Scientists are a step closer to defining when the age of humans officially began

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This next story poses big questions about the record that we as human beings are leaving on the world. Scientists study past eras by the record left behind in the earth - you know, fossils and other items. And now some scientists are aiming to define when we have added enough to that record that we should call this the age of humans. NPR's Rebecca Hersher reports.

REBECCA HERSHER, BYLINE: Humans have so profoundly changed the Earth that our soot and our plastics and our radioactive fallout have made it into the very rocks and ice and mud that form our planet. And for decades, scientists have debated whether that means we're officially in a new geologic time period, the Anthropocene. This week, geologists got one step closer to saying yes. They proposed that the age of humans began in the 1950s with the nuclear era.

NICHOLAS KAWA: I feel deeply ambivalent about that.

HERSHER: Nicholas Kawa is an anthropologist at Ohio State. He's ambivalent because geologists chose the time period when plutonium and other radioactive material from nuclear blasts shows up in the geologic record around the world. Colin Waters is the chair of the Anthropocene Working Group, which is in charge of making the new epoch official. He said there were other options, but radioactivity is the clearest global signature that humans have left on the planet. It first shows up in the 1940s, but...

COLIN WATERS: It's not really until you get to about 1952 with the big thermonuclear detonations that you start to see these appearing everywhere.

HERSHER: Basically, the early '50s was the moment that humans left their mark on the whole planet for the first time. To officially define a geologic epoch, scientists have to agree on one place that acts as a sort of calibration site. This week, Waters and his colleagues announced that site, a lake in Canada with layers and layers of undisturbed mud that have collected human pollution and radioactive elements. Kawa, the anthropologist, says defining the age of humans by atomic bombs raises some tough questions that go way beyond geology.

KAWA: What does this mean for how we understand ourselves in relationship to the planet, who we are as a species?

HERSHER: Formally naming the Anthropocene is a reminder of the other ways that humans have transformed the planet, he says, through climate change and ecological destruction.

KAWA: To say, hey, wake up, like, we have to really sort of grapple with the fact that we have changed the face of the Earth, and we have created conditions that might not be hospitable for ourselves or other species, you know, further down the road.

HERSHER: For their part, geologists will vote on the official Anthropocene designation next summer.

Rebecca Hersher, NPR News. Transcript provided by NPR, Copyright NPR.