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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)

Predicting the Future

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 Table of Elements was proposed, 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, despite the fact they hadn’t been discovered.

The 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 and Tyndall. Many scientist have refined the theory in the last century. Nearly all have reached the same conclusion: if we increase the amount of greenhouse gases in the atmosphere, the Earth will warm up.

What they don’t agree on is by how much. 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).

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 about 0.8 degrees C (1.4°F):

"According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS)…the average global temperature on Earth has increased by about 0.8°Celsius (1.4°Fahrenheit) since 1880. Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20°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.


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 outbound radiation. We can examine the spectrum of upward long-wave radiation in 1970 and 1997 to see if there are changes.


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

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 know CO2 absorbs and re-emits longwave radiation (Tyndall). The theory of greenhouse gases predicts that if we increase the proportion of greenhouse gases, more warming will occur (Arrhenius).

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.

These data provide empirical evidence for the predicted effect of CO2.

Basic rebuttal written by GPWayne

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Last updated on 1 August 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.

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 426 to 436 out of 436:

  1. devcarr @424... I believe that's referred to as emission altitude or effective emission height.

  2. Thanks. I see it is also called the Effective Radiating Level (ERL). But I can't find information on how much is has increased in altitude.

    I am surprised by the graph showing that temperature rise stops almost immediately when emissions stop. I would think temperature would rise until the Effective Radiating Temperature, at the ERL, caused enough outgoing radiation to balance the incoming radiation from the sun. Does that happen so quickly that it can be ignored?

    Is the ERL very close to the top of the troposphere, below it, or above it?

  3. Devcarr:

    The emission altitude varies with the wavelength of IR light that you examine.  Some IR is emitted from the surface while other wavelengths are emitted from 15 kilometers up.  The average emission altitude is about 10,000 meters which is near the average height of the troposphere.  The top of the troposphere also varies in height.

    The temperature in the troposphere varies according to the lapse rate.  The lapse rate is about 6C per kilometer (this also varies).  Temperatures have increased about 1.2C.   !.2 devided by 6 is .2 km or about 200 meters.  That is a very rough estimate.  I was very surprised when I first learned that the increase in the emission altitude is so small.  (the change is slightly different for different wavelengths of IR light).

  4. devcarr @427,
    I assume your surprise at the graphic @425Response showing the AGW temperature rise halting as soon as emissions stop because your questioning didn't expect the residual warming-yet-to-come to be pretty-much balanced by the reduction in climate forcing. Following the end of emissions, the GHGs are no longer being boosted by those emissions but instead falls as GHGs are naturally drawn out of the atmosphere.

    Perhaps your expected a response to your question @425 to be for the time for warming to end and equilibrium to be reached for a constant level of GHGs. This is of course a different question.

    The radiative imbalance is running at something like 1 Wm^-2 and if that were allowed to play out by keeping GHG levels constant, it would take a century or more before equilibrium is effectively achieved, with ECS=+3.0ºC suggesting an additional +0.8ºC. The time for this is uncertain as it is the longer-term processes that are poorly quantified when the ECS is assessed, and the value of ECS is famously poorly bounded.

    Hansen et al 2011 fig4

    The left-hand graphic here is Fig 4a from Hansen et al (2011) 'Earth’s energy imbalance and implications'. It shows a large part of the warming-to-come appearing in the first decade and that followed by a further large part in the following century. The big uncertainty is in the longer-term warming and this is the major cause of ECS being so poorly defined. And that long-term warming is not entirely a thermodynamic thing. Major longer term warming due in albedo can be caused by a minor and quite insignificant temperature imbalance acting over a long period. So any such long-term warming will result from the warming-already-achieved (+1.0ºC) as well as warming-to-come (+0.8ºC). And so if climate forcing were maintained, this longer term could then become quite significant.

  5. devcarr @427,
    You ask about the altitude of the planet's IR emissions out into space, this ERL as you term it. The physical altitude is dependent on wavelength so there is no single altitude.

    The graph below shows contours of absolute temperature and when these emission altitudes are within the troposphere, the altitude can be inferred from these temperatures. The big 'bite' out of the emissions spectrum at 666 Wavenumber is due to CO2, the smaller one at 1050 Wavenumber is ozone. Note the small central spike within each of these 'bites'. These are caused by the emissions altitude of the central region of these wave bands being so high that they are up in the stratosphere where the temperature starts to rise again. Thus the spike yields a higher emissions temperature than the rest of the 'bite'.
    Being up in the stratosphere means that as the central spike expands with additional CO2, this specific region of the affected wave band will act to cool the planet while other parts of the 'bite' expand to warm the planet. The net effect remains a warming one.

    emissions spectrum from Clive Best

    Significant change in CO2 concentration in the atmosphere will impact the shape of the 'bite' at 666 Wavenumber markedly, as well as kicking off other 'bites' (or perhaps 'nibbles') close to the ozone 'bite'. This is described in Zhong & Haig (2013) 'The greenhouse efect and carbon dioxide'.

  6. Can someone please comment on this article?  Why is overall radiation still constant over last couple decades?

  7. rkcannon,

    If Zoe Phin really thinks she has a better grasp than the Kramer et al (2021) team, she should go hack it in the litterature, I'm sure that GRL would be interested. In the meantime, the Kramer et al study is even more recent than Zoe's blog post, peer-reviewed and published, so it was picked up by the SkS team. It uses the data from CERES, whic includes the ERB instruments. Kramer et al find 0.5 W/sq meter increase just between 2003 and 2018.

    Kramer et al (2021)

    NASA's press release on the paper.


  8. Rkcannon, permit me to add a comment, as a non-expert in these matters.   Measuring the alterations in outgoing IR radiation from Earth, is a matter of measuring a very small quantity against the background of a very large quantity.   Rather like measuring your bodyweight on ordinary bathroom scales ~  with and without wearing your wristwatch.   It is hard to get an accurate assessment of the weight of your wristwatch, even though you repeat the measurements daily over many years!

    (Nevertheless, basic physics and common sense do combine to tell you that the wristwatch has a real positive weight, not a negative or zero weight.)

    Taking a step back and looking at the climate situation :-  over many decades, the observed surface temperature is rising, and the observed Ocean Heat Content is rising, and the observed planetary ice-sheets are melting, and the observed sea-evel is rising.   And these changes are in accord with our understanding of radiational basic physics, too.

    So only a fool (or scoundrel) would assert that Global Warming is not occurring.   (Despite the difficulties inherent in a situation of continual variations and distributions of planet-wide cloud types.)

    Speaking of which :- the NoTricksZone  website has an appalling track record of presenting distorted and/or misleading information.   It is clear that "NTZ"  has a strong agenda of presenting disinformation via misquotes and misinterpeting of scientific papers.   Yes, I am making an ad hominem comment ~  and it is a very well deserved ad hom in the case of NTZ  and its chief editor.   Whenever you see something "scientific" reported on NTZ  website, your own proper skepticism should immediately go to Triple Red Alert overdrive status.

    There are several versions of reporting circulating about an initial study (Kramer et al., 2021).    NTZ's  effort mentions a Zoe Phin, who is IIRC one of these "GreenHouse Effect does not exist" people ~  so again, your skepticism should result in a close examination of what's being put forward.  (Unless you wish to dismiss it all as a huge waste of time for you to investigate.  Just as you do when faced with a complicated "proof" of Flat Earth . . . or a new Perpetual Motion Machine . . . or a complicated screed of mathematics supplied by AGW-deniers like Christopher Monckton.)

  9. I would note that Zoe Phin's credentials are "B.S. Economics, M.S. Quantitative Finance, 10-year Wall Street veteran"

    These are most certainly excellent degrees and experience to have, but they are far from what you would want to rely on for topics like, changes in planetary energy balance and the earth's greenhouse effect.

    When you need an expert in economics and finance, you certainly don't go looking for a phycisist or a climate scientist. I'm not sure why people think the reverse is rational.

  10. Rob Honeyutt @434,

    Even though Zoe Phin's credentials are not that of a climatologist, I would suggest her work providing the basis for the 'article' presented @431 (her work is set out here) is so poor, she should approach her college and ask for her money back. Evidently her education has entirely failed.

    What Zoe Phin manages to show is no more than "Look!! Lots of numbers with decimal points. So I'm right and AGW is fake!!!" Or in her own  baseless words "The greenhouse effect hypothesis is simply incorrect and should be abandoned for the sake of empirical science."

    The first table of numbers presented by Zoe Phin are presumably taken from CERES and shows that high, uppermid & lowermid clouds have increased through the period 2003-19 while low cloud shows no statistical trend. However I don't see any use of this data within the analysis provided.

    The second and third tables (again presumably from CERES but quite where from CERES is a different matter as this is modelled data not measured data) shows annual global average values for surface upward & downward IR and TAO upward for clear-sky/all-sky and with/without areosols. These numbers are not the sort of thing that can be measured globally and further are a ridiculously good fit with GISS annual global average SAT which shows to anybody with half a brain who thought to examine them that the values are modelled numbers not measured numbers.

    The extent of the analysis provided by Zoe Phin is simply to compare the 2003-19 averages of these annual global average IR numbers and declare the upward surface IR do not vary enough between clear-sky/all-sky for clouds to play any role in any greenhouse effect. Thus Zoe declares AGW must be fake.

    The averaged clear-sky/all-sky TOA upward IR numbers vary by 25Wm^-2, enough to provide 20% of a theoretical +33ºC greenhouse effect, not a million miles from what would be anticipated. But the analysis dismisses the relevance of TOA IR. We are told "Less top-of-atmosphere outgoing radiation doesn’t cause surface heating."  So what does less energy leaving the planet do? Where does the energy that is now failing to exit planet Earth go? These are the questions Zoe Phin needs to answer before she continues to make a total fool of herself.

  11. MA Rodger,

    It may be too late for that. Down the comment thread at NTZ she seems to argue against space being a heat sink.

    Other commenters refer to G&T as if it held any value.

    It seems NTZ is every bit as much a waste of time as WUWT.

  12. Is there more continuous data available than the individual year comparisons of change in brightness temperature shown?

  13. Long Knoll:

    Google Scholar lists 233 citations of the source for figure 2 (Harries 2001). You could try looking through those to see if they provide additional data that meets your concerns.

  14. Long Knoll @437,

    There are papers that repeat the spectal comparison for a period of a decade or so as per Harries et al (2001), such as Chen et al (2007)Bantges et al (2016), Rentsch (2020). But measuring outgoing IR is difficult enough without the spectral aspect to the analysis so yearly data would be likely lost in the error bars. Feldman et al (2015), a paper I haven't read, do say they have managed to show a statistically significant result using downwelling surface IR.

  15. Thanks all! Will have a look as soon as I have the time!

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