Rachel: Have you ever thought about what it would feel like to witness the end of the world? Not in a flash of fire from an asteroid, but quietly, in your own backyard. You notice one summer that the bats you used to see at dusk are just… gone. The monarch butterflies that filled your garden last year? Vanished. Justine: It’s a chilling thought. We tend to see extinction as something ancient—dinosaurs and asteroids—or something happening far away, on a nature documentary. But Elizabeth Kolbert’s book, The Sixth Extinction, makes the terrifying case that the single greatest extinction event in 66 million years is happening right now. And the asteroid… is us. Rachel: Exactly. It’s a profound, paradigm-shifting idea. And today we'll dive deep into this from two powerful perspectives. First, we'll explore that shocking idea that we, humanity, are the new asteroid, changing the planet at a geologically unheard-of speed. Justine: Then, we'll uncover the silent but devastating crises happening out of sight—in the chemistry of our oceans and the heart of our most fragile ecosystems. It’s a story about running the history of our planet backwards, at warp speed. So, buckle up.

Rachel: So, Justine, that phrase—"we are the asteroid." It’s so dramatic, but what does it actually mean? The last big extinction, the one that wiped out the dinosaurs, was caused by a rock the size of a mountain hitting the planet. Our impact feels… different. Less concentrated. Justine: That’s the crux of it. It’s not about a single impact. It’s about the rate of change. Kolbert argues that if you’re a geologist millions of years from now, looking back at the rock layers, our entire human epoch—a few thousand years—will be compressed into a layer of sediment thinner than a piece of paper. The change will look instantaneous. It will look exactly like an asteroid strike. Rachel: And the engine of that change is carbon. The book explains this concept of "running geological history backwards," and it’s fascinating. For hundreds of millions of years, plants and marine organisms have been pulling carbon dioxide out of the atmosphere through photosynthesis. They die, get buried, and under immense pressure and time, they become coal, oil, and natural gas. They effectively locked away atmospheric carbon deep underground. Justine: It’s like the planet created a geological savings account. Over eons, it made these tiny deposits, slowly building up this vast reserve of sequestered carbon, which, by the way, also gave us the oxygen-rich atmosphere we need to live. Rachel: Precisely. And what have we done in the last 200 years? We invented machines to drill down, find that savings account, and we’re spending it all in one wild, weekend-long shopping spree. We are taking carbon that took 300 million years to store and pumping it back into the atmosphere in the span of a few centuries. Justine: That’s a terrifyingly good analogy. And the consequences aren't abstract. You mentioned the bats and butterflies at the start. That’s not just a poetic image; it’s a real story from the book, a way to make this global change feel personal. Rachel: It really is. Kolbert talks about her own backyard in the northeastern United States. For years, she’d see little brown bats swooping through the twilight. Then, within the last decade, they’ve been almost completely wiped out by a fungal disease. A disease that was almost certainly brought over from Europe by us, on our clothes or our gear. The local bats had no immunity. They were naive. Justine: And it’s the same with the monarch butterflies? Rachel: Exactly the same pattern. She talks about raising the caterpillars with her kids, watching them transform, this incredible, magical experience. And then, a few years ago, they just stopped coming. Their populations have crashed. These are not exotic, faraway creatures. These are the common, everyday animals of her home, and they are disappearing within a single human lifetime. Justine: So it’s not one big hammer blow. It’s death by a thousand cuts. We move a fungus here, we cut down a forest there, we change the temperature everywhere. Kolbert calls it creating a "New Pangea." The original Pangea was the supercontinent where all land was connected. Now, through our planes and container ships, we've effectively reconnected the entire world, moving species around with a speed and carelessness that nature never could. Rachel: And that creates chaos. You introduce a rat or a mongoose to an island where birds never had a predator, and the birds are gone in a generation. They just don't know how to react. We are constantly reshuffling the deck, and in a game with such high stakes, most species can't adapt to the new rules. Justine: Which is a perfect lead-in to the second, and maybe even more frightening, part of this story. Because it’s not just the things we move around. It’s the fundamental chemistry of the planet that we’re altering.

Justine: And Rachel, this leads to what might be the scariest part of the book for me, because it’s almost completely invisible. We talk a lot about climate change as warming, as heat. But Kolbert calls ocean acidification the "equally evil twin" of climate change. It’s what all that carbon is doing to the oceans. Rachel: It’s a silent crisis. About a third of all the CO2 we pump into the atmosphere doesn't stay there. It gets absorbed by the oceans. And when you dissolve carbon dioxide in water, you get carbonic acid. It’s basic chemistry. We are, quite literally, making the oceans more acidic. Justine: And that’s a problem because… why? I mean, the ocean is huge. A little change in pH, what’s the big deal? Rachel: It’s a huge deal if you’re one of the countless creatures that builds a shell or a skeleton to survive. Think of corals, clams, oysters, plankton. They build their structures out of a mineral called calcium carbonate. As the water becomes more acidic, the chemical building blocks they need become scarcer. It gets harder and harder for them to build. Justine: So for a coral or a clam, this is like us trying to build a house while someone is slowly dissolving our bricks. It's not just that it's getting hotter; the very building blocks of their world are becoming unstable. Rachel: That is the perfect analogy. And to see this in action, Kolbert takes us to this extraordinary place: One Tree Island. It’s a tiny, remote coral cay on the southern tip of the Great Barrier Reef. It’s a research outpost, a speck of land you can only get to with a permit. It feels like the end of the earth. Justine: A pristine, untouched paradise, you’d think. Rachel: You would. But the scientists there were studying the very thing we’re talking about. They were measuring the growth rate of the reef. And their findings were grim. They found that the rate at which the corals were building their skeletons—the process called calcification—had already dropped significantly. The reef was still alive, but it was struggling. It was building itself more slowly. Justine: So even in one of the most remote places on Earth, the signature of our industrial world is already there, dissolving the foundations of the ecosystem. Rachel: Exactly. And the book makes it clear that the outlook for reefs is dire. It’s very hard to imagine a future, even if we cut emissions drastically tomorrow, where coral reefs don’t suffer tremendous, catastrophic damage. They are vulnerable to warming, which causes bleaching, and they are vulnerable to acidification, which stops them from growing. It’s a one-two punch. Justine: And there’s an even more visceral example in the book, right? A place that gives us a terrifying glimpse into the future of our oceans. Rachel: Yes. A natural laboratory in the Bay of Naples, off the coast of Italy. Near the island of Ischia, there are these volcanic vents on the seafloor. But unlike most vents, these don’t spew heat or sulfur; they just bubble out pure, cool carbon dioxide. Justine: So they’re naturally creating the exact conditions of ocean acidification that we’re heading towards globally. It’s a time machine. Rachel: It’s a perfect time machine. Scientists can literally swim along a transect. Far from the vents, you have a typical, bustling Mediterranean ecosystem. But as you swim closer to the vents, into water with a pH level that we expect for all oceans by the end of this century, the world changes. The corals are gone. The sea urchins are gone. The snails are gone. The only things that thrive are some scraggly seagrass and jellyfish. About a third of the species just disappear. Justine: My god. So we’re looking at a preview of 2100, and it’s a world where a third of marine life could simply be… gone. Wiped out by an invisible chemical change. Rachel: That’s the projection. And this idea of vulnerability isn’t just in the ocean. It’s also a huge threat in the most diverse places on land: the tropics. We tend to think the poles are most at risk from warming, and they are warming fastest. But Kolbert argues the tropics might be in even greater danger. Justine: Which seems counterintuitive. How so? Rachel: It’s because of something called the "latitudinal diversity gradient." It’s a fancy term for a simple observation: biodiversity explodes as you get closer to the equator. For example, the entire boreal forest of Canada, a billion acres, has maybe 20 species of trees. In a single 50-acre plot in the Andes, researchers found a thousand tree species. Justine: That’s an incredible difference in scale. So the tropics are hyper-diverse. Rachel: Incredibly. But here’s the catch. That hyper-diversity comes with hyper-specialization. Tropical species tend to be adapted to very, very narrow temperature and altitude ranges. Kolbert describes walking down a mountain in the Andes with a botanist who tells her to pick out a distinctive leaf. He says, 'Watch for it. You’ll only see it for the next hundred meters of elevation, and then it will be gone.' Justine: Wow. So these species live on a knife's edge. If you change the temperature even slightly, their entire world, their perfect little niche, is gone. They have to move. Rachel: They have to move. But in the tropics, you have to travel a very long way to find a cooler climate. Or you have to move up a mountain. But what happens when you run out of mountain? The rate of change we’re causing is likely far too fast for most of these species to migrate or adapt. They are, in a sense, the most fragile of all.

Rachel: So, when you put it all together, Justine, the picture is staggering. On one hand, you have the sheer, unprecedented speed of our impact, turning us into a geological force. And on the other, you have these silent, cascading crises in the oceans and the tropics, unraveling the web of life in ways we can’t even fully see yet. Justine: It really is. And it brings to mind this brilliant psychological analogy Kolbert uses in the book. It’s about an experiment from the 1940s where psychologists showed people doctored playing cards—like a red spade, or a black heart. Rachel: I remember this. People got really uncomfortable, right? Justine: They got completely flummoxed. Their brains didn't know how to process it because it violated the fundamental categories they understood. A spade is supposed to be black. A heart is supposed to be red. So at first, they’d try to force it into the old boxes. They’d say, "Oh, that’s a purple spade," or "It’s a regular spade, just under a weird light." They would do anything but admit the card itself was wrong. Rachel: Until they couldn’t deny it anymore. Justine: Exactly. And that’s us. We are being shown a red spade. We’re seeing a world changing in ways that defy our old categories. We try to explain it away—'it’s just a weird summer,' 'it’s a natural cycle,' 'things will bounce back.' We are trying to shoehorn the terrifying new reality into our comfortable old paradigm. Rachel: But the book is essentially screaming at us that the paradigm is broken. The rules of the game have fundamentally changed, and we’re the ones who changed them. Justine: And that leaves us with the ultimate question. The book isn't a manual of solutions. It's a diagnosis. A very, very grim one. It doesn't tell us what to do. Rachel: No, it doesn't. It doesn't offer easy answers, because there aren't any. But it forces us to confront a profound truth and ask a critical question. Now that we know we are a geological force, with the power to create and destroy on a planetary scale, what kind of force do we choose to be? What will the fossil record of our era say about us? That’s the question that sits with you, long after you’ve finished the last page.