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.”
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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.