In the news today (September 2019) are the fires in the Amazon rainforest, with headlines like "the lungs of the planet are on fire", and claims that the rainforest makes 20% of the oxygen in the Earth's atmosphere. Since 21% of the Earth's atmosphere is oxygen, this is about 4% of the Earth's total atmosphere. This story was started by President Macron of France, although his claim was less dramatic, that 6% of the Earth's oxygen came from the Amazon rainforest.
The picture that comes across is that the Amazon rainforest converts the carbon dioxide that the rest of the world produces in excess, back into oxygen.
Let's look at the science. Plant life uses the energy in sunlight to convert carbon dioxide and water into glucose (a kind of sugar) by the following reaction:-
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| carbon dioxide & water gives glucose & oxygen 264 & 108 gives 180 & 192 |
Six molecules of Carbon Dioxide reacted with six molecules of water give one molecule of glucose and six molecules of oxygen. The oxygen is released into the atmosphere to the benefit of all animals. The glucose is kept by the plant, and linked to many other glucose molecules to make cellulose, lignin, and hemicellulose, the building blocks of wood. In flowering plants a tiny amount also goes into nectar. The numbers refer to the relative masses, so (say) 264 tonnes of carbon dioxide makes 180 tonnes of dried wood. The chemical compostion of dried wood varies from tree species to tree species, but is about 40% carbon.
A growing tree releases oxygen whilst it is making new wood or leaves. At night a tree's respiration will release a little carbon dioxide, but in the light it releases much more oxygen than carbon dioxide so the net effect is a surplus of oxygen.
When the leaves fall they are attacked by fungi, bacteria, insects and worms as a source of food, releasing carbon dioxide and water as the leaves are consumed.
The tree itself will eventually die, sometimes prematurely from attack by fungi or disease, or by being blown over in the wind. When it falls the wood is attacked by fungi, bacteria and insects such as termites and beetles. These break down the wood, eventually returning it the carbon dioxide and water (sometimes via the release of methane to the atmsosphere as with termites), and releasing its nutrients. By the time the wood is fully decayed the tree has released all the carbon dioxide it absorbed in its growth, back into carbon dioxide in the atmosphere.
An individual tree is essentially a fixed-term carbon store, storing carbon during its lifetime and lending us the waste oxygen it releases. When it dies after (say) a hundred years it takes all that oxygen back as it rots and converts it back to carbon dioxide. A forest is a carbon store made up of a lot of trees.
In an expanding forest new growth will dominate and carbon dioxide will be absorbed. In a naturally-shrinking forest rot will dominate and overall carbon dioxide will be released to the atmosphere. If humans come along and harvest the forest wood to build houses and furniture then leave the forest to regrow naturally, the carbon taken away in the wood remains trapped and the forest absorbs more carbon dioxide in regenerating itself. If however, the harvested wood is burnt, then the carbon dioxide is released to the atmosphere and the end result of the burning has the same effect as rotting.
In a forest of unchanging size with no harvesting the new growth replaces the old growth that dies and rots, and the carbon dioxide absorption of new growth is balanced by the carbon dioxide release of rotting wood, so the forest operates in a closed cycle. If such a forest was sealed off from the rest of the world's atmosphere it would have no effect on this cycle.
Animals are a part of the forest they inhabit. Their consumption of fruits and nuts from the forest speeds the return of those items back to carbon dioxide and water; digestion is faster than decay, even for digested greenery.
So how can we get an oxygen surplus from a stable forest?
For the forest to release a surplus of oxygen over time, it must have some mechanism to store carbon in the form of dead wood without it rotting, or at least rotting very slowly, and this store must be able to increase year-on-year.One store of carbon is the forest soil, if the soil is deep and trees are deep rooted. Here one can find wood residue from deep rooted trees, and leaf and wood litter on the surface, where it is more difficult for oxygen to reach. However, this reserve, although it can be large in deep soils with deep-rooted trees, is not of inexhaustible capacity and in established hardwood forests is unlikely to be increasing enough in capacity to provide any significant oxygen surplus. Old established forests of constant size operate very near to a closed cycle. In the Carboniferous Era the swamp forests conditions were such that trees did not rot but sank into the oxygen-free swamp, ultimately making our great coal deposits. The age of the swamp forests is hundreds of millions of years in the past and they no longer exist to any great extent.
The only way we can get an oxygen surplus from an established forest these days is to harvest its wood for some permanent use such as buildings, and allow the forest to regenerate itself to replace that lost wood, in the process replacing carbon dioxide with oxygen. That harvested wood becomes a permanent carbon store.
Do plants really thrive in a closed system?
In 1960 Davic Latimer plated a large glass carboy with spider plant seeds, letting them develop into plants before sealing the bottle. The plants thrive from photosynthesis, producing daughter plants. Plants sooner or later die and rot, releasing carbon dioxide to support further photosynthesis on the living plants. This closed ecosystem shows no signs of plant stress or failure. It is clear that even tiny closed ecosystems are perfectly stable and in fact, the Earth is just one immense sealed ecosystem.The Amazon Rainforest - the Numbers.
On that basis, this 20% figure might be fake news, as making 20% of the planet's atmospheric oxygen from carbon dioxide requires us to grow a new forest a thousand times the size of the Amazon rainforest. This figure of 20% clearly needs a more detailed explanation of how it was arrived at because on the face of it, there is a huge mismatch in the numbers.
The total carbon in Earth's biomass (land and marine plants and animals but excluding bacteria for which there is no clear figure) amounts to 5.5x10^11 tonnes. If it was all burned to carbon dioxide it would consume 1.47x10^12 tonnes of oxygen to give 2.0x10^12 tonnes of carbon dioxide. That is, 0.1225% of the available atmospheric oxygen (which is 21% of total atmospheric gases) giving a potential increase in the atmospheric concentration of carbon dioxide of 0.033% by volume. Now for every four parts of carbon dioxide that is added to the atmosphere, one part is absorbed by the oceans and one part is absorbed by the increased plant activity that results; since we have removed all the plants in this scenario one third enters the oceans and two thirds stays in the atmosphere. The current atmospheric carbon dioxide level is 0.0405% (2017 figure), and that brings it up to its maximum possible atmospheric concentration of carbon dioxide of 0.0625% if every last bit of bio-available carbon was burned.
There is just not very much available bio-available carbon in the world when compared with the amount of atmospheric oxygen, which is why atmospheric levels of carbon dioxide are much less that one part per thousand. Plants have to work hard to extract carbon dioxide from the atmosphere at the (2017) concentration of 0.0405%. Increase the concentration and they find life easier, growing significantly faster. Burning fossil fuels is slowly increasing the bio-available carbon.
Given all that, the claim that 20% of the Earth's oxygen - equivalent to 4% of total atmospheric gases - is sourced in the Amazon seems extraordinary and requires justification.
Just as a matter of interest, a human consumes 550 litres of oxygen a day, or 729 grams a day. That is 0.266 tonnes a year, and all the 7.53 billion humans on Earth consume 2x10^9 (two billion) tonnes a year between them. They would consume all the oxygen on Earth (if it were not replenished by plants' respiration, and if there were no other animals consuming it) in about 600,000 years - in practice they would die out sooner because they cannot survive low concentrations of oxygen or high concentrations of carbon dioxide. They would of course run out of food a lot sooner as food is made from carbon compounds, and once all the carbon had been converted to atmospheric carbon dioxide, that would be the end of life.
Is there anywhere on earth with an inexhaustible carbon sink?
If we look at bogs, where the plant matter cannot decay because of soil conditions with low oxygen levels, we find carbon can be locked up forever in the bog peat. Even animal tissue does not decay - you may have heard of "bog bodies", well preserved bodies of people who died in bogs thousands of years ago and whose bodies are well preserved. Bogs' accumulation of carbon is slow but inexorable, locking up carbon in peat. Burning peat or coal releases the carbon back to the atmosphere again as carbon dioxide.
In the oceans there are regions low in oxygen, where marine organisms fall to the ocean floor and are covered. In the lack of oxygen carbon cannot be oxidised to carbon dioxide and the oils in these orgaisms may become the precursors of crude oil. There is a lot of uncertainty over this process and whether it still works today as it has in the distant past. If it is still happening as it once did, then like the bogs the process of locking away the carbon is slow but inexorable. Again, as with peat and coal, burning it releases the carbon back to the atmosphere.
How old is the Amazon rainforest?
In the past there has been much speculation on the age of the Amazon rainforest in the absence of scientic knowledge on the subject. Some even claimed it was 55 million years old, and you can see that claim repeated across the internet.
However, since geologists such as Carina Hoorn from the University of Amsterdam have moved in and examined the geology of the region (see her article "The Birth of the Mighty Amazon" printed in Scientific American). The rainforest seems to have evolved from its early beginnings 23 million years ago, developing - with several setbacks - into almost its current form about 10 million years ago. The Amazon River itself came into existence only about 16 million years ago.
However, since geologists such as Carina Hoorn from the University of Amsterdam have moved in and examined the geology of the region (see her article "The Birth of the Mighty Amazon" printed in Scientific American). The rainforest seems to have evolved from its early beginnings 23 million years ago, developing - with several setbacks - into almost its current form about 10 million years ago. The Amazon River itself came into existence only about 16 million years ago.
What should we do about the Amazon rainforest?
First and foremost, we need to understand the ecology of the rainforests. From the claim that 20% of the Earth's oxygen is dependent on it, it is would appear that there is work to do here. Once we understand its unique ecological niche we can then decide how important it is to us and plan what to do. In 2007 Ecuador offered to forgo developing oil reserves worth $7.2 billion in the rainforest if the world could recompense it to half that value via a UN-administered fund. Five years later only $6.5 million had been raised, so Ecuador was forced to abandon the plan, and started extracting oil from the rainforest.
One estimate of the value of the Amazon rainforest to the world is $8.2 billion every year. This is highly speculative and the factual basis of this estimate is not clear. Nevertheless, poorer countries like Ecuador cannot afford to take on the cost of protecting it, to their own economic disadvantage. If it indeed has value to the world, then the world - not just the countries of the rainforests - needs to pay for protecting it, and not with paltry sums of the occasional ten or twenty million dollars.
Further reading:-
https://www.forbes.com/sites/michaelshellenberger/2019/08/26/why-everything-they-say-about-the-amazon-including-that-its-the-lungs-of-the-world-is-wrong/#12f489175bde
You can download the PDF of Carina Hoorn's Scientific American article from https://www.researchgate.net/publication/7074147_The_Birth_of_the_Mighty_Amazon
How do plants react to higher concentrations of carbon dioxide?
https://insider.si.edu/2014/06/strange-controversial-way-plants-trap-co2/
