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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. 


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!

Last updated on 26 September 2010 by climatesight.

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Comments 1 to 37:

  1. Bit of an odd looking cow!
  2. I'm a lecturer at a construction college of further education in the uk. Many of the courses we deliver have extensive sustainability units and of course co2 emissions are at the top of the list. This site and others like it have helped me build a picture of the whole issue for which I am most grateful.

    I was questioned the other day about this exact issue but the argument went... If we have gone from 1 billion to nearly 7 billion in 200 years and in that time have cut down massive quantities of vegetation without planting more then surely some of the rise is attributable to the rise in co2 - ie we have messed with the carbon cycle?

    Well yes, population is the elephant in the room said I. Then I pointed out all the evidence showing that we can attribute the rise in co2 to the combustion process (o2 levels dropping, c12/c13 ratio falling and of course how much we have burned compared to co2 rise).

    However it does seem to many an obvious equation...Less plants - less o2 is produced - more people combining carbon and oxygen - co2 has to increase surely?

    I suppose it comes down to how much GHG's humans bodily functions produce annually compared to how much is recycled by plants annually. I can feel myself beginning to answer my own question here!

    Does anyone have any figures relating to this topic?
  3. Johngee @2,

    All the CO2 we exhale comes from burning food. An expanding human population means more crops - otherwise we'd starve. So there is no nett contribution to GHG from human respiration.

    I would imagine most deforestation is for agriculture i.e. replacing one sort of vegetation with another. This is not harmless - forests are good "carbon sinks" (all that wood is "locking up" carbon) and they photosythesize all year round unlike crops. Forests may be more efficient at absorbing CO2 because of their height too.

    I have no figures - sorry, but to do this calculation you would need to take into account the changes in other animal populations (down for wild animals, up for domesticated) as well. I would doubt whether such comprehensive figures are available
  4. johngee@2 wrote: "I suppose it comes down to how much GHG's humans bodily functions produce annually compared to how much is recycled by plants annually."

    The figures shown below suggest that the CO2 flux into the atmposphere from terrestrial plants is about 60Gt C per year (which is vast)

    if you divide that 60Gt by the human population it would give the number of tons of carbon we would need to consume each year to balance the flux from land plants, I doubt any of us eats quite that much! ;o)
  5. You see... That's all I was missing!!! The carbon we breathe out comes from the food we eat via cell respiration! Sometimes it has to be said several ways before I get it. A very dead argument methinks.
  6. Johngee @2 said:
    I'm a lecturer at a construction college of further education in the uk.

    I think the construction industry has a hugely important role in reducing CO2 emissions, and I for one (and I'm sure many more here) are very happy to see people like you making use of sites like Skeptical Science.
  7. Breathing and bags of lettuce?

    From a USEIA report regarding the drop in US CO2 emissions in 2009:

    In 2009, energy-related CO2 emissions in the US saw their largest absolute and percentage decline (405 million metric tons or 7.0 percent) since the start of EIA’s comprehensive record of annual energy data that begins in 1949 ...

    Changes in CO2 emissions can be decomposed into changes in four major contributing factors: population, per capita GDP, energy intensity of the economy, and carbon intensity of the energy supply. All of these fell in 2009 except for population. Population grew 0.9 percent.

    So it would appear that what we do and how much of it we do are far more significant than what we eat.
  8. @phil. Cheers for that. It feels like your shouting on deaf ears most oif the time though... something called economics keeps getting in the way.
  9. Welcome Kate and I'm thrilled that you're contributing here!

    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...).
  10. Good qualitative picture. Now could you please add some quantitative data e.g. how much net CO2 on average does a single tree or a square mile of forest absorb through photosynthesis? How much net oxygen does it release?
    P.S. "net" to take into account plants respiration.
    Thank you very much.
  11. How much fossil carbon in the human carbohydrate diet? Is it negligible? If not, then breathing is not net zero. There's an increasing popular literature devoted to showing the synthetic origins of much of what we call "food." (See Michael Pollan's "In defense of food" for example.)
    This is not a comment about the carbon intensity of agriculture (a separate discussion) but rather the source of the carbon in the carbohydrate content of our diet. There are many chemicals (polyethylene glycol, for example) in our diet that are synthesized from petroleum. I would be interested to know what portion of the CO2 breathed out by humans has fossil C.
    I think this would rather strengthen the "real food" argument more than anything else.
  12. When I was a kid in the 1970s, there were 4 billion people. Now there is 7 billion. So there should be more CO2 due to breathing now than in the 1970s, not to mention the increased livestock.
    Response: (DB) Human respiration is part of the natural carbon cycle and has no net long term impact, unlike the bolus injection of fossil fuel CO2 into the carbon cycle.
  13. This article doesn't quite hit the nail on the head.

    For example, CO2 emmissions from burning coal or oil are also part of this cycle.. The plants that absorb the CO2 grow, die, fall to the ground, are pressurised for millions of years and then turn into Coal or Oil.

    So isn't burning fossil fuels just an extension of this carbon cycle?
  14. j87, #13

    "For example, CO2 emmissions from burning coal or oil are also part of this cycle."

    That carbon had been locked out of the carbon cycle for hundreds of millions of years. What we are doing is reintroducing it to the carbon cycle, at a very great speed, far faster than natural carbon sinks can absorb it. Therefore, atmospheric CO2 is steadily rising.

    "The plants that absorb the CO2 grow, die, fall to the ground, are pressurised for millions of years and then turn into Coal or Oil."

    Almost all of the plants that die now decay and ultimately release CO2 back into the cycle. They do not become coal or oil.
  15. J87 - No. We are referring to 'natural' carbon cycling. Fossil fuels were removed from the surface carbon reservoirs a long time ago, and played no part in the carbon cycle.

    Humans then came along, extracted, refined and burnt those fossil fuels, releasing all that 'additional' carbon back into the atmosphere. Hence we have a problem of humungous proportions.
  16. "When I was a kid in the 1970s, there were 4 billion people. Now there is 7 billion. So there should be more CO2 due to breathing now than in the 1970s, not to mention the increased livestock."

    We also now grow more food to feed the extra people. Any CO2 we breathe out was carbon we locked up in plants before eating them.
  17. Methane from livestock is a different matter.
  18. There are two ways to look at the effect of human breathing and CO2:

    1) what we exhale, and how we acquired that CO2 - fixed from the atmosphere by plants, so not a net contribution to atmospheric CO2

    2) don't try to estimate the fluxes in and out, and just look at the change in storage. On that basis, I'm fairly sure (99-44/100ths % pure) that the 7 billion people we have now store more carbon (i.e., weigh more in total) than the 4 billion in the 1970s, so humans represent a net sink of carbon, not a source.

    Once climate change is bad enough that we see large decreases in the human population, we'll become yet another source of positive feedback as the stored carbon is released back to the atmosphere.
  19. Factor in all the fuel and oxygen spent in agriculture, storage and transport. Then, calculate how deep each of our carbon sink is on average. The more local our food source, such as our back yards, the deeper your carbon sink. Can we dig the treasure of honest living on the beautiful Earth?

  20. So is James Lovelock wrong when he says in his book The Vanishing Face of Gaia (2009):

    I have mentioned several times before that breathing is a potent source of carbon dioxide, but did you know that the exhalations of breath and other gaseous emissions by the nearly 7 billion people on Earth, their pets and their livestock are responsible for 23 per cent of all greenhouse gas emissions?

  21. @dwdwclare If Lovelock is going to take exhalations as contributing to anthropogenic greenhouse gas emissions then you would need to consider the food we eat as being carbon uptake.  Respiration is carbon neutral, the carbon in the food we eat originally came from the atmosphere, so when we breathe it out again, we are just returning it to the atmosphere and it has no net effect on atmospheric CO2 levels.

    Taking carbon out of the lithosphere and putting it into the atmosphere does however affect atmospheric CO2 concentrations (and indeed increases the total amount of CO2 circulating through the carbon cycle).

    It could be that Lovelock is making a subtle point that requires greater context to be apparent.

  22. @Dikran Marsupial Here is the entire paragraph:

    It is surprising that politicians could have been so unwise as to agree on policies many decades ahead. Perhaps there were voices from scientists who warned of the absurdity of such planning, but if so they do not seem to have been heard. Even if we cut emissions by 60 per cent to 12 gigatons a year it wouldn’t be enough. I have mentioned several times before that breathing is a potent source of carbon dioxide, but did you know that the exhalations of breath and other gaseous emissions by the nearly 7 billion people on Earth, their pets and their livestock are responsible for 23 per cent of all greenhouse gas emissions? If you add on the fossil fuel burnt in the total activity of growing, gathering, selling and serving food, all of this adds up to about half of all carbon dioxide emissions. Think of farm machinery, the transport of food from the farms and the transport of fertilizer, pesticides and the fuel used in their manufacture; the road building and maintenance; supermarket operation and the packaging industry; to say nothing of the energy used in cooking, refrigerating and serving food. As if this were not enough, think of how farmland fails to serve Gaia as the forests it replaced did. If, just by living with our pets and livestock, we are responsible for nearly half the emissions of carbon dioxide, I do not see how the 60 per cent reduction can be achieved without a great loss of life. Like it or not, we are the problem – and as a part of the Earth system, not as something separate from and above it. When world leaders ask us to follow them to the inviting green pastures ahead they should first check that it really is grass on solid ground and not moss covering a quagmire.

    Let me also ask, if respiration is carbon neutral why would cow burps and farts add to the amount of greenhouse gasses in the atmosphere? Please don't for a moment think I'm trying to dispute the veracity of anthropogenic climate change being largely caused by the burning of fossil fuels. I'm just asking because I'm curious.

  23. dwdeclare - the issue with cow burps etc is methane - a potent GHG that would not normally be produced if grass were not eaten by a ruminant. Eventually methane oxidizes to CO2 but while present in the atmosphere it contributes strongly to the greenhouse effect. You will see that greenhouse gas inventories are expressed in terms of CO2e (CO2 equivalents) rather than CO2 though the accounting for methane in this method has some issues. Methane from ruminants is increasing only because the number of ruminants has been increased by intensive farming practices.

    I think the article should perhaps also mention that there is another way to track the source of the increase in the CO2 in the atmosphere - carbon isotopes. Fossil fuels have no C14. C13 ratios are also different for different sources.

  24. dwdeclare @22, it becomes clear from the broader context that Lovelock is in fact talking about (primarilly) fossil fuel emissions in the production of food rather than mistakenly considering respiration as a form of net emissions.  Because those fossil fuel emissions are for the most part normal industrial emissions, they can for the most part be eliminated by the same processes used to eliminate emissions from the rest of industrial civilization, and Lovelock's pessimism is largely unwarranted.

    There are a couple of subtleties involved, however, one of which you draw attention to.  Methane has a far higher forcing per unit carbon than does CO2.  Therefore, the emission of methane by cows in the form of burps and farts represents a transformation of CO2 into methane, and a net short term increase in radiative forcing.  Short term because after about 15 years (from memory), the methane in tha atmosphere has transformed back to CO2.  The same applies to methane released from swamps (natural) or rice fields (anthropogenic).

    The second subtley is far more important.  Most fertilizer used in modern agriculture is produced from methane plus components in the atmosphere.  The essential step is the Haber-Bosch process:

    N2 + 3 H2 → 2 NH3 (ΔH = −92.4 kJ·mol−1)

    The hydrogen is produced through one of several processes from methane, of which the two step process may be considered representative:

    CH4 + H2O ⇌ CO + 3 H2

    CO + H2O ⇌ CO2 + H2

    Combining these reactions, we see that for each 8 nitrogen atoms in fertilizer, 3 methane atoms are used in its production, and 3 CO2 molecules released to the atmosphere.  By mass, that means that for each 28 Kg of Nitrogen produced in fertilizer, 9 Kg of carbon is consumed as methane, or released to the atmosphere as CO2 in addition to that emitted in producing the energy to drive these reactions.

    How significant this is can be seen from wikipedia:

    "The Haber process now produces 500 million short tons (454 million tonnes) of nitrogen fertilizer per year, mostly in the form of anhydrous ammonia, ammonium nitrate, and urea. 3–5% of the world's natural gas production is consumed in the Haber process (~1–2% of the world's annual energy supply).  In combination with pesticides, these fertilizers have quadrupled the productivity of agricultural land:

    "With average crop yields remaining at the 1900 level the crop harvest in the year 2000 would have required nearly four times more land and the cultivated area would have claimed nearly half of all ice-free continents, rather than under 15% of the total land area that is required today."

    Due to its dramatic impact on the human ability to grow food, the Haber process served as the "detonator of the population explosion", enabling the global population to increase from 1.6 billion in 1900 to today's 7 billion. According to Howarth (2008), nearly 80% of the nitrogen found in human tissues originated from the Haber-Bosch process. Since nitrogen use efficiency is typically less than 50%, our heavy use of industrial nitrogen fixation is severely disruptive to our biological habitat."

    (My emphasis)

    As a rule of thumb, if 80% of nitrogen in human tissue originates from the Haber-Bosch process, then without that process the sustainable population will drop by 80%.

    Clearly the hydrogen in the Haber Bosch process could be collected by catalyctic processing of water.  That would need to be made significantly more economicly efficient, however, to compete with current methods of production.  One of the reasons we should rapidly convert electricity production to carbon neutral methods is just to allow more time before we need to modify the industrial manufacture of fertilizer, both in terms of emissions, and in terms of the availability of methane.

  25. Ah, Haber and Bosch, the two most influential people you've never heard of!

  26. Your argument is not wrong.  It does, however, fail to account for the population explosion from 1800 thru today (from approximately 1B persons to now over 7B persons).  While the CO2 is in a different form when exhaled from the human body (roughly 5-6% of total exhaled volume), it requires time for each molecule of CO2 to be absorbed and returned to plants, oceans, etc.  How much time is actually a variable based on numerous factors.  That is one of the primary changes that has taken place over the last 200 years.  Now with that said, should we all (including China and India) be responsible with how we manage our common resources?  Of course!  Let's just not take the approach that some have taken for the sake of publicity, wealth and fame (we all know who I'm talking about).  Rather, let's work together to ensure our home can be enjoyed for all the years to come.

    The other huge factor that I don't have time to go into depth about today are solar cycles.  It's a very big deal and it should be included in all our equations when we responsibly discuss global climate conditions.  Here's the bottom line - we need to learn as much as we can about the things that affect our environment.  But none of us have a handle on the enormity of components that make up the final equation.  Responsibly pursuing knowledge (not reacting to actors and politicians) is where we will find our long term solutions.  Let's start there and see how we do.


    [TD] The counterargument to the myth does indeed account for the increased population's CO2 exhalation, because the increased population's food grown also has increased to feed those people.  But you are correct that increased population increases net greenhouse gas emissions, because of large fossil fuel use to produce, process, transport, and cook/prepare the food for consumption, and in some cases replacement of carbon-sequestering plants (e.g., old forests) with cropland. 

    Regarding the Sun:  The Sun's "cycles" indeed are included in all our equations.  As a start, read the counterargument to the myth "It's the Sun."  After you read the Basic tabbed pane there, read the Intermediate and then the Advanced tabbed panes.  If you want to comment on that topic, do so over there, please.

  27. hlpump @26, Vitousek et al (1986) calculated that with a population of 5 billion, and assumed global average caloric intake of 2500 kcal per person, per day, that humans directly consume 0.76 Pg of organic material (0.35 PgC) annually.  The global populatin has since expanded to 7.2 billion, and and global average caloric intake is now estimated as 2940 kcal per person per day (2015 estimate).  Scaling accordingly, humans now directly consume 1.29 Pg of organic material (0.59 PgC) per annum.

    That represents just 0.5% of terrestial (not global) net primary activity, and 10.6% of emissions from fossil fuel use and cement manufacture.  Of course, all of that Carbon is drawn from the atmosphere originally, as noted in the OP.  You argue that the increase (< 0.025% of net terrestial productivity, and < 0.53% of antropogenic industrial emissions) represents a true increase in emissions.  However, the CO2 emitted in human respiration is still drawn from the atmosphere first by photosynthesis.  Therefore, the direct effect of the increase in human population is only to sequester an amount of carbon equal to the amount of carbon in the bodies of the additional population.

    TD (inline to your comment) notes that the impact of increased human population is to increase substantially anthropogenic emissions both through industrial (fossil fuel and cement manufacture) and non-industrial (Land Use Change) emissions.  That is correct.  Indeed, the increased sequestration in human bodies is almost certainly exceeded by reduced sequestration in forests.  However, all of those changes are already included in the accounting of anthropogenic emissions.  They are not additional, unaccounted for changes.  And they are not changes from human respiration.  

  28. Tom's statement that

    "Therefore, the direct effect of the increase in human population is only to sequester an amount of carbon equal to the amount of carbon in the bodies of the additional population."

    is key. I was going to point this out, but taking a quick glance over the existing comments I see that I already made such a comment two years ago, at #18.

    Ignore the fluxes in and out - the change in storage is all you need to look at to know if humans population growth is a biological carbon source or sink.

  29. Bob Loblaw @28, using the 3 billion increase in human population since the 1970s, and the global average adult mass of 62 Kg, we can calculate that human population growth represents a sequestration of not more than 0.034 Gigatonnes Carbon.  That is, it represents less than 0.0007% of anthropogenic emissions over that period, and indeed, less than 0.6% of annual industrial emissions.  The numbers are irrelevant except as trivia, but when you work them out it becomes absolutely plain that all these "objections" to AGW have never been worked out.  They are mere thought bubble objections - and yet they are treated seriously by many so-called skeptics of AGW.

  30. Tom:

    No dispute about the significance of the increased carbon sequestration.

    The storage approach to determining the source/sink question has a huge advantage over the flux approach, however. The flux approach can be argued to have relatively large error bars on individual components, which make it difficult to determine the net result when the individual fluxes are much larger than the change in storage. The storage approach is a direct measure of the net result.

    As an objection, the "humans breathing" is an utter fail.

    The same bogus argument is applied to the denial that the rise in atmospheric CO2 is the result of burning fossil fuels, when pseudoskeptics compare the fossil flux flux to natural fluxes. The "humans breathing out CO2" fails for the same mass balance reason you expressed in your Climate Change Cluedo post a couple of years ago.

  31. Of course, increasing human population requires occupation of land which would otherwise have been occupied by other life forms. The 'extra sequestration' in humans would thus also be offset by a decreased sequestration of carbon in non-human life forms. The net result would vary by the type of land taken over by the human population, but (as Tom noted) the values in play are so small that the entire exercise is meaningless.

  32. I'm sorry to bring up this subject again, but I've only just been introduced to this platform. I have had no previous opportunity of discussing whether " Breathing contributes to CO2 buildup "?

    When we replace forests bij vegetable fields a lot less carbon is stored in vegetable fields than in forests. The forest canopy is higher and permanent while vegetables are also a seasonal crop.

    The differrence in carbon storage has been burned and is introduced into the atmosphere. Therefore 7 billion people needing more crops than 4 billion people will subsequently have displaced carbon from storage in forests to the atmosphere. This is added to the GHG. Breathing will not further increase the amount of GHG as we have to keep growing crops to keep the cycle in balance.
    So yes there should be more GHG and I think you can feel this in your bones. But its in facilitating our food that crops are a far less functional temporary carbon sink.
    And breathing does NOT contribute to a buildup of CO2 as we will allways have to cultivate new crops to ensure a continual supply to enable future bre

  33. And of course I agree with what I have read somewhere that the extra 3 billion people are an extra sink of Carbon.

  34. Many of the above statements make sense within their own context.

    But just to ask a few more questions while making one or 2 Vital observations

    ALL C02 was here at some point thats how the fossil fuels were created by absorbing C02 - creating Oxygen, becoming fossil fuels and the massive population now burns them in every activity we do to keep the population increasing feeding a flawed econimic model that requires continual growth.

    The average tempeture is not the real issue, its the return to climate extreme´s that are causing panic amongst political powerhouses as it will impact on the their Economic gravy train.

    All C02 - Oxygen - organic matter, fossil fuels etc are part of the Carbon cycle over geological time.. nothing is exempt...

    Question...has any calculation or consideration been taken regarding the effect on temp that 7 Billion people and the remaining living creatures have on recorded or calulated temp.. we carry a lot of heat.

    Humans we ...

    Store a lot of water in our bodys and plastic bottles

    We are mobile heat cells that are warm blooded

    we displace a lot of air while creating hotspots.

    Unless we find a economic model that can work with natural population shrinkage we will never reduce the effect we have on the carbon cycle...

    The Climate will change as it always has and always will. with or without us.

    We need to adapt to a new way of living with each other and the planet we are guests on...

  35. CaptHillWalker

    Firstly, the heat our bodies produce is actually stored energy from the sun, captured by photosynthesis in plants, perhaps also transferred to animals. We then eat the plants and animals to generate the heat that comes out of our bodies. And this by and large isn't ancient sunlight; it was captured over the previous year, maybe several years in the case of large food animals such a cattle. So we are energy neutral in terms of our metabolisms - we just store sunlight for short periods then release it again.

    Similarly we are carbon neutral in terms of our metabolisms - we just store carbon captured from the atmosphere for short periods then release it again.

    Our burning of fossil fuels is a different matter - that is releasing energy & carbon captured 100's of millions of years ago.

    Some numbers to put these energy quantities into context:

    The human body, on average, consumes energy at around 100 watts. More when we are exerting but that is the average. So 7 billion humans consume energy at around 0.7 trillion watts.

    Worldwide energy consumption by our technologies in contrast is around 17 trillion watts. 24 times as much. Each human being has the equivalent of 24 slaves. For those in the developed world that figure is closer to 100 times; 100 slaves.

    Next, total energy flow from within the Earth, all geothermal heat, is around 44 trillion watts. So all human energy generation is around 40% of this. If human energy generation continues to grow at its long term growth rate, irrespective of whether that is from fossil fuels, renewables, nuclear, whatever, then by mid century human energy generation will have grown to match that.

    Then if we look at the measured buildup of heat in the climate system, primarily in the oceans, due to climate change, we are seeing a rise at a rate of around 300-350 trillion watts.

    • 7 to 8 times geothermal
    • 20 times total human energy generation
    • 500 times human metabolism which is energy neutral anyway.

    So the impact of anything purely metabolic is much, much smaller. And it is energy neutral anyway even if it is small

  36. I wonder if the treatment of human respiration here has ignored a couple of points. Two things to think about:

    1) Our collective breathing is a year-round phenomenon that is localized to inhabitted areas, whereas plant conversion of CO2 from the atmosphere to organic matter is seasonal and localized to less densly inhabited areas; the locales and anual timing of our breathing are different to the locales and seasonal resperation of growing plants. So that could mean an uneven distribution of atmospheric CO2, even if it is relatively transient due to weather patterns.   Is the warming effect of such transient areas of higher CO2 concentration more signficant than the assumed warming effect of plantery average CO2 concentration?

    2) Imagine a sealed, underinflated balloon containing water and air. If we heat the ballon up, the same amount of water still exists within it, but now more of the water is in the form of water vapour, not liquid water. There is still a steady state exchange of water vapour to liquid water and back, but more of that water is in vapour form when it is heated up compared to the oringinal underinflated balloon. Similarly, as breathing organisms, we have increased the volume of atmospheric CO2 because there are more of us breathing now than 200 years ago. If 5 or 6 billion of us stopped breathing permanently, then yes, plants would relatively quickly collect the atmospheric CO2 and return it to the soil as humus or store it in woody material. But as long as we keep breathing, we are like the heated balloon - more C is in gaseous form (CO2), and less is stored as organic matter.  And it may not be correct to state that our bodies compensate by sequestering CO2 in our own organic flesh and blood because we ourselves are not static - we grow and then we die, so in addition to the billions of us living, there are also billions of us decomposing, and as cemetary availability decreases, we will increasingly shoose cremation which releases our stored carbon instantaneously.

    So overall, I think the question of whether our breathing contributes to GW is still open.  I'm more inclined to think that it does contribute directly, and that contribution is not insignificant, yet other factors such as lifestyle are greater contributors.  

    Perhaps someone can do some numbers just for fun.  Assume: 1) all the current atmospheric CO2 due to human breathing can be approximated by five years of human exhalation - this is based on the arbitrary idea that over 5 years, global plant communities could convert our hot air to organic matter; and 2) all that human-respiration CO2 captured by plants would be held indefinitely out of the atmosphere due to the undisturbed process of plant growth and soil growth that would take place if we all stopped breathing.  With these assumptions, how much atmospheric CO2 do 7 billion humans generate over a period of 5 years, and how does that amount compare to the current total atmospheric CO2 quantity?

    Note: just a quick point on food production techniques. Soil degradation and subsurface compaction result in less organic matter (humus) in the soil, thus C that was previously stored in the soil has been and continues to be released as a consquence of our agricultural methods.

  37. rjs

    You are making this too complicated.  Human metabolism can only be a net source of  CO2 if human biomass decreases.  Its conservation of mass.

    But of course human biomass is actually increasing, so if anything human tissues are a net sink of CO2 from the atmosphere - i.e. we are taking CO2 up from the atmosphere into our bodies on the whole.

    Also, the amount of biomass in human tissues is miniscule (<0.1Gt C by my calcs) compared to that in trees and soil anyway (2000-3000 Gt C) or in the atmosphere (~800 Gt C), so any change in human biomass has little effect on the atmospheric CO2.

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