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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Does breathing contribute to CO2 buildup in the atmosphere?

What the science says...

Select a level... Basic Intermediate

By breathing out, we are simply returning to the air the same CO2 that was there to begin with.

Climate Myth...

Breathing contributes to CO2 buildup

"Pollution; none of us are supporting putting substances into the atmosphere or the waterways that might be pollutants, but carbon dioxide is not a pollutant. If Senator Wong was really serious about her science she would stop breathing because you inhale air that's got 385 parts per million carbon dioxide in it and you exhale air with about ten times as much, and that extra carbon comes from what you eat. So that is absolute nonsense." (Ian Plimer)

The very first time you learned about carbon dioxide was probably in grade school: We breathe in oxygen and breathe out carbon dioxide. Any eight-year-old can rattle off this fact.

It should come as no surprise that, when confronted with the challenge of reducing our carbon emissions from the burning of fossil fuels, some people angrily proclaim, "Why should we bother? Even breathing out creates carbon emissions!"

This statement fails to take into account the other half of the carbon cycle. As you also learned in grade school, plants are the opposite to animals in this respect: Through photosynthesis, they take in carbon dioxide and release oxygen, in a chemical equation opposite to the one above. (They also perform some respiration, because they need to eat as well, but it is outweighed by the photosynthesis.) The carbon they collect from the CO2 in the air forms their tissues - roots, stems, leaves, and fruit.

These tissues form the base of the food chain, as they are eaten by animals, which are eaten by other animals, and so on. As humans, we are part of this food chain. All the carbon in our body comes either directly or indirectly from plants, which took it out of the air only recently. 

This new animation published by Dr. Patrick T. Brown (Carnegie Institution for Science, Stanford University) in September 2018 explains the process:

Therefore, when we breathe out, all the carbon dioxide we exhale has already been accounted for. We are simply returning to the air the same carbon that was there to begin with. Remember, it's a carbon cycle, not a straight line - and a good thing, too!

Update information: This rebuttal was updated on September 12, 2018 to swap the graphic showing a - to some - "odd looking cow" with the new video we happened to notice on Twitter. Thanks to Patrick T. Brown for making it available on YouTube!

Last updated on 13 September 2018 by BaerbelW . View Archives

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Comments 151 to 151 out of 151:

  1. Antjrk:

    Why do you think that the Haber-Bosch process cannot be converted to renewable energy?  Smart Energy Europe, the OP a few days ago, provides a plan to generate all power using renewable energy.  Obviously you can obtain hydrogen by electrolysis of water and the remainder of the process can easily be electrified.

    More than half of current food supplies world wide are produced by small farmers who do not use any commercial fertilizers.  You need to reduce your claims of how many people are fed using artificial fertilizers.

  2. You can’t use the carbon cycle to prove that human respiration isn’t increasing CO2 levels in the atmosphere because the carbon cycle only describes the steady state. As others have already noted, the human population has grown exponentially over the last 100 years. It has almost quadrupled since 1920. That is not a system operating in the steady state or at long-term equilibrium.

    To put it simply, the carbon cycle describes five carbon reservoirs (vegetation, animals, soil, the ocean and the atmosphere) all of which also act as carbon pumps. Moreover, these five reservoirs are all interconnected, and the pumping capacity of each depends on their size. Generally, the bigger they are, the more carbon they pump. That means that changing the size of one will change the size of all the others in order to balance pumping rates and conservation of mass. This will happen as the system seeks to find a new equilibrium position. So an increase in the human population affects everything else. It changes the pumping rates and it changes the relative size of the other reservoirs. And the thing is, we can estimate what size this change might be.

    As the average 70 kg person generates about 1 kg of CO2 per day, that means they transfer 100 kg of carbon to the atmosphere every year. With nearly 8 billion people on the planet that equates to about 0.8 GtC per annum (GtC = gigatonne of carbon).

    But that is not all. The average person probably eats their own bodyweight in meat every year. So the growth in the human population since 1920 must be reflected in the growth in the number of farm livestock. If we assume 2 kg of livestock per 1 kg of human (i.e. a 2 year supply of meat in production), then the overall CO2 production from both is 2.4 GtC per annum. This is about a quarter of our fossil fuel CO2 output. So is this directly increasing atmospheric CO2 levels as some climate deniers might claim? The answer is no, or at least not directly.

    Some people have suggested that the increases in human and livestock CO2 emissions are offset by increased crop production. Their argument is that, as all the carbon we breathe out comes from crops, any increase in the CO2 produced by the human population will be offset by a commensurate increase in crop production required to feed the extra humans and their livestock. This is not true either.

    Increased crop production comes at the expense of other types of vegetation (e.g. forests). The total area under human cultivation may increase, but the total amount of land and vegetation won’t. Deforestation in the Amazon region to grow crops and farm cattle does not increase the rate of CO2 capture in the region. If anything, it decreases it.

    Increasing the number of animals does not increase the amount of vegetation or its growth rate. Instead it decreases the amount of carbon going into the soil. Animals eat plants before those plant can die and before they can decay in the soil. This means that animals replace the CO2 producing capacity of the soil. That is where the substitution occurs. And if the pumping efficiencies of both animals and the soil were the same then nothing much would change as the animal population increases. But they aren’t the same.

    The carbon pumping efficiency of the soil is only 4%. The soil contains over 1500 GtC but emits 60 GtC per annum. Humans store only 0.1 GtC but emit 0.8GtC per annum. That is an efficiency of 800%. It also means that the increase in CO2 production from humans and livestock is the same as that produced by about 4% of the Earth’s soil. That means that the total volume of soil must reduce by 4% over time as its pumping capacity is replaced by animals and as the volume of carbon entering the soil decreases. So 60 GtC will be lost from the soil while only 0.1 GtC will be transferred to animals and none to plants. There is only one other place that most of the 59.9 GtC can go: the atmosphere. This 59.9 GtC will increase the atmospheric CO2 concentration by about 30 ppm.

    So the human population increase could have increased atmospheric CO2 levels by up to 30 ppm over time, and about 20 ppm since 1920. Is this an upper estimate? Yes, probably. It assumes that the growth in the human population and farming livestock is a net gain and does not merely substitute for loss of other species. But we know this is not true. Humans and their livestock do displace other creatures to some extent. It also omits any additional loss of CO2 to the oceans and changes to vegetation volumes through loss of soil and increases in CO2. But what it does demonstrate is that when the human population changes, everything else changes.

  3. Slarty Bartfast @152,

    Perhaps you don't understand the concept of a 'cycle' when you talk of the 'carbon cycle'. Let me explain. The carbon moves from A to B to C to D and then, likely back to A again. That is a 'cycle'. It goes round and round.

    Now you are saying that a 70Kg human emits 100kg carbon a year. Given the weight of the annual carbon emission is greater than the weight of the human emitting, it should be telling you that the carbon must be coming from somewhere and into the human to allow the human to emit such a quantity. Within the waffle you present @152 I fail to see where you account for how humans source all this carbon. And until you do account for it, your attempts to analyse the impact of an increasing human population on atmospheric CO2 levels will remain no more than waffle.

  4. Slarty Bartfast @ 152:

    So many basic errors.

    1. No carbon cycle descriptions or modelling do not assume steady state or equilibrium. They iinclude reservoirs of carbon, and fluxes between reservoirs, and all can vary with time.
    2. "Pumping capacity" is a meaningless term. All reservoirs have multiple fluxes in and out of them, and those fluxes are the result of a variety of factors. There is no single "pumping capacity".
    3. Fluxes are not the result of the size of the reservoir. For example, soil carbon is lost to the atmosphere by decomposition, and this is highly dependent on temperature and biological activity. In tropical forests, carbon added to the soil by dying vegetation is rapidly decomposed and retruns to the atmosphere. The soils reservoir has little carbon because the flux is so high. In contrast, colder climates like the boreal forest accumulate large carbon stores because decomposition is very slow.
    4. MA Rdoger has already pointed out your egregious logic error in claiming that a 70 kg person is a carbon source of 100 kg/year. If a person remains at 70kg, then whatever flux of carbon to the atmosphere is being exactly balance by an uptake in carbon from other sources.
    5. An increase in the human population means an increase in carbon storage. See point #4.

    Your post is a distorted, misguided, uninformed outpouring. It bears little resemblance to reality.

  5. Slight correction to comment @152

    I forgot to include the farm livestock in some of the numbers in my previous comment (@152). So the penultimate paragraph of that comment should read as follows:

    The carbon pumping efficiency of the soil is only 4%. The soil contains over 1500 GtC but emits 60 GtC per annum. Humans store only 0.1 GtC but emit 0.8GtC per annum. That is an efficiency of 800%. It also means that the increase in CO2 production from humans and livestock (2.4 GtC per annum) is the same as that produced by about 4% of the Earth’s soil. That means that the total volume of soil must reduce by 4% over time as the volume of carbon entering the soil decreases and its pumping capacity is replaced by animals, if the ecosystem is to return to equilibrium. So 63 GtC will be lost from the soil while only 0.3 GtC will be transferred to the animal reservoir and none to plants. There is only one other place that most of the remaining 62.7 GtC can go: the atmosphere. This 62.7 GtC will increase the atmospheric CO2 concentration by about 30ppm.

  6. Slarty Bartfast @ 155:

    What you forgot to include was any relevant science. Your numbers are mere fantasy in the world of carbon cycle descriptions and science.

    "Pumping capactiy" is a meaningless term.

    Your "efficiency" calculation is meaningless.

    You are now talking about "returning to equilibrium" when in post 152 your were claiming that equilibrium was not a valid concept to use (and erroneously claimed that the existing science only describes the steady state).

    Frankly, you have no idea what you are talking about. You are asserting meaningless claims with no reference to any reputable source to support your position.

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