When we talk about carbon dioxide, the narrative is almost always that of a modern-day morality play. We hear about gigatons of CO2 emitted, about rising global temperatures and about the dire, unheeded warnings of climate scientists. In these tales, CO2 often seems less like a mute, inert molecule and more like an evil supervillain—a malevolent force that has been plotting for centuries to wreak havoc on our planet and ruin our lives.
But according to science journalist Peter Brannen, that dismal view is far too narrow. In his first book, The Ends of the World, Brannen chronicled Earth’s five major mass extinctions, charting the deep history of our planet’s greatest catastrophes. For his second, The Story of CO2 Is the Story of Everything (Ecco, 2025), he has higher ambitions, taking readers on dizzying jaunts through deep time to reframe our understanding of what may be the most vilified and misunderstood molecule on Earth.
Inspired and informed by conversations with leading planetary scientists, Brannen’s central argument is that CO2 is not merely an industrial pollutant but a key player in the four-billion-year-old drama of life on Earth. It is the molecule that built our planet, forming the global carbon cycle that has regulated climate, shaped geology and powered evolution for eons. He shows how the ebb and flow of atmospheric CO2 across Earth’s vast history has played a role in, yes, practically everything under the sun—from the primordial origins of life to the development of human civilization and our global economic system. From the ancient past to the present day, Brannen makes the case that to understand CO2 is to understand the very fabric of our world.
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Scientific American spoke with Brannen about what’s in his new book, how he came to see a simple gas as a character in a planetary epic and what the long history of CO2 can tell us about our precarious present moment—and our uncertain future.
[An edited transcript of the interview follows.]
How did this book come to be? That’s always a great way to start.
My previous book, The Ends of the World, was about the five biggest mass extinctions known in Earth’s history. And when paleontologists look at those events what they’ve found is, yes, a space rock seems to have triggered the most recent one, the Cretaceous mass extinction that wiped out the dinosaurs 66 million years ago. But the evidence is sparse for asteroid or comet impacts causing the others. Instead the other four—the Ordovician, the Devonian, the Permian and the Triassic mass extinctions—as well as the dozens of other minor mass extinctions in the fossil record, are most associated with major biogeochemical events, usually involving big spikes in atmospheric CO2. And these gigantic CO2 spikes are followed by extreme global warming and ocean acidification and all the other nasty climate change effects we’re understandably worried about today.
So this suggests the experiment we’re now running on the planet by burning fossil fuels has a lot in common with these really grisly planetary-scale events, which were literally the worst things that have ever happened in Earth’s history.
But in the course of researching that book, I realized—I’m not the first to think this, either!—that there’s a much bigger story to tell about CO2 because it’s not just some industrial by-product that spews out of smokestacks or spray cans such as methylmercury or chlorofluorocarbons, and so on. It’s fundamentally different—almost miraculously so. Life on Earth—what scientists call the biosphere—is carbon-based, and the source of that carbon is CO2. And when huge quantities of CO2 are suddenly injected into the atmosphere, this causes bad events, but in “normal” times, as it moves through the biosphere and between the air, the rocks and the ocean—the so-called global carbon cycle—CO2 is essentially the key thing that makes Earth a special, habitable place.
Venus is a near twin of Earth, right, almost the exact same size and bulk composition. But CO2 behaves very differently there in how it cycles around the planet; it’s built up so much in Venus’s atmosphere that it’s caused a runaway greenhouse effect, which is why Venus is a hell world while Earth is comparatively a great place to live, right next door. Or, you know, if all the CO2 in our atmosphere suddenly vanished, temperatures would rapidly plunge and before too long, glaciers would spread down to the tropics, and the oceans would freeze, and most of the biosphere would perish in a “snowball Earth” episode.
So it’s a good thing—essential, in fact—that there’s CO2 in Earth’s air, but we can absolutely have too much or too little, and the amount has fluctuated a lot over time. I wanted to reintroduce people to this thing they’ve heard about in the news and explain its vital role in shaping life on Earth.
I love the simplicity of the title you chose, The Story of CO2 Is the Story of Everything. But it really swings for the fences, doesn’t it? Like, “everything” is a very sweeping term! And I understand what you’re saying regarding planetary habitability and the fact of Earth’s carbon-based life. It’s harder, I think, to grasp and convey how humans come into the story, and one thing I love about your book is how adeptly you weave us into the fabric of this vastly bigger picture. Can you talk more about that?
Sure, and thank you. The idea is that to really understand the story of life on Earth, as well as what’s happening now with climate change, you need to understand this global carbon cycle I mentioned; life is etched in the flow of carbon all around our planet. And what I try to do in later parts of the book is describe how all of human history can be seen through this lens—so societies and empires, for instance, are composed of flows of carbon being organized in different ways. And the way our society and politics have developed across the past few centuries, it turns out, is closely connected with things such as how coal got into—and then how we got it back out of—the Appalachian Mountains or how oxygen got into Earth’s air. Seeing those connections can help explain how it was that humans came to be this geomorphological force on the planet—and how bizarre and important this moment in Earth history really might be.
What’s so extraordinary about our current moment is how one species on one branch of this gigantic tree of life has suddenly discovered this vast, ancient underground reservoir of carbon made by old life—and is lighting it all on fire. And that chemical reaction—burning carbon-rich organic matter with oxygen to make CO2 and release energy—is really the same thing that all aerobic life, all of Earth’s animals and plants, and so on, uses to drive its metabolism on a cellular level. We’re just doing this nightmarish, freakish version of it, where we’re suddenly combusting all of life’s leftover carbon from Earth’s history under our feet. We’ve sort of summoned these planetary forces into being by resurrecting the buried ghosts of all life that’s ever existed by bringing them back to the surface all at once.
This isn’t really a book about chemistry, but I need to add that the only way this all works at any scale is having lots of free oxygen in the atmosphere to react with the carbon. The air we breathe today is more than 20 percent oxygen, which is interesting because, for most of Earth’s history, there wasn’t nearly as much oxygen in the air. And it turns out that the rise of atmospheric oxygen isn’t as simple as some microbes figuring out photosynthesis a few billion years ago. You also have to constantly be burying a slow trickle of carbon—in dead plants and algae, in rocks and deep-sea sediments—to build the oxygen up in the air over hundreds of millions of years; otherwise the two react together, which draws oxygen back down. But if you lock that carbon up in the crust, oxygen will rise. Now, old plant stuff locked up in the crust, in those pockets where it’s economically exploitable, is better known as fossil fuel, right? I bet a lot of people don’t know that the reason why they can breathe is because there are fossil fuels under their feet.
I like to think of this interplay between carbon in the ground and oxygen in the air as making a big planetary-scale battery, where you get two parts of Earth—the really reactive, oxidizing atmosphere and the really reduced organic matter underground—out of equilibrium with each other, with lots of potential energy as a result. Then this weird fire creature suddenly shows up in the middle of these two reservoirs, and over the past few centuries, it’s learned how to reunite them to extract energy. So we’re talking about an almost instantaneous discharge of this huge planetary battery that took all of Earth history to build up.
So you’re saying we’re all basically fire imps dancing at the boundary between these two reservoirs, the oxygen-rich surface and the carbon-rich subsurface. And the development of human civilization really boils down to our getting better and better at discharging Earth’s battery, dissipating all the potential energy across this barrier.
Ha, sure, I guess that’s right. Imagine how aliens might see it, describing what different organisms on Earth actually do. They’d probably flag things such as nitrifying bacteria, bugs that pull nitrogen out of the atmosphere to fertilize the rest of the biosphere. But they’d also notice there’s this one remarkable creature that’s just moving all the carbon from within the crust into the atmosphere—and that’s us, obviously, the fire imps.
But I want to be careful: When you talk about it this way, it can seem like what we’re doing is just this inevitable, natural process, and I don’t think that’s necessarily true. This all sprung out of one particular part of the human population and is wrapped up in the details of human history—things such as the invention of the steam engine and the rise of capitalism. What we’re doing today is extremely unnatural in some ways, but I just find it eerie that it resembles this bigger picture: All life finds and dissipates free energy to maintain itself and grow. And human industrial civilization is doing this but at an almost unthinkable scale because it recently found the biggest source of free energy ever to exist on Earth.
What do you think happens next? Does Earth history tell us? Are we doomed to cause—and to suffer—another major mass extinction, or is there a way out? Easy questions, I know.
What Earth history tells us is that burning fossil fuels is not sustainable into deep geological time. There aren’t enough fossil fuel reserves to sustain us indefinitely, and there’s not enough margin in the carbon cycle to avoid disaster if we burn all we’ve got. Our fossil fuel era is like an explosion; it can’t last forever. So if we’re going to endure into the geological future, we need to very quickly find another source of energy at an equivalent scale to power society.
I think maybe the encouraging thing is that the public conversation, for most of the world, isn’t about debating the fundamental science anymore. The science is settled. It’s about different questions—of understanding the complexity and interconnectivity of the global carbon cycle and our place in it or of political economy and knowing where the levers are for us to pull in this system.
To understand the future and what’s going to happen, not only do you have to understand things such as the response of permafrost to warming or the ocean’s capacity to absorb carbon, you also have to understand humans as a component of these natural systems. This is why I think studying and communicating about climate change is the most interdisciplinary thing you can do because you can’t really divide these thorny issues of how we should organize society and how we should allocate resources from these broader, planetary questions. Climate change is such a huge, boundless phenomenon that everyone has to work on their specific parts. And I like to think my part, the worthwhile service I can provide with my storytelling, is to better illustrate just how big of a problem it really is.
You mentioned the science is settled, and I agree with that of course. But it seems to me there’s still a lot of climate denialism masquerading as “just asking questions” about scientific uncertainties, which can be pretty insidious—especially when the discussions involve geological timescales.
For instance, you write in your book about an unnamed smart and savvy nonscientist friend of yours who quite correctly noted to you that current levels of atmospheric CO2 are lower than they’ve been for most of Earth’s history and that they were dramatically higher tens of millions of years ago.
This is the sort of “talking point” that’s easily used to minimize and dismiss present-day concerns about climate change, right? Do you worry that this noble idea of offering a “big picture” view of our current moment in the context of Earth’s entire history can backfire?
So that exchange you mentioned was mostly about this period of time called the early Eocene, circa 50 million years ago, when CO2 was around 1,000 parts per million in the atmosphere, and Earth was about 12 degrees Celsius warmer—and there was still a thriving biosphere.
But to think that’s relevant for our situation doesn’t show an appreciation or knowledge of deep time—quite the opposite.
Like, yes, 50 million years ago, CO2 was much higher than it is today, and there were crocodiles and palm trees in the Arctic, and life was pretty happy. But if you ever so slightly poke at that “argument,” it just stops making sense because, for the past few million years, we have lived on a planet that has been in a weirdly low atmospheric CO2 regime—after a long, long decline in CO2 and temperature from the “greenhouse” world left over from the age of the dinosaurs. And that means most of the biosphere is now adapted to Earth being in an “icehouse” world that has ice ages. We’re technically still in an ice age, actually, because we still have polar ice caps. And we live on a planet that is presently partitioned by national borders and has more than eight billion people dependent on staple crops in certain special places where weather and climate allow. So okay, if we reverse these trends that have prevailed for tens of millions of years and, in just a century or two, get atmospheric CO2 levels as high as they were in the Eocene and suddenly live in a world where crocodiles can be comfortable in the Arctic—if you think our global civilization can withstand that shock, well, then you have more faith in humanity than I do.
There are precedents in the geological record for what’s happening now—and looking at them is pretty terrifying. We just accept as normal that we have a whole continent, Antarctica, that’s covered with kilometers-thick ice sheets. But that’s actually quite unusual in Earth history. One of the other times the world had similar “icehouse” conditions, such as an ice-covered continent, and then suddenly shifted to a greenhouse-style climate was the Ordovician mass extinction. And that was 445 million years ago, before the planet even had trees. That’s an alien world!
Or look at how much and how fast we’re injecting CO2 into the carbon cycle, into the atmosphere. The Permian mass extinction, the biggest one we know of—paleontologists call it the Great Dying—involved massive volcanic eruptions that pumped more CO2 into the air than we ever could even if we burned all available fossil fuels. And that really overwhelmed the carbon cycle and deranged Earth’s climate in all sorts of awful ways, and there was a huge warming spike and almost everything died. You might think, great, we can’t release as much CO2 as those eruptions did back then, even if we tried. But it’s not just the volume of CO2 released; the rate matters a lot. Those eruptions happened over tens of thousands of years. And right now, as far as we know, we’re emitting CO210 times faster than was emitted in the run-up to the Permian mass extinction. So what we’re doing right now is pretty geologically unprecedented; we really are in uncharted territory. That doesn’t mean we’ll necessarily spark another mass extinction on par with the Permian, but we really are leaping into the unknown.
This reminds me of one of my favorite quotes from your book: “Anyone who tells you they know what even four degrees of warming or more in a century will actually mean—or what that will look like on a planet gripped by ice ages for the past three million years—is full of shit. Especially if they’re an economist.”
I do wonder whether you’re personally more optimistic or pessimistic here—and how you think we might ever get to a world where we don’t burn any more fossil fuels.
Believe it or not, I’m less pessimistic now than I was when I started writing the book in 2020; a lot has changed in the past five years. I’ve recently started to better appreciate the amazing thing that’s happening now with solar power. It’s really getting astonishingly cheap, and China and many developing countries are prioritizing solar and things such as electric cars over fossil fuels just because it’s better technology. Solar doesn’t have as many awful geopolitical implications as fossil fuels or nuclear energy. There are still problems, such as supply chains for rare-earth minerals, for example. But, hey, the fact is: there’s a nuclear reactor in the sky called the sun that’s just beaming out free energy for us to use.
So you can imagine solar blowing away the fossil fuel world in the same way that the fossil fuel world blew away the world of horse-drawn carriages and plows—because it was a lot easier to buy cheap gas and fill your tractor—really, to dig seas of oil out of the ground and light them on fire—than it was for everyone to maintain stables of horses, right? That may be what happens next: solar gets cheaper and easier and just outcompetes fossil fuels in most domains.
But I don’t like this ethos that’s all too common, which is that you’re never supposed to give people doom and gloom about the climate. I think having the shit scared out of you isn’t always a bad thing—because, yeah, you can look at graphs showing the extraordinary progress in solar, and you might think we’re just on rails, and we’re inevitably moving toward this new, better world. But that’s probably wrong—it’s going to take concerted political intervention to stop burning fossil fuels, which is what we’re going to have to do.
I’m glad you brought up how much has changed in the past five years in terms of lower-carbon energy and why there may be reasons for optimism. But, to be a bit of a downer, what about things such as the rise of AI and the associated ramp-up in energy usage for data centers and computation?
Or, because you mentioned politics, how can you be optimistic, given the political situation in this country? It seems the ascendant view now at the highest levels of our government is that climate change isn’t real—and if it is real, it’s not a problem—and if it’s a problem, it’s not something we can solve, so we should just continue with business as usual. I mean, yikes!
It’s definitely not my area of expertise, but all these AI companies are burning through billions of dollars, and they’re not turning a profit, and it doesn’t seem like these tools increase productivity that much in most domains. Although they’re profoundly useful in some areas, such as biotech, where it seems like you can use them to do practically a year’s worth of research in an afternoon. And they all seem to wave away the fact that you basically need to build nuclear reactors to power these things, which is never going to be cheap to do. The point is that I imagine this will turn out a bit like the tech bubble of the 2000s: you’ll have some genuinely productive, game-changing applications, but most of the projects will go under because you can’t just keep losing billions of dollars per quarter forever while also facing ever increasing energy demands. It seems ripe for a massive adjustment.
The U.S. can barely manage its current electrical grid, much less completely rebuild it and add twice as much power. Like so many other things we do, at some point, the AI boom is going to run into constraints that collide with politics, economics or physics. So I’m skeptical there. Then again, it’d be great if we gained some energy breakthrough out of necessity because that’s usually when we figure stuff out. But I don’t think something such as fusion power is going to happen anytime soon, even with AI as a stimulus.
And as for politics, in the context of my book, I talk about these successive hegemonic powers over the past 500 years—which isn’t an idea I came up with. But yeah, you have the Dutch empire, and then the British empire and then the U.S. empire, each stepping up as the other gets senescent. And in that framework, looking at our political situation, it’s hard to avoid the conclusion that despite its many amazing advantages, the U.S. is somehow past its prime and has chosen to cede its global leadership. And the rest of the world is now figuring out how to make do without us. Maybe it’s just the death throes of a certain part of our society, having a spasm and tearing everything down while it still can. And maybe once the fever breaks, once we hit rock bottom, there’ll be nowhere left to go but up. But I’m not really optimistic about that, to be perfectly honest.
Talking about empires—nation-state superpowers—iteratively shaping the whole world reminds me: As a journalist covering “space,” I should note some people would argue many of the problems we’ve been discussing boil down to being confined to one planet. The idea is that by somehow making life multiplanetary and extending our economic sphere out into the wider solar system, we could reduce our negative impacts on Earth while also continuing the positive exponential growth trends we’ve seen in recent centuries.
I wonder if you think that there’s any validity to that thinking or that it’s all a naive, starry-eyed pipe dream.
I understand the vision and the logic behind it, and I think we need space exploration because it’s important to understand our place in the universe. But yeah, we’d have to do some shocking things in space to continue the exponential curve of the past couple centuries that you mentioned, such as transforming the entire solar system into a place that’s of more direct benefit to our human—or posthuman, I don’t know—society and industry.
Some people seem to view the past few centuries as sort of this new trajectory that’s going to go on forever. But when you appreciate how strange it is and what it required—exploding this planetary battery all at once—and how we never really see endless exponential growth in any complex natural system, that seems unlikely. Things usually instead display an S curve, which seems to be what we’re following now. In the book, I quote this scientist who’s done some back-of-the-envelope calculations showing the ridiculous things that must happen if you do take those exponential scenarios seriously. If there’s a conservative relationship between energy and economic growth, then within 400 years, you’re either boiling off the oceans with waste heat—which is never going to happen—or you’re covering the entire planet with solar panels, which isn’t going to happen, either. And if you extend that trend another 1,000 years or so into the future, you’d be using more energy than the sun emits in all directions—and that’s obviously never going to happen, either. So at some point the energy-growth relationship is going to change.
I don’t really know what the world looks like when that change happens; I don’t think anyone does. But it does suggest that, one way or another, in a few centuries, human society is going to be organized very differently—because right now, if growth falters even for a few years, this has major societal consequences: depressions and recessions and civil unrest and resource wars, and so on. I talk in the book about how, a few centuries ago, this thing called the Little Ice Age helped to knock out the agricultural foundations of a sort of feudal system in Europe and might have spurred things such as the Thirty Years’ War and lots of other horrific stuff. And that paved the way for capitalism and the modern nation-state, and all this stuff that still structures our world today. The point is: not that long ago society looked very different, and looking ahead about the same amount of time, you can foresee it will probably have to look very different again.
Back to space: More broadly, growing up, I was a total space nerd. I used to daydream about alien squid under the ice of Europa or about living in a settlement on Mars. But the more I’ve learned as I’ve gone on this journey into geoscience, the more I’ve come to appreciate what an absolute miracle Earth is. For better or worse, I’ve become increasingly uninterested in what’s “out there” because what we have “right here” is so strange and beautiful, and the history of Earth is so contingent and bizarre. Why would we ever want to go to Mars, you know?! We have Earth right here!
In planetary time, it’s like we woke up as human beings just yesterday, and everything in this world is made almost perfect for us somehow. And a lot of that ties back to, yes, this global carbon cycle that’s in balance just so—so that there’s a tiny bit of CO2 in the air to keep us barely warm enough but not so little that we freeze and not so much that it fries everything. And enough carbon is locked away in rock that we have oxygen to breathe. And the fact that we’re now trashing this incredible place and destabilizing this amazing system so quickly is sort of humiliating, as a society, that we’ve been such bad stewards of the planet.
We talked earlier about deep time and the way our inability to properly comprehend and integrate it into our lives blinds us to what we’re really doing to the planet. And this makes me wonder: How do you think people should feel, thinking about deep time and our place in it? More pointedly, having immersed yourself in all this for two books now, how do you feel? How has this journey changed you?
There can be fear in feelings of awe, in facing the sublime. And I like dwelling in that disconcerting space where you’re aware of and in contact with things and forces and scales that are so much bigger than you. It’s a nice reminder that you’re not really the main character, the star of the show.
I live in Massachusetts, where some of this geology is harder to see, but when I go out West, and I’m looking at some rock face that captures tens of millions of years of history, where the environment switches from the bottom of the ocean to a lagoon to a riverbed to a desert back to the bottom of the ocean, it’s humbling in the best way. It really chips away at your ego. And it can be consoling, even; given how out of control things feel today and how crazy you can make yourself refreshing your social media feed and keeping up with news, there’s something very peaceful in contemplating time at these gigantic scales. For me, knowing there’ve been so many chaotic and scary chapters in Earth history, it’s consoling to know that in a million years, everything’s going to be fine.
But even so, deep time doesn’t really have any obvious, direct relevance to your daily life. None of us get to live on geological timescales, and we can only care about the things that are in front of us. Your personal relationships and the people you love—those really are the most important things. I’ve struggled with this, I’ll admit. When I was writing the first book, my mom died, and I was grappling with losing her while I was also gaining this new, more cosmic perspective about Earth and our place in it. I never fully reconciled how one’s personal experience should inform this deeper, bigger view.
So I just come back to the recognizing the beauty of this world we all share. There’s beauty in being part of this long pageant of life. I think of my mom, and I think of how there’ve been countless mothers and their children that loved each other in countless ways, great and small, throughout Earth’s history. I think of how, today, you can see dinosaur trackways where—let’s say 93,871,252 years ago, during April—a dino mother and her child danced together for a moment on a sand flat. That’s really beautiful—all the more so when you think of all those moments that didn’t even make it into the fossil record. It’s a privilege to bear witness to that and to be a small part of this far greater story.
