state of Rondonia, Brazil, Aug. 24, 2019.
AP Photo/Eraldo Peres
Fires in the Amazon rainforest have captured attention worldwide in recent days.
Brazilian President Jair Bolsonaro, who took office in 2019, pledged in his campaign to reduce environmental protection and increase agricultural development in the Amazon, and he appears to have followed through on that promise.
The resurgence of forest clearing in the Amazon, which had decreased more than 80% following a peak in 2004, is alarming for many reasons. Tropical forests harbor many species of plants and animals found nowhere else.
They are important refuges for indigenous people, and contain enormous stores of carbon as wood and other organic matter that would otherwise contribute to the climate crisis.
Some media accounts have suggested that fires in the Amazon also threaten the atmospheric oxygen that we breathe.
French President Emmanuel Macron tweeted on Aug. 22 that,
The oft-repeated claim that the Amazon rainforest produces 20% of our planet's oxygen is based on a misunderstanding...
In fact nearly all of Earth's breathable oxygen originated in the oceans, and there is enough of it to last for millions of years.
There are many reasons to be appalled by this year's Amazon fires, but depleting Earth's oxygen supply is not one of them.
Oxygen from plants
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.
Nearly all free oxygen in the air is produced by plants through photosynthesis. About one-third of land photosynthesis occurs in tropical forests, the largest of which is located in the Amazon Basin.
But virtually all of the oxygen produced by photosynthesis each year is consumed by living organisms and fires.
As a result, net production of oxygen by forests - and indeed, all land plants - is very close to zero.
the terrestrial biosphere (green),
marine biosphere (blue),
lithosphere (Earth's crust, brown),
and atmosphere (grey).
Colored arrows show fluxes between these reservoirs.
Burial of organic material causes
a net increase in atmospheric oxygen,
and reactions with minerals in rocks
cause a net decrease.
Pengxiao Xu/Wikimedia, CC BY-SA
Oxygen production in the oceans
For oxygen to accumulate in the air, some of the organic matter that plants produce through photosynthesis must be removed from circulation before it can be consumed.
Usually this happens when it is rapidly buried in places without oxygen - most commonly in deep sea mud, under waters that have already been depleted of oxygen.
This happens in areas of the ocean where high levels of nutrients fertilize large blooms of algae. Dead algae and other detritus sink into dark waters, where microbes feed on it.
Like their counterparts on land, they consume oxygen to do this, depleting it from the water around them.
Below depths where microbes have stripped waters of oxygen, leftover organic matter falls to the ocean floor and is buried there.
Oxygen that the algae produced at the surface as it grew remains in the air because it is not consumed by decomposers.
Tiny phytoplankton in the ocean
generate 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...
It has hovered around 21% of the volume of the atmosphere for
millions of years.
For example, when iron is exposed to air in the presence of water, it reacts with oxygen in the air to form iron oxide, a compound commonly known as rust.
This process, which is called oxidation, helps regulate oxygen levels in the atmosphere.
Even if all
organic matter on Earth were burned at once, less than 1% of the
world's oxygen would be consumed.
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 bio-diverse and carbon-rich landscapes on Earth is reason enough to oppose it.