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Is the Amazon rainforest really the “lung of the planet”?

“The lung of the planet is on fire,” reads one on social media. “The Amazon, the lung of our planet, produces 20% of our oxygen,” Emmanuel Macron tweeted. Hundreds of fires have been eating away at the Amazon for several weeks. This environmental tragedy is due in part to drought, but especially to deforestation, encouraged by Brazilian President Jair Bolsonaro. Fires are caused in particular by slash-and-burn clearings used to transform forest areas into growing and livestock areas or to clean up areas that have already been cleared.

Activists around the world are mobilizing, and the health of the Amazon, which has sparked a diplomatic crisis between France and Brazil, will be among the topics on the menu at the G7 summit in Biarritz. But perhaps a more appropriate metaphor than “the lung of the planet” should be found to evoke this complex ecosystem, which produces oxygen, retains greenhouse gases and is home to unparalleled biodiversity.

By day, the Amazon does the opposite of a lung
“Inspire… Exhale…” When your lungs are functioning properly, and without you even being aware of it, they sort the air you breathe to supply your body with oxygen and eliminate what it doesn’t need: carbon dioxide (CO2). Plants also “breathe” continuously. But during the day, they do the exact opposite of our lungs.

Plants draw water and minerals from the soil to feed. With their leaves, they capture carbon dioxide (or carbon dioxide) from the atmosphere. Plants then use solar energy to oxidize water and reduce carbon dioxide to produce carbohydrates, and therefore energy, to live and grow. This is called photosynthesis. By the way, plants release oxygen (O2) into the air. But most of this oxygen is used for its own consumption. When photosynthesis stops at night, plants no longer emit O2, but they continue to breathe.

It does not produce “20% of our oxygen”
Nearly 6 million square kilometers, 16,000 different tree species… It is the largest rainforest in the world, the most famous without a doubt. It is often read, including on environmental NGO sites, that it produces “20% of our oxygen”. It’s making him bear a very heavy responsibility. “The formula is beautiful, but it is not scientific,” Philippe Ciais, a researcher at the Laboratory of Climate and Environmental Sciences, told Le Parisien.

Most scientists agree that Amazon produces between 5 and 10% of our oxygen. No more. On Twitter, Jonathan Foley, director of the Institute of the Environment at the University of Minnesota (USA) explains that his calculations allow him to reach “maximum 6%. Probably less.”

In fact, at least half of our oxygen comes from the oceans, where phytoplankton lives, made up of plant organisms that are suspended in the water. Its total biomass is significantly higher than that of forests. It is the world’s largest producer of oxygen and the largest CO2 trap, the “blue lung of the planet”.

This leaves the world’s forests with 50% of oxygen production. According to Jonathan Foley, the total tropical forest produces 24% of the oxygen produced on land and 12% of the total oxygen, “land and ocean included”. Assuming that the Amazon alone emits half of this 12%, Foley arrives at 6% of the world’s oxygen production. “It is biologically and physically impossible for the Amazon to produce 20% of the world’s oxygen,” he insists.

It consumes almost all the oxygen produced
Not to mention that the Amazon rainforest is not very young. While growing trees can emit a lot of O2, others, as they age and die, mostly emit CO2. In 2005, for example, a single storm killed 500 million trees in the Amazon, according to a 2010 study funded by Nasa. And these millions of dead trees have released all their CO2 reserves into the air.

“To put it simply, the balance of the forest itself is zero when it is in its state of equilibrium,” Pierre Thomas, professor emeritus at the Ecole Normale Supérieure de Lyon, told Le Parisien. “Sometimes a forest even emits more CO2 than it absorbs,” Alain Pavé, former director of the CNRS Amazon program, told HuffPost. Massive deforestation could have this consequence. Above all, the rainforest is a complex ecosystem, inhabited by billions of oxygen consumers. Mushrooms, bacteria, animals, and a few million humans.

The Amazon is more than just a stock of oxygen
So the Amazon is not quite “the lung of the Earth”, not just in any case. This would be “reductive,” Plinio Sist, who heads the Forests and Societies unit at Cirad, an international agronomic research organization, told L’Express. “It’s an invaluable source of biodiversity, it’s a carbon reservoir in the face of warming, it’s a climate regulator across the South American continent,” he says.

This rainforest is home to unique biodiversity: 40,000 species of plants including 16,000 tree species, 2.5 million species of insects, 3,000 freshwater fish, 1,500 birds, 500 mammals, 550 reptiles… And surely still much to discover. More than 2,000 new species have been identified and described since 1999, according to WWF.

The Amazon rainforest also regulates the entire climate of South America. It maintains moisture by producing water vapor. “If deforestation continues at the current rate, the region is at risk of serious drought problems,” warns Plinio Sist. With an inevitable impact on Brazil’s agriculture and energy production, which “relies in part on dams in the Amazon basin, threatened by a lack of rains with climate change.” But not only that. “Deforestation in the Amazon also influences rainfall from Mexico to Texas,” according to a 2005 Nasa study. “This does not change the amount of precipitation, but its distribution in the territory,” Nasa explains.

Deforestation and fires also have serious consequences for the global climate: from a CO2 trap, the Amazon could turn into a veritable chimney spitting out greenhouse gases. “The calculation is simple: a ton of trees go up in smoke, and almost two tons of CO2 immediately evaporate,” sums up Le Parisien. And this gas, the weakened forest is less and less able to reabsorb it.

“There are many reasons to be concerned about these peaks of deforestation in the Amazon,” concludes Jonathan Foley. Carbon, climate, water, biodiversity, humans… “But fortunately, at least, we don’t have to worry about oxygen.”

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