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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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How do human CO2 emissions compare to natural CO2 emissions?

What the science says...

Select a level... Basic Intermediate

The natural cycle adds and removes CO2 to keep a balance; humans add extra CO2 without removing any.

Climate Myth...

Human CO2 is a tiny % of CO2 emissions

“The oceans contain 37,400 billion tons (GT) of suspended carbon, land biomass has 2000-3000 GT. The atpmosphere contains 720 billion tons of CO2 and humans contribute only 6 GT additional load on this balance. The oceans, land and atpmosphere exchange CO2 continuously so the additional load by humans is incredibly small. A small shift in the balance between oceans and air would cause a CO2 much more severe rise than anything we could produce.” (Jeff Id)

Before the industrial revolution, the CO2 content in the air remained quite steady for thousands of years. Natural CO2 is not static, however. It is generated by natural processes, and absorbed by others.

As you can see in Figure 1, natural land and ocean carbon remains roughly in balance and have done so for a long time – and we know this because we can measure historic levels of CO2 in the atmosphere both directly (in ice cores) and indirectly (through proxies).

Figure 1: Global carbon cycle. Numbers represent flux of carbon dioxide in gigatons (Source: Figure 7.3, IPCC AR4).

But consider what happens when more CO2 is released from outside of the natural carbon cycle – by burning fossil fuels. Although our output of 29 gigatons of CO2 is tiny compared to the 750 gigatons moving through the carbon cycle each year, it adds up because the land and ocean cannot absorb all of the extra CO2. About 40% of this additional CO2 is absorbed. The rest remains in the atmosphere, and as a consequence, atmospheric CO2 is at its highest level in 15 to 20 million years (Tripati et al. 2009). (A natural change of 100ppm normally takes 5,000 to 20,000 years. The recent increase of 100ppm has taken just 120 years).

Human CO2 emissions upset the natural balance of the carbon cycle. Man-made CO2 in the atmosphere has increased by a third since the pre-industrial era, creating an artificial forcing of global temperatures which is warming the planet. While fossil-fuel derived CO2 is a very small component of the global carbon cycle, the extra CO2 is cumulative because the natural carbon exchange cannot absorb all the additional CO2.

The level of atmospheric CO2 is building up, the additional CO2 is being produced by burning fossil fuels, and that build up is accelerating.

Basic rebuttal written by GPWayne

Update July 2015:

Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 5 July 2015 by gpwayne. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.

Further reading

Both graphs from this page are taken from Chapter 2 of the IPCC AR4 report.

Real Climate goes in-depth into the science and history of C13/C12 measurements.

The World Resources Institute have posted a useful resource: the World GHG Emissions Flow Chart, a visual summary of what's contributing to manmade CO2 (eg - electricity, cars, planes, deforestation, etc).

UPDATE: Human CO2 emissions in 2008, from fossil fuel burning and cement production, was around 32 gigatoones of CO2 (UEA).


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Comments 226 to 250 out of 352:

  1. wsugaimd, The Earth has five short term reservoirs of CO2, and two long term reservoirs. The short term reservoirs are, in order of size: 1) The deep ocean; 2) The soil; 3) The surface ocean; 4) The atmosphere; and 5) The biosphere. Flows between these reservoirs is very large, and very rapid. The result is that soil, surface ocean, atmosphere and biosphere maintain equilibrium on a time scale of around a year, while the deep ocean maintains equilibrium on time scales of a century or so. These reservoirs are shown in this diagram from wikipedia (note, black ink indicates storage, blue ink indicates fluxes): Because fluxes between these reservoirs are rapid, equilibrium is maintained between them. Consequently the normal fluxes between these reservoirs cannot increase the total CO2 (or chemical derivatives such as cellulose) in the aggregate of the reservoirs. What is more, because equilibrium is maintained, except for circumstances which shift the equilibrium they will not result in changes in any particular reservoir. For example, increased aridity will result in less carbon being stored in soils and the biosphere. As a result, more carbon will be stored in the atmosphere and ocean. If it is a short term change in aridity, it will not effect the deep ocean, but if it is a change that lasts for a century or so, the excess CO2 (and derivatives) in the atmosphere and surface ocean will be depleted partially to re-establish equilbrium with the deep ocean. Similarly an increase in sea surface temperatures will result in diminished CO2 storage in the surface ocean, resulting in more CO2 being stored in the atmosphere, soils and biosphere (from the CO2 fertilization effect among other effects). Such changes in SST are responsible for much of the small changes in CO2 concentration prior to 1750 and visible in the graph @224 above. They are also responsible for the strong correlation between ENSO and the short term fluctuations in CO2 concentration as observed by modern instruments. In addition to the short term reservoirs, there are two long term reservoirs. They are sedimentary rocks, particularly lime stones but including coals and other fossil fuels, and the Earth's mantle. Transfer between these reservoirs and the short term reservoirs is normally very slow. This occurs by the formation of sedimentary rocks, which are then subducted to the mantle, which returns the CO2 to the surface through volcanos. Currently the rate at which volcanos return CO2 to the surface is about 1/100th of the rate at which humans emit CO2. In effect, humans have increased the transfer from the long term reservoirs to the short term reservoirs by a hundred fold. Because the short term reservoirs are in equilibrium, it is far more logical to consider that transfer as the cause of increase in CO2 storage which has been measured in all five of the surface reservoirs than to assume that the cause is a transfer from one surface reservoir to another (as you are doing). Indeed, it is only possible to consider the high relative flux from the biosphere to the atmosphere through respiration as the cause of the increase in atmospheric CO2 if you firmly ignore two points: a) The equally large flux from the atmosphere to the biosphere by photosynthesis; and b) The fact that the atmospheric CO2 has been effectively stable for 10 thousand years even in the presence of the large flux from biosphere to atmosphere, which as remained essentially unchanged.
  2. wsugaimd,
    I believe the C02 exhaled by...
    I see this a lot from skeptics.... sentences that start with "I believe." The thing is, of all things, the accounting of CO2 is done and solid. There is no wiggle room there. We know how, we know how much, we have multiple lines of evidence to prove that it cannot have come from any place else. I personally think the most complete link is this one but there are many others. Click the "View All Arguments" link below the thermometer at left, search the the page for CO2, and you'll find that every thing you have brought up has been thought of by others, and holds no weight whatsoever. Please take the time to study the information that is already available. You can "believe" what you choose, or you can educate yourself. The choice is yours, but falling back on your "belief" hardly entitles you to lay claim to the mantle of "skeptic."
  3. Wsugaimd's claim to be coming from a biology background and his apparent lack of understanding of the recycling of carbon through respiration and its role in the fast carbon cycle seem to be fundamentally irreconcilable.
  4. Dr Knorr from the University of Bristol ( showed that the rise in airborne CO2 since 1850 is statistically negligible, and hence that the oceans are absorbing more CO2 than previously thought. The article here states "While fossil-fuel derived CO2 is a very small component of the global carbon cycle, the extra CO2 is cumulative because the natural carbon exchange cannot absorb all the additional CO2." - well apparently it can, and this is what negative feedback means, when the level of CO2 rises, the 'system' reacts to counteract it. "ad hominem comments will be deleted.", unless they are directed against 'skeptics' it seems.
  5. huch44uk @229, the abstract of Kr Knorr's article reads:
    "Several recent studies have highlighted the possibility that the oceans and terrestrial ecosystems have started loosing part of their ability to sequester a large proportion of the anthropogenic CO2 emissions. This is an important claim, because so far only about 40% of those emissions have stayed in the atmosphere, which has prevented additional climate change. This study re-examines the available atmospheric CO2 and emissions data including their uncertainties. It is shown that with those uncertainties, the trend in the airborne fraction since 1850 has been 0.7 ± 1.4% per decade, i.e. close to and not significantly different from zero. The analysis further shows that the statistical model of a constant airborne fraction agrees best with the available data if emissions from land use change are scaled down to 82% or less of their original estimates. Despite the predictions of coupled climate-carbon cycle models, no trend in the airborne fraction can be found."
    The airborne fraction is the increase in atmospheric CO2 as a fraction human emissions. Fortunately for us, as Dr Knorr says, the increase in atmospheric CO2 is currently 40% of total emissions. Where it not for that, CO2 concentrations would not have increased by 100 ppmv since 1850, but by 250 ppmv - with disastrous results. It is widely predicted that as the oceans warm, the airborne fraction will increase, as warm oceans can absorb less CO2. Dr Knorr shows that as yet, evidence of that increase is not yet statistically significant. What he does not show, and does not purport to show, and indeed, explicitly disagrees with, is the claim that "the rise in airborne CO2 since 1850 is statistically negligible". On the contrary, if atmospheric CO2 has increased by 40% of total emissions since 1850 (Knorr's claim), then the increase is about 40% over 1850 values, which is certainly statistically significant. A word to the wise, moderation complaints are normally snipped on this site, and if there is no reason to preserve a comment, moderators will save themselves trouble by simply deleting the entire post. That is particularly the case when the moderation complaint is entirely specious - as yours is.
  6. Hutch: "[Knorr 2009] showed that the rise in airborne CO2 since 1850 is statistically negligible." Knorr said no such thing (see here. Hutch, are you actually reading the articles? Even if you only read the abstract from Knorr, you should understand that what you claim and what he claims are different. He points out that 40% of human emissions stays in the atmosphere. That's significant. Now, is the trend in airborne fraction of human-sourced CO2 increasing? You know what the mass balance argument is, yes? Yes, the natural sinks are trying to absorb the additional CO2, but they're not doing a very good job of it, and, worse yet, they're not going to do a better job in the future, because 1) land use changes will likely involve cutting down carbon-sucking forests and 2) the warmer the oceans get, the less able they are to hold their carbon. We are part of the system, and we are overwhelming the "negative feedback." What ad hominem are you talking about?
  7. Well, that's very instructive, just drop a single word (fraction) as hutch44uk did to flip the results upside down. Hopefully this was a honest mistake and hutch44uk will soon understand how biased was his reading of, maybe, just the title.
  8. "Man-made CO2 in the atmosphere has increased by a third since the pre-industrial era" This is wrong. "Man-made CO2 in the atmosphere" has increased by orders of magnitude since the pre-industrial era. What you mean is "Man-made CO2 has increased CO2 in the atmosphere by [more than] a third since the pre-industrial era"
  9. The statement that "Human CO2 is a tiny % of CO2 emissions" is arguably true but certainly misleading because as this article states, it ignores the fact that though much larger, natural CO2 emissions are balanced by natural CO2 absorption, while human emissions accumulate. However this statement is commonly misconstrued as "human beings produce 3 per cent of the carbon dioxide in the air" (Alan Jones 19 Oct 2012). That statement is not just misleading, it is false, because it concerns atmospheric CO2 levels, not emissions. Humans have increased "the carbon dioxide in the air" from 285 ppmv to 390 ppmv since 1850. That means human beings produced 27% per cent of the carbon dioxide in the air, not 3%. Note that I use the past tense produced while Jones used the present tense produce. Taken literally his statement is meaningless. However his intended meaning is clear: he is falsely claiming that CO2 from human sources is insignificant.
  10. jondoig @ 234 that Alan Jones statement would be hilarious, if it wasn't so serious. It is difficult to accept that people actually believe such untrue statements when they utter them, yet there it is in black and white. I do not imagine for a minute Jones would say this, if he was not confident it was true. It is a sad commentary on our education system that such gross misunderstandings can persist, but that is the rod we make for our backs when we believe in (relative) freedom of speech.
  11. Doug H @235, your faith in Alan Jones is touching.
  12. Tom Curtis @ 236, if I expressed what I really think of Alan Jones and his veracity, the comment would be moderated faster than a speeding bullsh*t bullet.
    Response: [DB] Likely, yes.
  13. While anthropogenic emissions are responsible for 100% of the post-industrial rise in atmospheric CO2, only about 3-4% IIRC of atmospheric CO2 is of directly anthropogenic origin. Thus Jone's statement is technically correct, but deeply misleading. The reason for this is that the vast natural exchange fluxes continually swap atmospheric CO2 (some of which is of directly anthropogenic origin) with CO2 from the oceanic and terrestrial reservoirs. These exchange fluxes have no net effect on atmospheric CO2 levels, but they do mean that relatively little CO2 in the air is of directly anthropogenic origin as it only takes on average 4-5 years for each molecule of CO2 to be exchanged with "natural" CO2. The thing that matters though is not whether the CO2 in the atmosphere is of directly anthropogenic origin, but what is causing total emissions (natural+anthropogenic) to be greater than total uptake, as that is what is causing atmospheric CO2 to rise, and the answer to that is "anthropogenic emissions".
  14. Science Question that I am struggling to find the answer to anywhere. About 50% of Humanity's output of CO2 is absorbed by carbon sinks, partly the biosphere. Obviously that photosynthetic activity uses solar energy to convert CO2 and H2O into carbonates, carbohydrates and (eventually) hydrocarbons. How much solar energy is absorbed in this process? Obviously that absorbed energy will not find its way out of the top of the atmosphere. Is that energy accounted for in the energy budgets and climate models?
  15. Mathew L @239, plants have a low albedo, meaning that an increase in foliage will increase the albedo of of a region. For rough figures, the albedo of desert (land without plants) is 0.4; for grassland and tundra it is 0.25; for deciduous forest it is 0.15-0.18; and for coniferous forest it is 0.08-0.15. That means the presence of plants increases the absorption of solar radiation substantially. Most of the increase becomes waste heat at the point of absorption. The rule of thumb from ecology is that only 10% of incident solar radiation is converted to sugars by photosynthesis. Most of that energy, however, is returned to the environment as waste heat as the sugar is used to power chemical reactions in the plant, or in some animal that has eaten the plant. A vanishingly small amount is fossilized to become a future fossil fuel. Consequently, the presence of plants will overall increase surface temperatures, but will even out surface temperature differences by causing some of the waste heat to be released at night, or early evening or morning when received solar energy is low. It may also cause the waste heat to be distributed over a wider region geographically as animals transport the chemical energy and release it at other locations; but the percentage so carried is small. Climate models certainly account for the change in albedo with changes in vegetation. I am not sure whether, or to what extent they account for the change in timing of the release of energy.
  16. A frequent ignorant comment is that CO2 makes such a tiny component of the atmosphere 'how can it change the climate?" Tiresome as it is I have tried various analogies but the final explanation that at least shuts them up is their acceptance of O3 as a UV shield and its low 8ppm, the other trump is their acceptance of volcanoes changing weather for years after huge eruptions of sulphur yet 20 million tonnes can bring years of poor summers. a tiny co2 link would be helpful to direct sceptics to

    [DB] Per SkS contributor Gary McGuigan, here is another interesting example of how trace concentrations can have a large effect:

    The Brazilian Wandering Spider (Phoneutria) or banana spider appears in the Guinness Book of World Records 2007 for the most venomous spider and is the spider responsible for most human deaths. This spider is believed to have the most potent neurotoxic venom of any living spider. Only 0.006mg (0.00000021oz) is sufficient to kill a mouse.

  17. Julesdingle, I usually go with; 'Without atmospheric CO2 all green plants would die. Without green plants nearly all animals would die. Trace? Yes. No effect? Don't be stupid.' The official SkS writeup for that myth (#76) is here.
  18. Or, consider the effects of the same concentration of HCN in the atmosphere.
  19. I have a question, is there any way of reducing some of the CO2 natural emissions or making other ways to absorve more CO2 preventing it from going to the atmosphere, such as planting more threes or something like that?

    My question is, the only way to stop globar warming is reducing the human CO2 emissions?


    (sorry for my english)

  20. Juanss, due to albedo changes, replacing the world's agricultural areas with trees will not necessarily be of any aid in stopping global warming. Scientist Ken Caldeira has shown that replanting all available boreal forests and even mid-latitude temperate forests will lead to warming.

    Only replanting all tropical areas with trees produces cooling. And there simply isn't enough of it to be effective (an area greater than the surface area of the United States must be replanted and no such sizable area exists). Only a drastic reduction in CO2 emissions will have any effects.

    First Link

    Second Link

    Third Link

    Fourth Link

  21. Following on from what Daniel wrote, it is worth adding that it would be a good idea to try and limit further deforrestation of the tropics, for many reasons, including CO2!

    There have also been attempts at seeding the oceans with nutrients to try and increase uptake by marine biota, but it didn't seem to help much.  The link below discusses a chance experiment following volcanic activity, but I seem to recall this type of seeding being tried deliberately as well.

    At the end of the day, cutting down fossil fuel use is likely to be easier and cheaper for the forseable future.

  22. So how does this work? Do the natural cycles segregate man made CO2 from the natural produced CO2? It is true that nature balances the levels when there are changes in CO2 production, whether it's man made or natural. "Balance" is a variable and it is relative over time. When the CO2 levels increase, the amount of flora increases as well. It's so basic I don't know how you can write such things about non-issues.

  23. Elysium, if you want to discuss science, then I would avoid starting your message "So how does this work?" and ending "It's so basic I don't know how you can write such things about non-issues.", which rather sounds like you have made up your mind about how it works.

    If you think something is a non-issue, then state exactly what you think is a non-issue and why, and maybe someone will discuss it with you.

    It clearly isn't "true that nature balances the levels when there are changes in CO2 production, whether its man made or natural".  If it were, CO2 levels would not have been rising sharply for the last century or so.

  24. Elysium...  If you believe it's "so basic" then you should have no problem citing the published literature that supports the point you're trying to make.

    I would first suggest that you read the comments policy before continuing the conversation.

  25. I am in search of an answer that I have not found in the comments tread or searching other references...relating to the isotope ratio signature that identifies ACO2 emissions. Is there also the signature of O18? The isotope signature that identifies Oxygen combustion either from natural events such as wild-fires or or anthropogenic events such as fuel combustion.

    What I want to know is if there is any signature that has O18 along with a C13-C12 ratio that would show combustion of older Carbon sources. It would appear to this layman that a signature directly related to the combustion of older Carbon would surely be the smoking gun of anthropogenic activity; not that any logical suspision really exist but another brick in the wall surely would be welcome.

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