What Is the Age Of Humans? : Short Wave Humans have changed the Earth in such profound ways that scientists say we have entered a new geological period: the Anthropocene Epoch.
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Debating When The 'Age Of Humans' Began

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Debating When The 'Age Of Humans' Began

Debating When The 'Age Of Humans' Began

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Hey, everybody. Maddie Sofia here with Emily Kwong.


SOFIA: So Emily...

KWONG: Yeah.

SOFIA: ...We have put out more than 350 episodes of SHORT WAVE since we launched this show.

KWONG: I am so proud and so tired (laughter).

SOFIA: I feel you. I feel you. And, you know, obviously, the coronavirus has gotten a lot of our attention. But after that, can you guess what topic we've covered the most and our listeners love?

KWONG: I don't know. I feel like all you talk about is bugs.

SOFIA: OK. Wow, wow, wow, wow. No. It's climate and the environment.

KWONG: Oh, yeah. Of course. It's one of the biggest stories of our lifetime.

SOFIA: Which is why we're dedicating this whole week to stories about climate change, starting today with a new episode about the debate over defining when humans really started impacting the planet. Oh, yeah. We're philosophizing over here.

KWONG: And then we're bringing back some of our favorites, like that episode about the slow creep of sea level rise in the U.S. and what the East Coast might look like in 2045.

SOFIA: Ooh, that was a good one - plus another one on the Supreme Court case going on right now over who should pay for climate change.

KWONG: And we followed some of the activists putting their lives at risk to protect the planet.

SOFIA: So tune in all this week.


SOFIA: You're listening to SHORT WAVE from NPR.


SOFIA: Hi, Rebecca Hersher.

HERSHER: So today I want to talk about the Anthropocene. Have you ever run across that term?

SOFIA: Yes, I have kind of.

HERSHER: I can tell from your tone that you don't want to elaborate.

SOFIA: No, no, no. Go ahead.

HERSHER: (Laughter) So the Anthropocene refers to the age of humans. And it's the part of Earth's history that we're living in right now. It's primarily defined by the impacts that humans have on the planet.

SOFIA: Ooh, human impacts on the planet, very appropriate for climate week. Becky, go on.

HERSHER: Yes. So the word Anthropocene, it was popularized about 20 years ago by an atmospheric chemist. But it's not an official geological time period yet.

SOFIA: OK. Why not?

HERSHER: Because we don't know when it started. Like, when would you guess that humans began really affecting the planet?

SOFIA: Like, maybe, like, mass agriculture, question mark?

HERSHER: Sure. Yeah. No, that's a totally reasonable answer. Here are some other reasonable answers - when humans started burning fossil fuels, or when colonialism began...

SOFIA: Ooh, that's a good one.

HERSHER: ...Or when humans started setting off atomic bombs. So scientists, specifically geologists, they've been trying to agree on one starting date. And they've been working on this for years. They're getting pretty close. And so the work is really starting to heat up.

SOFIA: So today on the show, when did the Anthropocene begin? And who gets to decide? I'm Maddie Sofia. And this SHORT WAVE, the daily science podcast from NPR.


SOFIA: OK. Becky, let's talk about the Anthropocene. What are the leading contenders for the starting date of this age of humans?

HERSHER: Well, it depends on who you ask. You know, for example, if you ask an anthropologist or a social scientist, they're likely to have a different answer than a geologist or a stratigrapher, people who specifically study geologic time by looking at how rocks are layered.

NICHOLAS KAWA: There's lots of different debates about the Anthropocene and where it starts.

HERSHER: That is Nicholas Kawa. He is an anthropologist at Ohio State.

KAWA: There are some people that see an old Anthropocene that could go all the way back to when we see humans, you know, controlling and using fire.

HERSHER: Other people feel like the turning point was when humans started large-scale agriculture, maybe a couple thousand years ago.

SOFIA: Yeah.

HERSHER: Many indigenous scholars feel that the age of humans began when colonialism started, like in the 15th century...

SOFIA: Oh, that makes sense.

HERSHER: ...And Europeans systematically disrupted people and land all over the world.

KAWA: And then, there's another camp that sees it as occurring more recently with industrialization. And so I think there's kind of an old Anthropocene and a new Anthropocene.

SOFIA: OK, OK. So anthropologists, historians, they are arguing for an old Anthropocene, so like, basically, starting earlier.


SOFIA: Who's arguing for the new Anthropocene that starts more recently?

HERSHER: The geologists. They are the ones who will get to make the official decision. And it makes sense that they'd be looking more recently when you think about what geologists care about.

SOFIA: They care about rocks, among other things. Rocks, right?

HERSHER: Yeah, totally. Geologists care about rocks. And industrialization - late 1800s, early 1900s - that's really when you start to see a clear line or layer in the rock that says humans were here. There are chemicals. There are tiny bits of plastic, carbon and nitrogen from burning fossil fuels. You even start to see radioactive elements from nuclear blasts. And all that stuff gets into the mud and soil and ice and ends up being part of the geological record.

SOFIA: And, Becky, why does it matter to geologists, like, to find a line in the rock?

HERSHER: Because in order for the Anthropocene to be an official geological time period, the world's geologists need to agree on a line in the rock or ice or mud that scientists can find almost anywhere in the world.

SOFIA: OK, OK. So it's important that you can find the line kind of everywhere.

HERSHER: Exactly. Yeah. And you need one reference point for that line. And that point in time, the place that epitomizes it, it's called the golden spike location. Every geological time period has one of these reference spots somewhere on Earth that perfectly captures what makes that period unique.

SOFIA: Ooh, I didn't know that, Becky. OK. So give me, like, a golden spike example.

HERSHER: So the golden spike location that defines the end of the age of dinosaurs is a good example.


HERSHER: So you know, a meteor hit Earth around that time. It made impact in what is now Mexico. And it sent debris all around the world. And you can see that layer of debris in rock or in ice.


HERSHER: But the place where the layer is most obvious is this one place in Tunisia, really far from Mexico. And so that location is the golden spike location for the end of the age of dinosaurs. So that's what geologists are looking for, a golden spike location but for the Anthropocene.

SOFIA: And when they find it, they'll be able to say exactly when the age of humans began?

HERSHER: Exactly.

SOFIA: OK. So what are places that are contenders for this new golden spike?

HERSHER: So right now there are 11 potential sites on five continents. There is an Antarctic ice core. There's mud at the bottom of bays in the U.S. and Japan, peat bogs and stalagmites in caves in Europe, mud from the bottom of a lake in China.

SOFIA: OK. So Hersher, I'm hearing maybe more mud than I was expecting, like, a lot of muddy options.

HERSHER: (Laughter) Yeah.

SOFIA: It sounds like - is that right?

HERSHER: Yeah. Totally. And I talked to the lead scientist at one of these mud sites. It's a reservoir in California. And basically, the dam for this reservoir was built in the late 1800s. The goal was to hold water. But really, what this reservoir has been great at collecting is sediment.

ALLISON STEGNER: And at this point, there's over 11 meters of sediment since 1892.

HERSHER: That's Allison Stegner.

STEGNER: It's a lot (laughter).

HERSHER: So she's a geologist at Stanford and the leader for the golden spike team at this reservoir.

SOFIA: So why is it so good to have so much mud to work with?

HERSHER: Well, you can think about the mud like a diary for the reservoir. So like, every winter when it rains, a lot of sandy sediment washes into the lake. And the next summer, when it's warm, a lot of plants grow in the lake. And then they die. And their plant carcasses sink to the bottom of the lake. And the next winter, there's more sandy beds. And then the next summer, there're more plant beds.

STEGNER: When we have a lot of sediment in a site like this, it gives us the ability to study what happened in the past at really fine resolution.

HERSHER: And maybe there are other things that end up in the mud along the way - right? - like tiny pieces of plastic or chemical residue, even radioactive elements. So you can use the layers in the mud to figure out exactly when those human-made things showed up.

SOFIA: So how exactly did this scientist and her team do that? I mean, it seems, like, kind of difficult to scoop the mud out of the bottom of this lake without messing with the layers. You know what I mean? It's mud.

HERSHER: Yeah. No. It's really hard. So they had to use a machine called a vibracore, which this is a fun fact - I thought it was fun - it's similar to what construction workers use to shake the bubbles out of cement. It vibrates really, really fast. And you can use it to push a pipe vertically down into the mud and then carefully remove that cylinder of sediment a few feet at a time. So in all, they've removed a three-story tall, 127-year-old cross section of the mud.


HERSHER: And they've scanned it for evidence of fossil fuel combustion, for radioisotopes that get deposited when nuclear bombs go off and a host of other evidence of humans. And now they're preparing their final argument, you know, laying out all their evidence, saying when they believe the Anthropocene began according to their mud.

SOFIA: And what's the answer, according to mud?

HERSHER: Well, Stegner's team is still working, as are the other 10 teams. They're on track for a final decision around 2023.


HERSHER: But there are hints about what the final answer will be. So the whole effort is being coordinated by the Anthropocene Working Group, which is an international group of geologists. And they say it's looking like the official beginning of the Anthropocene will probably be in the early 1950s.

SOFIA: 1950s? That's, like, recently, recently. Like - and also, why the early 1950s specifically? Like, humans started burning fossil fuels and manufacturing things a lot earlier than that, right?

HERSHER: Right? I had the same question. But those things didn't leave markers in the rock and mud and ice all around the world. Remember, it has to be everywhere.

SOFIA: Oh, right. Yeah.

HERSHER: What did leave markers all around the world is nuclear blasts.

SOFIA: Oh, OK. First of all, that's depressing. And second of all, didn't people start setting off atomic bombs in the '40s? Like, why do they say it starts in the '50s, Becky?

HERSHER: So the answer has to do with the size of the bombs, at least according to Colin Waters. He's the chair of the Anthropocene Working Group.

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

HERSHER: So yeah, the exact date won't be official for another couple of years. But it looks like people who were born in the 1940s were probably, technically, born in a different geologic period.

SOFIA: Nothing makes you feel old like being from another epoch. You know what I'm saying, Becky?

HERSHER: (Laughter).

SOFIA: But OK, OK, OK. Indulge me for a moment. Is that really the best geologists can do? Like, we're going to define the age of humans based on one of the most destructive acts of people.

HERSHER: See, now you're thinking like an anthropologist.


HERSHER: And those are really valid questions. So Nick Kawa, the anthropologist at Ohio State, has thought about this a lot.

KAWA: Where we see the starting point of the Anthropocene tells us a lot about how we see humanity.

HERSHER: So one thing that makes him a little queasy about the mid-century start date is that it ignores everything that came before.

SOFIA: Right.

HERSHER: You know, there's so much human history that just gets wiped away once the golden spike location is chosen and the date is made official.

KAWA: And so I think if we just look at the atomic bomb or whatever, we're ignoring this long lead-up that is linked to long histories of environmental degradation led by, you know, colonial powers and by, to be frank, market capitalism.

SOFIA: Whew.

HERSHER: He also said something that I think is really interesting because I also heard it from geologists. And that's that the Anthropocene is different. It's just different from other geological time periods. And that's because it comes with moral baggage, you know? Kawa said something that was really funny to me. He was talking about the meteor that helped finish off the dinosaurs. And he said, you know, we don't think about meteors in ethical terms. They just happen.

SOFIA: Yeah. Yeah. I get it. Atomic bombs and deforestation and extracting, burning oil and gas - humans do those things. And some humans do a lot more than others.

HERSHER: Yeah. Exactly. And there are questions of responsibility, of right and wrong. It's honestly kind of uncomfortable. But it can also be good. You know, if atomic bombs become the ultimate marker of humanity on Earth, Kawa says we should all take a moment and think about what that means.

KAWA: I think it's good for us to take ownership of that. And you think of so many nasty facets of human history that we like to kind of sweep under the rug. And it could actually be helpful in the sense that, like, OK, we're using an atomic bomb to define the human era. What does that say about us? And what do we actually - (laughter) what do we want future markers to look like, you know?


SOFIA: All right. Becky Hersher, that is a lot to think about. I - this was really interesting. Thank you so much for bringing it on the show.

HERSHER: Yeah, of course. Happy climate week.

SOFIA: Happy climate week. That's nice.


SOFIA: This episode was produced by Brit Hanson, edited by Gisele Grayson and fact-checked by Rasha Aridi. The audio engineer for this episode was Josh Newell. I'm Maddie Sofia. And you're listening to SHORT WAVE, the daily science podcast from NPR.


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