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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)

At a glance

Have you heard of Earth's carbon cycle? Not everyone has, but it's one of the most important features of our planet. It involves the movement of carbon through life, the air, the oceans, soils and rocks. The carbon cycle is constant, eternal and everywhere. It's also a vital temperature control-mechanism.

There are two key components to the carbon cycle, a fast part and a slow part. The fast carbon cycle involves the seasonal movement of carbon through the air, life and shallow waters. A significant amount of carbon dioxide is exchanged between the atmosphere and oceans every year, but the fast carbon cycle's most important participants are plants. Many plants take in carbon dioxide for photosynthesis in the growing season then return the CO2 back to the atmosphere during the winter, when foliage dies and decays.

As a consequence of the role of plants, a very noticeable feature of the fast carbon cycle is that it causes carbon dioxide levels to fluctuate in a regular, seasonal pattern. It's like a heartbeat, the pulse of the Northern Hemisphere's growing season. That's where more of Earth's land surface is situated. In the Northern Hemisphere winter, many plants are either dead or dormant and carbon dioxide levels rise. The reverse happens in the spring and early summer when the growing season is at its height.

In this way, despite the vast amounts of carbon involved, a kind of seasonal balance is preserved. Those seasonal plant-based peaks and troughs and air-water exchanges cancel each other out. Well, that used to be the case. Due to that seasonal balance, annual changes in carbon dioxide levels form regular, symmetric wobbles on an upward slope. The upward slope represents our addition of carbon dioxide to the atmosphere through fossil fuel burning.

Fossil fuels are geological carbon reservoirs. As such, they are part of the slow carbon cycle. The slow carbon cycle takes place over geological time-scales so normally it's not noticeable on a day to day basis. In the slow carbon cycle, carbon is released by geological processes such as volcanism. It is also locked up long-term in reservoirs like the oceans, limestone, coal, oil or gas. For example, the "37,400 billion tons of 'suspended' carbon" referred to in the myth at the top of this page is in fact dissolved inorganic carbon in the deep oceans.

Globally, the mixing of the deep ocean waters and those nearer the surface is a slow business. It takes place over many thousands of years. As a consequence, 75% of all carbon attributable to the emissions of the industrial age remains in the upper 1,000 m of the oceans. It has not had time to mix yet.

Fluctuations in Earth's slow carbon cycle are the regulating mechanism of the greenhouse effect. The slow carbon cycle therefore acts as a planetary thermostat, a control-knob that regulates global temperatures over millions of years.

Now, imagine the following scenario. You come across an unfamiliar item of machinery that performs a vital role, for example life support in a hospital. It has a complicated control panel of knobs and dials. Do you think it is a good idea to start randomly turning the knobs this way and that, to see what happens? No. Yet that is precisely what we are doing by burning Earth's fossil fuel reserves. We are tinkering with the controls of Earth's slow carbon cycle, mostly without knowing what the knobs do - and that is despite over a century of science informing us precisely what will happen.

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

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 a range of natural processes, and absorbed by others. The carbon cycle is the cover-all term for these processes. It has both fast and slow components.

In the fast carbon cycle, natural land and ocean carbon remains roughly in balance and has done so for a long time. We know this because we can measure historic levels of CO2 in the atmosphere both directly, in ice cores and indirectly, through proxies. It's a seasonal response to things like plant growth and decay.

In stark contrast to the fast carbon cycle, the slow version operates over geological time-scales. It has affected carbon dioxide levels and therefore temperatures throughout Earth's history. The reason why the slow carbon cycle is so important is because many of the processes that lead to long-term changes in carbon dioxide levels are geological in nature. They take place over very long periods and do so on an erratic basis. The evolution of a species that has deliberately disturbed the slow carbon cycle is another such erratic event.

Annually, up to a few hundred million tonnes of carbon pass through the slow carbon cycle, due to natural processes such as volcanicity. That's small compared to the fast carbon cycle, through which some 600 billion tonnes of CO2 pass to-and-fro annually (fig. 1). However, remember that the fast carbon cycle is a give-and-take seasonal process. The slow carbon cycle instead runs in one direction or another over periods typically measured in millions of years.

Global carbon budget

Fig. 1: Schematic representation of the overall perturbation of the global carbon cycle caused by anthropogenic activities averaged globally for the decade 2012–2021. See legends for the corresponding arrows and units. The uncertainty in the atmospheric CO2 growth rate is very small (±0.02 GtC yr−1) and is neglected for the figure. The anthropogenic perturbation occurs on top of an active carbon cycle, with fluxes and stocks represented in the background. Adapted from Friedlingstein et al. 2022.

Through a series of chemical and geological processes, carbon typically takes millions of years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. Because of these geological time-scales, however, the overall amount of carbon involved is colossal. Now consider what happens when more CO2 is released from the slow carbon cycle – by digging up, extracting and burning carbon from one of its long-term reservoirs, the fossil fuels. Although our emissions of 44.25 billion tons of CO2 (in 2019 - source: IPCC AR6 Working Group 3 Technical Summary 2022) is less than the 600 billion tons moving through the fast carbon cycle each year, it adds up because the land and ocean cannot absorb all of the extra emitted CO2: about 40% of it remains free.

Human CO2 emissions therefore upset the natural balance of the carbon cycle. Man-made CO2 in the atmosphere has increased by 50% since the pre-industrial era, creating an artificial forcing of global temperatures which is warming the planet. While fossil-fuel derived CO2 is a small component of the global carbon cycle, the extra CO2 is cumulative because natural carbon exchange cannot absorb all the additional CO2. As a consequence of those emissions, atmospheric CO2 has accumulated to its highest level in as much as 15 to 20 million years (Tripati et al. 2009). This is what happens when the slow carbon cycle gets disturbed.

This look at the slow carbon cycle is by necessity brief, but the key take-home is that we have deeply disturbed it through breaking into one of its important carbon reservoirs. We've additionally clobbered limestones for cement production, too. In doing these things, we have awoken a sleeping giant. What must be done to persuade us that it needs to be put back to sleep? 

Cartoon summary to counter the myth

Cherry picking

This Cranky Uncle cartoon depicts the "Cherry picking” fallacy for which the climate myth "Human CO2 emissions are small" is a prime example. It involves carefully selecting data that appear to confirm one position while ignoring other data that contradicts that position. Source: Cranky Uncle vs. Climate Change by John Cook. Please note that this cartoon is illustrative in nature and that the numbers shown are a few years old.

Last updated on 17 September 2023 by John Mason. View Archives

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Argument Feedback

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

Further reading

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).

Denial101x video

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


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Comments 26 to 50 out of 51:

  1. @#4: "Oceanic CO2 release decreases the acidity of sea water and carbonate fixing biota do better and lock up more CO2 allowing more CO2 to enter the oceans." This is wrong. CO2 absorbed by water generally INCREASES the acidity, thus lowering the ability of organisms to secrete carbonate. And where they do secrete it, it dissolves more readily once they are dead. The only saving grace here may be that calcium carbonate has an inverse solubility relative to temperature, i.e. as temperature goes up, solubility decreases.
  2. The "World GHG Emission Chart" is great, but I have to wonder: where does air-conditioning of cars, homes and commercial buildings fit in? If it was meant to be under "Other combustion", it sounds too small.
  3. Gincko....I think you misread the post....."if CO2 is RELEASED from ocean water, the acidity DECREASES.." that's what I said and that's what you said.. so the general ph declines, and biota do better and lock up CO2 as carbonate further diminishing dissolved CO2. Since the oceanic CO2 release is due to T rising, less atmospheric CO2 is absorbed so keeping ph down.. balanced by a diminution in solution which causes more atmospheric CO2 to dissolve.....and round it goes until T drops.
  4. Given that human CO2 emissions are significant why are we not discussing the elephant at the cocktail party? The world population is doubling every fifty years and the per capita CO2 output is nearly constant. Even casual inspection of the emissions flowchart makes it clear that bicycling to work and switching to LED lighting is just so much mental masturbation. Failure to confront exponential population growth is fatal. Could we at least have birth control changed from a sin to a sacrament?
  5. It's not an elephant FredT, it's a sacred cow. The more 'advanced' nations are showing a decline in birth rate that already threatens the continued viabilty of the indigenous population, and so to 'fill the gap' have to rely on immigration to maintain the society. In order to get people to produce less children you have to deal with a number of problems, not least is their standard of living. It's a complex subject, frought with difficulties - but you're right, deal with overpopoulation and the 'global warming problem' will fade away.
  6. @chris and mizimi Re: posts 14-17: I googled 'dynamic equilibrium' and was easily able to find not only many definitions but also a host of examples from biology and physics where it applies. I'm no expert on scientific communication, but I think it is critical to have a common conceptual and terminological corpus in order to exchange information with any degree of efficiency. It seems that many of the posters here rely on Chris to provide a rudimentary overview of the science (I mean the stuff that has survived peer-review and been published in reputable journals, not your uncle Jesse's theory of faerie-dust driven tropospheric warming, which he posted last Sunday after a few cold ones on some random website somewhere). There's nothing necessarily wrong with that, unless said posters are arguing passionately that the mainstream science is wrong. After all, what better way to undermine your own credibility than to take a vigorous stand against a position that 1. you do not really understand and 2. is supported by 90% of experts in the field - people who do actually do understand the science? On what basis can you disagree with the majority of professional scientists in a given discipline if you don't even have a handle on something as elementary as the terminology they use?
  7. This is a very good article, and it and comments answered to some of my questions. I hope you would post this content also in Wikipedia's [Carbon cycle] article, and link the article in wikipedia back to this page.
  8. To extend the 13C record back from 1981 see: Source: Zhao, X.Y., Qian, J.L., Wang, J., He, Q.Y., Wang, Z.L., Chen, C.Z. (2006). Using a tree ring δ13C annual series to reconstruct atmospheric co2 concentration over the past 300 years. Pedosphere,, 16(3), 371-379.
  9. The graph from New Scientist is a little bit rough. Isn't it funny that photosynthesis and respiration add for the exact same amount (pre industrial)? Do those figures have an error margin? how much? more than man-made emissions?. It looks a little bit like the pretended carbon neutrality of bio-fuels. A simplistic assumption that has provoked more CO2 emissions than fossil fuels. I can develop but Johnshon does it beautifully for me [Johnson, E. (2009) Environmental Impact Assessment Review, 29, 165-168]
  10. Hernandeath, the graphic states a balance which can be deduced from the atmospheric record. If the flows were not in balance, the atmosphere would not have kept roughly the same amount of CO2 for millenia. Now there may have been some give and take between land and sea but that does not change the conclusions. The systems have evolved towards a balance. The CO2 in the atmosphere is relatively tiny. Visualized as water, our atmosphere is about the same mass as 10m of water spread evenly. Out of that (by weight) the CO2 is currently about 6mm thick. Visualize a layer of glass (the greenhouse!) spread evenly. Now, it is easy to see this is tiny compared to the amount of carbon locked up in fertile soils, forests, or seas with carbonate rich muds. If those ecosystems were not finely balanced the atmosphere would have major fluctuations. But, before human large scale agriculture and industry, the records are of long constancy. And, really not so surprising that a mass of human activity reshaping our environment has produced a rapid change in the atmosphere - from bubbles in the Vostok ice cores, it seems we have produced a spill larger than any in a million years. So the New Scientist graphic may simplify, but it is basically the inescapable conclusion. The world has operated in rough CO2 balance, and we are the biggest change in the equilibrium for a very long time.
  11. Lord Monckton is quoted as saying that if every nation were to cut emissions by 30% over the next 10 years, "the warming forestalled would be 0.02 degrees celsius, at a cost of trillions". Is this true?
  12. Your statement, "atmospheric CO2 is at its highest level in 15 to 20 million years (Tripati 2009)" is not justified by the reference. The Tripati et al CO2 time series estimates do not have the time resolution to say if any millennium's CO2 concentration might have exceeded the current levels of 2010. The time-averaging inherent in their technique will mask the peaks and valleys of CO2 concentration that occur in time periods shorter than their time resolution (which, according to Figure 2A/B, varies between roughly 100,000 and 1000,000 years). You could say that Tripati et al suggest that current levels are higher than the average of the last 15~20 million years.
  13. Fimblish wrote: Lord Monckton is quoted as saying that if every nation were to cut emissions by 30% over the next 10 years, "the warming forestalled would be 0.02 degrees celsius, at a cost of trillions". Is this true? It's not clear what Monckton even means by that. Does he mean that we cut the total 2010-2020 emissions by 30%, but then for the rest of the century our emissions are back up to the "business as usual" trend? If so, the reduction in warming would be relatively small. But that's an absurdly unrealistic scenario. If he's talking about gradually reducing emissions starting in 2010 by enough to put us 30% below BAU in 2020, then staying 30% below the BAU trend for the rest of the century, then he's wrong -- that would yield a much, much greater reduction in warming than 0.02C. In my experience, many people dramatically overestimate the difficulty of changing course while also underestimating the impacts. See Pacala and Socolow (2004) for a good demonstration that effective reductions in CO2 are very feasible, or google "stabilization wedges".
  14. arthuredelstein, are you aware of a natural process that pours so much CO2 in the atmosphere in such a short time? I don't know any and none has been seen from when the time resolution of paleo data is good enough (hundreds thousands years). We can make any hypothesis, but it needs to be supported by facts or known science.
  15. That carbon cycle from the IPCC AR4 graphic? Looks a bit different from another one from our friends at the UN: In short, how did 6 gigatonnes a few years ago now become 26 gigatonnes of human CO2 releases?
    Response: The UN graphic uses units of carbon. I use units of carbon dioxide. The difference is fairly simple - 1 gigatonne of carbon equals 3.66 gigatonnes of carbon dioxide. I explain the conversion process in more detail at Comparing CO2 emissions to CO2 levels.
  16. I'm looking for a rough estimate of net human CO2 emissions as a percentage of net natural emissions. (I know. Meaningless. But I'm checking a claim by a respected climate scientist who thought it worthwhile to scare some NZ brewers with such an estimate. His was 10%.*) This page looked like a likely source but I can't get your numbers to behave. Please tell me what I'm doing wrong. Net(?) human emissions: 29 Gt Net natural emissions: (220+220+332)-(450+338-0.4x29) = -4 Gt Which gives a net annual increase of 25 Gt. That's nearly twice the number you quote in your 'What the science says...' section and five or six times times the number offered by the Mauna Loa observatory. (+2 ppm CO2 pa is about +4 gigatonnes CO2, no?) What's occurring? *Salinger actually wrote that 'Human inputs are about 10% of the natural cycle', which is gibberish. If he meant 'about 10% of natural inputs', he's clearly wrong. If he meant 'net human inputs are about 10% of net natural inputs'... That's what I'm trying to find out. Incidentally, in the same presentation he also claimed that 'Human energy use [is] nearly half of total solar input to Earth'. He was off by about four noughts with that one. Or is it three? Enough to get him sacked, anyway. I dunno Alarmists!
  17. Vinny Burgoo writes: Which gives a net annual increase of 25 Gt. That's nearly twice the number you quote in your 'What the science says...' section and five or six times times the number offered by the Mauna Loa observatory. (+2 ppm CO2 pa is about +4 gigatonnes CO2, no?) You might be making the same error that oracle2world made in the comment immediately preceding yours. According to CDIAC, "1 ppm by volume of atmosphere CO2 = 2.13 Gt C" But 1 GT C = 3.67 GT CO2. So +2 ppm a^-1 is about +15.6 GT CO2.
  18. I'm having trouble reconciling the values presented in this article vs the CO2 amount measured in: and (cited by the CO2 article in wikipedia) They are orders of magnitude different! Am I missing something here?
    Response: What I'm displaying in my carbon cycle graph is the flux of carbon dioxide. What you're looking at in the CDIAC graph is the flux of carbon. To convert carbon to carbon dioxide, you multiply by 3.66 (I explain the process in more detail here - and actually use the CDIAC data from your link). So for example, the CDIAC graph finds that our current rate of CO2 emissions is around 8000 million metric tons of carbon. This is around 8 gigatonnes of carbon which equates to 29 gigatonnes of carbon dioxide.

    I opted to use units of carbon dioxide in my carbon cycle graph because I thought it would be less confusing - people relate to carbon dioxide emissions, not the carbon element of the carbon dioxide molecule. I've regretted it ever since because the convention is to use carbon and hence much confusion has ensued. I will update my carbon cycle graphs with units of carbon sometime down the track (when I get the time).
  19. nocompromise, not sure what numbers you're looking at. The data you link are fossil fuel carbon emissions which correspond to the data shown in fig.2 here. Where is the orders of magnitude difference? If instead you need to reconcile fig. 1 (29 GTons) and 2 (8 GTons), it's due to the diffent mass of C and CO2, a factor of 3.6.
  20. It appears that 'billion tonnes' and GTons are interchangeable?
  21. nocompromise, yes, G (Giga) is a prefix. There are many more indeed.
  22. I am please to find this site as I have been working on building up a Balance Sheet and "C" Flow for the period 2000 to 2010 and you have filled in some gaps. It seems to me that ocean temperatures must be rising. If they were static then the oceans would absorb any amount of CO2 due to equalisation of partial pressures given thay there is 50 times as much CO2 in the oceans as in the air. How much has the average ocean temperature changed from 2000 to 2010.
  23. You have referred me to "working out climate sensitivity by satilite measuements" as a response. While it is not conclusive on most points it is conclusive on the fact that no one has a handle on global sea temperatures. There seems to have been a concensus developed that average atmospheric temperatures have increased by 0.7C over the last century but there is none on average seawater temperatures. The reason I am interested is that on an holistic basis it seems that the solubility curve of CO2 would require the oceans to give up 4% of their CO2 for a 1.0C temperature increase. ie it would take a 0.03C increase in average seawater temperature from 2000 to 2010 to explain the 43Pg's/GT's increase of atmospheric carbon over that ten year period.
  24. Just a matter of sematics but I have a problem calling most of the carbon sinks "sinks". To the lay person, a "sink" implies an essentially non-reversible storage system. In other words, once the carbon is absorbed into a "sink", it will never come out. In reality we know that there are very few essentially irreversible carbon storage systems out there. Rather most of what we call "sinks" are very reversible and are indeed one of the reasons why our system has a feedback to rising temperatures (e.g., increased methane production from bog, release of methane from thawing permafrost, increased release of methane from ocean methane hydrate deposit, etc.). For clarity, I would suggest that we start calling reversible carbon storage systems "reserviors" and reserve the term "sinks" to only those systems that are essentially irreversible (e.g., deposition of carbon to deep ocean sediments).
  25. thpritch #49 "To the lay person, a "sink" implies an essentially non-reversible storage system. In other words, once the carbon is absorbed into a "sink", it will never come out. In reality we know that there are very few essentially irreversible carbon storage systems out there." Really, than why do we now have coal and oil to burn, and how by not burning them will we beable to prevent the release of CO2? If these sinks are essentially non-reversable, and the same mechanizms that produced these fuels are currently going on today.

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