Carbon Dioxide Defines Our Terrifying, Exhilarating World—And Always Has

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 a 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 latest book, The Story of CO₂ Is the Story of Everything (Ecco, 2025), he reframes 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 practically everything under the sun—from the primordial origins of life to the development of human civilization and our global economic system. Brannen makes the case that to understand CO₂, from the ancient past to the present day, is to understand the very fabric of our world.

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

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 [published in 2017], was about the five biggest mass extinctions known in Earth’s history. And when paleontologists have looked 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 are 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 one of the industrial by-products that spew out of smokestacks or spray cans, such as methylmercury or chlorofluorocarbons. 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. When huge quantities of CO₂ are suddenly injected into the atmosphere, it causes bad events, but in “normal” times, as it moves through the biosphere and 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.

If all the CO₂ in our atmosphere suddenly vanished, temperatures would rapidly plunge. Before too long, glaciers would spread down to the tropics, the oceans would freeze, and most of the biosphere would perish in a “snowball Earth” episode.

So it’s good—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.

One thing I love about your book is how you weave humans 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’s 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 they can breathe is that 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’s history to build up.

Basically you’re saying we’re all 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.

Hah, 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.

Life on Earth—what scientists call the biosphere—is carbon-based, and the source of that carbon is CO₂.

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’s 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’s 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 separate 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 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 kind 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.

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 currently partitioned by national borders and has more than eight billion people dependent on staple crops in certain special places where weather and climate allow. Okay, so 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’s 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 tremendous 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 try. 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 CO₂ 10 times faster than it was emitted in the run-up to the Permian mass extinction. So what we’re doing is 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 are definitely leaping into the unknown.

In terms of the future, are you more optimistic or pessimistic—and how do 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 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 the same way the fossil-fuel world blew away the world of horse-drawn carriages and plows. 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 artificial intelligence and the associated ramp-up in energy usage for data centers and computation?

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 it 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.

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 it makes me wonder: Having immersed yourself in all this for two books now, how do you feel? How has this journey changed you?

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. 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’s history, it’s consoling to know that in a million years everything’s going to be fine.

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 care only about the things that are in front of us. Your personal relationships and the people you love—those 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 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 have been countless mothers and their children who 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.

Editor’s Note (1/5/26): This article was edited after posting to correct the illustration.