The Amazon Is Not Earth’s Lungs

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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.”

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.”

Read: When a killer climate catastrophe struck the world’s oceans

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.

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.

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.

Read: The Amazon cannot be recovered once it’s gone