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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)

At a glance

We, and almost all of our relatives in the animal kingdom, are aerobic. That means we all depend on this simplified equation in order to function:

glucose + oxygen → carbon dioxide + water + energy

We breathe in oxygen and that oxidises carbohydrates in our body's cells. That chemical reaction gives us the energy required to perform all the varied tasks we do, from blinking to running a marathon. The products of the process are carbon dioxide and water. While the air we breathe in contains just under 420 ppm CO2, what we breathe out contains 40,000-50,000 ppm CO2, a hundredfold increase due to the simplified equation above.

Because we are breathing constantly, this rapid gas-exchange with our surroundings is also constant and, while each of us live, is perpetual. We are part of the fast carbon cycle that involves the movements of carbon through the living world. Of course, the living world also includes plants. Plants take in carbon dioxide to react in the presence of sunlight with the water in their cells. That, in a nutshell, is photosynthesis, the process responsible for the plant-based carbohydrates we eat.

We are vastly outnumbered in terms of carbon biomass by the plant kingdom. Of the estimated nearly 500 billion tonnes of biomass carbon on Earth, the animals account for just 0.4% whilst the plants represent 90%. No wonder that the graphs of measured CO2 levels show an annual fluctuation, forming a symmetrical wobble. The wobble represents the Northern Hemisphere seasons because that's where most of Earth's land masses are found. In the growing season when the plants are busy photosynthesising, CO2 falls, only to rise again in the dormant season. The annual wobble is like the heartbeat of the planet, a regular rhythm along the rising slope that represents our emissions from fossil fuel burning.

Let's imagine a world without fossil fuel-burning. The annual wobble from the seasonal growth and dormancy of plants would be superimposed upon a near-flatline of CO2 levels over human lifetimes. Only occasional events, occurring over tens of thousands to many millions of years, would perturb that near-flatline. That's because there is a second, slow carbon cycle that operates over geological time-scales. In the geologic past, sudden changes in CO2 levels have occurred, primarily due to volcanism on a scale no human, living or dead, has ever witnessed. The fossil record tells us the outcome has never been good.

Fossil fuels are part of the slow carbon cycle. They represent one of several long-term geological reservoirs in which carbon gets locked away. But because we are digging or pumping fossil fuels from the ground and burning them, it is the slow carbon cycle that we are interfering with. No other species has ever intentionally interfered with the slow carbon cycle: this is a first on Planet Earth in its 4.5 billion year long existence. The person quoted in the myth box above is a geologist. He should know better.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

The very first time you learned about carbon dioxide was probably at school, where you were taught that we breathe in oxygen and breathe out carbon dioxide. The process, known as aerobic respiration, is something the vast majority of animals do. In our cells, the following enzyme-controlled reaction is taking place:

C6H12O6+6O2 → 6CO2+6H2O

It's a bit more complicated than that, but the equation is a representative overview. Carbohydrate is oxidised to carbon dioxide and water. The reaction is exogenic - meaning it releases energy at around 3000 Kilojoules per mole of glucose. And while we breathe in air with almost 420 ppm CO2 (2023 figure), it should come as no surprise that the air we breathe out contains 40,000-50,000 ppm (4-5%) CO2, representing a hundredfold increase. That's the product of aerobic respiration.

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!"

If someone makes such a statement, they are missing two crucial points. Firstly, our respiration doesn't matter in the big scheme of things. In terms of carbon biomass, we are dwarfed by the plant kingdom. Animals only account for a paltry 0.4% of the estimated near-500 billion tonnes of biomass carbon on Earth. Plants make up 90%.

Through photosynthesis, plants take in carbon dioxide and release oxygen, in a chemical reaction that is essentially the opposite to our aerobic respiration. Plants do perform some respiration, because they need to metabolise as well, but it is outweighed by the photosynthesis. The carbon they collect from the CO2 in the air, converted by photosynthesis into carbohydrates, forms their tissues - roots, stems, leaves, fruit and so on. Such tissues are eaten by all sorts of animals, which in turn are eaten by other animals. We humans 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. When we breathe out, all the carbon dioxide we exhale is simply being returned to the air. We are simply giving back the same carbon that was there to begin with. In doing so, we are actively participating in the fast carbon cycle. But our participation is tiny compared to that of plants.

The Keeling Curve (fig. 1) is the graph showing rising CO2 levels as measured at Mauna Loa and other observatories. On it, the plant world's participation in the fast carbon cycle can be seen. Due to photosynthesis, CO2 levels show an annual fluctuation, forming a regular wobble. The downward part of the wobble represents the Northern Hemisphere growing season. Since that's where most of Earth's land is distributed, it's where most of the CO2 drawdown takes place. In the Northern Hemisphere winter, when most plants are dormant, you get the upwards part of the wobble. The wobble, like a planetary heartbeat, is a regular rhythm superimposed upon the rising slope that represents our emissions from fossil fuel burning.

 The Keeling Curve

Fig. 1: The Keeling Curve - monthly mean CO2 concentration data (with the occasional volcanic anomaly filtered out), Mauna Loa Observatory, 1958-2022. Inset shows the annual 'wiggle' caused by seasonal plant-growth and dieback in the Northern Hemisphere. Image licensed under the Creative Commons Attribution-Share Alike 4.0 International licence.

Secondly, fossil fuels are the remnants of the fast carbon cycle, fortuitously preserved at various points along the geological time-line. That burial and preservation locked them out of the fast carbon cycle, putting them into the long-term storage part of the slow carbon cycle. Normally the slow carbon cycle operates over geological timescales. Thus, some of the coal we've mined has been more than 300 million years in storage, belonging, appropriately enough, to the Carboniferous period.

Forget about breath. Our carbon emissions from the slow carbon cycle are a) colossal and b) geologically unique. No other species in Earth history has deliberately disturbed the slow carbon cycle. But it has been disturbed - occasionally - by geological processes. Magma has occasionally cooked coal-deposits, as has been observed in Siberia (fig. 2). That rapid release episode, at the end of the Permian period 250 million years ago, didn't work out well. Biodiversity took a massive hit. It recovered – but the recovery took around ten million years.

Masses of coal caught up in basalt. 

Fig. 2: masses of coal caught up in basalt, Siberian Traps Large Igneous Province, from Elkins-Tanton et al. 2020. The rising magma interacted with and thoroughly cooked a major coal-basin, releasing a colossal amount of fossil carbon over a few thousand years. The result was catastrophic with the largest mass-extinction of the entire fossil record. Photo: Scott Simper, courtesy of Lindy Elkins-Tanton.

Weathering, plate tectonics, deformation and metamorphism of rocks have all affected CO2 levels - over millions of years. And that's the point. We are doing to our atmosphere, in a few centuries, what most geological processes could only accomplish over millions of years. Through fossil fuel burning, we are performing a unique, vast and uncontrolled experiment with our home planet – the only one we have.

The animation below was published by Dr. Patrick T. Brown (Carnegie Institution for Science, Stanford University) in September 2018, to explain how human respiration fits in to the overall process.

Last updated on 3 December 2023 by John Mason. View Archives

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Comments 151 to 160 out of 160:

  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.

  7. Your non-empircal assumptions are incorrect: Assuming dry air (which exhaled air is not) at 1bar and 288K, current atmospheric CO2 content amounts to around 0.000490 mol (0.04% by volume) - this is what humans generally inhale. Exhalation post metablism has a molar content of 0.04 mol (4% by volume). Given the molar mass of CO2 that is a mass increase of 1.7388 g/mol.
    Assume a population of 6bn split equally by men, women and children of an average respiration volume of 6,000, 4,000 and 2,000 ml and 10 respirations per minute. In a 24-hour period for that population exhaled air has a volume of 5,702,400,000 cubic metres.
    The relative weight of inhaled CO2 is 1.2288% for one cubic meter of air at 1.225 kg/m3 = 15g.
    The relative weight of exhaled CO2 is 6.0763%. That is an increase of 4.945 times making the increased weight of CO2 in exhaled air 19.945 g. So the increased mass of exhaled CO2 per respiration is approximately 5 g.
    For the assumed population the mass of the exhaled CO2 increment is therefore 28.5Mt per day, or 10.4Gt per annum.
    Where does that 10.4Gt feature in your 'carbon cycle' mass balance given that, at least for 2019, that figure is greater than the 'land-use emissions' and amounts to around a third of industrial and fossil fuel emissions? Let's not even talk about what fraction of exhaled CO2 is radiation transparent rather than reflective.

  8. 157:

    Where does that extra carbon we breathe out come from if not from the food we have eaten?
    And if you agree that it does come from the food, where does that carbon come from if not directly or indirectly from plants extracting that carbon from the atmosphere recently while they were growing? (you have heard about the photosynthesis, right?)

    And there is no specific fraction of CO₂ that is radiation transparent since all CO₂ molecules absorb very strongly at some infrared wavelengths but weaker or hardly not at all at other wavelengths. There are slight variations in the absorption depending on the isotopes of carbon and oxygen, but almost all CO₂ molecules are made of carbon-12 and oxygen-16.

  9. Chris.Shattock @157,

    I would suggest that your estimate for the exhaled flux of CO2 from humanity of 10.4Gt(CO2)/yr is of the correct order of magnitude but a lot higher than most estimates which put it at something like 1kg(CO2)/human/day. (If you have a scan up-thread, there will surely be references to this finding.) So 6 billion humans would be breathig out perhaps 2.2Gt(CO2)/year. Mind today's human population is a little higher than 6 billion.

    As for this exhaled flux increasing the level of atmospheric CO2, as explained @158, our breath is part of a cycle, CO2 taken from the atmosphere and then returned to the atmosphere. This cycle is quantified as "net primary production" which totals about 60Gt(C)/yr = 220Gt(CO2)/yr from land and a similar amount within the oceans. So humanity is exhaling just 1% of a cycle that does not actually add to atmospheric CO2.

    And the biggest impact hmanity has on the scale of this cycle is reducing its size through cutting down trees and repalcing the forest area with farmland. According to Harberl et al (2007), humanity has hus reduced the cycle by about 10%.

  10. Please note: the basic version of this rebuttal has been updated on December 3, 2023 and now includes an "at a glance“ section at the top. To learn more about these updates and how you can help with evaluating their effectiveness, please check out the accompanying blog post @

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