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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)

At-a-glance

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.

Denial101x video

Comments

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Comments 451 to 451 out of 451:

  1. The only [snipped] wavelength of Earth's radiation the Co2 can effectively absorb is the 14-16 micron Band.  The other two bands are in an area where Earth radiation is minimal.

    NASA data now available shows that the 14-16 band energy is already totally absorbed at present levels of CO2.  (NASA Technical Memorandum 103957, Appendix E)

    Therefore more CO2 can not result in more energy absorption, thus can have no effect on Warming.

    This information is only recently available.  

    It is time to quit trying to find complicated verbal arguements trying to get around the facts.

    Response:

    [BL] Strike five. As this user seems to have nothing else to say except to repeat this snippet, he will no longer be participating in this forum.

     

  2. I have had this publication and this one cited against me in argument. Has anyone come across them? Essentially they are being used to argue that C02 > 400ppm concentration will not trigger further heating.

  3. Dominic68:

    Both those papers are published in "journals" from Scientific Research Publishing (SCIRP), which has a pretty bad reputation. In addition to the Wikipedia link, note that SCIRP is listed on Beall's list of predatory publishers. Not a good start.

    The first one also mentions Hermann Harde in the abstract, who is a known crank.

    Third strike: the abstracts both refer to "backscatter" of IR radiation. CO2 does not scatter IR radiation - it absorbs it. The absorbed energy heats the air via collision with other gas molecules, and that warmer air leads to increased radiation (as IR, for earth-atmosphere temperatures). Any competent climate scientist understands the difference between absorption and later emission of IR radiation (what greenhouse gases cause) versus scattering of radiation (which happens to sunlight in the visible spectrum).

    The experiments they propose are not worth looking at.

    The second paper was previously discussed in comment on this post at SkS.

  4. Addendum:

    As noted in the other SkS post I linked to in my previous comment, there is also discussion of the first Seim and Olsen paper on this SkS thread.

  5. The problem with the articles by Seim, et al., is that the poor experimental design is inadequate to simulate an Earth/Atmosphere environment. It does not simulate and cannot demonstrate the mechanism of global warming. This is a result of an incorrect description of the mechanism provided by the authors, who apparently do not understand the descriptions offered by their references. The authors fail to describe the global energy balance properly by focusing on backscatter (an incorrect term for re-radiation) or back radiation. Instead, the correct mechanism for global warming is a result of reduced energy lost to space emitted from the cold atmospheric layer at the bottom of the stratosphere. This is neglected in both the description and the experimental design. Reduced energy loss to space emitted by CO2 from this layer is obvious from the spectrum. Global warming is a result of upsetting the steady state global energy balance as increasing CO2 further reduces emittance to space. To learn more about the mechanism, please read my guest post at: Introducing-an-Atmospheric-Radiation-Model-to-Learn-About-Global-Warming.html
    This is in addition to the comments made on Seim’s paper a year ago, as pointed out by Bob Loblaw @453, by Bob, MA Rodger, and myself. MA Rodger also uses the MILIA model in those comments.

  6. Yes, Charlie Brown's guest post on the MILIA (aka MODTRAN) model is definitely worth reading. You'll often see references to MODTRAN in comments her at SkS, and Charlie's post give a good introduction and encourages people to try experimenting with the model.

    Thanks to Charlie for taking a deeper look into the Seim et al papers. For me, the egregious errors displayed in the abstracts were enough to not want to make me waste my time on the main parts of the papers. Sometimes you can tell a book by its cover.

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