In recent days, fires raging in the Amazon rainforest have captured attention worldwide. Brazilian President Jair Bolsonaro, who took office in 2019 has followed through with his campaign promise of reducing environmental protection and increasing agricultural development in the Amazon. The resurgence of forest clearing in the Amazon, which had decreased more than 80% after peaking in 2004, is alarming for numerous reasons. These tropical forests are home to many species of plants and animals found only here. They are vital refuges for indigenous people and hold enormous stores of carbon as wood and other organic matter that would otherwise contribute to the climate crisis.
Some media outlets have even suggested that fires in the Amazon also threaten the atmospheric oxygen that we breathe. On Aug. 22, French President Emmanuel Macron tweeted that “the Amazon rain forest—the lungs which produce 20% of our planet’s oxygen—is on fire.”
This oft-repeated claim that the Amazon rainforest produces 20% of our planet’s oxygen is, in fact, based on a misunderstanding. Actually, nearly all of Earth’s breathable oxygen originates in the oceans, and there is plenty of it to last for millions of years. There are several reasons to worry about due to this year’s Amazon fires, but diminishing Earth’s oxygen supply is not one of them.
Oxygen From Plants
Scott Denning, Professor of Atmospheric Science, Colorado State University says, “As an atmospheric scientist, much of my work focuses on exchanges of various gases between Earth’s surface and the atmosphere. Many elements, including oxygen, constantly cycle between land-based ecosystems, the oceans and the atmosphere in ways that can be measured and quantified”.
He says that nearly all free oxygen in the air is produced by plants through photosynthesis. Around one-third of land photosynthesis occurs in tropical forests, the largest of which is located in the Amazon Basin. But almost all of the oxygen produced by photosynthesis annually is consumed by living organisms and fires. Trees constantly shed dead leaves, twigs, roots and other litter, which feeds a rich ecosystem of organisms, mostly insects and microbes. The microbes consume oxygen in this process.
He continues, “ Forest plants produce lots of oxygen, and forest microbes consume a lot of oxygen. As a result, net production of oxygen by forests—and indeed, all land plants—is very close to zero”.
Oxygen Production In The Oceans
According to him, in order to accumulate oxygen in the air, some of the organic matter that plants produce through photosynthesis must be removed from circulation before it can be consumed. This usually happens when it is rapidly buried in places without oxygen i.e. most commonly in deep-sea mud and underwaters that have already been depleted of oxygen. This occurs in areas of the ocean where high levels of nutrients fertilize large buds of algae.
Mr. Denning explains that dead algae and other detritus sink into dark waters, where microbes feed on it. Similar to their counterparts on land, they consume oxygen to do this, depleting it from the water around them. Beneath depths where microbes have stripped waters of oxygen, leftover organic matter is left and buried on the ocean floor. Oxygen produced by the algae at the surface while it grew remains in the air because it was not consumed by decomposers.
He added, “Tiny phytoplankton in the ocean generates half of the oxygen produced on Earth. This buried plant matter at the bottom of the ocean is the source of oil and gas. A smaller amount of plant matter gets buried in oxygen-free conditions on land, mostly in peat bogs where the water table prevents microbial decomposition. This is the source material for coal. Only a tiny fraction—perhaps 0.0001%—of global photosynthesis is diverted by burial in this way, and thus adds to atmospheric oxygen. But over millions of years, the residual oxygen left by this tiny imbalance between growth and decomposition has accumulated to form the reservoir of breathable oxygen on which all animal life depends. It has hovered around 21% of the volume of the atmosphere for millions of years”.
According to the professor, some of this oxygen returns to the planet’s surface through chemical reactions like oxidation with metals, sulfur and other compounds in Earth’s crust.
He concludes, “Even though plant photosynthesis is ultimately responsible for breathable oxygen, only a vanishingly tiny fraction of that plant growth actually adds to the store of oxygen in the air. Even if all organic matter on Earth were burned at once, less than 1% of the world’s oxygen would be consumed”.
According to him, Brazil’s reversal on protecting the Amazon does not meaningfully threaten atmospheric oxygen. Even a massive increase in forest fires would produce changes in oxygen content that are difficult to measure.
He asserts, “There’s enough oxygen in the air to last for millions of years, and the amount is set by geology rather than land use. The fact that this upsurge in deforestation threatens some of the most biodiverse and carbon-rich landscapes on Earth is reason enough to oppose it”.