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And when you get right down to it, humans have existed for a very short time relative to the history of the Earth. Still, some geologists say Homo sapiens have had such a huge impact on the Earth that the human era should be marked in the official timeline of the planet, the same way scientists use terms like Jurassic or Cretaceous period to talk about the age of dinosaurs. NPR's Nell Greenfieldboyce reports this idea has kicked off a vigorous debate.
NELL GREENFIELDBOYCE, BYLINE: The Earth is about 4 and a half billion years old. Scientists have divided up that vast stretch of time into categories and subcategories. There's eons and eras and periods and epochs.
JAN ZALASIEWICZ: And currently, formally, we live in something called the Holocene Epoch.
GREENFIELDBOYCE: Jan Zalasiewicz is a geologist at the University of Leicester in the U.K. He says the push to give an official name to the time of human domination began about 15 years ago. A prominent atmospheric chemist was at a meeting where people kept referring to environmental change in the Holocene.
ZALASIEWICZ: And he said stop it, you know, things have changed so much. We're no longer in the Holocene. We're in the - and he was looking for words - the Anthropocene. And then that started people talking.
GREENFIELDBOYCE: The term Anthropocene has caught on; people do use it, but it's not official. Zalasiewicz is part of a working group that's considering whether it should be. The group hopes to soon make recommendations to the international scientific organization that decides such things, but defining the Anthropocene is tricky. Consider this - when did it start? One early idea was that it began with the Industrial Revolution.
ZALASIEWICZ: But the majority of us are beginning to favor a more recent time in the mid-20th century.
GREENFIELDBOYCE: He says that's when human activities first left clear marks on the geologic record. Atom bomb tests produced fallout that can be detected in ice cores, soils and sediments all over the planet. Similarly, that's when there's signs of a huge increase in burning fossil fuels.
ZALASIEWICZ: Since the mid-20th century, the whole world has been sprinkled with fly ash - tiny carbon particles.
GREENFIELDBOYCE: Geologists like to see those kinds of signals in the rock to mark the beginning of an official time period. But some researchers say, wait a second, humans actually began to transform the planet thousands of years earlier with agriculture. Bill Ruddiman is a climate scientist at the University of Virginia.
BILL RUDDIMAN: If the Anthropocene began in 1945, then the entire story of changing the surface of the Earth by cutting forest and plowing prairies is - occurred before the Anthropocene. Does that make sense?
GREENFIELDBOYCE: In the current issue of the journal Science, he and some colleagues argue that the Anthropocene is a useful concept, but that trying to formally define it isn't helpful. Now the geologic time periods in use today were originally developed as scientists try to figure out Earth's history by looking at rocks and fossils because rocks and fossils were all they had. Stan Finney is a geologist at California State University in Long Beach. He points out that we can trace humanity's impact in a totally different way. We have things like written records.
STAN FINNEY: Why do we have to go find something in the rock record?
GREENFIELDBOYCE: And he says other organisms besides humans have had an even bigger impact. When the first plants evolved to grow on land, there were dramatic changes to the Earth's surface and atmosphere.
FINNEY: We don't have a name for that incredible revolution and period in Earth's history.
GREENFIELDBOYCE: So he asks, do we really need a special name for the time of humans? Nell Greenfieldboyce, NPR News.
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