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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 we know more CO2 is causing warming?

What the science says...

Select a level... Basic Intermediate Advanced

An enhanced greenhouse effect from CO2 has been confirmed by multiple lines of empirical evidence.

Climate Myth...

Increasing CO2 has little to no effect

"While major green house gas H2O substantially warms the Earth, minor green house gases such as CO2 have little effect.... The 6-fold increase in hydrocarbon use since 1940 has had no noticeable effect on atmospheric temperature ... " (Environmental Effects of Increased Atmospheric Carbon Dioxide)


To make a statement like, "minor greenhouse gases such as CO2 have little effect", is to ignore 160 years of science history. So let's look at who figured out the heat-trapping properties of carbon dioxide and when.

Experiments involving various gas mixtures had demonstrated the heat-trapping properties of water vapour, CO2 and methane in the 1850s. But those effects were yet to be quantified - there were no meaningful numbers. It was to be another 40 years before that happened.

Swedish scientist Svante Arrhenius (1859-1927) was the person who crunched the numbers. The results were presented in a remarkable paper, "On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground", in 1896.

The many calculations in the 1896 paper include estimates of the amounts of CO2 increase or decrease required to drive the climate into a different state. One example used was the Hothouse climate of the Cenozoic, around 50 million years ago. Another was the glaciations of the last few hundred millennia.

To get a temperature rise of 8-9°C in the Arctic, Arrhenius calculated that CO2 levels would have to increase by 2.5 to 3 times 1890s levels. To lower the temperature 4–5°C to return to glacial conditions, he calculated a drop in CO2 was needed of 0.62-0.55 times 1890s levels.

We know CO2 levels in the 1890s from ice-core data. They were around 295 ppm. Let's do the sums. A reduction factor of 0.55 to 0.62 on 295 ppm gives 162.2-183.9 ppm. Modern ice-core measurements representing the past 800,000 years show that in glacial periods, CO2 levels fell to 170-180 ppm.

What we now know due to additional research since 1896 when Arrhenius worked on this, is that CO2 was an essential 'amplifying feedback'. That means changes triggered by long term, cyclic variations in Earth's orbit cause warming or cooling and CO2 release or entrapment in turn. Those changes in CO2 levels affected the strength of Earth's greenhouse effect. Changes in the strength of the greenhouse effect then completed the job of pushing conditions from interglacial to glacial - or vice-versa.

Arrhenius also made an important point regarding water vapour: "From observations made during balloon voyages, we know also that the distribution of the aqueous vapour may be very irregular, and different from the ideal mean distribution." This statement holds true today: water vapour is a greenhouse gas but because water exists in gas, liquid and solid forms in the atmosphere, it is continually cycling in and out of the air. It is distributed in a highly uneven fashion and is uncommon in the upper atmosphere. That's where it differs from CO2.

Once CO2 is up there, it's up there for a long time. As a consequence it has a pretty even distribution: 'well-mixed' is the term. As Arrhenius quantified all that time ago, once it's up there it constantly absorbs and re-radiates heat in all directions. That's why dumping 44 billion tons of it into our atmosphere in just one year (2019 - IPCC Sixth Assessment Report 2022) is a really bad idea.

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

Good scientific theories are said to have ‘predictive power’. In other words, armed only with a theory, we should be able to make predictions about a subject. If the theory’s any good, the predictions will come true.

Here’s an example: when the Periodic Table of the chemical elements was proposed in 1869, many elements were yet to be discovered. Using the theory behind the Periodic Table, the Russian chemist Dmitri Mendeleev was able to predict the properties of germanium, gallium and scandium prior to their discovery in 1886, 1875 and 1879 respectively. His predictions were found to be correct.

The effect on Earth's greenhouse effect of adding man-made CO2 is predicted in the theory of greenhouse gases. This theory was first proposed by Swedish scientist Svante Arrhenius in 1896, based on earlier work by Fourier, Foote and Tyndall. Many scientists have refined the theory since Arrhenius published his work in 1896. Nearly all have reached the same conclusion: if we increase the amount of greenhouse gases in the atmosphere, the Earth will warm up.

Where there is less agreement is with respect to the exact amount of warming. This issue is called 'climate sensitivity', the amount the temperatures will increase if CO2 is doubled from pre-industrial levels. Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F). A key reason for this range of outcomes is because of the large number of potential climate feedbacks and their variable interactions with one another. Put simply, some are much better understood than others.

What Goes Down…

The greenhouse effect works like this: Energy arrives from the sun in the form of visible light and ultraviolet radiation. The Earth then emits some of this energy as infrared radiation. Greenhouse gases in the atmosphere 'capture' some of this heat, then re-emit it in all directions - including back to the Earth's surface.

Through this process, CO2 and other greenhouse gases keep the Earth’s surface 33°Celsius (59.4°F) warmer than it would be without them. We have added 42% more CO2, and temperatures have gone up. There should be some evidence that links CO2 to the temperature rise.

So far, the average global temperature has gone up by more than 1 degrees C (1.9°F):

"According to an ongoing temperature analysis led by scientists at NASA’s Goddard Institute for Space Studies (GISS), the average global temperature on Earth has increased by at least 1.1° Celsius (1.9° Fahrenheit) since 1880. The majority of the warming has occurred since 1975, at a rate of roughly 0.15 to 0.20°C per decade."

The temperatures are going up, just like the theory predicted. But where’s the connection with CO2, or other greenhouse gases like methane, ozone or nitrous oxide?

The connection can be found in the spectrum of greenhouse radiation. Using high-resolution FTIR spectroscopy, we can measure the exact wavelengths of long-wave (infrared) radiation reaching the ground.

Greenhouse spectrum

Figure 1: Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapour is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

Sure enough, we can see that CO2 is adding considerable warming, along with ozone (O3) and methane (CH4). This is called surface radiative forcing, and the measurements are part of the empirical evidence that CO2 is causing the warming.

...Must Go Up

How long has CO2 been contributing to increased warming? According to NASA, “Two-thirds of the warming has occurred since 1975”. Is there a reliable way to identify CO2’s influence on temperatures over that period?

There is: we can measure the wavelengths of long-wave radiation leaving the Earth (upward radiation). Satellites have recorded the Earth's outgoing radiation. We can examine the spectrum of upward long-wave radiation in 1970 and 1997 to see if there are changes.

Change in outgoing radiation

Figure 2: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature (Harries et al. 2001).

This time, we see that during the period when temperatures increased the most, emissions of upward radiation have decreased through radiative trapping at exactly the same wavenumbers as they increased for downward radiation. The same greenhouse gases are identified: CO2, methane, ozone and so on.

The Empirical Evidence

As temperatures started to rise, scientists became more and more interested in the cause. Many theories were proposed. All save one have fallen by the wayside, discarded for lack of evidence. One theory alone has stood the test of time, strengthened by experiments.

We have known CO2 absorbs and re-emits longwave radiation, since the days of Foote, Tyndall and Arrhenius in the 19th Century. The theory of greenhouse gases predicts that if we increase the proportion of greenhouse gases, more warming will occur.

Scientists have measured the influence of CO2 on both incoming solar energy and outgoing long-wave radiation. Less longwave radiation is escaping to space at the specific wavelengths of greenhouse gases. Increased longwave radiation is measured at the surface of the Earth at the same wavelengths.

Last updated on 16 July 2023 by John Mason. View Archives

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Further reading

A good summation of the physics of radiative forcing can be found in V. Ramanathan's Trace-Gas Greenhouse Effect and Global Warming.

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Comments 126 to 131 out of 131:

  1. @Tom, many thanks for correcting my intuition, getting things wrong and having ones understanding challenged is an excellent way to learn! Forgetting that the water vapour is in the lower trophosphere should have been obvious even to me. ;o)
  2. Hello, I'm new to this place and don't really want to cause any fuss and this may have been covered elsewhere, however I am very interested in all of the topics covered so far. Not wanting to post a link to anything that may be biased whatever a persons personal belief is, I still believe scientific work deserves credit where it is due and discredit if it can be falsified. Please any input on the findings of Roy. Clark on the questioning of radiative forcing models and techniques would be much appreciated. I realize he is very much what you would consider a skeptic but any confirmation of his observations and/or conclusions, contradictions, corrections would be handy. I am currently unaware if he has tried to submit this work for peer review as I doubt he believes the system is currently without faults or bias itself. Thanks
  3. As a newcomer, Welcome! Quite frankly, anyone who runs around claiming "fraud!" like Roy does is already 1 foot in wingnutville. Add to that the usual gibberish about "2nd Law" violations and he takes the next step all the way in. Also speaking frankly, your whole linked site is a Gish Gallop of epic proportions. If you would like to select the 1 specific item that you feel Roy's whole case rests upon, do so and someone here will engage you on that. On the appropriate thread. One way to best utilize this site is by looking at the argument structure via taxonomy: Or you can just use the Search Function in the UL corner of every page. Just plug in a term like "2nd Law" and you'll get something like this. BTW, Roy is welcome to come here openly. We don't bite.
  4. photeki @127, the linked article immediately demonstrates that Roy Clark is yet another in the long line of supposed "skeptics" who attempt to refute the theory of the greenhouse effect with out first bothering to learn what it is. Specifically, he attempts to show that the CO2 effect is saturated by discussing back radiation only. The greenhouse effect is not based on back radiation, and any discussion that assumes it is shows the author to be in complete ignorance of the theory they purport to refute. You will find a basic introduction to the greenhouse effect here. Read it carefully. Notice how the relevant factors are the Top of Atmosphere radiative balance, and that no mention of back radiation is needed. And for the record, if you work out the radiative physics for an increase of CO2 at the TOA, it does result in significant changes in radiative forcing, of 3.7 W/m^2 per doubling of CO2.
  5. photeki, Hint... anyone who includes lots of words and graphs and numbers, but no actual mathematics, is being lazy and trusting to common sense and "thought processes" to qualify everything while quantifying nothing. More directly... his perspective and opinions are totally worthless. Is it not peer-reviewed because he's afraid the process is flawed, or just because he's so demonstrably wrong? How often do people get to walk onto the floor of the U.S. Mint and demand their fair share of freshly printed bills, because the money-review system is flawed and unfair and they deserve more?
  6. Bob Loblaw, over here I said I answered "yes" to the question: ""Do you think human activity is a significant contributing factor in changing mean global temperatures?"" which I claimed put me in agreement with 97% of climate scientists. My thought in a nutshell was that CO2 is represents the largest forcing change over the 20th century, so "significant contributing factor" makes sense. You asked "What kind of model could be used to support a claim of "significant contributing factor", but would not also have an estimate of sensitivity built into it?" Using a model without sensitivity built in: the rise in CO2 is 6% per decade so the rise in forcing from CO2 per decade is 5.35 * ln (1.06) which without any feedback (lambda is 0.28 K/W/m2) means 0.087C per decade rise due to CO2. The observed rise per decade is 0.2C Now the question boils down to: is 43% of the rise in temperature per decade due to CO2 assuming no feedback a "significant contributing factor in changing mean global temperatures"? My answer is yes, that is a significant contribution.
    Response: [DB] Please take the model "hammer" discussion to the "Models are unreliable" nail.
  7. DB, I will take any model discussion there. I should point out here that using the same simple (no model) equations I get 2.4C per doubling of CO2. I'm sure someone else will point this out, but fast feedback in my post above disproves my claim of low sensitivity that I made on the other thread.
  8. Eric, I would also point out that simultaneous to adding CO2 to the atmosphere, we are seeing the sun enter a quiet phase, and we are adding dimming aerosols to the atmosphere (a short lived exercise in "global cooling" that will end when fossil fuel consumption ends, while the CO2 will remain). These two factors represent negative forcings which are holding the rate of warming down to a mere 0.2C per decade. This suggests that when the sun wakes up and the dimming aerosols (pollution) are no longer being added to the atmosphere, actual warming will be even greater than the 2.4C per doubling that you are currently calculating. For more on this look at Huber and Knutti.
  9. Eric: as per the moderator's comment, I have posted a reply over in the models thread.
  10. Sphaerica, I have replied on the "it's the sun" thread here Regarding aerosols, the stratospheric variety are very low and I would like to know on an appropriate thread if that is more or less important than tropospheric aerosols that you refer to.
  11. I'm on a quest to find peer-reviewed papers specifically testing the enhanced greenhouse effect by raising CO2 levels in a volume of atmosphere in the lab. I'm looking for a fistful of more tightly controlled examples of those experiments you see on youtube. Not OLR or DLR in the atmos, not spectroscopic tables, just the meat and potatoes physical lab test. Checked every citation for Tyndall's papers. Nothing. Tried various search terms in google scholar. Nothing. I begin to imagine that this experiment has never been submitted for peer review! If anyone knows of such papers, or can tell me the right search terms to use to find them, that would be much appreciated. I plan to offer them up to Ari for a new list @, and to have them standing by for the undead hordes.
  12. barry: "the meat and potatoes physical lab test." I think you need to explain just exactly what it is you mean by this. For example, if what you want is a simple demonstration that CO2 really does absorb IR radiation, then there are gazillions of commercially-available systems for measuring CO2 in air using IR spectrometry. e.g., Licor.
  13. Do you have anything yet, barry? Maybe I'm wrong, but I'm not sure people do that kind of stuff anymore. They certainly don't publish it. You might ask over at science of doom.
  14. barry strictly speaking, there's no lab experiment possible on the enhanced greenhous effect. It is like asking a lab experiment on the gravitational collapse leading to the explosion of a supernova. Climatology shares with astronomy and other fields of science the characteristic of reproducible and controlled experiments being impossible. But then, knowing the radiative properties of GHGs and the structure of the atmosphere you know what to expect. Whoever knows even a little bit of the two does not question the atmospheric greenhouse effect, self-styled skeptics only question its magnitude.
  15. I have an idea for an experiment, and have identified a suitable industrial partner, however I have been unable to attract sufficient funding from the Natural and Environmental Reseach Council. They say I should just use a model instead. More seriously, to perform a physical experiment you would need to have a colum of atmosphere tall enough to exhibit a significant lapse rate, which pretty much rules out any lab based experiment. The basic mechanism was first suggested by Calendar and the first quantative analysis performed by Gilbert Plass in the 1950s. That is probably the closest you will get.
  16. By the way, David Archer and Raymond Pierrehumber (of RealClimate) published a book a while back called "The Warming Papers", which is essentially a collection of some of the key historical landmark papers on climate change, with commentary from the authors. So if you want to find out the historical undepinnings of climate change research, the Warming Papers is an excellent place to start. ALL skeptics should get a copy of it and read it so they know the true depth of what they are skeptical of.
  17. Following up several points in one place... Although a controlled experiment of "the greenhouse effect" is not possible (we're doing an uncontrolled one, however), many, many parts of atmospheric science are amenable to controlled lab experiments - such as IR characteristics of CO2 and other gases, scattering effects of aerosols, and much other physics-related issues. There isn't any simple "global climate theory" - it's a compendium of many aspects of physics and biology; some with extremely strong evidence, others with less. A specific question can be answered with specific details, but if the question asked is "What's the Answer to Life, the Universe, and Everything", then answering "42" is probably not an answer to the question you really want to know. I know I've seen the table of contents of "The Warming Papers" on-line. Many of the papers contained in it are freely available for download, too. (Try Google Scholar.)
  18. Coming from a geological background, I would also point out that climate theory is pretty successful at predicting the surface temperatures of other planets/moons given atmospheric concentration, TSI and albedo. Regard each planet as a lab experiment.


    The data from The European Project for Ice Coring in Antarctica shows results that are inconsistent with those presented here.  I did a scatter plot of CO2 vs. temperature for the 800,000 years represented by that study.  The first thing I noted was that the current point is way, way, way far away from the other 799.  That suggests by itself that there is a fundamental structural difference in what’s going on today vs. the past. Next, I plotted the climate equation using the maximum value of λ, 1.2.  It was way below the actual data points.  Then I estimated the value of λ which  would best fit the historical data (leaving out the current point because it is so far off).  The result was a value of 20.7 which is an order of magnitude larger than the maximum value stated here of 1.2.  Finally, I fit the best linear relationship.  It’s R square, .79, was better than that of the log fit, .76.  Can anyone give me a scientific explanation of what’s going on here?


  20. Regarding post # 64 by Stephen Baines:

    I asked a question about the law of conservation in a different thread and rceived some very interesting and compelling - to me - answers from Tom Curtis and Bob Loblaw.  They were clear and very helpful!

    Philosophically, I think it is important to be skeptical AND curious because the two fundamentally drive knowledge.  The two above mentioned men have been extremely helpful in presenting science as I remembered it, but with much more fidelity.

    I can only add here, questioning the limits our predictive powers, is that mother nature freely associates.  I trust the science and I consider models as useful tools to help point to a condition that is possibly very harmful to our future.  I cannot ignore, however, what I see...and for me it is sobering. 

  21. I did not read this entire thread, so forgive my question.

    There was a discussion above about co2 sequestration.  I understand that the ocean's ability to 'buffer' additional co2 can be helpful at first blush.  But buffering mechanisms, to my way of thinking, are trade-offs if you like.  From my own botany and soil science background I understand that the chemical equation will 'buffer' in the other direction as well if conditions permit.  Doesn't the buffered co2 bother us?  I don't see it as a long term benefit; if we stop introducing co2 into the air today, the level can drop to a point where some of the co2 is re-released into the atmosphere...or is my science off?

  22. Not sure its been covered yet, Is a 250ppm co2 laden atmosphere thinner than a 400ppm co2 laden atmosphere.

    Is it thicker in depth and does it let the same amount of solar radiation through?

  23. vonnegut, This is covered by the post from Tom Curtis at 125, CO2 is not a big absorber of visible light or ultra violet or the wavelengths of much of the IR we recieve from the sun.

    Incidentally, see my post at 126 for an example of how to deal with getting something wrong.

  24. I’m interested in the effects of CO2 on outgoing radiation, I have some questions.

    1. If an infinitely thin line were extended out perpendicular from the earth, how far would it extend before it reached a 99.9% probability (aprox.) of coming in contact with a CO2 molecule?  At 280 ppm and 400 ppm.

    2. When a CO2 molecule re-radiates, what is its efficiency? I.e. energy in vs energy out.  An analogy would be a number of steel balls in a row, each ball does not impart all of its energy to the next. After a certain number of balls the output is almost zero. The energy is absorbed as heat in the balls.

    3.  Would the long wave radiation in the CO2 bands ever reach the top of the atmosphere by the relay mode?

    4. What other gases emit long wave radiation when heated. I.e. the IR we see at the top of storm clouds.

    5. How much would the earth’s temperature rise if all long wave radiation (CO2 bands) were filtered and showed up as heat in the lower atmosphere?

  25. Others might have the numbers at their fingertips but here is some very quick answers.

    1/ To make any sense of this question, I think you are really meaning, how far would a photon of suitable wavelength travel before it hit a CO2 molecule. I dont know the exact no.s but pretty much 100% by 10m.

    2/ you are mixing up macroscopic (heat) and microscopic here. Think what "heat" means in the collision between photon and CO2. As such, the "efficiency" is 100%.

    I think you get a better idea of what is going on if you look up Radiative Transfer Equation for the diffusion model, and also Beers Law (Beer's-Lambeth law). A bit much for a comment reply.

    3/ Yes - pretty much the only way IR exits the atmosphere.

    4/ Not sure I follow you but do you mean what are the other greenhouse gases, then prominent are water vapour, CH4, O3,NO.

    5/ Dont really follow this at all. What CO2 does is absorb outgoing IR and then when it reradiates some of that re-radiation is going down (and by same path warms the surface). The gas itself doesnt heat.

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