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How do we know that recent CO2 increases are due to human activities?

Filed under: — eric @ 16 December 2004

An updated version of this post is now available.

The fact that CO2 increases in the past 150 years are due virtually entirely to human activities is so well established that one rarely sees it questioned. Yet is is quite reasonable to ask how we know this.

There are actually multiple, largely independent lines of reasoning, discussed in some detail in the IPCC TAR report, Chapter 3. One of the best illustrations of this point, however, is not given in IPCC. Indeed, it seems not all that well appreciated in the scientific community, and is worth making more widely known.

Carbon is composed of three different isotopes 14C, 13C and 12C of which 12C is the most common and 14C (used for dating purposes) is only about 1 in 1 trillion atoms. 13C is about 1% of the total.

Over the last few decades, isotope geochemists have worked together with tree rings experts to construct a time series of atmospheric 14C variations over the last 10,000 years. This work is motivated by a variety of questions, most having to do with increasing the accuracy of the radiocarbon dating method. A byproduct of this work is that we also have a very nice record of atmospheric 13C variations through time, and what we find is that at no time in the last 10,000 years are the 13C/12C ratios in the atmosphere as low as they are today. Furthermore, the 13C/12C ratios begin to decline dramatically just as the CO2 starts to increase — around 1850 AD. This is no surprise because fossil fuels have lower 13C/12C ratios than the atmosphere.

The total change is about 0.15%, which sounds very small but is actually very large relative to natural variability. Although it has proved quite challenging to do the analyses, there are a limited number of measurements of the 13C/12C ratio in ice cores. The results show that the full glacial-to-interglacial change in 13C/12C of the atmosphere — which took many thousand years — was about 0.03% 00 or about 5 times less than that observed in the last 150 years. The ice core data also agree quite well with the tree ring data where these data sets overlap.

I will put a couple of plots up when I get a chance. For those who are interested, some relevant references are: Stuiver, M., Burk, R. L. and Quay, P. D. 1984. 13C/12C ratios and the transfer of biospheric carbon to the atmosphere. J. Geophys. Res. 89, 1731�1748. for tree rings, and
Francey, R.J., Allison, C.E., Etheridge, D.M., Trudinger, C.M., Enting, I.G., Leuenberger, M., Langenfelds, R.L., Michel, E., Steele, L.P., 1999. A 1000-year high precision record of d 13Cin atmospheric CO. Tellus 51B, 170�193.

31 Responses to “How do we know that recent CO2 increases are due to human activities?”

  1. 1
    GeniusNZ says:

    but – to complete the chain of logic – are CO2 increases DEFINITLY the cause of the warming?
    Is that the only major reason (or a perfect indicator) for climate change on earth? (this is an honest question – for all I know maybe it is)

  2. 2
    Wil Burns says:

    This article is not that helpful for those totally unitiated in atomic physics. For example, there are probably many readers who do not know what an “isotope” is; thus, this explanation would be totally confusing. Also, it’s not made clear why an analysis of atmospheric 14C readings yields 13C data.

  3. 3

    The decrease in the 13C/12C ratio can also be observed in century long time series of corals and sclerosponges from tropical reefs. This illustrates how the anthropogenic CO2 slowly invaded the oceans.
    Sclerosponges are sponges that build a massive skeleton of aragonite and can live for many centuries. Our carbon isotope time series from Caribbean sclerosponges start in the 15th century and show only very small variations until the early 19th century. With the start of industrialization the 13C/12C values begin to drop precipitously, compared to the small variability of the preceeding centuries.
    Reference: Böhm, F., Haase-Schramm, A., Eisenhauer, A., Dullo, W.-Chr., Joachimski, M.M., Lehnert, H., Reitner, J. (2003), Evidence for preindustrial variations in the marine surface water carbonate system from coralline sponges. Geochem. Geophys. Geosystems, 3/3, DOI 10.1029/2001GC000264.

  4. 4
    Scott Nance says:

    Not being an expert on atmospheric chemistry, or even a scientist at all, I, found both this post and Florian’s comment to be extremely enlightening. To wax just a bit philosophical, I notice that the conclusion regarding C12/C13 ratios is based on statistical analysis and probabilities. Statistically, for example, the difference between 0.15% and 0.03% is huge, although the absolute difference in magnitudes isn’t that great. Statistics and probability is of course the basis of most modern physics, and plays a huge role in climate science. There seem to be some people who intuitively understand the meaning of probabilities, and those who don’t. For the former, a change in the ratio of C13 to C12 is solid evidence that the increase in CO2 since 1850 was anthropogenic; not “proof,” because absolute certainty is impossible, but a high enough probability to serve as a basis for conclusions and action. To people who don’t understand statistics and probability, it’s just more scientific mumbo-jumbo, “so we don’t really know, do we?” I’m not sure how much this post will do to convince the latter, while to the former, it’s quite useful. Realizing that this site is not about politics, I still must wonder, how many of our government leaders really understand statistics and probability?

  5. 5
    eric says:

    I’m glad this post was enlightening (comment #4).
    However, given several comments to the effect that my post is “over our heads” I will also write an update explaining some of the terms.
    All of this ought to be a bit easier to understand (for those unfamiliar with isotopes) once I put a graph up, showing the data.

  6. 6
    Tim Jones says:

    I’m eargerly awaiting your answer to the first question “are CO2 increases DEFINITLY the cause of the warming?

    Response This is addressed elsewhere on the site. See the various posts under the category “greenhouse gases”.

  7. 7

    So how do we know the C02 rise is man made?
    RealClimate How do we know that recent CO2 increases are due to human activities? The RealClimate Blog takes on the nagging question–given the fact that we see C02 rises in the atmosphere in the last 2 centuries, how do…

  8. 8
    GeniusNZ says:

    There seems to be a lot to sift through there – I may have missed it but most of the posts (and work in general) seem to just assume the link in the chain I am looking for.
    Here is the sort of missing link proofs that would help

    1) A accurate regression of CO2 in the atmosphere against temperature change resulting in a strong correlation between the two (This might be made more difficult by a time lag effect). Maybe this is what they did to create their climate models?
    (If there is a very strong correlation you have a “slam dunk” case not only in academia but also in the public arena – I wonder why one does not hear of it.)

    If there is insufficient data to do the above then

    2) a fairly beyond dispute analysis of exactly how much CO2 should effect the earths temperature in theory checked of course against the approximate fuel burnt and temperature rise.

    The aim being

    Man burns Carbon – CO2 increases (hard to dispute) – CO2 increases mostly due to humans (also hard to dispute but proven as above) – CO2 heats earth (this is generally accepted and the proof seems quite straight forward) – heating of the earth is mostly due to CO2

    So then assuming “mostly” = 100% (resulting in errors that are not too large) –

    Man burns X carbon -> earth heats up by X deg

    This allows the appropriate cost benefit analysis (maybe this is getting to far into politics) that should be significantly more useful for the main debate than these temperature predictions that we have and takes many unpredictable factors out of the equation and if we have a full chain of logic it should be easier to find – because time as opposed to amount of carbon related models leave you asking questions like “what will happen to technology”

    Brief response: These are all excellent questions and thoughts. The answers are not single-sentence answers though. I’ll take the time to respond in more detail when I get a chance, but meanwhile note that many of these questions are addressed elsewhere on this site. And indeed, I encourage you to read the IPCC summary for policymakers. -Eric

  9. 9
    Steve Funk says:

    Is the variation .15% of all carbon or .15% of the carbon 13? i.e., has carbon 13 increased from 1% to 1.15% of the total carbon, or from 1% to 1.0015%?

  10. 10
    eric says:

    Response to #9: The ratio has changed by 0.15%. Suppose the ratio were exactly 1/1000 (1 atom of 13C for every 1000 12C atoms) in 1850. Then the ratio today would be 0.15% lower than that, or (1 – 0.0015*1) = 0.9985 atoms of 13C for every 1000 atoms of 12C. Again, these numbers sound small, but they are actually large (i.e. very easily measureable) values. Most labs can measure 12C/12C ratios to a precision of about 0.005 percent. That’s about thirty time smaller the observed change.

  11. 11

    The atmosphere is not homgoenous, laterally or horizontally. It varies in pressure, density, heat, composition hourly, daily, millenially, earth events such as volcanoes,earthquakes both on land and sea have local and wide area effects. Cosmis events such as sun spots and perhaps cyclical changes in the stratosphere have also affected climate.
    One component (C12/13 ratio) appears to have varied over a tiny fraction of geological time.
    CO2 is a small fraction of sea level gases, so far all models, calculations do not include water vapour or particulates which act in the atmosphere and in reducing the ice cap/snoe level albido effect.
    Whilst your presentation is persuasive, it forgets that the volcano Thera (The Greek Isle of Santorini) upset the North temperate climate for decades, led to massive migrations etc., which can be seen in the cie cores, dendrochronologies of Irish bog oaks etc.,
    To make the leap from this very minor cline to the alleged changes of the climate today is too much.
    Well for me it is.
    ResponseThere is nothing in the post about isotope ratios that has anything to do with climate! The point is simply that this is one of the many ways that we know that the increased CO2 in the atmosphere in recent times is due to burning of fossil fuels and other (largely anthropogenic) sources. Whether CO2 affects climate is an entirely different subject. Incidentally, virtually ALL climate models do include each of the items you suggest are not included.

  12. 12
    D says:

    #11 is repeating an oft-parrotted argument: “GCMs do not look at WV”. For further enlightenment on what GCMs actually do, one should look at primary sources, rather than interpretations by non-climate scientists.

    The home page for a group that looks at all major GCMs and checks them against one another.



  13. 13

    How Do We Know That CO2 Increases Come From Human Activity?
    RealClimate has an interesting post up today explaining how climatologists can say with some certainty that the observed increase in carbon dioxide concentration in the…

  14. 14
    dave says:

    There is no known natural climate forcing that can explain the rapid rise in CO2 (ppmv) since 1850. Although this post is very informative – I did not know about the C13/C12 results – it raises a question that is really already answered to a very high degree of certainty. This can be a dangerous thing to do, as some of the comments here show.

    So, I think the original post should have also addressed the lack of other climate forcings (volcanism, solar insolation fluxes) that might have resulted in climate feedbacks affecting the carbon cycle that would have, in turn, resulted in the on-going increase of CO2 (ppmv) in the atmosphere. The volcano Pinatubo (erupted in 1991) forced the climate a bit for a year or so but outside of that, in the recent record (since 1970, say), there has been no other natural forcing observed in any data of any kind that could have caused these CO2 increases (via feedbacks). Of course, one could posit an unknown forcing (the F factor?) just like “the ether” in the earlier days of physics.

    Anthropogenic (human-caused) emissions have increased CO2 in the atmosphere and this in itself constitutes a forcing on climate like volcanism has in the past. No natural variability is implicated. Natural events (like Solar energy fluxes, the Arctic Oscillation or El Nino/La Nina) are observed and taken into account. For anyone doubting all this, it would be nice to see an alternative plausible, testable hypothesis. And by the way, I am not aware of any event in the Paleoclimate record where CO2 in the atmosphere rises this much this fast although, to be fair, paleoclimate data from the distant past often lacks the fine resolution required for such a demonstration. Interestingly, there seems to some evidence that this could have occurred with methane (also a greenhouse gas) associated with the Paleocene/Eocene Thermal Maximum 55 mya. due to release of methane gas hydrates. But for CO2, as far as I know, there’s nada.

    I’m sorry to some if I’ve just stated the obvious.

  15. 15

    Just like to continue (pile onto?) geniusNZ’s line of reasoning.

    One can show there is a correlation between CO2 and global warming. But there is also a correlation between CO2 and many other human-related parameters, indeed any parameter which closely tracks human population. Urbanization, deforestation, etc. How does one determine (prove?) causality?

    Also, there is a nice graph elsewhere on your site showing temperature variation over the course of the past thousand(s?) of year, which is basically a straightish line with some zigs and zags and then an upwardly sloping graph. Forgive my scepticism, but this seems to be one of the easiest types of graphs to model (essentially an exponential curve). You just continue the upward moving line. But the world’s climate seems to have feedback mechanisms which tend to attenuate extremes – put very simply (and perhaps incorrectly), if it gets too hot, it rains and cools everything down. How does one determine whether the true temperature curve is exponential and not third-order polynomial with a maximum in five years?

    And, forgive my off-threadness, just a basic question. Where is the testability in the theory? Are climatologists offering a prediction of the global temperature next year? The average over the next five years? (Who wants a weatherman who can only explain why it rained/was sunny yesterday?) Are there any experiments, even on the fringes of the theory, which can be held to either verify or refute?

  16. 16
    Chris O says:

    No climate scientists simply argue that they “see a correlation” between CO2 and temperature, and therefore CO2 is causing the temperature increase we are seeing. It is the physics they are looking at, of all parts of the climate system they can accurately (or even approximately) model. To begin:

    CO2 affects the radiation balance of the earth, *all things being equal*, causing it to heat. BUT, of course, there are feedbacks.
    These feedbacks are the primary source of uncertainty in how much the earth will warm (side note: the question that most climate scientists who study the forcing due to CO2 try to answer is, how much will the long-term globally averaged surface temperature of the earth rise due to an rapid rise of CO2 to twice its industrial level, that is, 270 ppm to 540 ppm; it is currently about 380 last time I checked, and rising at ~ 3ppm/year, although this rate of change appears to be accelerating). There are quite a few feedbacks, and the important question to ask about each one is, is the feedback positive or negative? A positive feedback will re-inforce the trend and cause surface temperature (globally/temporally averaged, of course) to rise even faster than it would have without the feedback. Negative feedbacks due the opposite, causing the system usually to either slow its rate of temperature increase or even causing it to level off, albeit at a new higher average temperature.

    So, what are the feedbacks, and due to the negative ones outway the positive ones? Well, there are both. The ice-albedo effect leaps to mind. Basically, as the ice melts near the polar caps (greenland, etc), the underlying earth/ocean underneath it absorb more and reflect less sunlight than the ice did, so it gets even warmer as a result. Also, as the earth warms evaportation from the ocean increases, leading to a general increase in the water vapor in the air. Now, water vapor is also a greenhouse gas (pop-quiz, why?: it’s because it has a dipole moment as it is NOT a symmetric molecule, like O2 or N2, so it has bending modes that can be excited by infrared radiation); more water vapor = more greenhouse warming, again *all other things being equal*. [Editor’s note: please see comment #25 below for a correction to this explanation.]

    The catch is, *all other things* are generally never equal, so to do a good job, these guys really have to have all the relevant feedbacks included in their models, and when they run them, they, well, let all the feedbacks “feed back”, and just see what happens. You can get some crazy behavior, like in any highly nonlinear model with a bunch of feedbacks (like, say, the stock market).

    Beyond that, I highly suggest you buy a book on the THEORY behind climate change (a non-politically charged one, ideally) and read it. This really is a pretty big topic. Or just read as much as you can on the IPCC website. Personnally, I recommend IPCC 2001 “Third Assessment Report”, “The Scientific Basis”. This is about a 900 page book, but it is actually really GOOD.

    And, about your temperature graphs here are links to both the CO2 increase and the tempearture increases for the past 1000 years.

    GHG increases:


  17. 17
    geniusNZ says:

    What is the consensus on the size of the feedbacks?
    A) Do the positive (making hotter) ones outweigh the negative ones? (Over the range that we can expect in the next 100 years or so)
    B) What sort of percentage effect do they have?
    C) Do they cause a significant difference to the atmosphere per million tonnes (an arbitrary amount) of CO2 burnt now vs. 1 million tonnes burnt 50 years ago?

    If there is consensus here we should be getting close.

    My uninformed guess – which may reflect general opinion, is

    A) No – I would have thought all things being equal the odds would be that we were close to some sort of stable point. (Statistically you should spend most of your time near such a point).
    B) I would have guessed significantly less than 100% (I could easily be wrong). This will help with getting a valid number for the amount of temperature rise assuming the effect of CO2 is possible to calculate and the feedbacks are somewhat harder to calculate.
    C) I would guess not much difference – I have seen no particular evidence for that.

    Ideally I would like to take all the highly non linier feedbacks out of the equation if possible because they totally distort your “what will the world be like in 2100” questions. If you had the perfect model, you might say “in 2100 the temperature will be below what it is now because that will be the down swing of a really extreme non linier feedback mechanism”. But that means the number is misleading because there is (in a sense) also a hidden amount of “temperature rise potential” hidden within the feedback mechanisms. Anyway, we are already ignoring short term in regards to “what will happen next year”

    If that is the case then the statistics you want to know is “what is the capacity of the buffering (or multiplying) effect of the feedback – i.e. when will it run out and what is the “potential” it has been storing up (if that is the case).

    Another way of answering the question might be “the very long term equilibrium temperature of a planet with X amount of CO2 and H2O in this orbit is X”. Of course that might give us ridiculously long time scales…

  18. 18

    So has evaporation been increasing as predicted?

    Also, do the climate change models correctly predict the temperature gaps between day and night — or does that not matter?

  19. 19
    Paul D Farrar says:

    I like the argument William Connolley gave on sci.environment. It’s one that’s easy for most people to get:

    The increase in atmospheric CO2 is a good bit less than the amount humans release. Therefore, natural processes are not increasing atmospheric CO2; they are decreasing it.

  20. 20
    GeniusNZ says:

    we should have lots of information to test our scenario’s against – so our trend prediction excluding human factors like there being a new technology for creating energy or variations in economic activity (or rate of growth of economic activity) should be pretty good. I suggest taking out these two factors when presenting data.

    Unless we are in (or, less likely, are about to pass through) a buffer zone of some sort in which case this temperature prediction stuff is missing the point – what matters is defining the bufferzone and the exact amount of CO2 it can buffer.

    I am also interested to know what happens when we burn ALL the oil, and natural gas and coal (depending on the low oil supply and high oil supply theories) as opposed to just what happens in 2100 this defines the magnitude of the potential threat and allows us to define exactly where our line in the sand is. I know this introduces some big assumptions but one of the useful ones is the same people who propose continuing as is tend to believe there is a very large supply and thus a larger effect. This could (if hte effect is large enough) point out a contradiction in holding those two views simultaniously. Or if the oil runs out before the environment is too damaged point out that the problem is self containing. Either way it would reveal an important part of the graph.

  21. 21
    Eli Rabett says:

    Before you update and simplify your answer take a look at
    which is old, but good and simple. No need to post this, but do go and read

  22. 22
    GeniusNZ says:

    Paul –
    this raises two questions
    how much less?
    and how much CO2 total can these natural forces absorb
    – ie the carbon must be going somwhere where is that and is it somwhere that can absorb the full amount of carbon released by all the fossil fuel or will it “run out of storage space”.

  23. 23
    Eli Rabett says:

    You got it GeniusNZ, it does appear that the carbon sinks are saturating. pre%20project%20products/Falkowski2000.pdf Look at page 2.

  24. 24
    Steve Funk says:

    Could you define “aerosol” in your glossary. I’m not sure whether it refers to material such as freon gas, or is a synonym for particulate matter.

    [Done (using IPCC) – William]

  25. 25
    bob says:

    This may seem pedantic, but on a physical sciences-based site I think
    it’s important to get the basic science right. I quote from contribution
    no. 16 to the original post from eric on 16th December,(How do we know that
    recent CO2 increases are due to human activities?) :

    “Now, water vapor is also a greenhouse gas (pop-quiz, why?: it’s
    because it has a dipole moment as it is NOT a symmetric molecule, like O2 or
    N2,so it has bending modes that can be excited by infrared radiation);
    more water vapor more greenhouse warming, again *all other things being

    Sorry, but NO! and NO! The dipoles do not behave like this, and it’s
    not the fault of a vibration.

    1) The villain of the drama, CO2, has no dipole when “at rest” (those
    who know quantum mechanics can translate this into a fancier language).
    This does not stop the bending mode from being excited- it is this very
    excitation, involving a transient dipole, which is responsible for CO2
    blocking much of the window left by the water absorption.

    2) The dipole of water means that unlike linear CO2 and tetrahedral
    CH4, water has a strong pure rotational absorption spectrum, and this
    rotational absorption, sitting on top of most of the 277K black body
    radiation from a “naked” earth, is what is responsible for the natural
    warming of about 20K. Because of the very complex set of rotational
    energy levels of an “asymmetric top” molecule, and pressure broadening, this
    absorption is effectively continuous. All vibration frequencies of
    water are far too high to affect the black body emission (bad vibes?).

    Finally,just for the record, it’s not because of symmetry that N2 has
    no bending modes- you can’t bend a diatomic molecule.

    Professor DR Lloyd
    Emeritus Fellow
    Chemistry Department
    Trinity College
    Dublin 2

  26. 26
    Ramon Guirado says:

    Hi, I have one idea, perhaps the decrease in the amount of CO2 is like a countdown for the live on Earth, I do not know the consecuences of an Earth without CO2 but perhaps could be the end of opportunities of life on Earth. Where can I find a graph with data of concentrations of the different gasses in the history of the planet?


  27. 27
    Alexandre Cout de Andrade says:

    Dear Mr. Steig,

    I read the updated version of your text “How do we know that recent CO2 increases are due to human activities?” I found it very good. However I have some doubts:
    1) You explained how tree rings may be used to determine the concentration of CO2 in the atmosphere during the tree lives. Since ancient times, as human presence in the world increased, the area occupied by forests decreased drastically. This means that vegetal biomass nowadays is smaller than ever, and less CO2 is absorbed by the trees altogether. This results in a higher CO2 natural concentration in the atmosphere. How are these facts considered in the calculations? What about the algae in the oceans, how is it possible to estimate their quantity in ancient times?
    2) Tree rings grow more in summer and during day time. How does this affect the calculations?
    3) Have you ever read a book called “The skeptical environmentalist”(Lomborg, Bjorn – Cambridge University Press)? Amongst several other issues the author writes about global warming. He says that the models used to calculate climate changes are very inaccurate, and that “the noise from the models is bigger than the signal we are supposed to formulate policy from.” Is this true?
    4) If you know the book mentioned above, what do you think about it (concerning global warming)?
    Thank you very much.
    Best regards,

    Alexandre Couto de Andrade

  28. 28
    Jryan says:

    What I stick on with all my readings on this subject and responses to such findings is that either the layman is having trouble accurately deciphering the information or the reporter (or scientist) is doing a very bad job of relaying information.

    What seems to be the break between scientists and the average person is that the average person believes that 100% of the current warming trend is man made (as if the Earth has never gone through one before). Most scientists aren’t really claiming that. On one hand there is the group that focuses on the increase of greenhouse gasses… or cause, and a wholey other group that takes such data and tries to translate that information into effect.

    At this point in the process, anyone who tells you they have the final answer on the effect of anthropogenic CO2 on Global temperature is trying to sell you something. :)

  29. 29

    Jyran (#28) There is a current idea that up to 30% of the warming last century was due to solar effects, but even if it was 50% that does not help. We can live with the current warming, and it is unlikely that the solar effect will continue to increase.

    However, the warming is going to get worse because CO2 levels are rising and because of “commitment”, which is the technical term for the increase in temperature due to current levels which have not yet taken effect. In other words, it is the future warming, which is the real danger, and that will be 100% due to anthropgenic greenhouse gases.

    I don’t have the final answer about whether 30%, 10%, or -10% of the current warming is due to solar. OTOH it is pretty obvious to me that so long as we increase the amount of CO2 in the atmosphere the planet is going to heat up, until it becomes unbearable.

  30. 30
    jryan says:

    Future warming is 100% due to anthropogenic CO2? You may want to check that figure as it is 100% wrong, or at the very least unmeasureable. The actual deviation from the heating that would have otherwise occurred in the future absent anthropogenic CO2 will maybe be 100% due to anthropogenic CO2. Or, in other words, the heating cause by anthropogenic CO2 will be 100% caused by anthropogenic CO2. Now, the question that we need to answer: how much is that?

  31. 31
    James Turner says:

    This is an attempt to make wishy/washy data look much more reliable than it really is.

    I do agree that global warming is taking place but it is my position that the level of CO2 in the atmosphere is growing BECAUSE of global warming. I have yet to see any reasonable argument that can challenge my proposition.