Humans could burn every living thing on the planet and still not dent its oxygen supply.
As tongues of flame lapped the planet’s largest tract of rain forest over the past few weeks, it has rightfully inspired the world’s horror. The entire Amazon could be nearing the edge of a desiccating feedback loop, one that could end in catastrophic collapse. This collapse would threaten millions of species, from every branch of the tree of life, each of them—its idiosyncratic splendor, its subjective animal perception of the world—irretrievable once it’s gone. This arson has been tacitly encouraged by a Brazilian administration that is determined to develop the rain forest, over the objections of its indigenous inhabitants and the world at large. Losing the Amazon, beyond representing a planetary historic tragedy beyond measure, would also make meeting the ambitious climate goals of the Paris Agreement all but impossible. World leaders need to marshal all their political and diplomatic might to save it.
The Amazon is a vast, ineffable, vital, living wonder. It does not, however, supply the planet with 20 percent of its oxygen.
As the biochemist Nick Lane wrote in his 2003 book Oxygen, “Even the most foolhardy destruction of world forests could hardly dint our oxygen supply, though in other respects such short-sighted idiocy is an unspeakable tragedy.”
The Amazon produces about 6 percent of the oxygen currently being made by photosynthetic organisms alive on the planet today. But surprisingly, this is not where most of our oxygen comes from. In fact, from a broader Earth-system perspective, in which the biosphere not only creates but also consumes free oxygen, the Amazon’s contribution to our planet’s unusual abundance of the stuff is more or less zero. This is not a pedantic detail. Geology provides a strange picture of how the world works that helps illuminate just how bizarre and unprecedented the ongoing human experiment on the planet really is. Contrary to almost every popular account, Earth maintains an unusual surfeit of free oxygen—an incredibly reactive gas that does not want to be in the atmosphere—largely due not to living, breathing trees, but to the existence, underground, of fossil fuels.
Shanan Peters, a geologist at the University of Wisconsin at Madison, is working to understand just how it was that our lucky planet ended up with this strange surplus of oxygen. At a presentation in June, at the North American Paleontological Convention in Riverside, California, he pulled up a somewhat unusual slide.
“What would happen if we combusted every living cell on Earth?” it asked. That is, Peters wanted to know what would happen to the atmosphere if you burned down not just the Amazon, but every forest on Earth, every blade of grass, every moss and lichen-spackled patch of rock, all the flowers and bees, all the orchids and hummingbirds, all the phytoplankton, zooplankton, whales, starfish, bacteria, giraffes, hyraxes, coatimundis, oarfish, albatrosses, mushrooms, placozoans—all of it, besides the humans.
Peters pulled up the next slide. After this unthinkable planetary immolation, the concentration of oxygen in the atmosphere dropped from 20.9 percent to 20.4 percent. CO2 rose from 400 parts per million to 900—less, even, than it does in the worst-case scenarios for fossil-fuel emissions by 2100. By burning every living thing on Earth.
“Virtually no change,” he said. “Generations of humans would live out their lives, breathing the air around them, probably struggling to find food, but not worried about their next breath.”
To understand why requires a short tour of our planet. The two most abundant gases billowing out of volcanoes are water and carbon dioxide. Photosynthesizers—whether plants, algae, or cyanobacteria—use this raw material to pull off Earth’s greatest magic trick: harnessing photons from a giant thermonuclear explosion 93 million miles away (that is, sunlight), to strip that H2O of its H’s, and add them to that same volcanic CO2, to make the stuff of life. That’s namely stuff with lots of C’s, H’s, and O’s in it, like sugars and carbohydrates, wood and leaves. The O2 left over from this sorcery is released to the environment as waste.
Of course, that’s not all that happens on our planet. All but a hundredth of a percent of this photosynthetic stuff is consumed by creatures like us (and many quite unlike us) in the exact reverse process. That is, we use up that same waste oxygen in order to burn that photosynthetic organic matter, whether by munching leaves, or flesh reconstituted from leaves, to steal that chemically captured sunlight for ourselves. We then release the original CO2 and H2O back to the environment as exhaust.
This process of respiration, this metabolic burn, represents a perfect and complete reversal of photosynthesis. That is, after a tree spends a lifetime producing oxygen and tree stuff, that oxygen is used up in its own undoing—by decomposers such as fungi, or by bugs and animals that eat its leaves, and by bacteria that respire dead stuff in the earth. In the ocean, this unraveling is carried out by creatures that skim algal muck from the surface of the sea, and big things that eat one another, and bacteria that feed on the snowfall of tiny carcasses sinking through ocean depths, or larger ones resting on the seafloor. Organic carbon stuff does not like to sit around for long.
In the long run, and from the perspective of oxygen, it’s a wash. As much is consumed as is created—and not only by life. Free oxygen likes to react with almost everything on the planet, whether that’s rocks at Earth’s surface, or sulfur in volcanic gases, or iron in ocean crust. Left to its own devices, oxygen will disappear all by itself.
On their own, then, trees—and even entire forests and seas of plankton—are not enough to fill the atmosphere with a surplus of oxygen. If 99.99 percent of the vast reservoir of oxygen created by the living world is consumed by the living world, that gets you an atmosphere with 0.01 percent oxygen, not our modern 20.9 percent. Photosynthesis is a necessary but not sufficient condition for a world that is hospitable to white-hot oxygen-burning furnaces like us.
“The notion that we owe the breath we breathe to the rain forest, or the [phytoplankton] off the rain forests’ coasts, is just a little bit misinformed on the long timescale,” says Peters.
You don’t get to 20.9 percent, or an atmosphere that can host animal life, without geologic time, and without the fossil record. The tiny remainder of photosynthetic stuff that isn’t consumed and respired again by life—that 0.01 percent of plants and phytoplankton that manages to escape from this cycle of creation and destruction—is responsible for the existence of complex life on Earth. It’s the organic carbon that, once created, doesn’t get consumed again. Somehow this rounding error of plant stuff gets shuttled away after it dies, and is shielded from decomposition before it can be undone by the oxygen it produced in life. By not getting destroyed by oxygen, this conserved plant stuff gifts a tiny surplus of the unused gas to the atmosphere above. On the time scale of tens of millions of years, such meager gifts can accumulate—apparently to 20.9 percent.
This minuscule leak out of the system requires a rare confluence of conditions. If a tree or mat of cyanobacteria or swirling hurricane of phytoplankton dies, and is quickly buried by sediment, or comes to rest at the bottom of a putrid, anoxic sea, it can escape underground before it’s unraveled. Over large swaths of time, this tiny trickle of organic carbon into the earth can swell to become a vast reservoir of buried life. And far above—at the surface world this life once inhabited—it leaves behind an equivalent gift of oxygen.
These reservoirs of organic carbon underground—in places where they’re sufficiently concentrated that they’re useful to industrial civilization to dig up and burn—are called fossil fuels. Far more of this organic carbon exists in the geologic record, from all the life that ever lived, than in the thin, pulsating organic film of life at Earth’s surface. And so we breathe in not merely the thin wisp of oxygen created by living trees on Earth’s surface, but also the ancient oxygen gifted to us by these tens of millions of years of preserved forests and plankton blooms (coal, oil, and natural gas) that now rest under our feet.
Underneath West Virginia and England are vast sleeping jungles, more than 300 million years old, filled with centipedes the size of alligators and scorpions the size of dogs. Under West Texas is a tropical coral reef from a 260-million-year-old ocean, visited, in its day, by sharks with circular saw teeth. Under Saudi Arabia are whole seas of plankton that pulsed with the seasons and sunbathed under the waves in the age of dinosaurs.
This is what we are burning at Earth’s surface today. We’re not just burning down the Amazon. We’re burning down all the forests in Earth history that we can get our hands on. For every worrying part per million that CO2 goes up from burning fossil fuels, atmospheric oxygen goes down an equivalent amount, and then some. As a result, oxygen is dropping far faster from burning fossil fuels, and their untold forests, than it is from burning just the trees available on the planet’s surface. We’re reversing tens of millions of years of photosynthesis all at once.
Luckily, unlike CO2, we measure oxygen not in parts per million, but in parts per hundred. In other words, we have been gifted such an absurd surplus of oxygen by deep geological time, and by strange ancient life we’ll never know, that it won’t soon run out by our own hand, whether by deforestation or industry. Thankfully, most of the organic carbon in the Earth can be found not in easily recoverable reservoirs of fossil fuels, available to feed our industrial appetites, but in rather more rarefied deposits—small whispers of this life diffused in mudstones throughout Earth’s crust. There’s plenty of oxygen. For now.
Nevertheless, our geological bonfire illustrates just how unusual the project of humanity is. We are trying to retrieve, burn down, and metabolize all the forests and sea life ever buried, from alien worlds long past. We’re not merely lighting a match to the Amazon and imperiling everything that lives in it with extinction, but also summoning creatures long dead to return to Earth’s surface and give up the ancient energy they took to the grave. This global industrial metabolism, this heedless combustion of the life at the planet’s surface and throughout its history, is a new phenomenon on the face of the Earth. It is a forest fire of the eons.
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PETER BRANNEN is a science writer based in Boulder, Colorado. His work has appeared in The New York Times, The Washington Post, and Wired. He is the author of The Ends of the World: Volcanic Apocalypses, Lethal Oceans, and Our Quest to Understand Earth’s Past Mass Extinctions.