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The 72 million Google results for "Amazon world lungs" (unquoted) is evidence enough that there's a common perception that the Amazon rainforest is the "world's lungs". God knows I grew up believing that.

At one point I read that this was incorrect, since the forest consumes just as much oxygen as it produces. But then I was told that was just anti-environmental propaganda.

Now with the Amazon fires all over the news, I've started hearing this once again (for example, this Forbes article). So, is the Amazon a (significant) net source of oxygen?

If not, how adequate are the substitutes mentioned in that article (i.e. soy farms and cow pastures)?

(And, if this doesn't extend the scope of the question too much: if (rain)forests aren't net sources of oxygen, what is?)

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I'm not sure where have I first stumbled upon this information, but I think it was during my childhood when media was not centered on the rain forests so much. The primary oxygen production on Earth is actually happening in oceans. Here are few articles I found:

This article says that scientists prefer term oxygen turnover. The term production is very misleading. Rain forests actually produce about as much as they consume because of decomposing plants and animals.

Although media nowadays may say the Amazon is the lungs of our planet, I wouldn't justify burning it just because the forest is not an oxygen producer. It is still a part of nature and burning it can narrow the diversity of both plants and animals, which can have consequences on the whole planet.

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    It's worth noting that mature trees are a natural carbon sink, as their growth removed a lot of carbon dioxide from the atmosphere, storing it in the form of cellulose and other ligneous matter. So while the recent fires have little effect on the forest's ability to remove atmospheric carbon, they just released a whole bunch of it. And since new (young) trees are likely not going to be planted because of Brazil's economic situation, this atmospheric carbon is not going to go away any time soon. – Dungarth Aug 27 at 1:33
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    @Dungarth That would make for a great additional answer (with some sources) – Mars Aug 27 at 6:30
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    @Dungarth recent, more comprehensive research indicates this may not be true. The idea of trees "maturing" is based on a single old study that hasn't stood up to scrutiny. Take a look at the answer I wrote on Sustainability.SE discussing the research. – LShaver Aug 27 at 14:00
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    @Dungarth What you elaborate is correct, but your terminology is not. A (current) carbon sink is something that is at present actively removing carbon from the atmosphere -- the same way a sink is draining water --, which a grown tree does not. Old trees were sinks when they were growing; i.e. young, growing trees are carbon sinks. Old trees are carbon storage (as you elaborate correctly). – Peter - Reinstate Monica Aug 27 at 15:26
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    @PeterA.Schneider Thank you for the precision! English isn't my native language and I'm less familiar with the proper terms. – Dungarth Aug 27 at 19:10
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The Forbes article asserting the Amazon contributes net zero is correct. The link below shows the environmental science to back it up and also adds an important conclusion about our sources of oxygen.

http://www.yadvindermalhi.org/blog/does-the-amazon-provide-20-of-our-oxygen

The oxygen levels in the atmosphere are set on million year timescales by the subtle balance of geological, chemical and biological processes. They are not set by the short term (short term equals anything less than hundreds of thousands of years) activities or existence of current biomes.

A final point to make is that the atmosphere is awash with oxygen, at 20.95% or 209,500 ppm (parts per million). Carbon dioxide, by comparison, is around 405 ppm, over 500 times less than oxygen, and rising by around 2-3 ppm per year. Human activity (around 90% of which being fossil fuel combustion) has caused this oxygen concentration to drop by around 0.005% since 1990, a trivial amount. In parallel, the same activities have caused carbon dioxide concentrations to rise by by 37 ppm since 1990, or 10%. This is a much more substantial percentage because there is so little carbon dioxide in the atmosphere to begin with, so human activities that emit or absorb carbon dioxide can make a major difference. This is why we need to worry about the increase of carbon dioxide in the atmosphere (and its resulting impact on climate), and why we don't need to worry about running out of oxygen

Edit: A new article in The Atlantic explains this in much simpler language.

https://amp.theatlantic.com/amp/article/596923/

This is what we are burning at Earth’s surface today. We’re not just burning down the Amazon. We’re burning down all the forests in Earth history that we can get our hands on. For every worrying part per million that CO2 goes up from burning fossil fuels, atmospheric oxygen goes down an equivalent amount, and then some. As a result, oxygen is dropping far faster from burning fossil fuels, and their untold forests, than it is from burning just the trees available on the planet’s surface. We’re reversing tens of millions of years of photosynthesis all at once.

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    Be aware that, when talking about the "lungs of the world", the "production of oxygen" is equivalent to "reduction of CO2". Also note that suffocation in a closed system usually happens by means of CO2 poisoning, not lack of oxygen. Actually, your blog then goes on to show that, indeed, the rainforests are where the majority of land-based photosynthesis happens. So it basically wants to keep its cake and eat it... – DevSolar Aug 27 at 7:14
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    (ctd.) As for the point being missed, yes, the oxygen - carbon dioxide - oxygen cycle is, effectively, a zero-sum affair. There is no big "carbon sink" on this planet (because if there were, plants would die, as they need CO2). That's why adding carbon to the atmosphere and removing what photosynthesis capacities there are is such a disastrous thing. So saying "we don't need to worry" can't be "a correct answer" by any stretch of the imagination. – DevSolar Aug 27 at 7:21
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    @DevSolar : the quote includes "we need to worry about the increase of carbon dioxide in the atmosphere", which seems to be in agreement with your comment. – Evargalo Aug 27 at 7:45
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    Not the fault of the answer, but it seems to be unnecessarily hard to compare "0.005%" for O2 and "37 ppm" for CO2 in the quote's figures. If I'm counting correctly, 0.005% is the same as 50 ppm. – Henning Makholm Aug 27 at 14:30
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    @DevSolar "the rainforests are where the majority of land-based photosynthesis happens". This does not make them "the world's lungs" since the oceans release more oxygen than forests, maybe up to 80% of the total. – CJ Dennis Aug 28 at 0:33
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tl;dr - yes, but there are doubts that it will keep being "world's lungs" in the future.

The removal of athmospehric CO2 and the production of oxygen are two sides of the same process, while the former is much easier to estimate and measure. And terrestrial ecosystems play an important role in the CO2 absorption, taking up roughly as much CO2 as ocean biomes, despite their smaller area:

enter image description here

(figure from Balancing the Global Carbon Budget, R.A. Houghton, 2007)

However, it's important to note that rainforests sequester the carbon primarily in the biomass, as opposed to biomes deemed effective carbon storages, such as swamps or mangroves:

enter image description here

Practically this means that while tropical forests are very effective short-time CO2 absorbers, they relatively quickly reach an equilibrium after which they lose nearly as much CO2 from decay as they gain from photosynthesis.

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    Out of curiosity, do you have data on total tCO2eq, i.e. taking into account the area actually covered by the respective biomes? – DevSolar Aug 27 at 14:10
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    How does a summary of "yes" derive from the fact given in your last paragraph? The Amazon is a forest, not just a group of trees. CO2 and O2 use is balanced. – fredsbend Aug 28 at 5:16
  • @DevSolar No, I haven't seen such data yet. One could pick area figures from Wikipedia and multiply. Mangroves are quite widespread but typically cover a very narrow piece of land (seashore) compared to rainforests. – Dmitry Grigoryev Aug 28 at 6:31
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    @Alchimista Mangroves store a lot of carbon in the soil, like swamps do. Growth and decomposing will always balance out in the end, but a mangrove will store 2000 ton/ha of CO2 at that point, compared to 700 ton/ha for a tropical forest. – Dmitry Grigoryev Sep 19 at 10:55
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    @Alchimista Those probably sequester CO2 forever (on a human scale) if they happen to fall on the seabed. This happens very slowly though: as the first picture shows, land ecosystems absorb CO2 much quicker. – Dmitry Grigoryev Sep 19 at 11:07

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