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Does breathing contribute to CO2 buildup in the atmosphere?

Posted on 26 September 2010 by climatesight

Guest post by Kate from ClimateSight

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!

This post is a new rebuttal to the skeptic argument 'Breathing contributes to CO2 buildup' (written by Kate from ClimateSight). This blog post is the Intermediate version - she's also written a Basic Version that features a simple graphic explaining the carbon cycle from a respiration point of view.

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Comments 51 to 72 out of 72:

  1. FreeDubay @ 50

    Fossil fuels are from plants, its true, but that carbon was removed from the air hundreds of millions of years before humans ever walked the earth. So releasing that carbon isn't neutral from the perspective of human history or civilization. In addition, burning all of the fossil fuels would return us to a truly ancient pre-human atmospheric chemistry, with all its attendant consequences for climate, in only a only century or two, whereas it took many millions of years to stash away that carbon in the earth in the first place.  

    The CO2 we exhale, on the other hand, was usually taken from the atmosphere in the last year or two. The uptake of that carbon from the air by food crops contributes fractionally to the regular annual fluctuations in atmospheric CO2. Over scales longer than a year, the net effect on climate of carbon dioxide uptake by food crops and exhalation of CO2 by people is essentially zero.

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  2. An interesting point, though, FreeDubay.  I'm set on cremation, but perhaps this is not the best way to go.  Maybe burial in a place designed for human sequestration.

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  3. So, if I understand what you are saying about this being a cycle. If human population growth completely stopped and remained constant as well as every other CO2 producer/source (all remain constant) and at the same time every square inch of the Earth became covered in lush vegetation, increasing the amount of photosynthesis occuring by a factor of 100 let's say, according to your "cycle" theory, the amount of CO2 in the atmosphere would never decrease? I think that is pretty much incorrect. Yes it is a cycle, but the amount in the atmosphere depends on how much is being put into the atmosphere (sources) and how much is being taken out of the atmosphere (sinks). So...you're incorrect.

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  4. “Does breathing increase CO2 in atmosphere?” is the question. The direct answer is humans inhale on average about 11,000 liters of air per day. The current concentration of CO2 in atmospheric air is about 410 parts per million (ppm). We exhale on average about 11,000 liters of air per day with a concentration of CO2 at about 40,000 ppm or one hundred times the concentration we inhale. My calculations indicate that we intake about 3.6 kg of CO2 per day and exhale about 360 kg of CO2 per day or a net increase of CO2 in the atmosphere of 355 kg per day. Therefore, in a year, an average human on earth increases the amount of CO2 in the atmosphere by about 130,000 kg or about 65 metric tons. With about 7.5 billion people on earth today, this equates to approximately 488,000 million metric tons per year. In 2014, it is estimated that the world produced about 36 billion metric tons of CO2 by burning fossil fuels. Therefore, breathing increases CO2 in atmosphere by about 1 percent that produced by burning fossil fuels.

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  5. Geotim,

    Since CO2 is 12/44 carbon if I exhaled 355 kilograms of CO2 that would be about 100 kg of carbon or more than my entire body mass.  About 60% of my mass is water.  Perhaps you need to review your calculations.

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  6. I agree and thanks for keeping me honest.  Assuming 1 ppm CO2 by volume in air is equal to 1.94 miiligrams per cubic meter, my corrected calculations indicate that we intake about 8.7 g of CO2 per day and exhale about 854 g of CO2 per day or a net increase of CO2 in the atmosphere of 845 g per day. Therefore, in a year, an average human on earth increases the amount of CO2 in the atmosphere by about 308 kg or about 0.15 metric tons. With about 7.5 billion people on earth today, this equates to approximately 1.2 billion metric tons per year.  In 2014, it is estimated that the world produced about 36 billion metric tons of CO2 by burning fossil fuels. Therefore, breathing increases CO2 in atmosphere by about 3 percent that produced by burning fossil fuels.

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  7. GeoTim @56 , best if you forget your calculation altogether.  It is irrelevant whether human out-breath is 1.2 billion or 12 billion tons of CO2 annually.

    If you had read & understood the OP, then you would know that the nett contribution into the atmosphere from human out-breath — is very close to zero.

    Okay, there would be a relatively microscopic contribution of fossil fuel CO2 — from the bubbles in the Coca-Cola and other soda-pop that you drink.  But in realistic terms, that's mighty small.  I would be interested to see what figure you can calculate that to be !

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  8. Eclectic @57,

    I think we waste our breath on this GeoTim. That the implications of the conservation of mass (which apparently holds the positon of a law of physics) set out by Michael Sweet @55 was ignored and only prompted a revision of what are obviously error-filled calculations; in my book this demonstrates trollish tendancies as well as innumeracy. (For the record, the calculation presented @56 for annual human CO2 exhalation looks about right, except is there really 2,000kg in a tonne?)

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  9. MA Rodger @58 , as you well know, the "huge human bodily contribution of CO2 to our planetary atmosphere" is one of the many falsehood memes which is deeply imbedded in certain sections of the community, and is one which is a very uphill matter to correct.  Not impossible to correct, but quite difficult.

    Apropos nothing: I was looking through the Curry blog "ClimateEtc" just the other day, and found a comment that will amuse you.  It was by "Russell Seitz" (regarding The Hockey Stick and its later replications/confirmations) :-  "We are all indebted to [Mr X.] for so vividly illustrating the hazards of ignoring the climate science literature for decades on end."

    ~ Mr X. was one of the more intelligent of the crackpots to be found often in the blog's comments columns . . . but really, almost any denialist's name could have been inserted in its place.

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  10. Thanks MA Rodger.  I woke up thinking I had bungled that calculation again.  308 kg is about 0.308 metric tons which results in approximately 2.4 billion metric tons per year or about 6 percent of the contribution produced by burning fossil fuels.  I just wanted to present the calculation to show that we are CO2 producing systems.  One day when we stop using fossil fuels (ha ha) someone will say that humans and animals are the biggest contributors to CO2 increase.  I wonder what dinosaur CO2 production was.

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  11. Geo Tim @60.

    Don't thank me. You are still away with the fairies.

    May I correct you once again. "Humans and animals" are not "the biggest contributor to CO2 increase" being part of the carbon cycle, as the OP explains. But in terms of ranking sources of exhaled CO2, as the biomass of domestic animals is 70% greater than human biomass (see graphic here which is Fig 1 from Bar-On et al (2018) 'The biomass distribution on Earth'), we can expect the breathed CO2 from livestock (mainly cattle) to exceed that from us humans. But the biggest biomass shown on that graphic is that of plants (7,500-times greater than human biomass) and plants also breathe CO2 (as well as absorb it for photosynthesis). And even though plants are quite sedentary and don't run around like animals, collectively they do exhale a lot of CO2, over ten-times the CO2 that mankind's fossil-fuel-use is responsible for, according to the research of Huntingdon et al (2017) and clearly described here by one of its authors.

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  12. I do understand that the plants inhale and exhale CO2.  I read that studies indicate the CO2 being inhaled by plants will reach a saturation point resulting in only CO2 exhalation.  I assume that the plants that are being cut down to make way to grow the food for us humans (including to feed the animals we eat) are not as efficient at being net absorbers of CO2.  I notice NASA has a satellite measuring CO2 of the earth which states that the highest concentrations of CO2 are over metropolises not forests.  It would be interesting to compare metropolises in the developed areas (lots of burning of fossil fuels) to metropolises in underdeveloped areas (less fossil fuel use).  I think the more populated countries were at the top of NASA'a list for CO2 concentrations.

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  13. Geo Tim,

    You said, "My calculations indicate that we intake about 3.6 kg of CO2 per day and exhale about 360 kg of CO2 per day or a net increase of CO2 in the atmosphere of 355 kg per day. Therefore, in a year, an average human on earth increases the amount of CO2 in the atmosphere by about 130,000 kg or about 65 metric tons"

    Here is the flaw in your calculations. CO2 doesn't magically derive from thin air. It comes from the food we eat. Now that carbon did come from the air.

    So no matter how much CO2 we breathe out, it always comes directly or indirectly from food that fixed it out of the air to begin with. The net over a lifetime will always be almost exactly zero.

    You can never have it move towards being a net emissions source ever, it breaks all kinds of laws of physics and conservation of mass etc... In rare cases it could end up being a very small net sink, due to the possibilty of your dead body being preserved and fossilized. (or in some cases your solid waste being used to sequester carbon in the soil) But it can NEVER EVER be a net emissions source because we all must eat food to grow. Never is it possible we manufacture new mass. If you ever do a calculation that makes it appear as if you created mass, then you simply made an error somewhere in your calculation, or missed an input.

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  14. There are about 3 to 7 quadrillion tons of CO2 (depending on source) in the atmosphere.  Photosynthesis removes only about 150 billion tons of CO2 per year and can only remove about half of the CO2 resulting from burning fossil fuels.  I don't think I can honestly state that all of the CO2 we breath out is recycled back into food.

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    Moderator Response:

    [JH] Unless you can cite sources for your opinions, they are of little value on this venue. We are all about science-based discussions and not personal opinions that have no scientific underpinnings. 

    Please note that posting comments here at SkS is a privilege, not a right.  This privilege can be rescinded if the posting individual treats adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Please take the time to review the policy and ensure future comments are in full compliance with it.  Thanks for your understanding and compliance in this matter.

  15. GeoTim,

    According to Wikipedia, approximately 3200 gigatons or 3.2 trillion tons of CO2 are in the atmosphere.  That is a factor of 1000 less than your 3 quadrillion tons.  If I use your unsupported number of 150 billion tons per year, it would take about 20 years for all the carbon to cycle through plants. It appears you have misplaced a decimal again.  Note that I have linked my sources of information so you can check them if you want to. 

    We have not discussed the much larger carbon reservoir in the ocean.  The atmospheric and oceanic CO2 exchange and affect the calculation.  Perhaps your number is all the carbon dioxide.  In that case it would take longer for all the carbon to cycle through plants.  It would all cycle through if you wait long enough.  150 billion tons/yr for plants seems low to me but I could not find a reference.

    Essentially all carbon in food comes from the atmosphere.

    I conclude (as is stated in the OP) that all the carbon we breath out is recycled into plants.  I suggest you stop trying to do your own calculations and accept that scientists actually know what they are doing.

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  16. michael sweet @65,

    The error @64 surely has to be a simple decimal point issue. The graphic below (from here - it's a bit out-of-date as today's atmosphere with 408ppm CO2 contains 870 billion tons carbon) shows 57 trillion tons of carbon to be found on planet Earth, enough to make just 212 trillion tons of CO2. That's a long way short of the 3,000 to 7,000 trillion tons mentioned @64.

    DOE Carbon Cyycle

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  17. MA Rodgers,

    Your illustration is carbon while GeoTim is stating CO2.  Your 57 trillion tons must be corrected by 44/12 is approximately 210 trillion tons CO2: still much less than 3000 trillion tons.

    The entire point is that it is easy to make an error.  Your illustration is made by a professional and peer reviewed so the chance of error is much less.

    It is easy to point out errors.  It is difficult to do calculations exactly correctly.  I always try to get a reference to eliminate errors.

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  18. GeoTim may be interested in this short video which clearly explains why humans breathing out CO2 really isn't an issue:

    www.youtube.com/watch?v=Ah09gPZDymg

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  19. Read about the Haber Process (Wikipedia) which has enabled doubling the global population and consumes about 5% of annual natural gas supplies to produce artificial nitrogen.  Nearly 50% of the nitrogen found in human tissues originated from the Haber-Bosch process.  One half the food we eat or one half the food the world eats is based on fossil fuels.

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  20. GeoTim @69 , the Haber-Bosch-type processes are an important artificial source of nitrogen in agriculture (for production of vegetation, and ultimately human food).  Nitrogen, not carbon.

    As in many industrial activities, fossil fuels are currently extensively used to power the process.  But it ain't necessarily so.  In a low-carbon emissions scenario, the hydrogen used for nitrogen fixation (into ammonia) could come from hydrolysis of water — the electric power supplied by renewables or nuclear.

    That is easily done.  GeoTim, you would be better turning your attention to how CO2 emissions could be phased out from the cement production processes.

    GeoTim, you have still not shown your calculation of the human out-breath containing fossil carbon content deriving from the fizzy bubbles in CocaCola & other soda-pop.  This is the real human source which threatens the collapse of world civilization.  CocaCola is the hidden culprit, the fatal weakness in modern society.

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  21. With all due respect, may I ask whether we have the capacity to provide the needed energy using renewables or nuclear power if fossil fuels were gradually terminated?  The objective, of course, would be to reduce the amount of CO2 generated each day (including humans') to a level that can be consumed by current and future consumers (photosynthesizing plants, coral, and ocean water, etc.) and not adversely effect the biosphere; in other words to a sustainable level.  I do not fill energetic enough (and I make mistakes) so I will take the suggestion to not perform any more calculations and leave you with the idea of sustainability.  I would love to read the plan someday.

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  22. GeoTim @71 , you are now wandering way off-topic for this thread.

    Please find one of the many threads discussing the replacement of fossil carbon fuels by "zero-carbon" energy sources, and express your well-considered thoughts there.  The change-over to "zero nett carbon" could be achieved in around 20 years using present-day technology — let alone the likely more efficient solar technology which will be under development currently.  "Nuclear" is also possible, but is very slow to construct, and is very expensive (see the impressively large subsidy donated by RossAtom to the Finland economy for the new reactors there).

    Considering that most present-day coal/gas electricity generation stations will be expiring of old age in 20 or so years (and most of today's cars and trucks will also "age out") . . . it turns out that it is little or no additional cost to replace them with zero-carbon machinery.

    The main exception is airplanes and ships — they can use kerosene/diesel made by vat fermentation ~ but this technology is not yet in large-scale economic production.  But likely will be reasonably cheap in 20 or 30 years' time.

    Additional threads can be found that discuss reducing atmospheric CO2 by returning carbon to the soil (soil micro-organisms can sequester large amounts of carbon compounds, with better farming/grazing management methods).

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    Moderator Response:

    [PS] Indeed offtopic. Please move any further response to this thread.

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