Tourists visit the the Mendenhall Glacier, in Alaska. Geologists are considering whether humans’ impact on the planet has been significant enough to merit the naming of a new epoch. PHOTOGRAPH BY MATTHEW RYAN WILLIAMS/THE NEW YORK TIMES/REDUX

The duties of the Anthropocene Working Group—a thirty-nine-member branch of a subcommission of a commission of the International Union of Geological Sciences—are both tedious and heady. As the group’s chairman, Jan Zalasiewicz, whom Elizabeth Kolbert wrote about, in 2013, says wryly, “People do not understand the very slow geological time scale on which we work.” Yet the A.W.G.’s forthcoming recommendations may bring an end to the only epoch that any of us have ever known—the Holocene, which began after the last ice age, about twelve thousand years ago, and lasts to this day. The group’s members are pondering whether the human imprint on this planet is large and clear enough to warrant the christening of a new epoch, one named for us: the Anthropocene. If it is, they and their fellow-geologists must decide when the old epoch ends and the new begins.

In a paper published today in the journal Nature, Simon Lewis, of the University of Leeds, and Mark Maslin, of University College London, propose that the Anthropocene’s “golden spike”—the line between it and the Holocene—be set at either 1610 or 1964. Geologic time periods are usually bounded by markers in rock or ice: for instance, the beginning of our current era, the Cenozoic, is identified by a dusting of iridium that fell across the globe about sixty-six million years ago. (The element, otherwise rare in Earth’s crust, may have been dropped here by the same asteroid that purportedly killed off the dinosaurs.) The year 1610 is distinguished in Antarctic ice cores by a dip in atmospheric carbon dioxide. In the decades after the Europeans—and their germs—arrived in the Americas, some fifty million people died; huge swaths of abandoned farmland reverted to forest, and the trees absorbed more carbon than the crops. The year 1964, meanwhile, is discernible in rock layers by its high proportion of radioactive isotopes—fallout from nuclear-weapons testing.

Some members of the Anthropocene Working Group approve of aligning the birth of the new epoch with that of the atomic age, which coincides conveniently with the mid-twentieth-century explosion in human population and industrialization. Lewis and Maslin, neither of whom are A.W.G. members, lean toward the 1610 date, arguing that the movement of species across the Atlantic around that time caused a “clear and permanent geological change to the Earth system.” Zalasiewicz notes that there are many variables to consider. “We have to try to understand humans as a driver of geology, just as we’ve previously understood massive volcanic outbursts and meteorite impacts,” he told me. “Those are very simple, by comparison.” The important thing, he stressed, is that the epoch be defined using traditional scientific criteria. After Paul Crutzen, a Dutch atmospheric chemist and Nobel laureate, coined the expression, in 2000, Zalasiewicz said, “People were using ‘the Anthropocene’ as if it were a real geological term—without inverted commas, without any sense of irony. We had to do something about it.”

The Anthropocene Working Group will deliver its recommendations to the International Commission on Stratigraphy next year. But millennia from now, when the remnants of our civilization are crushed into thin layers of rock, it will hardly matter where the Anthropocene’s golden spike lies—in 1610, in 1964, or, as Crutzen originally proposed, in the late eighteenth century, when James Watt designed his steam engine. The key question is how a formally designated human epoch will shape scientific thought in the meantime. As Naomi Oreskes, a science historian and A.W.G. member, put it, “It is geologists saying, ‘We are witnesses to this profound and problematic transition. And we want the world to talk about it.’ ” The challenge of the Anthropocene is to learn to see ourselves not at the open end of Earth’s timeline but within its bounds, as fossils in the making.