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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

Comments

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Comments 126 to 150 out of 208:

  1. No mention of green house gas emissions from volcanic sources. Why is this?
  2. #126: "No mention of green house gas emissions from volcanic sources. Why is this?" Because volcanic GHG emissions are tiny compared to other sources - about 130 to 150 times less per year than what we release burning fossil fuels, for instance.
    Response: Indeed. For details see "Volcanoes emit more CO2 than humans."
  3. Well, the story sounds inconvincing for me. 1. It is assumed, the "natural" sources/sinks of CO2 are still in a perfect equilibrium. Who has proven that? 2. The humans emit just 5% of the total CO2 influx. This is nothing and could increase the CO2 percentage in the air by the same 5%, no more. The CO2 concentration not far from equilibrium behaves as d(CO2)/dt = Source - (CO2)/tau where tau is the relaxation time. We know that tau is somewhere between 5 and 10 Years. The equilibrium concentration CO2 = Source*tau. If you add extra (antropogenic) source Sa, the equilibrium concentration changes to CO2_new = (Source + Sa)*tau Because Sa = Source*0.05, we have new equilibrium CO2_new = 1.05*CO2 (natural). That is a negligible change. Result: antropogenic emissions cannot possibly account for observed CO2 rise. No way. However, if there is a significant change in the relaxation time "tau", the CO2 concentration can change dramatically. The relaxation time is defined by the CO2 absorption by the ocean surface and its consumption in there by the phytoplankton. The biological consumption is very fast, comparable with the abiotic purely ionic exchange with the atmosphere! It is known that due to pollution with fertilizers and pesticides large regions of ocean lose phytoplankton. According to some "studies", we have already lost up to 40% of phytoplankton since mid-1900. If true, this has certainly a big influence on the relaxation time "tau" and can easily explain the CO2 rise. Another certain reason for CO2 rise is the observed warming of the last century, because the water temperature explicitely influences the relaxation time tau. These are the two main reasons for CO2 rise, not the tiny emission by the fuel combustion. When you bathtube is overflowing, you should either check whether you opened the tap too far, or you check the sink. And very often it is the case that you must call a plumber to clean the sink.
  4. Bugai, Look up Suess effect, in reference to the work of Hans Suess, for independent, empirical confirmation that the CO2 is from fossil fuel combustion. I apologize for not providing a direct link, as this is being tapped out on the phone.
  5. bugai wrote: "1. It is assumed, the "natural" sources/sinks of CO2 are still in a perfect equilibrium. Who has proven that? No, the natural sources and sinks were in approximate equilibrium prior to the industrial revolution. Anthropogenic emissions have disturbed this equilibrium and hence the natural carbon cycle is in a state of imbalance in an attepmt to restore the equilibrium. This is demonstrated by the fact that atmospheric CO2 is rising at a rate of about half anthropogenic emissions, which implies that the natural environment is a net sink, taking in more CO2 than it emits. 2. The humans emit just 5% of the total CO2 influx. This is nothing and could increase the CO2 percentage in the air by the same 5%, no more. The flaw in this argument is that while humans emit only 5% of the total CO2 influx, the natural environment is responsible for 100% of the outflux, which we know to be in excess of natural emissions. The annual growth in atmospheric CO2 depends on the difference between total emissions and total uptake. Anthropogenic emissions are large comparted to the natural net flux, and hence are what currently governs the increase or decrease of atmospheric concentrations. where tau is the relaxation time. We know that tau is somewhere between 5 and 10 Years." This is incorrect, the residence time is about 5 years, but for CO2 the residence time and relaxation time (known as "adjustment time" in climatology) are not the same. For the initial response (governed by uptake into the thermocline) the adjustment time is of the order of 74 years, the full response takes much longer. See the articles on residence time for discussion (an advanced version is in preparation).
  6. Antwort an Bibliovermis. The Suess effect does not show that the CO2 increase is due to emissions. It only shows that we do emit CO2. This we perfectly know without Suess. Antwort an Dikran Marsupial. Your claim is that the mankind influences the natural CO2-sinks via CO2 emission. This is very shaky as the antropogenic source is negligibly small. Why should it so drastically change the natural sink? Especially around 1850 when the antropogenic source was even some 10 times smaller. It is much more reasonable to assume the change in the CO2 sink is due to a different, more powerful influence, like pollution with fertilizers. For this, there is no more balance between NATURAL CO2-sources and CO2-sinks. And - the 5 years IS the relaxation time for extra CO2 (experimentally proven). Concerning the 74 years or 1000 years for thermocline - this is only if you neglect photosynthesis by phytoplankton.
  7. 111, Briago1, No one seems to have replied to you (because you packed too many off topic arguments into a single post), but your misconceptions are pretty easy to address, so I took the time to help point you in the right directions. Response to your first point is on the runaway warming thread. In response to your second and third points (which are relevant on this thread):
    2) On the whole 29GT does not sound like much
    The carbon cycle involves a constant flow in and out (refer to this diagram here). What matters is not how much flows in and out, but the net difference. In this context, the system has been pretty much in near perfect balance for thousands of years. We are now shifting it out of balance by 29GT per year, which slowly but surely increases the concentrations both in the atmosphere and the ocean.
    3) ...a measly 4% increase in plant/algae life would more than make up for the difference
    But it doesn't. Scientists have actually measured where the carbon is going, and how much is going into increased vegetation, the ocean and the atmosphere. We don't need to argue about what should or might happen here, because we've pretty much measured it and we actually know exactly how much is going where. Response to your fourth point is on the CO2 lags Temperature thread. Response to your fifth point is on the Has Arctic Sea Ice Recovered thread.
  8. Bugai wrote: "Your claim is that the mankind influences the natural CO2-sinks via CO2 emission. This is very shaky as the antropogenic source is negligibly small." The error in this line of reasoning has already been pointed out, anthropogenic emissions are not negligibly small. Whether CO2 rises or falls depends on the difference between total emissions and total uptake, the volume of the flux is irrelevant. Anthropogenic emissions are large compared to the difference between natural emissions and natural uptake, which are quite closely balanced compared to the magnitude of the fluxes involved. Around 1850 the difference between natural emissions and natural uptake was also much smaller than it is now. If you look at the annual increase in atmospheric CO2 you will find it has been roughly 45% of anthropogenic emissions, going back as far as 1850. So as anthropogenic emissions have increased, the difference between natural emissions and natural uptake has increased with it and the natural carbon sink has been strengthening. "For this, there is no more balance between NATURAL CO2-sources and CO2-sinks." I have already pointed out that there is no balance between natural sources and sinks. You would make more progress if you paid better attention to the replies to your posts. Regarding residence time and adjustment time, I suggest you read the glossary of the IPCC report, which explains the distinction very clearly. Carbon cycle models do not neglect photosynthesis by phytoplankton, however they also do not neglect respiration by oceanic biota. Here is a challenge for you, if you think that anthropogenic emissions are not the cause of the observed rise then explain (i) how the annual rise in atmospheric CO2 can be less than anthropogenic emissions unless the natural environment is a net sink or (ii) how the natural environment can be the cause of the observed rise if it is a net sink. Nobody has risen to the challenge so far.
  9. 131, bugai, Your position is untenable because the accounting has been done and is very straight forward. We know how much carbon we burn. This is carbon that has been sequestered under the ground for hundreds of millions of years. It can't get back there on it's own. We take it out, we burn it, it must go somewhere. There are three places that it can go; into the air, into the oceans, or into vegetation. We have been measuring the level in the atmosphere. We know it is increasing. We have been measuring the level in the ocean. We know that it is increasing. We know that the balance (amount burned - added to the atmosphere and ocean) is going into vegetation. This has all been studied, measured, worked out, and is incontrovertible. Hence your conclusion:
    antropogenic emissions cannot possibly account for observed CO2 rise. No way.
    is untenable. Beyond this, you have two problems to solve: First, if anthropogenic CO2 is not going into the oceans and atmosphere, then where is it going? Second, what is the source of the CO2 that is causing the increase in the atmosphere? It's not the oceans, because CO2 is increasing there. Where is this added CO2 coming from?
  10. bugai#131: "the antropogenic source is negligibly small" If that is true, you will need to explain why the rate of increase in global atmospheric CO2 has dropped noticeably in years immediately following global recessions - when emissions decrease. In addition, explain how locally measured atmospheric CO2 concentrations mirror diurnal cycles (increase during higher traffic hours) as well as weekly cycles (drop on weekends).
  11. Answer to Sphaerica: sorry, I am not a "believer". I want to understand. Your "argumentation" does not do it in any way." Answer to Dikran. Or, better, a question. You say: "natural emissions and natural uptake .. are quite closely balanced compared to the magnitude of the fluxes involved. " Who balanced that? Why you think they are still balanced? How to describe the balancing process? My equations (see first posting) do that. The small extra antropogenic source cannot possibly change the balance. We do change the system by different means: pollution.
  12. bugai Sorry, refusing to answer a question and asking another one in its place is transparent evasion and not something I am going to encourage by indulging such behaviour. If you really want to get to the truth, I suggest we go through the arguemnt step by step and you can point out where the error lies when we get to it. Step #1: Do you agree that conservation of mass applies to the carbon cycle, in other words the annual increase in atmospheric CO2 is equal to total emissions minus total uptake?
  13. To muon calculon: 1. "the rate of increase in global atmospheric CO2 has dropped noticeably in years immediately following global recessions": please, provide a reference for this statement. 2. "CO2 concentrations mirror diurnal cycles (increase during higher traffic hours) as well as weekly cycles (drop on weekends)." Is off-topic, as diurnal changes are (i) local and (ii) too fast to equilibrate with anything.
  14. To Dikran: Step #1: Do you agree that conservation of mass applies to the carbon cycle, in other words the annual increase in atmospheric CO2 is equal to total emissions minus total uptake? Answer: trivially true.
  15. Bugai, "It is known that due to pollution with fertilizers and pesticides large regions of ocean lose phytoplankton. According to some "studies", we have already lost up to 40% of phytoplankton since mid-1900. If true, this has certainly a big influence on the relaxation time "tau" and can easily explain the CO2 rise." I would be interested to see these studies. Could you provide a reference please?
  16. To Hyperactive Hydrologist: I am not an expert, but here is what Wiki writes: http://en.wikipedia.org/wiki/Dead_zone_(ecology) "Use of chemical fertilizers is considered the major human-related cause of dead zones around the world."
  17. bugai good, so step #2, do you agree that we can write this more formally as dC = E_a + E_n - U_n where: dC is the annual increase in atmospheric CO2 in GtC/year E_a is annual emissions from anthropogenic sources in GtC/year E_n is annual emissions from natural sources in GtC/year U_n is annual uptake by natural sinks in GtC/year Technically there ought to be U_a, which is annual uptake by anthropogenic sinks, but this is effectively zero as we are not yet achieving any significant carbon sequestration. Again, this should be fairly obviously true as it is just a restatement of step 1 with total emissions divided into anthropogenic and natural. Do you agree with this?
  18. To Dirkan: I prefer the full differential equation: dC/dt = E_a + E_n - U_n Do you agree with that? It is better to have a true differential equation, because we have then something to solve.
  19. bugai You may prefer a differential equation, however as I will shortly be introducing annual observations into the discussion it makes far more sense to look at annual changes rather than instantaneous rates. So, do you agree with step 2 or not? I have solved the differential equations as well, so I am happy to discuss those later once we have agreed on the basics.
  20. To Dirkan: I'd like to stick to the ODE. You may think of dt=1 year, if you wish. 1 year is "instantaneous".
  21. bugai @ 141, I had a quick search of the literature and found this paper. They do in fact show a -6.3% change in ocean primary productivity between the 79-86 and 97-02 periods. I'm not sure this is sufficient to account for the increase in atmospheric CO2 concentrations over that period.
  22. bugai Sorry, you are just obfuscating. How about this, I will carry on with the dC and you can read it as dC/dt with t=one year if you really want to (although that would be an abuse of notation it would be your error not mine). Do you agree with step #2?
  23. To Hyperactive Hydrologist: Well, to account for the observed CO2 raise from 280 ppm around 1750 to 380 ppm now, we need 100*(1 - 280/(380-280)) = 26% in productivity change. The observed 6.5% over 20 years can be easily extrapolated to 26% over the laser 250 years.
  24. To Dikran: will go any step further? I get tired with you.
  25. bugai wrote : "To Dikran: will go any step further? I get tired with you." I have noticed over the years that the difference between those who are genuinely interested in the science and those who are deniers or trolls is that those who are interested in the science will follow a step by step explanation, and those that are not either obfuscate or abandon the discussion when it becomes apparent that they have backed themselves into a corner where they will soon be forced to admit that they were wrong. They normally get further than step #1 though. The ball is in your court, if you want to show that you are interested in the science, then either agree with step #2 or show why it is incorrect.

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