by Deborah Blum
Such was Lyell and Darwin’s influence that for more than a century, even as it became increasingly clear that “whole families or orders” had indeed at various points suddenly been eliminated, geologists eschewed any account of these episodes that might be construed as Cuvierian. This reluctance extended into the 1980s, when it was proposed that an asteroid plowing into the earth at the end of the Cretaceous period, 65 million years ago, was what had done in the dinosaurs, along with the plesiosaurs, the mosasaurs, the pterosaurs, the ammonites, most birds, and a significant proportion of mammals. The impact hypothesis was resisted until the 1990s, when the existence of a huge impact crater formed precisely at the end of the Cretaceous was confirmed. The crater lies off the Yucatán Peninsula, buried under half a mile of newer sediment.
While the discovery of the impact crater didn’t exactly invalidate Lyell and Darwin’s model, it revealed their dismissal of catastrophe to have been itself “unphilosophical.” Life on earth has been “disturbed by terrible events,” and “living organisms without number” have been their victims. What is sometimes called “neocatastrophism,” but is mostly now just considered mainstream geology, holds that the world changes only very slowly, except when it doesn’t.
As best as can be determined, the rate of change today is as fast as it’s been at any time since the asteroid impact. This is why Zalasiewicz believes that the stratigraphers of the future should have a relatively easy time of it, even though who or what was responsible for the sudden alteration of the planet may not immediately be clear. At one point he mused, “It may take them a little while to sort out whether we were the drivers of this, or if the cats or the dogs or the sheep were.”
After everyone had changed into dry clothes, we met in the sitting room of the B & B for tea. Zalasiewicz had brought along several papers he had recently published on graptolites. Settling back in their chairs, Condon and Millar rolled their eyes. Zalasiewicz ignored them, patiently explaining to me the import of his latest monograph, “Graptolites in British Stratigraphy,” which ran to sixty-six pages and included illustrations of more than 650 species. In the monograph, the effects of the extinction event showed up more systematically, if also less vividly, than on the rain-slicked hillside. Until the end of the Ordovician, V-shaped graptolites were common. These included species like the Dicranograptus ziczac, whose tiny cups were arranged along arms that curled away and then toward each other, like tusks; and Amphigraptus divergens, which was shaped like a bat in flight. Only a handful of graptolite species survived the end-Ordovician extinction, which, it’s now believed, was caused by the sudden glaciation of the supercontinent Gondwana. (No one is entirely sure what caused this glaciation.) Eventually the surviving graptolites diversified and repopulated the seas of the Silurian. But Silurian graptolites had a streamlined body plan, more like a stick than a set of branches. The V shape had been lost, never to reappear. Here, writ very, very small, was the fate of the dinosaurs, the pterosaurs, and the ammonites—a once highly successful form now relegated to oblivion.
That evening, when everyone had had enough of tea and graptolites, we went out to the pub on the ground floor of Britain’s narrowest hotel, which is twenty feet across. After a pint or two, the conversation turned to another one of Zalasiewicz’s favorite subjects: giant rats. Zalasiewicz pointed out that rats have followed humans to just about every corner of the globe, and it is his professional opinion that one day they will take over the earth.
“Some number will probably stay rat-size and rat-shaped,” he told me. “But others may well shrink or expand. Particularly if there’s been epidemic extinction, and ecospace opens up, rats may be best placed to take advantage of that. And we know that change in size can take place fairly quickly.” I recalled once watching a rat drag a pizza crust along the tracks at an Upper West Side subway station. I imagined it waddling through a deserted tunnel, blown up to the size of a Doberman.
Though the connection might seem tenuous, Zalasiewicz’s interest in giant rats represents a logical extension of his interest in graptolites. When he studies the Ordovician and the Silurian, he’s trying to reconstruct the distant past on the basis of the fragmentary clues that remain—fossils, isotopes of carbon, layers of sedimentary rock. When he contemplates the future, he’s trying to imagine what will remain of the present once the contemporary world has been reduced to fragments—fossils, isotopes of carbon, layers of sedimentary rock. One of the many aspects of the Anthropocene that he believes will leave a permanent mark is a reshuffling of the biosphere.
Often purposefully and just as often not, people have transported living things around the globe, importing the flora and fauna of Asia to the Americas and of the Americas to Europe and of Europe to Australia. Rats have consistently been in the vanguard of these movements, and they have left their bones scattered everywhere, including on islands so remote that humans never bothered to settle them. The Pacific rat, Rattus exulans, a native of Southeast Asia, traveled with Polynesian seafarers to, among many other places, Hawaii, Fiji, Tahiti, Tonga, Samoa, Easter Island, and New Zealand. Encountering few predators, stowaway Rattus exulans multiplied into what Richard Holdaway, a New Zealand paleontologist, has described as “a grey tide” that turned “everything edible into rat protein.” (A recent study in the Journal of Archaeological Science concluded that it wasn’t humans who deforested Easter Island; rather, it was the rats that came along for the ride and then bred unchecked. The native palms couldn’t produce seeds fast enough to keep up with their appetite.)
When Europeans arrived in the Americas and then continued west to the islands that the Polynesians had settled, they brought with them the even more adaptable Norway rat, Rattus norvegicus. In many places, Norway rats, which are actually from China, outcompeted the earlier rat invaders and ravaged whatever bird and reptile populations the Pacific rats had missed. Rats thus might be said to have created their own “ecospace,” which their progeny seem well positioned to dominate. The descendants of today’s rats, according to Zalasiewicz, will radiate out to fill the niches that Rattus exulans and Rattus norvegicus helped empty. He imagines the rats of the future evolving into new shapes and sizes—some “smaller than shrews,” others as large as elephants.
“We might,” he has written in The Earth After Us (2008), “include among them—for curiosity’s sake and to keep our options open—a species or two of large naked rodent, living in caves, shaping rocks as primitive tools and wearing the skins of other mammals that they have killed and eaten.”
Meanwhile, whatever the future holds for rats, the extinction event that they are helping to bring about will leave its own mark. Many evolutionary lineages have recently come to an end; many, many more are likely soon to follow. Extinction rates today are hundreds of times higher—for some groups, such as amphibians and freshwater mollusks, perhaps thousands, or even tens of thousands, of times higher—than they’ve been since mammals took over the ecospace emptied by the dinosaurs. For reasons of geological history, the current extinction event is often referred to as the “sixth extinction.” (By this accounting, the event recorded in the rocks at Dob’s Linn is the first of the five major mass extinctions that have occurred since complex animal life evolved.) Whether the sixth extinction will turn out to be anywhere near as drastic as the first is impossible to know; nevertheless, it is likely to appear in the fossil record as a turning point. Climate change—itself a driver of extinction—will also leave behind geological traces, as will deforestation, industrial pollution, and monoculture farming.
Ultimately, most of our carbon emissions will end up in the oceans; this will dramatically alter the chemistry of the water, turning it more acidic. Ocean acidification is associated with some of the worst crises in biotic history, including what’s known as the end-Permian extinction—the third of the so-called Big Five—which took place roughly 250 million years ago and killed off something like 90 percent of the species on the planet.
“Oh, ocean acidification,” Zal
asiewicz said when we returned to Dob’s Linn the following day. “That’s the big nasty one that’s coming down.”
In recent years, a number of names have been proposed for the new age that humans have ushered in. The noted conservation biologist Michael Soulé has suggested that instead of the Cenozoic, we now live in the “Catastrophozoic” era. Michael Samways, an entomologist at South Africa’s Stellenbosch University, has floated the term “Homogenocene.” Daniel Pauly, a Canadian marine biologist, has recommended the “Myxocene,” from the Greek word for “slime,” and Andrew Revkin, an American journalist, has offered the “Anthrocene.” (Most of these terms owe their origins, indirectly at least, to Lyell, who, back in the 1830s, coined the names Eocene, Miocene, and Pliocene.)
The word “Anthropocene” was put into circulation by Paul Crutzen, a Dutch chemist who, in 1995, shared a Nobel Prize for discovering the effects of ozone-depleting compounds. The importance of this discovery is difficult to overstate. Had it not been made—and had the chemicals continued to be widely used—the ozone “hole” that opens up every spring over Antarctica would have expanded until eventually it encircled the entire globe. One of Crutzen’s fellow Nobelists reportedly came home from his lab one night and said to his wife, “The work is going well, but it looks like the end of the world.”
Crutzen once told me that the word “Anthropocene” came to him while he was in a meeting. The meeting’s chairman kept referring to the Holocene, the “wholly recent” epoch, which began at the conclusion of the last ice age, eleven and a half thousand years ago. According to the International Commission on Stratigraphy, or ICS, which maintains the official geological time scale, the Holocene continues to this day.
“‘Let’s stop it,’” Crutzen recalled blurting out. “‘We are no longer in the Holocene; we are in the Anthropocene.’ Well, it was quiet in the room for a while.” At the next coffee break, the Anthropocene was the main topic of conversation. Someone came up to Crutzen and suggested that he patent the term.
Crutzen wrote up his idea in a short essay, titled “Geology of Mankind,” which ran in the journal Nature. “It seems appropriate to assign the term ‘Anthropocene’ to the present, in many ways human-dominated, geological epoch,” he observed. Among the many geologic-scale changes people have effected, Crutzen cited the following:
Human activity has transformed between a third and a half of the land surface of the planet.
Many of the world’s major rivers have been dammed or diverted.
Fertilizer plants produce more nitrogen than is fixed naturally by all terrestrial ecosystems.
Humans use more than half of the world’s readily accessible freshwater runoff.
Most significant, Crutzen noted, people have altered the composition of the atmosphere. Owing to a combination of fossil-fuel combustion and deforestation, the concentration of carbon dioxide in the air has risen by more than a third in the past two centuries, while the concentration of methane, an even more potent greenhouse gas, has more than doubled. Just a few more decades of emissions may bring atmospheric CO2 to a level not seen since the mid-Miocene, 15 million years ago. A few decades after that, it could easily reach a level not seen since the Eocene, some 50 million years ago. During the Eocene, palm trees flourished in the Antarctic and alligators paddled around the British Isles.
“Because of these anthropogenic emissions,” Crutzen wrote, the global climate is likely to “depart significantly from natural behavior for many millennia to come.”
Crutzen published “Geology of Mankind” in 2002. Soon the Anthropocene began migrating into other scientific journals. “Global Analysis of River Systems: From Earth System Controls to Anthropocene Syndromes” was the title of a 2003 article in the journal Philosophical Transactions of the Royal Society B. “Soils and Sediments in the Anthropocene,” ran the headline of a piece from 2004 in the Journal of Soils and Sediments.
Zalasiewicz noticed that most of those using the term were not trained in the fine points of stratigraphy, and he wondered how his colleagues felt about this. At the time, he was head of the Geol Soc’s stratigraphy committee, and during a meeting one day he asked the members what they thought of the Anthropocene. Of the twenty-two stratigraphers present, twenty-one thought that the concept had merit.
“My response was it’s a very interesting and powerful idea,” Andy Gale, a professor at the University of Portsmouth, told me. “I felt it was worthwhile to pursue, because it’s an important tool for making people think.”
The group decided to approach the concept as a formal problem. Would the Anthropocene satisfy the stratigraphic criteria used for naming a new epoch? (To geologists, an epoch is a subdivision of a period, which, in turn, is a division of an era; the Holocene, for instance, is an epoch of the Quaternary, which is a period in the Cenozoic.) After a year’s worth of study, the answer that the group arrived at was an unqualified yes. Among other things, the members observed in a paper summarizing their findings, the Anthropocene will be marked by a unique “biostratigraphic signal,” a product of the current extinction event, on the one hand, and of the human propensity for redistributing life, on the other. This signal will be permanently inscribed, they wrote, “as future evolution will take place from surviving (and frequently anthropogenically relocated) stocks.”
Or, as Zalasiewicz would have it, giant rats.
Just as in the early years of the Geol Soc, stratigraphers today spend a lot of time arguing about borders. A few years ago, after much heated discussion, members of the ICS voted to move the start of the Pleistocene epoch from about 1.8 million to about 2.6 million years ago. This decision was part of a broader, and even fiercer, debate about whether to do away with the Quaternary, the period that spans both the Pleistocene and the Holocene, and fold it into the Neogene. (The elimination of the Quaternary was vigorously—and ultimately successfully—resisted by Quaternary stratigraphers.)
The debate over the Anthropocene’s borders is complicated by the fact that the geology of the epoch is, at this point, almost entirely prospective. The way stratigraphers usually define boundaries—once they’ve stopped arguing about them—is by choosing a particularly fossil-rich sequence of rocks to serve as a reference. These reference sequences are colloquially known as “golden spikes”—technically, they’re called Global Boundary Stratotype Sections and Points, or GSSPs—and they’re scattered around the world (though a disproportionate number are in Europe). The striped rocks at Dob’s Linn have been designated the golden spike for the start of the Silurian period. For the base of the Carboniferous, the golden spike is near the town of Cabrières, in southern France, and for the start of the Triassic it’s in the hills of Meishan, China. (The Chinese have tried to turn this last golden spike into a tourist destination, with a manicured park and a statue of a tooth from a once common eel-like creature known as a conodont.)
Since the rocks of the Anthropocene don’t yet exist, it’s impossible to choose an exemplary sequence of them. To stratigraphers, then, a key, but also rather vexing, question is what could serve instead of the traditional golden spike. In 2009 the ICS set up an Anthropocene Working Group to examine this and related issues; not surprisingly, Zalasiewicz was appointed chairman. At the time of our visit to London, he told me that he thought there were many possible ways that the start of the epoch could be designated. One would simply be to choose a date—1800, say, or 1950. This is how geological periods of the deep, pre-fossiliferous past are defined; what’s known as the Neoproterozoic era, for example, is said to have begun precisely one billion years ago.
Another possibility would be to use nuclear fallout. The aboveground tests of the mid-twentieth century dispersed radioactive particles all around the globe. Some have half-lives of more than a thousand years; in a few cases, like uranium-236, the figure is in the tens of millions. To future geologists, the fallout will thus present a novel radioactive “spike” (assuming, that is, that the future does not hold a nuclear war). This sort of geochemical
marker is used to define the end of the Cretaceous. The impact that occurred during the final seconds of the period left behind a thin layer of sediment containing anomalously high concentrations of the element iridium—the so-called iridium spike.
Yet another possibility is to use the world’s subway systems, an idea that also has precedent in deep time. Geologists refer to the outlines of burrows that creatures left behind in the sediments as “trace fossils.” The start of the Cambrian period, some 540 million years ago, is defined as the point when the first complex burrows appear; these left impressions in the rocks that resemble scattered grains of rice. (No one is sure what the animals that made the burrows looked like, as their bodies have not been preserved.) London’s subway system, the world’s oldest, will leave behind an enormous set of trace fossils, as will New York’s and Seoul’s and Paris’s and Dubai’s.
“All the great world cities have underground systems now,” Mark Williams, a stratigrapher who teaches at the University of Leicester and is a member of the Anthropocene Working Group, noted. “They’re extensive, they’re fairly permanent from a geological perspective, and they’re a very, very good indicator of the complexity that’s come to characterize the twentieth and twenty-first centuries.”
Williams told me that the response to the idea of formalizing the Anthropocene had “generally been very positive.” (Just in the past few months, three new academic journals focusing on the Anthropocene have been launched.) But, as is to be expected from a group that can sustain a decade-long disagreement about the status of the Quaternary, there’s still plenty of dissent. Some critics argue that humans have been altering the planet for thousands of years already, so why get all worked up about it now!’