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Many times we see in articles that the Amazon rainforest creates 20% of the oxygen of the world . Even french president Macron stated this in a tweet Macron tweet

According to this Amazonas size Amazon size is 5.5 millions km². But according to this List of countries by forest area

World forest area is between 39 millions to 43 millions km2, depending if you believe the world forest area they give, or the added forest area of each country (which surpasses 43 millions km2) . I suppose other data is taking into account to say Amazonas create 20% of the oxygen.

Why is it said that Amazon creates 20% of the oxygen production of the world, when it accounts with less than 14-12.8% of the forest area?

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    $\begingroup$ I have just now hear at news about that 20% on news about recent fires,but seriously, make the calcles. I read on google 1 of each 2 mollecules of O2 are produced by unicells on oceans. So then Amazonia produce 40% of Oxygen from land forests? More than dubvious and I think oceans produce more than the 50%. Unicells are always underestimated on those studies, a bit alarmists, coming sometimes from ecologists. $\endgroup$ – user12525 Aug 23 at 19:52
  • $\begingroup$ @Pablo: Not all forests can be expected to equally productive when it comes to photosynthesis. Some receive more hours of sunshine during a year, for example. $\endgroup$ – njuffa Aug 23 at 23:50
  • $\begingroup$ @njuffa but if they come with a figure they need to have a detailed explanation which forests produce more O2 and why. Otherwise it would be speculation $\endgroup$ – Pablo Aug 24 at 4:46
  • $\begingroup$ @njuffa this is not correct all forrests get the same number of sunlight hrs during the year. $\endgroup$ – trond hansen Aug 24 at 4:49
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    $\begingroup$ @trondhansen Last I checked, sunshine is interfered with by clouds and fog, for example, and there are huge differences within the hours of sunshine received in various places in the US, e.g. Yuma has 4000 hours of sunhine per year, while Juneau has only 1300. In terms of the energy received from the sun and available to plants for photosynthesis, other effects play a role as well, such as the angle at which the sunlight reaches the ground. See this map of solar irradiance in the US for example. $\endgroup$ – njuffa Aug 24 at 5:03
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I cannot find any language in peer-reviewed literature (as far as publicly accessible) that makes the 20% claim reported in the question. I therefore consider this claim to be of obscure and dubious origin.

About 50% to 55% of the oxygen produced via photosynthesis is estimated to come from the world's oceans, as reported in the following two papers:

Curtis A. Suttle, "Marine viruses -- Major players in the global ecosystem." Nature Reviews Microbiology 5 (10): 801-12 (2007)

The oceans cover more than 70% of the Earth’s surface. They control the climate, provide a significant amount of the protein that is consumed globally and produce approximately half of the Earth's oxygen.

A. Yu. Borisov, L. O. Björn, "On oxygen production by photosynthesis: A viewpoint", Photosynthetica (2018) 56: 44

We also find that the ocean produces 22% more oxygen than the land surface.

According to an expert quoted by the BBC, about 16% of the land-based oxygen production from photosynthesis occurs in the Amazon basin:

A large proportion of the world's oxygen is produced by plankton, explains Professor Malhi. He says of the oxygen produced by land-based plants, about 16% comes from the Amazon.

Is this claim plausible? Based on the reported size of Amazonian forests at 6-8 million km2 and the world's total forested area of about 40 million km2, I am inclined to answer in the affirmative, although not all of the land-based oxygen production occurs in forested areas.

Yale's Global Forest Atlas:

Amazon basin is the largest tropical rainforest in the world, covering a size approximately equal to the lower 48 United States. 6-8 million square kilometers of forest house approximately 10% of the world’s biodiversity and 15% of its freshwater.

Based on data from the FAO suborganization of the UN

Forests cover 31 percent of the world’s land surface, just over 4 billion hectares.

The area of a forest and its oxygen production are unlikely to be in strict linear proportion to one another, as the rate of photosynthesis should depend on such factors as solar irradiation (which varies with latitude and cloud cover) and temperature (in boreal forests) or the seasons (for deciduous forest in moderate climates). The density of a forest will likewise be influenced by the availability of water and nutrients.

How do scientists estimate the amount of oxygen produced via photosynthesis? Best I can tell from a perusal of the literature this is done indirectly by estimating total biomass, which itself is determined by a combination of satellite data and on-the ground measurements. Such estimates carry a non-trivial amount of uncertainty.

Nowak, David J., Robert Hoehn, and Daniel E. Crane. "Oxygen production by urban trees in the United States." Arboriculture & Urban Forestry. 33 (3): 220-226. (2007)

The net amount of oxygen produced by a tree during a year is directly related to the amount of carbon sequestered by the tree, which is tied to the accumulation of tree biomass.

S.S. Saatchi, R.A. Houghton, R.C. Dos Santos Alvala, J.V. Soares and Y. Yu, "Distribution of aboveground live biomass in the Amazon basin", Global Change Biology (2007) 13, 816-837

To determine the spatial distribution of forest biomass of the Amazon basin, we report a method based on remote sensing metrics representing various forest structural parameters and environmental variables, and more than 500 plot measurements of forest biomass distributed over the basin. [...] We estimate that the total carbon in forest biomass of the Amazon basin, including the dead and belowground biomass, is 86 PgC with ± 20% uncertainty.

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    $\begingroup$ Nice answer, but you are missing rainfall as a key contributor to the productivity of a given chunk of land. Over 60 millimeters per month of rainfall, even in the dry season, and over 2000 mm per year in an equatorial setting are what make Amazonia so fertile. But you (along with those who claim Amazonia provides 20% of the Earth's oxygen) are also missing organic decay. That incredibly high fecundity makes for a lot of detritus, which absorbs oxygen. The net is nearly zero. $\endgroup$ – David Hammen Aug 24 at 8:36
  • $\begingroup$ @David Hammen I mentioned "availability of water" as a factor in forest density, surely that covers rainfall? I quoted the "net zero" fact for Amazonian forest in an earlier comment on the question and debated with myself whether I should add it to my answer or not; I finally decided it was too tangential to the question asked, which inquired about oxygen sources without considering sinks. $\endgroup$ – njuffa Aug 24 at 8:41
  • $\begingroup$ Macron claimed it on twitter. I don't know if it is the original source as journalism agencies distributed that data in several countries. twitter.com/EmmanuelMacron/status/1164617008962527232?s=20 $\endgroup$ – user12525 Aug 24 at 9:14
  • $\begingroup$ @Universal_leaner Thanks for the pointer. Twitter? That certainly counts as highly dubious in my book. It would also explain why I had never encountered this 20% claim before: I don't use social media. $\endgroup$ – njuffa Aug 24 at 9:20
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Do we need to worry about oxygen?
No. Although some reports have claimed the Amazon produces 20% of the world’s oxygen, it is not clear where this figure originated. The true figure is likely to be no more than 6%, according to climate scientists such as Michael Mann and Jonathan Foley [Twitter links]. Even if it were accurate, the crops being planted in the cleared forest areas would also produce oxygen – quite likely at higher levels. So although the burning of the rainforest is worrying for many reasons, there is no need to worry about an oxygen shortage.

Guardian

Why is it said that Amazon creates 20% of the oxygen production of the world Yadvinder Malhi explains:

Below I lay out the science of where this number comes from, and why it is incorrect when you have a whole-ecosystem view of the Amazon. The 20% figure comes from a partial understanding of the global oxygen cycle. The tropical forests account for about a 34% of global land surface photosynthesis. This is shown in the figure below (Beer et al. 2010, Science). The figure shows the global land distribution of photosynthesis - the rainforests are the big red patches and the Amazon accounts for about one half of the world's rainforests. Tropical rainforests photosynthesise so much because they have a year-long growing season not constrained by winter or drought.
enter image description here
...
a bigger point that is often missed is that the Amazon consumes about as much oxygen as it produces. This is shown in the diagram below. Plants produce oxygen through photosynthesis (green arrow). However, the the same plants consume the equivalent of over half the oxygen they produce in their own respiration (blue arrows: my own team's research suggests this is more like 60%). Plants metabolise just as animals do, just at a slower rate, and at night when there is no photosynthesis forests are net absorbers of oxygen. The remaining 40% of the Amazon oxygen budget is consumed mainly by microbes breaking down the dead leaves and wood of the rainforest, a natural process called heterotrophic respiration (dark blue arrows). These process of plant and heterotrophic respiration are effectively the reverse of the photosynthesis equation above.
enter image description here

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The reason is: human nature. People like to exaggerate to make a point, and quite often they are such excellent liars that they believe their own exaggerations. Clearly the Amazon does not create 20 percent of the worlds oxygen. Most of the oxygen in the atmosphere is fossil oxygen, produced by photosynthetic organisms long before there were any rainforests, and as Universal Learner says, most of the CO2 disposal and oxygen production takes place in the oceans.

By saying this, I don't mean to downplay the importance of the Amazon rainforest (some of it is rainforest and some is the more inflammable monsoon forest). It is full of desirable wild life, some of it rare and endangered. It is tragic to see so much of it disappearing. Nevertheless, naturalists, ecologists and environmentalists should try to be more objective and stick to the facts. Although the Amazon tropical forest doesn't provide as much oxygen or dispose of as much CO2 as is sometimes claimed, it still makes a valuable contribution.

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    $\begingroup$ Some reliable references to justify your answer would be a very good thing to add to this answer. But you don't have any such references, do you? $\endgroup$ – David Hammen Aug 24 at 1:57
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    $\begingroup$ AFAIK nobody has claimed that 20% of the oxygen in the atmosphere is from the Amazon rainforest. To my understanding the claim is that 20% of the oxygen generated right now comes from that region. I don't know whether that's true, but it doesn't seem impossible. $\endgroup$ – Semidiurnal Simon Aug 24 at 4:22

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