As the news of fires raging in the Amazon spread across the world last week, so did a misleading yet oft-repeated claim about the rainforest’s importance: that it produces 20 percent of the world’s oxygen.

Some have taken it to mean that we’re at risk of jeopardizing the world’s oxygen supply. “We need O2 to survive!” former astronaut Scott Kelly tweeted last week.

However, the figure—which has earned the forest the title “lungs of the Earth”—is a gross overestimate. As several scientists have pointed out in recent days, 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.

Physically Impossible

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.

AMAZON FIRES 'SO MUCH WORSE THAN BEFORE,' SAY LOCALS A hymn describes Porto Velho's sky as forever blue, but now it's grey with smoke from the Amazon fires.

Think about it: For every batch of carbon dioxide molecules trees pull out of the air, they push a comparable number of oxygen molecules back out. 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.

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, who detailed his calculations in a recent blog post.

That percentage sinks to 9 percent when taking into account the oxygen produced by phytoplankton in the ocean. (Climate scientist Jonathan Foley, who directs the non-profit Project Drawdown which researches climate change solutions, arrived at a more conservative estimate of 6 percent).

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. So during the night when there’s no sun around for photosynthesis, they’re net absorbers of oxygen. 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. Instead, 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, at 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. The processes that determine how much oxygen is found in the atmosphere on average occur over vast geological timescales and aren’t really influenced by the photosynthesis going on now, Denning explains in an article in The Conversation.

Cradle of biodiversity

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.”

Obviously, none of this is to say that the Amazon isn’t important. In its pristine state, it makes a significant contribution to pulling 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—one of our most powerful in mitigating climate change, alongside other tropical forests in central Africa and Asia—some of which are also currently burning.

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

“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, and 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.