THE AMAZON DOESN’T PRODUCE 20% OF THE WORLD’S OXYGEN

The Amazon rainforest is often called the “lungs of the Earth” and credited with producing some 20 percent of the world’s oxygen. But that’s often a repeated, yet misleading claim. In fact, it’s a gross overestimate. The Amazon’s net contribution to the oxygen we breathe likely hovers around zero.

“There are a number of reasons why you would want to keep the Amazon in place, oxygen just isn’t any one of them,” remarks Earth systems scientist Michael Coe, who directs the Amazon program at the Woods Hole Research Center in Massachusetts.

To Coe, the claim “just doesn’t make any physical sense” because there simply isn’t enough carbon dioxide in the atmosphere for trees to photosynthesize into an entire fifth of the planet’s oxygen.

Given that the atmosphere contains less than half a percent of carbon dioxide, but 21 percent oxygen, it’s not possible for the Amazon to generate that much oxygen.

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Several scientists have come up with more accurate estimates. Yadvinder Malhi, an ecosystem ecologist at Oxford University’s Environmental Change Institute, bases his calculations on a 2010 study that estimates tropical forests are responsible for around 34 percent of photosynthesis occurring on land.

Based on its size, the Amazon would account for about half of that. That would mean the Amazon generates around 16 percent of oxygen produced on land, explains Malhi.

But that’s not the whole story. Trees don’t just exhale oxygen—they also consume it in a process known as cellular respiration, where they convert the sugars they amass during the day into energy, using oxygen to power the process. During the night when there’s no sun around for photosynthesis, they’re net absorbers of oxygen.

(How to erase 100 years of carbon emissions? Plant trees—lots of them.)

Malhi’s research team reckons that trees inhale a little over half the oxygen they produce this way. The rest is probably used up by the countless microbes that live in the Amazon, which inhale oxygen to break down dead organic matter of the forest.

“The net [oxygen] effect of the Amazon, or really any other biome, is around zero,” he explains. Because of this balance between oxygen production and consumption, modern ecosystems barely budge oxygen levels in the atmosphere.

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Which biome produces most of Earth’s oxygen?

The oxygen we breathe is the legacy of phytoplankton in the ocean that have over billions of years steadily accumulated oxygen that made the atmosphere breathable, explains Scott Denning, an atmospheric scientist at Colorado State University.

This oxygen could only accumulate because the plankton became trapped at the bottom of the ocean before they could rot. Otherwise, their decomposition by other microbes would have used up that oxygen.

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Nevertheless, the 20 percent myth has been making the rounds for decades, though it’s unclear where it originated.

Malhi and Coe reckon it stems from the fact that the Amazon contributes around 20 percent of the oxygen produced by photosynthesis on land—which may have erroneously slipped into public knowledge as “20 percent of the oxygen in the atmosphere.”

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Why the Amazon is still important

Obviously, none of this is to say that the Amazon isn’t important. In its pristine state, it makes a significant contribution to pulling tons of carbon dioxide out of the atmosphere. Coe likens it not to a pair of lungs, but to a giant air conditioner that cools the planet.

Indeed, the Amazon is one of nature’s most powerful tools for mitigating climate change, alongside other tropical forests in central Africa and Asia—which, like the Amazon in recent years, have also been vulnerable to fires.

The Amazon also plays an important role in stabilizing rainfall cycles in South America and is a crucial home for groups living in the Amazon, including Indigenous peoples, as well as countless animal and plant species.

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“Very few people talk about biodiversity, but the Amazon is the most biodiverse ecosystem on land, and climate change and deforestation are putting that richness at risk,” notes climate scientist Carlos Nobre with the University of São Paulo’s Institute for Advanced Studies.

For its importance to the world, the Amazon might as well be a metaphorical pair of lungs. This analogy may have been helpful in galvanizing action around deforestation. But to most researchers, it doesn’t make much sense—not least because actual lungs inhale oxygen rather than exhaling it.

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“If people want to relate it to a fundamental part of their body that maintains stability and maintains life, maintains wellbeing—symbolically, you can make some kind of association,” says Nobre. “But physically speaking, [the Amazon rainforest is] not really the lungs of the world, no.”