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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 60% 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). [Paragraph updated July 2022, to correct information on % of additional CO2 that is absorbed.]

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 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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