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What is causing the increase in atmospheric CO2?

What the science says...

There are many lines of evidence which clearly show that the atmospheric CO2 increase is caused by humans.  The clearest of these is simple accounting - humans are emitting CO2 at a rate twice as fast as the atmospheric increase (natural sinks are absorbing the other half).  There is no question whatsoever that the CO2 increase is human-caused.  This is settled science.

Climate Myth...

CO2 increase is natural, not human-caused

that atmospheric CO2 increase that we observe is a product of temperature increase, and not the other way around, meaning it is a product of natural variation...it may be the Emily Litella moment for climate science and CO2 – “Never mind…” (Anthony Watts)

Simple Accounting

The easiest way to prove that the atmospheric CO2 increase is man-made is through a simple accounting approach (i.e. see Cawley 2011).  The equation for the change in atmospheric CO2 (ΔCatm) is

This says that if we ‘emit’ a ton of carbon by, say, triggering a volcano then the atmosphere will gain a ton. If we ‘absorb’ a ton of carbon by growing a tree, then the atmosphere loses a ton.  We can expand the equation by counting human emissions (HE) and absorption (HA) and natural emissions (NE) and absorption (NA) separately.

This works because carbon is additive. If a volcano emits a ton of carbon and a factory emits a ton then the atmosphere has gained two tons. This is a very simple balance sheet for the carbon cycle and fortunately there are ‘accountants’ who have measured some of these values for us.

Recently the amount of CO2 in the atmosphere has been rising at ~2 parts per million per year, or around 15 billion tons/year. Meanwhile  human emissions excluding land use change (like clearing or planting forests) are 30 billion tons per year. In billions of tons per year we have:

We can rearrange this:

Humans are also clearing rainforests and changing land use, but here we'll assume that human effects on absorption (HA) are not much different from zero, i.e.

So Natural Absorption (NA) must be bigger than Natural Emissions (NE). Nature is absorbing more CO2 than it is emitting. It is not causing atmospheric CO2 to rise at all - in fact it is acting to try and reduce atmospheric CO2, and thus the long term rise is entirely because of humans.

Ocean Acidification

The oceans are the Earth's largest carbon storage medium, so if the atmospheric CO2 increase were "natural", it would likely be coming from the oceans.  But we know the CO2 increase is not coming from the oceans, because the pH of the oceans is dropping (a.k.a. ocean acidification).

When CO2 is absorbed into a solution, it binds with a water molecule to form a molecule of carbonic acid:

CO2 + H2O = H2CO3

H2CO3 has a rather strong acidifying effect in that 95% of it turns into HCO3-.  This loss of an H+ ion causes the ocean pH to decrease (for more details on ocean acidification, see the OA no OK series).

In short, the fact that the pH of the oceans is decreasing tell us that they are absorbing more carbon than they are releasing, not vice-versa.

Oceanic CO2 Rising Fastest at the Surface

If CO2 were being driven into the ocean from the air, the oceanic concentration would rise fastest at the surface.  If CO2 were being expelled from the oceans, we would expect to see the opposite - decreasing concentrations at the surface.

The World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS) has observed that as we expect for CO2 being driven into the oceans, concentrations of CO2 in the oceans are rising fastest at the surface.

Atmospheric O2 is Decreasing

Burning carbon requires oxygen (O2), and when we burn an atom of carbon, the required oxygen becomes part of the CO2 molecule.  So if the CO2 increase is caused by burning carbon (fossil fuels), we would expect atmospheric O2 levels to decrease at the same rate.  And that's indeed what we observe (Figure 1).

Cape Grim O2

Figure 1: Atmospheric Oxygen Concentration observed from Cape Grim, Tasmania

There's no reason to expect that a natural release of CO2 would have any effect on atmospheric O2 levels.  On the other hand, the O2 concentration is changing exactly as we would expect from a fossil-fuel driven CO2 increase.

CO2 Rise is Smoother than Temperature

Some, most recently Murry Salby, have argued that the CO2 rise is in reponse to the temperature rise.  However, the temperature rise has been quite erratic (because there are many factors which impact the average global temperature, especially in the short-term).  If atmospheric CO2 changes were in response to temperature changes, then we would expect to see an erratic rise in CO2 as well.  Instead, the atmospheric CO2 increase is very smooth, similar to the increase in human CO2 emissions.

CO2 emission vs concentration

Figure 2: Human CO2 emissions (blue, left y-axis, Source: IEA) vs. atmospheric CO2 concentration (red, right y-axis, Source: Mauna Loa record)

Isotopic Signature

Carbon is composed of three different isotopes: carbon-12, 13, and 14.  Carbon-12 is by far the most common, while carbon-13 is about 1% of the total, and carbon-14 accounts for only about 1 in 1 trillion carbon atoms in the atmosphere.

CO2 produced from burning fossil fuels or burning forests has a different isotopic composition from CO2 in the atmosphere, because plants have a preference for the lighter isotopes (carbon-12 and 13); thus they have lower carbon-13 to 12 ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same carbon-13 to 12 ratio – about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average carbon-13 to 12 ratio of the atmosphere decreases.

Reconstructions of atmospheric carbon isotope ratios from various proxy sources have determined that at no time in the last 10,000 years are the carbon-13 to 12 ratios in the atmosphere as low as they are today. Furthermore, the carbon-13 to 12 ratios begin to decline dramatically just as the CO2 starts to increase — around 1850 AD. This is exactly what we expect if the increased CO2 is in fact due to fossil fuel burning beginning in the Industrial Revolution.

carbon 13 ratio

Figure 3: Atmospheric carbon-13 ratio observations from Cape Grim, Tasmania

These isotopic observations confirm that the increase in atmospheric CO2 comes from biogenic carbon, not from the oceans or volcanoes.  Some "skeptics" like Murry Salby argue that the carbon-13 ratio isn't unique to fossil fuels.  However, because the carbon-14 ratio has also decreased significantly (Figure 4), we know it's from old (fossil fuel) sources, not modern sources.  This is not new science either, it's something we've known for over half a century (Revelle and Suess 1957), and there  have been many studies confirming these results.  For example, Levin & Hesshaimer (2000):

"It has been erroneously argued that the observed atmospheric CO2 increase since the middle of the 19th century may be due to an ongoing natural perturbation of gross fluxes between the atmosphere, biosphere, and oceans. That the increase is in fact a predominantly anthropogenic disturbance, caused by accelerated release of CO2 from burning of fossil fuels, has been elegantly demonstrated through 14C analyses of tree rings from the last two centuries (Stuiver and Quay 1981; Suess 1955; Tans et al. 1979)."

14C ratio

Figure 4: Temporal change of carbon-14 ratio in tree rings grown at the Pacific coast (Levin & Hesshaimer 2000)

Settled Science

As you can see, there are many lines of evidence showing that the increase in atmospheric CO2 is due to human fossil fuel combustion.  Each one of these lines of evidence is very conclusive on its own, and when all put together, it's abundantly clear that the science is settled on this issue.

Last updated on 1 March 2012 by dana1981. View Archives

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Comments

Comments 1 to 14:

  1. {snip}
    Response: If you wish to discuss your egregious misinterpretation of Dr Knorr's article, do so in the thread in which you originally raised the point. There is no need to spam multiple threads. This is a first warning.
  2. Question on the part on "simple accounting":

    I assume if we set HE to 0, then NE-NA would result in a deltaC of 0, representing the pre-industrial equilibrum. This puzzles me: Why is nature (oceans, plants, soils) suddenly able to absorb 15 billion tons more CO2 with an atmosphere with ~400ppm, as opposed to the pre-industrial equilibrium with ~200ppm?

  3. Falkenherz @2.

    If we say there was pre-industrial equilibrium with 280ppm CO2 by volume, NE-NA=0, but the natural carbon cycle is still at work. NE=NA= 770 GtCO2 pa.

    Now we are at 400ppm having pumped something like 2,200 GtCO2 into the atmosphere over the previous century or so and seen a little over half of it get absorbed by the oceans and biosphere, if we stopped emitting tomorrow (HE=0), equilibrium will not be achieved for a millenium or so. The eventual level of atmospheric CO2 would be somewhere near 340ppm.

    Archer et al 2009 is usually seen as a pretty definitive study on the subject.

  4. Falkenherz to add to what MA ROger has already said, if you want a specific mechanism, the transport of carbon dioxide between the atmosphere and surface oceans is proportional to the difference in partial pressure of CO2 between air and ocean. Therefore if we increase the partial pressure of CO2 in the atmosphere (e.g. by burning fossil fuels) then this difference increases, and more CO2 passes from atmosphere to the ocean than in the other direction.  This causes the oceans to take up more CO2 until the partial pressures are in equilibrium again.

    David Archer has written a very good primer on the carbon cycle, which is well worth a read.

  5. Thanks for the answers and sorry for the double-post, no idea how that happened.

    Response:

    [Dikran Marsupial] no problem, easily fixed.

  6. Oh Dear......

    1) http://climate.nasa.gov/evidence

    2) http://oilprice.com/The-Environment/Global-Warming/Carbon-Isotopes-Prove-Humans-Have-Caused-Global-Warming.html

    3) or maybe it is greed: http://www.the-one-project.net/survival_of_human_beings_and_the_danger_of_economic_growth.htm

  7. SIMPLE ACCOUNTING REVISITED "IT’S THE ANIMALS"

    Simple Accounting Revisted "It's the Animals"


    The simple accounting demonstration that CO2 increase is manmade is pure crap.
    Animals and plants produce more than 220 GT of CO2 per year. Let’s just change a few words. Let humans be part of the nature term NE (as we are) and let a group of animal species A be the one that produce some extra CO2 by an amount of 30 GT (we can certainly find some species to be blamed). Then we have the same result,
    NE-NA = -15
    but now the added CO2 is blamed on animals, not humans. Why should humans be solely responsible for all CO2 production added to the atmosphere? Is this some sort conspiracy against humans? Why not share the blame among all species, animals and humans included?


    In any way, the NE and NA terms will balance in the future (CO2 will stabilize as it always did in history) and humans and animals will keep on living. As is well known, CO2 levels have been much higher in the history of the planet and life kept growing despite of it. I'm just sick of alarmists and skeptics bashing against those who have another perspective about this whole topic and who, ironically, are also skeptics. Skeptics against skeptics. How uglier can it get?

    Response:

    [TD] Please do not use all caps, because it is the web equivalent of yelling.  Instead use the bold and italic formatting controls.

  8. Chris, plants take carbon out of the atmosphere.  Animals eat plants.  Animals breathe out CO2.  In general, animals are carbon neutral, just as human breathing is carbon-neutral.  Humans, however, are also digging up and burning billions of tons of carbon that has been stored in the Earth over hundreds of millions of years.  We're adding the carbon of the past to the present (and future).  

  9. Chris636

    You are not doing the accounting correctly.  It's not enough to simply produce CO2. You have to have a net exchange from one reservoir to the other.  

    So, for your example to work, the respiring organisms would have to cause a net loss of plant carbon to the atmosphere.  To match the observed  increase in atmospheric CO2, you would have to move about 250 petagrams of carbon from the terrestrial biosphere to the atmopshere (current - preindustrial atmsopheric CO2 = 850 Pg - 600 Pg).  

    That is 40-60% of current living terrestrial plant biomass (terrestrial plant C ~450 - 650p petagrams C according to the IPCC.)  About 2/3s of that deline in terrestrial biomass would have occured since 1970.  There is large uncertainty around estimates of plant biomass for sure, but you bet we would have noticed such a massive decline over such a short period of time.

    As indicated in the post, the CO2 accumulating in the atmosphere is highly depleted in 14C. We know it is therefore tens of thousands of years old because that is how long it takes for 14C to decay completely.  That age rules out everything except fossil fuels.  

    It is simple accounting in the end.  Really, scientists are not so stupid to miss something so obvious.  If individuals had been, you can bet their competitors would take them to task!

  10. Chris636:  Of course the CO2 level eventually will stabilize--when humans eventually run out of fossil fuels to burn.  The problem is that the ill effects of those high CO2 levels will get much, much worse, for a very, very long time in the time scale of human lives, civilization, and even species.  See RealClimate for a couple of relevant posts.

  11. Chris626 wrote "The simple accounting demonstration that CO2 increase is manmade is pure crap."

    so all of the worlds carbon cycle specialists are wrong, no hubris there then! ;o)

    "Let humans be part of the nature term NE (as we are)"

    This is just silly, if you define humans as part of nature the word "anthropogenic", "artificial" and ultimately "natural" have no meaning. 

    " let a group of animal species A be the one that produce some extra CO2 by an amount of 30 GT "

    The flaw in this argument is obvious.  The carbon dioxide that animals produce through respiration is directly (in the case of herbivores) or indirectly (in the case of carnivores) derived from plant matter, which is contructed from carbon dioxide taken out of the atmosphere.  Thus all animals merely return to atmosphere the carbon that was taken out of it via photosynthesis, and hence are essentially carbon neutral.  Now if you can identify an animal species where this is not the case and is increasing the amount of carbon moving through the carbon cycle, then you might have the beginings of an argument.

    "Why should humans be solely responsible for all CO2 production added to the atmosphere? Is this some sort conspiracy against humans? Why not share the blame among all species, animals and humans included?"

    because humans are the only animals that introduce additional carbon into the carbon cycle by extracting it (in the form of fossil fuels) from the lithosphere and burn it, which puts it in the atmosphere.

    "How uglier can it get?"

    Rather ironic thatyou should ask that, given you started your post by calling the work of eminent scientists "crap".

    It is sad that there are skeptics that can't even accept that the increase in atmospheric CO2 is due to anthropogenic emissions when the evidence is unambiguous and unequivocal.  If the natural environment (including all the animals) were a net carbon source, the atmospheric CO2 would be rising faster than anthropogenic emisssions as both nature and mankind would be net sources.  However we know for sure that this is not the case, the observations rule that out completely.

  12. I dont want to dogpile, but Chris626 is only looking at one of the lines of evidence presented (getting the accounting wrong). If it is "crap" then explain the isotopic evidence and ocean acidification.

    While the planet might have had higher levels in past, (but also different solar input) the problem is rate of change (because adaption takes a long time), not the final temperature.

  13. All: Please do not dogpile any future comments posted by Chris626. Dogpiling is both unnecessary and unseemly. Furthermore, it is prohibited by the SkS Comments Policy.  

  14. chris626: The first comment you posted on this thread was deleted by me because it did not comply with the SkS Comments Policy.

    Please note that posting comments here at SkS is a privilege, not a right. This privilege can be rescinded if the posting individual treats adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.

    Please take the time to review the policy and ensure future comments are in full compliance with it. Thanks for your understanding and compliance in this matter.

     

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