Save the burning Amazon — and save Earth’s lungs.That’s been the call to action from officials including France’s Emmanuel Macron, activists and celebrities in much of the media coverage to date of the dry-season wildfires engulfing this singularly valuable ecosystem, including from MarketWatch.The “lungs” warning is based largely on an underexplained figure holding that some 20% of the globe’s oxygen is produced by the lush Amazon basin that falls largely inside the borders of Brazil, where a pro-development president in Jair Bolsonaro is expected to turn down fire-fighting international aid.
Bolsonaro wants Macron and the rest of the world to let him decide how many farmers, ranchers, loggers and indigenous people get to use and clear the rainforest, and how often.And: Tim Cook donates nearly $5 million to charity, and Apple pledges aid for Amazon rainforestBut the oft-cited and overly edited 20% oxygen-production figure doesn’t tell the whole story and risks becoming a refutable stat that can spoil a basket of reasons why a burning Amazon rainforest is troubling, say scientists.As Allison Mills, associate director of research communications at Michigan Technological University, told Newsweek:‘I have seen this 20% all over the place on social media. It doesn’t really make much sense.
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.’The Amazon may produce a lot of oxygen, by some measures, but its plants and animals then use up most of it.A study in the Journal of Ecology shows that while free oxygen in the air comes mostly from plants (photosynthesis), most forests actually use almost all the oxygen they create (respiration). By the time the plants and animals use the oxygen they need, they send only about 1% to 6% in reserve into the atmosphere.Instead, nearly all of Earth’s breathable oxygen originated in the oceans.Over millions of years, the residual oxygen left by a tiny imbalance between growth and decomposition of ocean plant matter has accumulated to form the reservoir of breathable oxygen on which all animal life depends, said Scott Denning, professor of atmospheric science at Colorado State University. (Read more of Denning’s explanation.)
That oxygen level has hovered around 21% of the volume of the atmosphere for millions of years. (Here’s more on the role of the ocean’s phytoplankton, and oceans in general, in the production and storage of oxygen.)“Even a huge increase in forest fires would produce changes in oxygen that are difficult to measure. There’s enough oxygen in the air to last for millions of years, and the amount is set by geology rather than land use,” Denning said.
“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.”Mills, Denning and other scientists assert that a bigger liability for Earth from an Amazon in ashes is the impact on carbon dioxide, among the major greenhouse-gas emissions. Lost forest means a diminished ability to soak up emissions from the fossil fuels that humans burn.“Recent increases in Amazon clearing are part of a larger pattern of conversion of tropical forests to pastures and farms, all of which releases atmospheric CO2,” Denning said in the comments section of his piece. “Our best estimates are that all global land clearing currently releases about 10% the amount of CO2 released by burning fossil fuels into the air. It’s too early to say how much the recent Amazon fires will increase this number. They are certainly a substantial source of CO2, but fossil-fuel combustion is more than 10 times greater.”