The plight of the Amazon has received widespread attention over the past week or so as reports surfaced that Brazil—which hosts around 60 percent of the world's largest tropical forest—has experienced a significant spike in the number of wildfires this year.
Amid this coverage, many media outlets, charities, celebrities and even world leaders repeated the claim that the Amazon produces 20 percent of the world's oxygen supply. The implication here is that the destruction of the rain forest poses a threat to this oxygen supply.
But is this true? Experts say the real figure is actually smaller, and furthermore, this way of thinking is misleading given the true nature of the Amazon's effect on global oxygen levels.
"I have seen this 20 percent all over the place on social media, it doesn't really make much sense," Scott Saleska from the University of Arizona, told Newsweek. "There are many, many reasons to be concerned—nay, terrified—by the resurgence of deforestation and burning of Amazonian forests, but a risk to the world's oxygen supply is not one of them."
In fact, the world's oxygen levels are actually quite stable and are not dependent on rain forests, which use up as much of the gas as they produce in the long run, according to Philip Fearnside, a professor at Brazil's National Institute of Amazonian Research.
"It was a surprise to see the claim that 20 percent of the world's oxygen comes from Amazonia surface on mainstream media, Fearnside told Newsweek. "Amazonia is not a big source of oxygen because trees respire, just like animals. Trees use up most of the oxygen that they produce though photosynthesis."
In photosynthesis, plants capture and store solar energy, using it to convert carbon dioxide in the air into sugar molecules which they use for food, producing oxygen as a byproduct.
"There is a net release of oxygen while the tree is growing and storing carbon in its wood, but when the tree dies the wood rots, removing the same amount of oxygen from the air to form carbon dioxide (CO2) from the carbon in the wood," he said.
"A net release of oxygen occurs only if the carbon sequestered through photosynthesis is buried in a place where it cannot combine with oxygen to form CO2. On a global scale, the main location for this is at the bottom of the ocean, where some of the organisms sink to the bottom when they die and are buried in the sediments."
Essentially, this means that the net effect of the Amazon rain forest on the amount of oxygen in the global atmosphere is "virtually nothing," since the photosynthesis to produce new plant matter is (almost) balanced by microbes decomposing dead plant material, according to Saleska.
"The amount of oxygen in the atmosphere is 20.95 percent, and it is not changing very much," he said. "From this perspective, the Amazon could burn up and blow away and the amount of oxygen in the atmosphere would not be much affected—CO2 is another story though."
Saleska notes that since 1990, the level of oxygen in the atmosphere has dropped by 0.005 percent—hardly at all.
"This is scientifically detectable and very interesting and useful, but practically it is negligible," he said. "The drop is due to the burning of fossil fuels (mostly) and roughly 10 percent is due to biomass burning associated with deforestation worldwide. If Amazon deforestation were half of all deforestation—it's probably not quite—then just 5 percent of the overall 0.005 percent net decline in oxygen would be due to Amazon deforestation."
So if we lost the entire Amazon forest—it would only change atmospheric oxygen—which is thought to weigh 1.2 million gigatons in total—by a small amount, much less than 1 percent.
But even if the Amazon has a very small effect on the overall level of oxygen in the Earth's atmosphere, can can we still estimate how much it produces? Saleska says that tropical vegetation is responsible for about 25 percent of the oxygen that is produced by photosynthesis on land, via trees, shrubs, grasses and other plants.
"The Amazon is, generously, half of the tropics—and certainly somewhat less—so that means, at most, 12 percent of the oxygen produced each year from land photosynthesis comes from the rain forest. However, photosynthesis on land is only about half of global photosynthesis—the other half is in the ocean. So, at most, 6 percent of the oxygen from photosynthesis comes from the Amazon," Saleska said. "Keep in mind that even the gross flows of oxygen from photosynthesis are very small, compared to the very large amount of oxygen in the atmosphere."
Andrei Lapenas, a professor of climatology from the University at Albany, SUNY, speaking to Newsweek, put a figure on the Amazon's oxygen production, estimating that the forest consumes and emits about 32 gigatons of oxygen per year.
However, just because the destruction of the Amazon will not affect the world's oxygen supply does not mean that it won't have other significant impacts. For example, the loss of trees in the Amazon through deforestation is concerning with regards to the global climate because of the role these plants play in acting as a carbon sink.
"The Amazon is a carbon sink, which slows the rate of carbon dioxide build up in the atmosphere, and thus climate warming," James Randerson from the University of California, Irvine, told Newsweek.
"Deforestation and fire-driven forest degradation affect the carbon cycle in two ways. First, there is a direct release of carbon dioxide into the atmosphere in the conversion process. Second, the loss of forest reduces the ability of the forest as a whole to absorb carbon. More forest fires in the Amazon will accelerate the buildup of greenhouse gases and we will have higher levels of global warming," he said.
Ecologist and author Sandra Steingraber from Ithaca College said that around half of the carbon dioxide that is pulled out of the atmosphere by the earth's biosphere on land is sucked up by tropical forests. But in the face of deforestation, the Amazon, at least, may be losing this ability.
"The Amazon is a big carbon sink but its ability to scrub carbon dioxide from the atmosphere is declining," she said. "This will contribute to climate chaos, turning tropical forests from a global carbon sink to a global carbon source is perhaps the most important consequence of destroying the Amazon."
