Amazonia Does Not Really Produce 20% of the World’s Oxygen
Among the many important issues related to the fires that plough in the world’s largest rainforest, oxygen is not one of them.
An aerial view of the Amazon near Porto Velho, in the Brazilian state of Rondônia, on …
An aerial view of the Amazon near Porto Velho, in the Brazilian state of Rondônia, on August 21, 2019.
As news of the fires in the Amazon spread, so did a misleading – but often repeated – statement about the rainforest: the Amazon produces 20% of the planet’s oxygen.
This claim has appeared in news coverage by chains such as CNN, ABC News, Sky News, and others, and in publications by politicians and celebrities on social networks such as French President Emmanuel Macron, US Senator and presidential candidate Kamala Harris, and actor and environmentalist Leonardo DiCaprio.
Some people believe that we are at risk of compromising the world’s oxygen supply. “We need O2 to survive!” former astronaut Scott Kelly said on Twitter on August 22.
However, these numbers – which have earned the rainforest the title of “lung of the planet” – are an estimate far above reality. As has been pointed out by several scientists in recent days, the Amazon’s contribution to the oxygen we breathe is probably around zero percent.
“There are several reasons why we should want to keep the Amazon intact, but oxygen is not one of them,” notes systems scientist Michael Coe, who runs the Amazon program at the Woods Hole Research Center in Massachusetts.
For Coe, this claim “makes no physical sense” because there is not enough carbon dioxide in the atmosphere for the photosynthesis of trees to feed one fifth of the planet’s oxygen.
That is, for each batch of carbon dioxide molecules that trees extract from the air, they expel a comparable number of oxygen molecules. But since the atmosphere contains less than half the percentage of carbon dioxide, and 21% of oxygen, it is not possible that the Amazon can generate so much oxygen.
There are more precise estimates advanced by many scientists. Yadvinder Malhi, an ecosystem ecologist at Oxford University’s Institute for Environmental Change, bases his calculations on a 2010 study that estimates that tropical forests account for about 34% of the photosynthesis that happens on land. Based on its size, the Amazon would account for about half of that. This means that the Amazon generates about 16% of the oxygen produced on land, explains Malhi, who detailed his calculations in a recent publication on his blog.
That percentage drops to 9% when the oxygen produced by phytoplankton in the ocean is taken into account. (Climate scientist Jonathan Foley, who directs the non-profit Drawdown project and researches solutions to climate change, published a more conservative estimate of 6%).
But there are more data to consider. Trees don’t just exhale oxygen – they also consume it in a process known as cell respiration, where they convert the sugars that accumulate during the day into energy, using the oxygen to fuel the process. Then, during the night, when there is no sun for photosynthesis, they absorb oxygen. Malhi’s research team estimates that in this way trees inhale just over half the oxygen they produce. The rest is probably used by the countless microbes that live in the Amazon, which inhale oxygen to decompose dead organic matter from the forest.
“The overall effect of oxygen from the Amazon, or any other biome, is around zero percent.”
Because of this balance between oxygen production and consumption, modern ecosystems change very little oxygen levels in the atmosphere. Instead, the oxygen we breathe is a legacy of phytoplankton in the ocean, which has accumulated oxygen over billions of years, making the atmosphere breathable, explains Scott Denning, an atmospheric scientist at Colorado State University.
This oxygen only accumulated because the plankton was trapped at the bottom of the ocean before decomposing – otherwise, its decomposition by other microbes would have consumed that oxygen. The processes that determine the amount of oxygen found in the atmosphere occur, on average, on vast geological time scales, and are not really influenced by the photosynthesis that is happening now, Denning explains in an article in The Conversation.
Cradle of biodiversity
Even so, the myth of the 20% has been circulating for decades, although its exact origin is not known. Malhi and Coe calculate that this is because the Amazon contributes about 20% of the oxygen produced by photosynthesis on land – something that may have mistakenly entered the public domain as “20% of oxygen in the atmosphere”.
It is obvious that none of this interferes with the extreme importance of the Amazon which, in its pristine state, contributes significantly to the collection of carbon dioxide from the atmosphere. Coe does not compare the Amazon to a few lungs, but to a giant air conditioner that cools the planet – one of the most powerful systems we have for mitigating climate change, along with other rainforests in Africa and Asia, some of which also face fires at the moment.
And the Amazon also plays an important role in stabilising rainfall cycles in South America, as well as being crucial for indigenous peoples and countless species of animals and plants.
“There are few people talking about biodiversity, but the Amazon is the most biodiverse terrestrial ecosystem, and climate change and deforestation are putting that wealth at risk,” notes climate scientist Carlos Nobre of the Institute for Advanced Studies at the University of São Paulo.
Given its importance to the world, the Amazon may well be a metaphorical pair of lungs, and this analogy may have been useful in galvanizing actions against deforestation. But for most researchers, it doesn’t make much sense – not least because the real lungs breathe in oxygen rather than out.
“If people want to relate the Amazon to a fundamental part of our body, a part that maintains stability, life and well-being – symbolically, they can make that kind of association,” Nobre says. “But, physically speaking, the Amazon is not of the entire lung of the planet.”
POR KATARINA ZIMMER
This article was originally published in English at www.nationalgeographic.com