin Linke took place in the main exhibit area, with enlarged images of the scientists at work around the globe: underwater among coral reefs, encased in snow and ice, spelunking into the recesses of the Earth, and coring Searsville Lake. Hadly, Barnosky, and Stegner contributed a 39.4-foot-long photographic banner of Core JRBP2018-VC01B. The CT scan was laid out across the exhibit space dated at intervals. A colorized X-ray illustrated the differences in sediment density. But how could individual museumgoers interact with such a document? “All of us contribute in some way to the processes defining the Anthropocene,” says Zalasiewicz, “but we struggle to grasp the totality of the complex planetary changes now underway, and quite how we relate to them.” At Hadly’s suggestion, museumgoers marked important years in their own lives on the banner’s time line. People eagerly scratched in births, deaths, immigrations. The monumental moments in people’s lives appeared as minuscule slivers against the core. Yet its time line points to a destiny we share with the Earth.
Mary Ellen Hannibal is a former western media fellow at Stanford and winner of the university’s Knight-Risser Prize in Western Environmental Journalism. Her most recent book is Citizen Scientist: Searching for Heroes and Hope in an Age of Extinction. Email her at stanford.magazine@stanford.edu.
