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.
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.
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.
PETER BRANNEN is a science writer based in Boulder, Colorado.
His work has appeared in The New York Times, The Washington Post, and Wired.