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?

Question

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?

Answer 1

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.

Answer 2

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.

...
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.

Answer 3

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.