Does Breathing Contribute to CO2 Buildup in the Atmosphere?

I was recently honoured to join Skeptical Science, a comprehensive database of rebuttals to common climate change misconceptions, as an author. Here I am republishing my first article regarding the common myth that breathing out contributes to the buildup of atmospheric carbon dioxide. It is the Intermediate version, and I have also written a very similar Basic version, which includes a diagram by John Cook. Enjoy!

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.

More specifically, the mitochondria within our cells perform cellular respiration: they burn carbohydrates (in the example shown below, glucose) in the oxygen that we breathe in to yield carbon dioxide and water, which we exhale as waste products, as well as energy, which is required to maintain our bodily processes and keep us alive.

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

carbohydrates + oxygen → carbon dixoide + water + energy

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.

Therefore, when we breathe out, all the carbon dioxide we exhale has already been accounted for. By performing cellular respiration, 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!


28 thoughts on “Does Breathing Contribute to CO2 Buildup in the Atmosphere?

  1. Nice job, Kate. Kind of a funky looking cow in the basic version though :-)

    Yeah, we had fun choosing cow pictures in the review process! -Kate

  2. Unfortunately Kate, a good percentage of the human population has a big carbon footprint. Your chemical balance equation is correct, but it doesn’t account for where the carbohydrates came from. If we all went out and picked our fruit and vegetables from our gardens that we planted and tilled without the benefit of fossil fuel, and if we went into the jungle and chased down rabbits and antelopes for meat, without the benefit of a helicopter or land rover, then our carbohydrates would have no carbon footprint. But most of go to the supermall and buy food that was produced hundreds or thousands of miles away by heavy farm machinery and processed and shipped by other machinery. The net result is that each calorie we consume costs on average about [citations needed] calories of fossil fuel to get to our mouths. Unless we consume food with no carbon footprint, every time we breathe out we will also have a carbon footprint. It just won’t show up in your equation. So this is one time the skeptics are usually right.

    Yes, food has an indirect carbon footprint, but this too is due to fossil fuels – not breathing out. The carbon in our bodies is from the food we eat. The carbon we use to get the food we eat is another matter entirely. There’s a nice discussion about this over at Skeptical Science, check the link at the beginning of this post. -Kate

  3. This makes it so much clearer: So only CO2 released from burning fossil fuels is poisoning our atmosphere.

    Now of course IF you are an evolutionist and believe that the dinosaurs spent million of years eating lots of those plants whose bodies then became the oil deposits that are with us today, then burning it is just putting it back into the air like our breathing, right?


    Fossil fuels were actually formed by forests (coal) and phytoplankton/zooplankton (oil), not dinosaurs. The fossil fuel deposits we access today predate the dinosaurs. Regardless, as this was a different geological era, the carbon has been locked up (and subsequently prevented from cycling through the biosphere) for millions of years. Therefore, their contribution is net positive to atmospheric CO2. The only way it could be net zero would be if fossil fuels were replenishing themselves at the same rate at which we are extracting them, something that they’re definitely not doing!

    The existence of dinosaurs is not a matter of belief. Please read the comment policy in the sidebar to avoid having your comments flagged. Thanks. -Kate

  4. Kate,

    Sorry I did not mean to say I was questioning their existence, though we might debate about when they walked this planet.

    So even if you move the equation back before the dinosaurs there was a time when this carbon was not locked up as you put it, so why is releasing it again today a problem?


    It has been removed from the biosphere for millions of years, so the carbon cycle in the biosphere (which is fairly short) has long shifted its equilibrium to discount it. Greg Craven, a high school science teacher, has a more comprehensive explanation in a three-part video starting here – links to the other 2 can be found in “Related Videos”:

    How is the dating of dinosaur life a matter of debate? Please provide appropriate citations. -Kate

  5. John Stoos

    Releasing it again is not a problem, if you don’t mind going back to the conditions that applied millions of years ago, ie 200 foot higher sea levels for one.

  6. Most of the ancient carbon is buried in carbonate rocks, not coal and oil.

    The coal and oil (especially the coal) was laid down, largely, during the Carboniferous — 300-360 million years ago. (Hence the name :-)

    The issue with releasing 60 million years worth of carbon burial in a couple of hundred years, 300 million years later, is that the sun has gotten brighter over that span. The sun gets brighter as it ages, so to keep the climate more or less stable over geologic time, the atmospheric CO2 levels has to continue dropping.

    That sets up one of the end of life on earth scenarios — 500 to 1000 million years from now, even a 0 level for CO2 will not be enough to keep the place comfortable against the increasing solar brightness.

    Kate: two sources to look at for information about young earth creationism and their arguments are

    They’re much more useful than, say, evolutionary biology literature as YEC arguments (for instance, age of the earth) don’t have much to do with that. (I’m a long-time member of both groups.)

  7. John Soos, you said in your post “Now of course IF you are an evolutionist and believe that the dinosaurs …”.

    Science is not based on belief. There isn’t a credible scientist alive who ‘believes there were dinosaurs’. Scientists know they walked the earth because we have abundant evidence of this – fossil skeletons (some with skin and feathers, and even intact muscle fibres, trackways, even coprolites! That they walked the earth is fact. If you are uncomfortable with this knowledge, then I suspect that there isn’t a argument that will persuade you otherwise, on that point, or on any point regarding human-causation of climate change.

    But the point of Kate’s responses is this: all the coal, and oil and gas we are burning has been buried for millions of years – and we’re putting all that CO2 back in the atmosphere over a span of decades – the world will respond to this massive change in ways that are unfavourable to our civilization.

    The age of the fossil fuel deposits varies; some of North America’s coals are as ‘young’ as 45 million years, but most of it is 100-150 million years old (much of that mined in Alberta) or as old as 300-350 million years old (most of that mined in the Appalachians)( Oil, as Kate points out, is mostly marine in origin and derived from microscopic algae and zooplankton (but not exclusively so:

    Tellingly, the geological record shows that when such large changes in the amount of CO2 in the atmosphere happened in the geological past (after the dinosaurs, but before humans) that the climate changed rapidly resulting in large-scale disruption to life across the world; 55 million years ago in what geologists call the Paleocene-Eocene Thermal Maximum, natural processes released massive amounts of greenhouse gases (probably initially methane); see here for one take on this: The PETM is seen by many geologists as a possible analog for the ‘unplanned experiment’ our burning of fossil fuels has started.

  8. Can’t really add a lot to what has been already commented on.
    But another point might be that the carbon cycle hundreds of millions or just millions of years ago would have been balanced for the conditions that existed at the time. Changes took thousands of years to happen, not a few hundred.

    Carbon was ‘locked up’ in a less stable format called trees and vegetation (which is why we have a lot of coal now). If John hasn’t noticed, we have fewer trees and forests around today. I know because where I live used to be a huge forest, as little as a few hundred years ago. The only part of it left is small pockets of park land amongst large tracts of farmland and housing estates.
    So add old stored carbon in the form of CO2 to a new environment with a reduced number of carbon sinks and you end up with an imbalance.

  9. The sequestered carbon that is fossil fuel was put down there by the action of biological organisms. It originated from volcanoes venting gases from non biological sources within the earth. Without the biological action (and rock weathering) sequestering the excess CO2, the gas would have continued to build up in the atmosphere over millions of years.

    Digging up the sequestered carbon and sticking it back into the atmosphere is reversing a vital regulatory function whereby life keeps the climate fairly stable for itself to thrive in.

  10. Kate, as it seems John Stoos has an issue with the age of the earth, and because this causes him some misunderstanding over the role played by burning fossil fuels in AGW, this website may be helpful as it is written from a religious perspective and generally speaking those who question the earth being billions of years old often do so because of their religious beliefs. The website explains radiometric dating, including carbon dating, which of course is of no use in dating dinosaurs (only useful back about 40,000 years; other isotopic methods are used for rocks millions to billions of years old).

    Sorry, that one is my fault, I paraphrased John’s comment to carbon dating, while I really should have been more general and said radioactive dating. -Kate

  11. John Stoos:
    Carbon dating is not used for the age of the earth. It’s only good for 50-60,000 years, tops. Doesn’t help with dating something that turns out to be 4.5 billion years old.

    Two handy references on the topic are:
    a) the age of the earth FAQ at
    b) Mark Isaak’s Index to Creationist Claims at

    Sorry, that one is my fault, I paraphrased John’s comment to carbon dating, while I really should have been more general and said radioactive dating. -Kate

  12. Kate, I don’t think you should apologise. Pastor John Stoos’ church holds this position (copied without editing from their web site: ):

    “Article XII
    We affirm that Scripture in its entirety is inerrant, being free from all falsehood, fraud, or deceit.

    We deny that Biblical infallibility and inerrancy are limited to spiritual, religious, or redemptive themes, exclusive of assertions in the fields of history and science. We further deny that scientific hypotheses about earth history may properly be used to overturn the teaching of Scripture on creation and the flood.”

    In short, Pastor John Stoo’s and his congregation are bible literalists / young-earth creationists and I would argue that the comments posted by myself, Robert and Nick are reasonable responses from our scientific perspectives.

    A note of caution for anyone who visits Pastor Stoo’s website; there is html code embedded there that prevents a visitor from using the back button to leave the web site.

    • David,

      You reflect our views correctly and thank you for pointing out the glitch in our site. I can assure you it was not intentional and will have our web master fix the problem.


  13. I’ll post my comment on SkSci here as well, since you have a “notify” feature (something that it would be nice for John to add – not that I want to multiply his already massive workload).

    Thanks for your work on this and I’m excited that you’re contributing to SkSci, which is truly becoming more and more excellent as time goes on.

    My only hesitation is that I wonder whether this piece could be improved with some more figures on the exact amount of CO2 that human respiration contributes. I get the idea that we’re only exhaling the carbon that has first been photosynthesised out of the atmosphere, but from one point of view, the origin of the CO2 is irrelevant, what matters is the total amount. So I wonder whether this argument could be supplemented with a consideration of the total contribution of human respiration to CO2 emissions (for completeness, perhaps we would also need to consider human CH4 emissions…).

  14. Byron Smith – human consumption of transported food (depends how it is transported) increases the carbon footprint of the food as of now, hence ‘eat local food’ is better, this is one of the differences between ‘developed’ and ‘developing’ countries.

  15. I think the inclusion of figures might result in the subject widening to include other sources of greenhouse gases and the answer being diluted.

    Humans generally contribute nothing. Any ‘contribution’ would be a result of an imbalance in sources and sinks. The only way humans would contribute CO2 is by the population growing significantly and/or the demise of sinks such as forests and oceans. Those are environmental issues as well which have their own set of deniers, and the increase in population growth required to make a significant contribution in CO2 would probably dwarf the resulting CO2 contribution.
    eg. humans would have wrecked the planet so much that the physical damage would register long before ‘breathing’ emissions registered as a threat.

    Fossil fuels are different because they have a level of independence from the natural cycle and hence inject CO2 into the system.

  16. @Warmcast – Yes, I’m aware of this and entirely agree that the “human respiration” concern is largely spurious, but putting some numbers of it would (presumably) demonstrate just how spurious.

  17. Since I was a child the human population of the planet has grown significantly. In 1918 the earths human population registered 1.8b which took since the beginning of time humans first walked the earth until 1918 to develop. Last year in 2012 human population reached a crazy 7b. Now that massive unsustainable increase must account for planet warming as we see it today. Both Human exhaust and industrial exhaust are accountable. Reduce one and you reduce the other. Without the masses of humans we wouldn’t have the masses of industrially produced causes such as the main use of fossil fuel (coal) for Electricity production or secondly fossil fuelled vehicles (oil) etc.

    You have only spoken here of CO2 exhaust from human breathing. I feel you need to also add Methane as a Human Exhaust to your theories and arguments. Thereafter overweight people exhaust far greater amounts than normal body mass people as their breathing is faster and food intake greater therefor Methane exhaust also higher. I guess an average per human can be worked. I personally feel, prove me wrong, that the overall increase in population thereby human bodily emitters of greenhouse gases on this planet is indeed the correct reason for increases in atmospheric Methane and CO2. An over 5b increase in human population since 1918 cant be wrong. Imagine the population in the next 50 years now that we are starting with 7b not 1.8b breeders.

    As to the argument that human CO2 exhaust is countered by nature itself who first took the CO2 from the air through Photosynthesis and earth through its roots where after we ate it and exhausted it via our exhalation process, I would also add that we exhaust it via flatulation, then perhaps we can clarify this by asking a) how much CO2 do we breathe in each lungful b) how much CO2 do we breathe out each lungful c) what is the difference d)is the difference soley residue from the plants we eat or does it include the animals and fish and birds we eat.

    And here is a curly one…Does the CO2 exhausted by cigarette smokers equal the CO2 found in the amount of tobacco they smoke??

  18. Please may I also ask if anyone can produce the figures of how much CO2 and Methane do humans produce on a daily and annual basis versus versus how much all industry (coal/oil users) produces. Then perhaps we can also find someone with a total figure on how much CO2 and Methane the planet or nature naturally produces. Thereafter we need to know how much good gases the planet naturally produces. Perhaps then and only then can we easily work out what population size is maximum to the planets wellbeing before all O2 is spent and the impasse is reached.

  19. Unfortunately, you are not considering that the carbon dioxide from the coal was underground. When it is burned for electric production and expelled by humans, it becomes a part of the air (and is no longer a solid). Those particulates and greenhouse gases, then change the planet and weather patterns.

    It never has been about making new or destroying things. It is about how our presence and activities have impacted the planet Earth in negative ways.

    And, quite frankly, I doubt very seriously that God would agree with anyone, who thinks we should make excuses for destroying the only planet HE gave to us.

  20. Appreciate the lesson, but I have a question. Cannot the carbon cycle be overtaxed by an exponentially growing population vs. a dwindling photosynthetic engine (due to deforestation, drought and desertification)? The cycle exists; but could it not be thrown catastrophically out-of-balance?

  21. I recently put the “CO2 from breathing” proposition to a geologist friend of mine. He thinks it’s misleading to suggest that CO2 originates in the atmosphere – and humans are simply returning it through eating carbohydrates and respiration.

    To paraphrase, more correctly, all human biomass originates from photosynthetic biomass – as does all other non-photosynthetic biomass, so it’s indeed possible that human respiration from population growth over the past 100 years has increased CO2 in the atmosphere. This is because we’ve converted photosynthetic biomass into non-photosynthetic (human) biomass, and it’s the ratio of the two that ultimately determines atmospheric CO2.

    However, respiration aside it’s other human activities – such as deforestation that has reduced photosynthetic biomass substantially and this is even more important with fossil fuel derived CO2 now having entered the carbon cycle.

    Even if we halt all fossil fuel burning tomorrow we have depleted the only means of extracting the fossil fuel derived atmospheric CO2 – namely photosynthetic carbon, and a sustained population of >7 billion will serve as a significant impediment to our efforts to grow the photosynthetic pool.

  22. given the amount of atmospheric CO2 and our population growth and need for energy is it even possible to reduce CO2 to a level that would impact global warming or are we blowing smoke?

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