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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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Climate Hustle

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 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 skeptickev. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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.

It's not particularly relevant to this argument but World Resources Institute have posted such a great resource, I had to put it somewhere. It's 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 301 to 301 out of 301:

  1. Uhhhh....  Thanks for burying me in reading material! :)

  2. Tom Dayton @ 297: "Your understanding that "the current warming cycle is releasing more naturally sequestered carbon into the atmo than mankind is emitting" is incorrect."

    There is an argument that warming is forcing carbon release. My understanding of the argument as simply phrased above is correct. That doesn't mean I'm a proponent of that argument.

    Tom Dayton: "The amount we release is enough to outstrip the abilities of the natural sinks to absorb it."

    That is also my understanding of this argument. From the 'intermediate pane':

    "Therefore human emissions upset the natural balance, rising CO2 to levels not seen in at least 800,000 years."

    Obviously, temperatures, ocean levels, and CO2 concentrations have varied over the millenia. Because that was the case then, doesn't mean that humans now are or are not forcing the climate beyond what is thought to have been a natural balance.

    There is no question in my mind that humans have burnt off a lot of fossil fuels that otherwise would have stayed in the ground. The climate will seek a new balance, but that new balance would also include warmer temperatures and different coastlines, among several other effects.

    I looked at:

    "But in today's world, the greatly increased partial pressure of CO2 from fossil fuel emissions causes a flux of CO2 from the atmosphere to the oceans."

    Ai chihuahua. 400 ppm is a "greatly increased partial pressure? As compared to 270 ppm in 1750? The pressure relationship is not defined solely by 400/270. I could use some education on this matter.

    Still, "Hocker begins his analysis by calculating the first derivative of the CO2 data", which doesn't make sense to me either. It seems more like he's hindcasting.

    I also looked at:

    "Caveat: Land use and biomass changes certainly soak up a lot of CO2, some [of] it [is] simply regrowth of forests etc, but the point is that the increasing CO2 in the atmosphere clearly demonstrates that they do not soak up enough." [a small amount of editing for clarity added]

    Woah, in that, the same care in studying carbon sequestration by plant life has not been included in the calculations. My three acres is sequestering more carbon than either an equivalent area in Manhattan or the Sahara. Land based plant life must be included for the sake of accuracy.

  3. John Fornaro: It seems you have overlooked or misunderstood the mass balance evidence of humans being responsible for the rise in CO2. It's just algebra.

  4. John Fornaro @302:

    "400 ppm is a "greatly increased partial pressure? As compared to 270 ppm in 1750?"

    Importantly, the current CO2 concentration is 400 parts per million by volume, ie, ppmv - not parts per million by mass.  That hooks it into a number of important equivalencies.  Specifically:

    1)  pi/p = ni/n where pi is the partial pressure and p the total pressure, and ni the moles of the individual gas and n the total moles of the gas; and also

    2) Vx = Vtot x pi/p = Vtot = ni/n, where Vx is the partial volume and Vtot is the total volume of the gas.

    The second equation is why the ratio of molecules of CO2 to the total number of molecules in dry air is expressed as ppmv.

    It follows from the above that an increase of 42.9% in concentration will result in approximately a 42.9% increase in partial pressure, any slight variation being due to a variation in the total pressure.  That, as the article says, is a "greatly increased partial pressure".

    "Woah, in that, the same care in studying carbon sequestration by plant life has not been included in the calculations."

    The change in plant life is given fairly precisely by the change in C12/C13 ratio once adjustment is made for the contribution of fossil fuels to that change.  It is also given some what less precisely by the change in O2 levels, in that the total change in O2 level, ignoring ocean outgasing, is the original total, minus the amount combusted with fossil fuels, plus the extra amount from CO2 that has been photosynthesized, with the carbon being retained in plant matter.  Detailed local surveys (which have been conducted across a number of ecosystems) are necessary to determine in what form the retained carbon is stored (living plant tissue, or dead plant tissue, or soil organic carbon) but not to determine the total extra amount stored. 

  5. The following study published in Nature, April 5th 2017, shows a 31% ± 5% plant growth since the beginning of the industrial revolution. This would counter the claim that "sinks" are static and cannot process the comparatively tiny increase in carbon emissions due to human activity.

    Large historical growth in global terrestrial gross primary production

    Large historical growth in global terrestrial gross primary production:  "Growth in terrestrial gross primary production (GPP)—the amount of carbon dioxide that is ‘fixed’ into organic material through the photosynthesis of land plants—may provide a negative feedback for climate change1, 2. It remains uncertain, however, to what extent biogeochemical processes can suppress global GPP growth3. As a consequence, modelling estimates of terrestrial carbon storage, and of feedbacks between the carbon cycle and climate, remain poorly constrained4. Here we present a global, measurement-based estimate of GPP growth during the twentieth century that is based on long-term atmospheric carbonyl sulfide (COS) records, derived from ice-core, firn and ambient air samples5. We interpret these records using a model that simulates changes in COS concentration according to changes in its sources and sinks—including a large sink that is related to GPP. We find that the observation-based COS record is most consistent with simulations of climate and the carbon cycle that assume large GPP growth during the twentieth century (31% ± 5% growth; mean ± 95% confidence interval). Although this COS analysis does not directly constrain models of future GPP growth, it does provide a global-scale benchmark for historical carbon-cycle simulations."


    [DB] As others have noted, you will need to furnish a source citation for this claim:

    "This would counter the claim that "sinks" are static"

    Hotlinked DOI.  An openly accessible copy is here.

  6. Pattio,

    Can you provide a reference for your claim that someone says sinks are static?  I am underthe impression that most of the sinks and sources of carbon respond to changes in the environment around them.

    While you article is interesting, it is clear from the measured increase in CO2 in the atmosphere that natural sinks have not been able to absorb all the CO2 humans release.  That may change in the future although it is not clear if the sinks will increase or decrease.

  7. Pattio: as Michael says, please do provide a reference to support your claim that others hold the position that sinks are static.

    The sources that I am familar with (e.g., the IPCC) pretty clearly recognize that about half of what is emitted to the atmosphere (by burning fossil fuels) is abosrbed by the oceans and biosphere (the "sinks"), which directly contradicts two of the claims you made in your opening paragraph:

    1. ...that others claim the sinks are static (unsupported because others feel that sinks have increased to absorb half of what is emitted)
    2. ...that your argument counters the claim that the sinks cannot process the increase in emissions (they can't, as evidenced by the fact that they can only process half, with the other half still residing in the atmosphere).
  8. Pattio: The airborne fraction of CO2 has been fairly constant, despite the growth in the rate of anthropogenic CO2 emissions. Therefore the natural sinks are not static. That determination has been made by scientists who, therefore, do not in reality believe the sinks are static.

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