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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 26 to 50 out of 160:

  1. 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.

  2. 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.  

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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

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

  9. 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...

  10. 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

  11. 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.

  12. 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.

  13. RJS

    I liked your water baloon analogy.

    The debate in some of the climate threads in cyberspace over whether the respiration of 7 million human inhabitants adds significantly to AGW is obviously alive and well, particularly among skeptics. 

    On the face of it the total emissions from animal respiration are considerable but of course we know that the carbon cycle is also a closed system.

    Having given this some thought though I'm of the view that respiration from 7B people must increase atmospheric CO2 if for no other reason than the fact that we're net emitters rather than net absorbers of it, unlike plants. 

    To maintain an equilibrium it's obviously necessary for respired CO2 to be offset by photosynthesis in plants, which requires us to grow synthesising plants at the same rate that we're consuming them.  We all agree on that I'm sure.  

    However, the net effect is to amplify the carbon cycle in much the same way that higher tropospheric temperature amplies the water cycle.

    In the same way that the atmospheric transportation of water increases with temp so too does the transportation of CO2 into the atmosphere from increased animal respiration, resulting in an increase in the static level of atmospheric CO2 that's proportional to the population.   

    Since the laws of physics can't be broken the only way that this is possible is by converting O2 into CO2 and by transporting sequestered carbon (from trees and plants) into the atmosphere. Although humans grow crops to maintain the equilibrium they've needed to remove existing trees and plants to accomplish it, so effectively the amount of carbon that's stored in plants must have decreased as population has increased.  

    To what measnureable extent humans contribute to an altered ratio of OC and CO2 in the atmosphere is impossible to measure but I'm sure that it's dwarfed by CO2 borne from fossil fuels, but nonethless I don't think we can or should claim that human respiration from 7 billion people does not increase atmospheric CO2 to some degree.

  14. Art Vandelay - "...the fact that we're net emitters (of CO2) rather than net absorbers of it, unlike plants"

    Um, No. Aside from our fossil fuel emissions any stable population of organisms is carbon neutral, as the carbon in them comes from the environment and upon death returns to the environment. While carbon-containing food comes from the environment and is itself cycled back as (among other things such as fertilizer) CO2

    Now, if a population increases, there is a corresponding sequestration of carbon in biomass - and in that fashion the 7 billion people on the Earth represent a carbon sink, not a carbon source. Breathing, however, is simply not a net cause of rising CO2

    But then we go and burn fossil fuels that have been sequestered for hundreds of millions of years, and we go straight from carbon neutral to immense net emitters... sigh.

  15. KR @39

    Now, if a population increases, there is a corresponding sequestration of carbon in biomass - and in that fashion the 7 billion people on the Earth represent a carbon sink, not a carbon source.

    But this may be offset by a decrease in other animal populations. Admittedly the rise in human population may have also produced a concomitant rise in domestic animals, however the population of wild animals has crashed over the past century. The population of non-photosynthesising organisms is dependent on the population of photosynthesising ones, and that in turn is dependent on the land mass available to them, along with the "ingredients" for photosynthesis. Of course land use change (specifically de-forestation) has been a significant carbon source, offsetting any increase in other forms of biomass.

  16. Art Vandelay @38, any carbon in animal matter has first existed as carbon in plant matter.  Any carbon in plant matter was in turn first extracted from the atmosphere by photosynthesis.  The food chain may be extended in that, for instance plants (planckton) may be consumed by carnivorous planckton, which may in turn be consumed by planckton eating fish, that may be consumed by low level carnivous fish, that may be consumed by a top predator like Tuna, that may in turn by consumed by humans - but that does not alter the fact that every molecule of carbon in humans was first extracted from the atmosphere by photosynthesis (ignoring plastic based prosthetics).  Therefore your argument fails because it reverses the logical order of the process, assuming in effect that the CO2 in humans comes into existance by a creative act and needs to be then, later extracted by photosynthesis.

    Given this, there are only two ways that human respiration can increase atmospheric CO2.  The first is if the whole cycle ceases, so that CO2 respired is not then taken up by plant matter.  The second is if the whole cycle changes its time constants so that the carbon in the cycle spends relatively less time in biomatter and relatively more time in the atmosphere.  For both of these, because they involve the whole cycle and not just human consumption it is not particularly appropriate to look at it in terms of respiration.

    Of the two methods mentioned above, it is known that the first is not occuring.  The second, however, is occuring, but is already accounted for in carbon budgets under the label of Land Use Change (LUC).  Land Use Change, however, includes a large number of inputs in addition to changes in the relative rate of respiration.  It includes, specifically, deforestation for the timber industry; and deforestation for land clearing (in which the timber is simply waste, and does not enter the human carbon cycle).  These components dominate the LUC budget, so it is not possible to extract from the LUC figures any meaningful estimate of the change in relative times of carbon in the atmosphere from those figures (SFAIK).  

    One thing that is known, however is that net biosphere productivity is a sink for CO2.  That is, once you add up all of the effects of deforestation, changes in agricultural practises, growth of urban areas, draining of swamps etc, and subtract from that the effects of increased plant growth due to increased humidity, the fact that human timber is better protected from decay than natural equivalents, (and hence precipitation) and any carbon dioxide fertilization effect, the total biosphere is absorbing more CO2 from the atmosphere than it is emitting.

  17. Phil @40, below is the most recent IPCC estimate of the various fluxes in the carbon cycle (with changes to the process since the preindustrial shown in red):

    If you look closely you will see that "Gross photosynthesis"  minus "Total respiration and fire" results in a positive flux of 2.6 +/-1.2 Petagrammes Carbon per year out of the atmosphere.  That is an increase over the preindustrial value by more than the 90% uncertainty.  Ergo it is simply false that "Of course land use change (specifically de-forestation) has been a significant carbon source, offsetting any increase in other forms of biomass."  (As an aside, "respiration" in these terms includes natural decay.)

    FYI, these values are known fairly well because they can track the decline in O2 in the atmosphere, which excedes the corresponding increase in CO2 from fossil sources, even after allowing for ocean uptake (CO2) and outgassing (O2).   

  18. @KR "Now, if a population increases, there is a corresponding sequestration of carbon in biomass - and in that fashion the 7 billion people on the Earth represent a carbon sink, not a carbon source. Breathing, however, is simply not a net cause of rising CO2."

    I agree with the latter - that a static human population cannot on its own cause CO2 to rise.

    However, on the former point, 7 billion humans came into existence in a very short time frame in geo terms, and as we know, every carbon atom in every human is plant borne, which means that there must be less plants to sink the 'respired' CO2 with 7 billion people than there was before the 7 billion people existed. IOW, the carbon that was sequestered in plants is now sequestered in humans, and unlike plants human are combustion engines needing carbon and oxygen to produce energy and expelling CO2 as waste in the process.

    Consider the position if only plants existed. Their growth would be ultimately limited by insufficient CO2 in the atmosphere from photosynthesis.

    And now consider paradoxical the position if human population rises to the point where all vegetation is consumed as food. 

  19. Art Vandelay - In terms of the balance of biomass shifting from plants to humans with population growth, carbon has simply moved from one set of organisms to another - and not added to atmospheric concentrations. Again, breathing is carbon neutral, as what is exhaled simply cycles between food and CO2 and back again. 

    Atmospheric concentrations can only have net changes over the long term if more carbon enters or leaves the biological carbon cycle - from volcanic activity (no net long-term change), from weathering (slow drawdown, not relevant over century time-scales), fossil fuel burning (the relevant cause of change over the Industrial Age), etc. Breathing does not, and can not, influence long term atmospheric CO2 concentrations. It's a complete red herring in the discussion of climate change. 

  20. 44@KR

    "Atmospheric concentrations can only have net changes over the long term if more carbon enters or leaves the biological carbon cycle"

    Yes, I agree, so it follows that baseline atmospheric CO2 will increase if either respiration increases or if photosynthesis decreases.  

    See Tom's attached IPCC flux diagram above which shows the exchange of CO2 in and out of the atmosphere.  It shows that total respiration has increased since pre-industrial times - which is interesting.  

    "Breathing does not, and can not, influence long term atmospheric CO2 concentrations. It's a complete red herring in the discussion of climate change."

    Well I do agree that it's a "red herring" in the dicussion of climate change because animal respiration is dwarfed by the burning of fossil fuels but I don't agree that human and other animal respiration cannot alter atmospheric concentations, and as Tom's diagram shows, respiration and fire are the only natural land based inputs into the cycle. 

    My argument isn't that animal respiration is in any way significant as a driver of global climate, but that doesn't mean that via modulation that it cannot or does not alter CO2 in the atmosphere over a given time scale.

  21. Note that CO2 in respiration and in carbon in plant and animal biomass is still in the biological carbon cycle. Increased respiration and CO2 output requires balancing increased food consumption and carbon input into the breathing organisms - changing the speed of carbon cycling between plant and animal, but not adding or subtracting from the sum carbon in circulation. 

  22. "changing the speed of carbon cycling between plant and animal, but not adding or subtracting from the sum carbon in circulation"

    See Tom's diagram. The land based inputs are respiration and fire, so the 'sum carbon in circulation' (as you put it) is always equal to the 'respiration and fire' that's fed into the system.

    However, that aside, I can see why we're in disagreement and it's because we're presenting two different arguments.

    Your proposition (argument) that human breathing (respiration) does not increase C circulation in the cycle refers to a static population, whereas I'm proposing that human population growth increases the input 'C' into cirulation in the system and therefore increases atmospheric CO2.

  23. Carbon in biomass is also "in circulation", which is the primary point I think you're overlooking. And CO2 in respiration comes from food consumption, with food still being part of the carbon cycle: carbon input = carbon output for any organism over both short terms and its entire lifecycle. 

    Side note: if total incorporated biomass has been raised by population growth (which requires an assumption that plant biomass isn't displaced) that sequestration of carbon in organisms can only decrease atmospheric CO2. 

  24. By def, Biomass (animal and plant) is considered to be the total mass within the carbon cycle at any given time so you can't change 'total' biomass, only its composition.  

    But again, the confusion here is due to my misunderstanding the original proposition - which specifically does not refer to human population growth perse. 

    Also, my proposition that population growth increases atmospheric CO2 assumes that it results in increased 'total respiration' and that is not necessarily the case - because human population growth is likely to be at the expense of other species. 

    Nonetheless, it would be interesting to know exactly what effect the rise of human population to 7 billion has had on the carbon cycle.

  25. Art Vandelay @49, biomass is the total mass in animal and plant and microbial matter.  It does not include CO2 in the atmosphere or oceans, nor soil carbon in the form of humus.  This, it is not the total mass of carbon within the carbon cycle and can change over time.

    As to human respiration, you can look at it several ways.

    You can consider just humans and their respiration, in which case yes human respiration increases atmospheric CO2.  Such a view however is transparently incomplete for it does not account for where the human carbon comes from in the first place.  It in effect treats human biology as an ex nihilo creator of carbon.

    So, we add the human carbon input, which comes from food and all of which comes initially from the atmosphere.  You then have a cycle in which the ouput (human respiration) is very slightly less than the input (human consumption of CO2 indirectly drawn from the atmosphere).  It is very slightly less because the total mass of humans in increasing through population growth in the third world and obesity growth in the first.

    You can then look at how the food is produced, and look at effects on total biomass from human food production.  If you do, it is probable that the entire process of human food production reduces biomass, mostly by deforestation.  However, when you do that you are no longer looking at emissions from human respiration alone, but emissions from human respiration plus human food production.  More importantly, the emissions from human food production are already accounted for by estimates of emissions from LUC.  So, looking at that does not find a form of emissions that were not previously accounted for. 

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