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Thomas L. Friedman, the New York Times columnist, wrote yesterday:

These intact forests — from those in the Amazon and Congo River basins to ones in Canada, Russia and Ecuador — are the world’s life-support system. They sponge billions of tons of carbon dioxide out of the atmosphere, generating oxygen [...].

A forest would, over longer time periods, only be a net carbon sink — and corresponding oxygen source — if its biomass increases and/or if dead organic material is removed from the global cycle of matter, e.g. by forming peat underground.

But, for example, parts of the Amazon rain forest stand basically on a relatively thin layer of humus above infertile quartz sand (here is a popular science article); since the rain forest is millions of years old, there is no significant sequestering of organic matter over time in the Amazon.

Of course I may have overlooked something, and rain forests in Africa or Asia, let alone forests in temperate climates, all in places with very different geologies, may indeed sequester carbon. After all, our fossil fuels once were plants.

Are forests overall significant carbon sinks or should I write a letter to the editor? I suppose that Friedman should at least remove the Amazon basin from the sentence.

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  • $\begingroup$ I kind of agree with GdD, the emphased sentence is accurate: forests do sponge billions of tons of carbon dioxide out of the atmosphere. It would have been more honest to add: "and this carbon is slowly released when the trees decompose, or quickly released when the trees are burnt". But as it is, the sentence is factual. $\endgroup$ Sep 29, 2022 at 10:40
  • $\begingroup$ @Jean-MariePrival Saying "a forest generates oxygen" is like saying "a glass pane generates light". $\endgroup$ Sep 29, 2022 at 11:48
  • $\begingroup$ It is stating the world's forest's part in the carbon cycle. Whether it invites the uninformed to assume it is carbon negative or not or maybe the author wasn't clear on it, I don't think there is a valid complaint. $\endgroup$
    – Ken Fabian
    Sep 30, 2022 at 10:35
  • $\begingroup$ @KenFabian The Amazon forest has no observable part in the global carbon cycle. You can, as I said in another comment, remove it from Earth without any impact on the CO2 or O2 budget. It is simply not needed for anything concerning CO2 or O2. Of course, if you burn it (or if you let it decay) the contained carbon is released as CO2, but that's a separate issue that the author does not address here. $\endgroup$ Sep 30, 2022 at 10:45
  • $\begingroup$ And don't get me wrong: The forests are of enormous importance for all kinds of reasons: As habitat for indigenous people, for atmospheric water transport, cooling, biodiversity, water filter and probably other functions that are unknown or just don't come to my mind. But the global CO2 and O2 circulation and budget is not one of them, at least not in the Amazon. Whether that's different for the other forests is exactly my question. $\endgroup$ Sep 30, 2022 at 10:49

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From a quick search online using your question, the consensus is forests emit carbon dioxide as well as absorb carbon dioxide, but overall they absorb more than they emit.

From NASA Finds Good News on Forests and Carbon Dioxide, dated 30 December 2014.

A new NASA-led study shows that tropical forests may be absorbing far more carbon dioxide than many scientists thought, in response to rising atmospheric levels of the greenhouse gas. The study estimates that tropical forests absorb 1.4 billion metric tons of carbon dioxide out of a total global absorption of 2.5 billion -- more than is absorbed by forests in Canada, Siberia and other northern regions, called boreal forests.

Forests and other land vegetation currently remove up to 30 percent of human carbon dioxide emissions from the atmosphere during photosynthesis. If the rate of absorption were to slow down, the rate of global warming would speed up in return.

On 12 February 2021, the World Economic Forum stated:

New research, published in Nature Climate Change and available on Global Forest Watch, found that the world’s forests sequestered about twice as much carbon dioxide as they emitted between 2001 and 2019. In other words, forests provide a “carbon sink” that absorbs a net 7.6 billion metric tonnes of CO2 per year, 1.5 times more carbon than the United States emits annually.

The Climate Council website, similarly stated, more that 3 years ago,

When forests are cleared or burnt, stored carbon is released into the atmosphere, mainly as carbon dioxide. Averaged over 2015—2017, global loss of tropical forests contributed about 4.8 billion tonnes of carbon dioxide per year (or about 8-10% of annual human emissions of carbon dioxide).

Whilst forests are important carbon sinks, meaning they draw down carbon dioxide from the atmosphere, the carbon stored in these sinks is part of an active, relatively quick carbon cycle. As living things (including trees) die and decay, the carbon that they once stored is released back into the atmosphere.

The article, Explainer: how much carbon can the world’s forests absorb?, from 2013, has a similar narrative: despite being both a carbon emitter and absorber, overall forests, particularly tropical forests are an overall absorber of carbon dioxide.

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The article is doesn't say forests are carbon sinks, the quote says forests convert large amounts of CO2 into oxygen, which is completely correct. I didn't read the article but I suspect the point the author is making is that cutting down forests will accelerate CO2 buildup in the atmosphere, which is also correct.

Saying that forests process billions of tons of CO2 is right, that doesn't mean they are converting those billions of tons into sequestered carbon every year, it's being processed into food to sustain the tree and provide for growth. The natural process of carbon sequestration is actually pretty slow.

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  • $\begingroup$ It is, as far as I can see, not correct for much of the Amazon rain forest. A forest like that which is not growing or sequestering dead biomass underneath can not be a long-term net carbon sink. Yes, a growing tree uses CO2 to build its biomass and exudes the excess oxygen. Then it lives, huge, for decades or centuries, being neither sink nor source; then it dies, and the process is reversed during its decay: The (micro) organisms oxidize all the carbon back to CO2. There is, obviously, a huge turnaround, but it's a zero sum game. $\endgroup$ Sep 28, 2022 at 22:35
  • $\begingroup$ Oh, and "sponge billions of tons of carbon dioxide out of the atmosphere" in this context must imply " ... and do not return it for a long time", or it would not have any climate impact. $\endgroup$ Sep 28, 2022 at 22:39
  • $\begingroup$ That isn't right. The natural cycle is that living processes create CO2, and trees and other photosynthesizing organisms convert it back to O2 as part of their life-cycle. Just by doing what they naturally do they convert CO2 to O2, that quote isn't implying anything about the long term. $\endgroup$
    – GdD
    Sep 28, 2022 at 22:41
  • $\begingroup$ A forest (and the "forest" is the trees and the animals and the fungi and the bacteria) in a dynamic balance (not growing, not shrinking) does not produce any net oxygen over the year, and does not take out any CO2 either. Not one molecule. You could teleport it to Alpha Centauri without any impact on Earth's CO2 budget or trajectory. $\endgroup$ Sep 28, 2022 at 22:54
  • $\begingroup$ @GdG The statement is not even accurate in the short term: Stephen C. Wofsy, et al., "Carbon Dioxide in the Atmosphere Over the Amazon Basin", Journal of Geophysical Research, Vol. 93, No. D2, February 1988, pp. 1377-1387: " The daily cycle of atmospheric CO₂ in forested regions reflects exchange of metabolic CO₂ with vegetation and soils. Photosynthesis provides a net sink for atmospheric CO₂ during daylight hours; respiration and decay represents net sources of CO₂ at night. " Various sources differ as to the magnitude of these two effects and I don't have the latest data at hand. $\endgroup$
    – njuffa
    Sep 29, 2022 at 10:23
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Mr. Friedman failed to consider aspects of cell respiration and loss to environment, these two factors are enough to make short term plant photosynthesis not that useful for sequestering carbon dioxide, as lots of plants release carbon during the night (cell respiration), however if you look at the long term aspects plants can use CO2 to build plant tissue, sadly though eventually the plant would die (lets use a tree as an example) and the microorganisms will turn lots of it into carbon dioxide and the rest gets buried underground to undergo the process of being turned into coal.

One additional problem would be deforestation, the Amazon rain forest is losing vegetation at a rapid rate which would also release more CO2. So with all this evidence in mind the Amazon having met all the CO2 loss requirements might actually release more CO2 than sequester. I feel like the article notes only 1 half of the "Gas exchange" process and leaves out the respiration and decomposition processes. Hopefully this answers your question.

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  • $\begingroup$ Well, what you describe is what I wrote, right? $\endgroup$ Feb 26, 2023 at 15:36

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