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We are, according to scientists, in the midst of a new epoch of Earth history—one we forced upon ourselves.

According to members of the Anthropocene Working Group, humans have so severely altered the Earth’s geology that we’re no longer in the Holocene epoch. Instead, they say, we’re in the Anthropocene epoch. And now they’re trying to figure out when, exactly, it started, and how to prove it’s a real thing.

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Dr. Jan Zalasiewicz, who serves as the chair of the Anthropocene Working Group (AWG) explained to Fusion in an email that living in the Anthropocene epoch “means that things won't go back to 'normal' if, say, humans disappeared overnight tomorrow.  We've already pushed the planet along a different course.” Cool.

The AWG, Zalasiewicz explained, “Is analyzing Anthropocene to see whether it might be formalized to become part of the Geological Time Scale—which would be a very big step for geologists.” Part of this analysis includes figuring out when, exactly, the Anthropocene epoch would have started. Dr. Colin Waters, also a member of AWG, told Fusion in an email that “Geologists typically define the start of new time intervals by identifying a single reference point (or ‘golden spike’), at a specific location, that marks the lower boundary of a succession of rock layers as the beginning of the time unit.”

Now, members of the AWG are trying to figure out what that “golden spike” is. Waters explained that different researchers have different ideas about when the Anthropocene epoch likely began. Theories range from thousands of years ago, with the rise of agriculture, animal domestication, and more, to the industrial revolution—closer to 200 years ago.

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But Waters and Zalasiewicz, along with others, proposed a more recent timeline: A new study, suggested that the epoch began somewhere around 1952. Waters, the lead author, explained:  “What our paper argues is that the larger fusion ('thermonuclear' or 'hydrogen') weapons tests, which started in 1952, produced higher-altitude fallout which dispersed over the entire Earth surface.”

He added that the nuclear tests don’t in themselves merit a new epoch, but that they signify a greater change which does:

“The advent of the nuclear age in itself does not merit the identification of a new epoch. It is just that the signature of weapons testing coincides with a range of human-driven changes that have produced stratigraphic signals that indicate a dramatic shift in the Earth system around the mid-20th century.”

This, he said, is referred to by researches as the “Great Acceleration,” and includes: “population growth, economic development, industrialization, mineral and hydrocarbon exploitation, the manufacturing of novel materials such as plastics, the emergence of megacities, and increased species extinctions and invasions.”

Zalasiewicz added that “there is still a lot of work to do to characterize and define the Anthropocene more fully… nevertheless, it is already clear that the Anthropocene has geological reality.” And there’s no turning back.

Danielle Wiener-Bronner is a news reporter.