Rain forests, oxygen and carbon

Factual Questions
1

I’m seriously confused. All my life I’ve been taught that the rainforests are the lungs of the Earth. Animals breathe oxygen and produce carbondioxide, plants breathe carbondioxide and produce oxygen (yes, I know oxygen is a part of carbondioxide), thus maintaining the atmosphere. Then Blake comes along and tells me that I’ve been misinformed for two decades.

In this post, Blake says (providing cites) that the rainforests consume as much oxygen as they produce, and that they even act as an oxygen sink at times. He suggested I ask in General Questions if I wanted to know more, so I will.

Two questions:

  1. Is this true? If it is, why have I been lied to all my life?

  2. What does happen to the carbon I breathe out every day?

2

Well I can answer two of those questions. I just said I couldn’t explain where all the oxygen came from in the first place. That’s a geologist or chemists job.

Is it true? Yes.

What happens to the carbon you breathe out? It mostly gets cycled. The carbon you breathe out is absorbed by various processes. Most of it probably gets dissolved into water in the ocean. From there it can either chemically combine with inorganic minerals and get lost more or less for good. Or it can be utilised by plants during photosynthesis. Or of course it can be absorbed by plants on land.
The plants combine the carbon atoms t produce sugars and various other substances. Most of these sugars are burned rapidly, within a few days and the carbon is released as carbon dioxide again.

See, plants respire just like animals do. Although plants do ‘breathe carbondioxide and produce oxygen’ they also ‘breathe oxygen and produce carbondioxide’, which is something that is often glossed over in high school biology for some reason. The plants use sunlight to produce sugars, but they then burn those sugars to provide energy just like animals do. It’s simply a way of converting solar energy into chemical.

During daylight hours the amount of sugar being produced is greater than the amount being burned. However as light levels fall the amount of sugar being produced falls and eventually ceases, while the amount being burned remains far more constant. As a result the plant becomes a net oxygen consumer and is busily producing carbon dioxide.

The carbon dioxide plants breathe out simply enters the cycle just like the stuff you breathe out.

The small amount of sugar that isn’t burned ad converted back into CO2 immediately to provide energy for the plant gets converted into the plant body. It is transformed into starch, cellulose, fats, proteins and all the stuff that plant are made of. As the plant body is shed in the case of limbs and leaves it begins to rot. The bacteria and fungi burn the wood and ‘‘breathe oxygen and produce carbondioxide’, releasing the carbon back into the atmosphere. When the plant dies the roots and trunk suffer the same fate.
The carbon dioxide plants breathe out simply enters the cycle just like the stuff you breathe out.

Some of the plant is of course eaten by animals. This includes you, and this is where all the carbon you breathe out comes from. They break down the plant material into carbon dioxide.

The carbon dioxide animals breathe out simply enters the cycle just like the stuff you breathe out.

Many plants and plant parts are burned in forest fires, barbecues etc. The fire converts the plant body into carbon dioxide.

The carbon dioxide animals produced by fires simply enters the cycle just like the stuff you breathe out.

So you see, you don’t produce any carbon at all, nor do you consume oxygen. You just act as a carbon conduit between one plant and the next. You prevent the plant from storing carbon by ensuring that it is all converted to CO2 and ready to be used by the next plant. Bacteria, fungi and fires do the same the same thing.

Mature forests can’t produce oxygen for this reason. The oxygen only exists until it is recombined with carbon, and that happens as soon as the tree decays or is eaten or burned. In other words the forest needs to exist as a solid in order to keep the oxygen as oxygen rather than CO2. In order to produce oxygen the mass of the forest would need to be increasing, which it clearly isn’t in a mature forest.

The only way to get s forest to produce oxygen/store carbon is to cut it down and let it regrow. A growing forest is of course increasing in mass and so producing oxygen. Provided the forests products aren’t eaten or allowed to burn or decay the forest will be genuinely producing oxygen.

Rather ironically for conservation groups one of the best ways to do this is to pulp the forest for newsprint. Most papers end up in landfills, ad once buried decay essentially ceases, with the paper very, very slowly becoming peat. The oxygen is then never recombined with carbon.

This is a potted version of a very complex biogeochemical cycle, and ignores a lot of important carbon pools, like the soil and ocean sediments, but hopefully it answers the question.

3

Blake is right- the oxygen was at one time combined with carbon to make carbon dioxide, during the hot early period of the Earth’s coalescence-
This carbon dioxide has been progressively dissociated into carbon and oxygen over several billion years, with the oxgygen content exceeding 3% of present levels by 600Myr b.p.
The carbon which was dissociated out of the atmosphere has either become part of the biosphere or buried in sediments, as oil, coal, graphite and traces of carbon minerals in many sedimentry rocks…
the oxygen in the atmosphere today is mostly balanced by rock-borne carbon, with only a small proportion heldin the biosphere.

This means that even if you gathered all the forests and all the biomass in the world it would only decrease the oxygen 3-5%
(but it would increase the carbon dioxide by a much greater percentage, simply because carbon dioxide is such a small part of the atmosphere overall). The atmosphere would be in bad shape if we did this, but there would still be plenty of oxygen.

And, as Blake says, the rainforest contains many respiring organisms, and in a stable state neither contributes or extracts oxygen.

SF worldbuilding at
http://www.orionsarm.com/main.html

4

This means that even if you gathered all the forests and all the biomass in the world * and burnt them*it would only decrease the oxygen 3-5%
is what I meant to say

SF worldbuilding at
http://www.orionsarm.com/main.html

5

Hey,

, you wouldn’t have a reference handy for that 3-5% figure would you? I read somehere that it would be less than 1%. Not that the actual figure matters much, I’d just like a reliable reference in case this ever comes up again.

6

Answering the second part of your question is complicated.

Much education of children is, strictly speaking, lies. We simplify things, tell them things that are untrue but produce the desired result, give them only one side of the story, or even tell them things that we thought were true when the textbook was written (or the teacher learned it) but has since been changed. Compare the type of things you are taught in grade school with the stuff you get in University - often they flatout contradict each other.

This lungs of the earth label being attached to the rain forests has part of its origin in this type of thing. Children are taught that green plants produce oxygen, but they don’t usually get told that they also consume it. And if they do, the impression is left that the amount of oxygen produced is greater than the oxygen used. Which is quite true, until the plant dies (or it gets dark). Many adults never study enough science to get a more balanced picture, so they spread this fable.

[cynicism warning]
In this case, I suspect that this image was so useful for the various ecological groups that they have deliberately chosen to keep and enhance it, even though the scientists among them should have known it was wrong. Certainly I have seen this imagery used in publications by groups that should have known better.
[/cynicism warning]

DancingFool

7

Sorry, no.
I quoted it from memory, in a bit of a rush as always…
I remembered it from a lecture in climate geology twenty years ago,
(by my old prof,Dr C Curtis, Sheffield University)
and a more recent figure of 3% from a respectable scientist which I googled up recently but can’t find now.
Nevertheless, I’ll have a look around…

SF worldbuilding at
http://www.orionsarm.com/main.html

8

Thanks for that.
I tend to agree with Dancing Fool. Put “lungs of the Earth” or “forests produce oxygen” into Google, anf the majority of the sites returned are either ecological groups, or people with links to ecological groups. AS DF says, groups like WWF and Greenpeace are perpetuatng this ignorance on their websites, but certainly have no excuse for doing so given the scientific expertise at their disposal.

There is clearly interest amongst these groups in encouraging people to believe this lie.

9

I’ve never quite understood why the “lungs of the Earth” phrase is supposed to make me think forests are good, anyway. After all, what do lungs do? Why, take in oxygen and chuck out, er, CO[sub]2[/sub]…

10

OK, I’ve thought about this a little more and talked it through with Pricegal. I have one further question.

Some years ago, I read about a project called Biosphere. A dome was created containing an entire ecosystem separate from the outside world. Plants, rivers, animals, you name it. And eight or so humans that were going to live in there for a few years.

Now, it was, until now, my understanding that since animals exhale carbondioxide and plants exhale oxygen, the air supply inside this dome would last more or less forever. If plants are consumers of oxygen as well, are you telling me that this ecosystem would only last until the air inside the dome ran out?

11

No.

For starters the plants were mostly added young. Young trees will grow for 50 years+. Growing trees do indeed produce oxygen.

Secondly the plants were an internal food supply for the people and livestock. They had to eat something. Had there been no plants in their they would have had to import food. As the food was digested CO2 would have been released. This would of course have led to a CO2 buildup.

Instead they grew and ate their own food plants. They ate the bananas, the CO2 was released and the banana plant used it to produce more bananas which could again be eaten. At no point was more carbon added to the system, and so no carbon dioxide was added.

The reverse is true of the oxygen. The carbon dioxide the animals and peolpe breathed out was used by the plants, releasing oxygen, which was then converted into CO2 again when the animals ate the plant, and was in turn convrted back into oxygen as the plant produced more fruit. And so ad infinitum. No oxygen lost, none gined. Just one big merry-go-round.

This is what plants do. They use sunlight to convert carbon dioxide into more plant. We then ‘steal’ that solar energies for ourselves. But we only borrow the carbon. That gets cycled straigh back through the plants, then back into the animals in an endless loop.

The same is true of the oxygen. Every time it passes through us it gets detroyed, and every time it passes through the plant it gets reformed. (well ignoring that it’s mostly getting destoryed and reformed within the plant itself).