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Scientists have been debating the Anthropocene theory for years, ever since some of them began arguing that urbanization, pollution, nuclear fallout and other marks of human existence on the planet should mark the start of a new epoch, or period of geologic time. In response to the ongoing debate, the International Union of Geological Sciences created a subcommission of scholars (dubbed the Anthropocene Working Group) to decide by 2016 whether to officially declare that the Holocene is over and the Anthropocene has begun—and, if a new epoch has begun, to assign it an official start date.

In a new study, published in the journal Nature this week, two British geologists make the case for two possible start dates: 1610 and 1964. According to them, each of these years is a clear turning point, or so-called “golden spike,” that has been captured in the Earth’s rocks, sediment or ice. As study co-author Prof. Mark Maslin of University College London, tells BBC News: “We look for these golden spikes – a real point in time when you can show in a record when the whole Earth has changed. If you look back through the entire, wonderful geological timescale, we have defined almost every boundary in that way.”

By 1610, the world had been fundamentally altered by the collision of Old and New Worlds that occurred a century earlier. European conquest and colonization of the Americas revolutionized global trade and farming. Entire species jumped between continents, as maize (corn) and potatoes native to Central and South America began to be grown in Europe and China, while European settlers brought wheat and sugar to the Americas. At the same time, waves of disease killed some 50 million people in the Americas, many of them farmers. As abandoned farmland turned back into forest, the trees absorbed enough carbon to cause a marked dip in carbon dioxide in the Earth’s atmosphere, as measured in Antarctic ice cores.

On the other hand, the researchers said, the start of the Anthropocene Epoch could reasonably be dated three-and-a-half centuries later. In 1964, a nuclear test ban was enforced that put an end to the tests of the 1940s, ‘50s and early ‘60s. While the testing was going on, radioactive carbon in the atmosphere—as measured in the Earth’s rock layers—increased dramatically, followed by a sharp drop in the wake of 1963’s Partial Test Ban Treaty.

In addition to 1610 and 1964, some scientists point to a third “golden spike”—the impact of the Industrial Revolution in the late 18th and early 19th century—as a contender for the start date of the Age of Man. Still others say efforts to pinpoint the start of a man-made epoch are premature, and that such a momentous change can only be seen with thousands (or even millions) of years of hindsight.

With so many competing opinions, it’s clear that the 39 members of the Anthropocene Working Group have their work cut out for them. They are currently reviewing the evidence and will offer their conclusions—including a preferred start date—next year.

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