And can you say again how it happened, how this thermostat broke?

Yeah.

So I just came from the annual Geological Society of America meeting.

And it seems like, you know, for a while people were wondering what is the mechanism of getting into snowball earth.

And it seems like they've sort of converged on this explanation.

So this is sort of weathering gone wrong.

I just said how weathering is so great it's kept the planet from heating up too much.

But then what happened to that plant?

That's Q-U-I-N-C-E .com/nbid.

[upbeat music] Hello, my name is Peter Brannen, and I'm a science writer who focuses on what we can learn from Earth history as in our own time we push the planet into uncharted territory.

So in my first book, The Ends of the World, I spent a few years hanging out with paleontologists and geochemists, going on fieldwork and joining them in the lab, where these scientists were trying to tease out the story of what caused the biggest mass extinctions in all of Earth history.

The upshot of that book was that it turns out that other than the asteroid wiping out the dinosaurs, in recent years, geologists have discovered that each of the other mass extinctions, including the very worst by far, were intimately related to severe changes in climate driven by changes in CO2.

In the course of writing that book, though, I realized there was a much bigger story to tell about the ways that CO2 has shaped life on Earth over its entire history, which led me to this book, The Story of CO2 is the Story of Everything.

[upbeat music] So I'm gonna share five of the key insights with you from my new book.

And the first insight is that over Earth history, CO2 has been the primary knob on the Earth's temperature.

[instrumental music] So these big swings of carbon dioxide is kind of what propels evolution, then eventually, you know, the evolution of humans, right?

Yeah.

Well, it seems like Snowball Earth is this crucible for evolution.

So, uh, when CO2's high, which I'm sure people are familiar with, like the age of dinosaurs, I think people think of this sort of warm jungle world.

And then since they went extinct 66 million years ago, when they did get hit by an asteroid, um, we inherit this greenhouse world from the dinosaurs, where it's high CO2, it's really warm, and we evolved in that world.

And then for the last 50 million years or so, there's been this slow decline of CO2, and it's getting cooler and cooler.

Around three million years ago, y- CO2 finally declines enough that the planet loses enough of its blanket that we start going in and out of these crazy ice ages.

That kind of put a check on growth, is what you're saying.

So these big swings of carbon dioxide is kind of what propels evolution, then eventually, you know, the evolution of humans, right?

Yeah.

Well, it seems like Snowball Earth is this crucible for evolution.

So over the age of, uh, animal life, you have periods when there's ice ages, like the ones that we recently just evolved from, and then you have some really warm greenhouse periods, uh, when CO2 is high, which I'm sure people are familiar with.

Like the age of dinosaurs, I think people think of this sort of warm jungle world.

And then since they went extinct 66 million years ago when they did get hit by an asteroid, um, we inherit this greenhouse world from the dinosaurs, where it's high CO2, it's really warm, and we evolved in that world.

And then for the last 50 million years or so, there's been this slow decline of CO2, and it's getting cooler and cooler.

That kind of put a check on growth is what you're saying.

Does that mean that the Earth was in a relatively stable state in that time? What's the explanation for that?

Well, I mean, so some people will point to, there's this thing, the great oxygenation event, um, two and a half billion years ago that some people will say, oh, this was the first mass extinction.

Cause, um, you had all these anoxic microbes that suddenly there was oxygen in the air.

And so there's this huge mass extinction, but that is more of a pop science story than it actually validated by our sort of understanding of earth history.

Cause, uh, nothing went extinct during that extinction, maybe for a couple of reasons.

Oxygen probably only topped out at like 1% of the atmosphere during the Great Oxygenation Event.

So it's sort of overrated in the... at least the way it's talked about sometimes.

Yeah.

Where was CO2 then? Two and a half million years ago, roughly?

I mean, it's where you start getting into this 280, 180 world that we've been seesawing in and out of around then.

Around 300 parts per million is, I think, where people put the threshold.

But then finally, your blanket that you're wearing as a planet is thin enough that these very regular changes to the orbit and the tilt of the planet that haven't really mattered in the same way in Earth history suddenly become hugely relevant because they...

CO2 is low enough that you can have ice forming.

If you have lower sunlight in the northern latitudes during summer, then you can build up ice sheets over year, over year, and suddenly you can go in and out of these crazy ice ages.

And that incredibly volatile environment, which has switched between 40,000 year, then 100,000 year ice ages, um and brief sort of springtimes like the one we're in today this is like an interglacial that if we weren't here we'd have another ice age probably coming at some point in the next few tens of thousands of years is thought to have shaped humanity because it was an incredibly difficult and volatile world it might have selected for things like the use of fire which then totally transformed our anatomy and allowed us to have these energy hogging brains and have things like culture and language and technology but Things like agriculture in this volatile world were very difficult, both because you're having these rapid climate swings where if you want to have some generational homestead, it's a bad idea if next year a sandstorm can come and sweep it away.

Can you give any thoughts