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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.

Denial101x video


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Comments 1 to 25 out of 456:

  1. The graph you show is not "The resultant change in outgoing radiation was as follows" as you state. What it is the data that has been manipulated to highlight the drops in radiance in the regions of the spectra that are absorbed by CO2 and CH4 (and other trace gases). The fact that no area of the graph goes above the 0 point (dotted line). The real observed data shows a large region of the spectra above the 0 line. You can see this in the original Harris 2001 paper (unfortunately subscription) or Fig 3 in the Chen paper (free access). What this says is there are other parts of the spectra which are letting more energy escape from the planet in 2006 than in 1970. if you take the CO2 part of the spectra in isolation this would suggest greater energy retention (global warming). If you highlighted just the positive areas (say spectra from 800-1000) you would conclude greater energy radiance (global cooling). If you took the whole of the spectra I'm not sure whether you'd conclude greater or less radiance in more recent years. Why don't people look at the whole of the spectra and what would be the explanation for greater radiance at other wavelengths? I accept that this analysis might be way of highlighting the CO2 'signature' in the spectra I don't see how you can conclude global warming without analysis of the whole wavelength spectra.
    Response: Harries 2001 does look at the full infrared spectrum except for wavelengths less than 700nm (which happens to be where a large portion of the CO2 absorption occurs). The observed changes in the spectrum from 1970 to 2006 are consistent with theoretical expectations. As the atmosphere warms, more infrared radiation is radiated to space. However, less infrared radiation escapes at CO2 wavelengths. The net effect is that less total radiation escapes out to space.

    This is independently confirmed by surface measurements which find the net result is more longwave radiation returning back to the Earth's surface (Philipona 2004, Evans 2006). It's also confirmed by ocean heat measurements which find the oceans have been accumulating heat since 1950 (Murphy 2009).
  2. re comment 1 (HumanityRules), referring to apparent increases in radiation in the 800-1000 part of the spectrum in the Harries paper (the full version of which you can find through Google Scholar): on page two, 2nd column, that paper does indeed offer a potential explanation, viz. incompletely-cleared artifacts in the data (due to ice-cloud absorption). Considering the study is the first of its kind, its finding of reduced radiation precisely in the wavelengths associated with increased GHG concentrations remains remains highly suggestive, no?
  3. Re #1 (Humanity Rules). My understanding is that Earth emissions are modeled as black body radiation. Given the earths temperature of ~298K that means the IR radiation emitted by the earth peaks at around 600cm-1 and tails off around 1500cm-1. There is this on-line model that allows you to play with the earths emissions. This is Climate science 101 so it may be hopelessly naive !
  4. Re #2 (Hugh) Considering the study is the first of its kind, its finding of reduced radiation precisely in the wavelengths associated with increased GHG concentrations remains remains highly suggestive, no? Suggestive that there is a slight increase heat absorbance of CO2 and MH4 - but only if you cherry pick the data. Any atmospheric gas that absorbs infrared is considered a GHG. If you look at the full data you will see that although there is some increase in the heat absorbance of CO2 and MH4, you are seeing more heat escape at wavelengths different from CO2 and MH4. When talking about the causation of global warming, what does that suggest about the "greenhouse effect"?
  5. jabberwockey, it is nature that cherry picks wavelength as far as molecular absorption is concerned. On the contrary, thermal emission is broad band and it depends on temperature. Hence what you see in the full spectrum (the background) reflects the increase of surface temperature.
  6. Some quesitons. 1) Graph one is derrived from the difference between two satelites launched 26 years apart. How are their sensors callibrated so that they would give the same readings on the same day? 2)Graph 2. Why is there no downward radiation for CFCs, HNO3 NO2 etc, as there is for CO2, when the first graph shows that their energy is being 'trapped' in the atmosphere? Additionally. from the picture showing solar light penetrating the atmosphere but terrestrial IR being trapped, what happens to the solar IR? The sun produces aproximately 400000 times as much IR at the frequency absorbed by CO2 as the earth. The atmosphere must therefore absorbs and re-radiate half of this back into space. Also given the far larger solar IR radiation the CO2 will be saturated. Additional IR from the earth is a tiny amount in comparison. Answers appreciated.
  7. 1) different satellites and/or instrumets are always calibrated against one another in a better way than just the reading on a single day. 2) the first graph is a difference between spectra taken in two different points in time; the second graph is just a point in time. The difference depenss on how much the relative contrbution changed over time. IR from the sun is indeed absorbed by CO2. When you calculate an energy balance in a layer of the atmosphere you take both the incoming and outgoing energy into account. The tiny amount of energy (one and something W/m2) taken up by increasing CO2 will not make the earth look like Venus but it's enough to increase the temperature by a couple of degrees, maybe three by the end of the century. That's unfortunately enough to produce a significant change in the biosphere. There is no saturation effect to help us. The lifetime of the CO2 excited state is short enough for the CO2 molecules to be ready to absorb more of the incoming photons.
  8. @Ricardo. Point #2. That doesnt answer my question. Graph 1 shows energy being absorbed by all the gasses mentioned. Graph 2 shows enery be emitted by only some of them. Thats my point. How is it selective so that only some of the gasses emmit energy back o the surface which is what the second graph implies? Point #3. Energy ballance. So an increase in CO2 will absorb, and re-emit to space more IR from the sun as well as absorb and re-emit to earth more IR from the earth. Since the sun produces more IR than the earth how does the extra CO2 cause warming and not cooling? Point #4. Excited state duration. If the duration is less than 12 hours then all the energy absorbed by CO2 during the day will be lost at night. How does this generate net warming over a period of days-years? I am not being argumentative, I am geneuinely interested in the science behind GH gasses, it is just that logically there seem to be problems with the theory. I am glad you mentioned excited state durtation by the way because I could not find any information on this online and to me it is a critical factor in heat storage.
  9. matt sykes, #2 fig. 1 is a difference spectrum between 1976 and 1990. It shows only the changes during this period of time. If you want to compare the spectrum in fig. 3 (an emission spectrum), you need an absorption spectrum from space looking down. You can play with this using calculated spectra #3 there's much more energy in the visible than IR coming from the sun. It is this that warms the earth, not the IR. As far as CO2 absorption is concerned, it's not true that the IR around the 700 cm-1 band is much more than what the earth emits at the same wavelength. Integrating it over all the IR is wrong. Then the earth emits in the IR and this emission happens to peak around a CO2 absorption band; part of it is trapped producing warming. This is the very basics of the greenhouse effect. #4 The lifetime of the excited state is of the order of nano- to -micro- seconds depending on temperature and pressure. The extra energy can be released in two ways, by re-emission of a photon or by thermalization by collision (warming) of the surrounding air molecules; the re-emitted part will be absorbed again by other CO2 molecules and the process repeats itself until the pressure is so low that the photons have a high probability to escape to empty space. This is the way it works, again the basics of the greenhouse effect. If you increase CO2 concentration you slow down the process. You clearly still get cooling at night but not all of the energy absorbed during the day. It is not going to happen even if you do not increase CO2. A good example is the moon which, having no atmosphere, have enormous temperature gradients across the day/night line. I would more easily believe in your genuine interest had you not said "that logically there seem to be problems with the theory". Given that dozens of really smart people have worked on this for decades you'd better ask yourself "am i missing something?".
  10. @Ricardo. #2 OK, lets ignore thins since no one understands my quesiton. #3 According to this,, the sun emmits far more IR then the earth at the frequencies absorbbed by CO2. Surely all this IR saturates the CO2 in the atmopshere with energy, dwarfing the IR coning form the earth? #4 OK, so you say that the CO2 primarially rleases energy as a photon of light at the same frequency it absorbed it at, and that due toi the volume of CO2 it takes more than 12 hours for a photon on average to exit the atmosphere to space. Interesting you mention the moon, its daytime high is 105`c. Is it not the case that the reason the earth has a lower daytime high is because gasses in the atmosphere, including CO2, reduce the energy that strikes the surface? As for the throry of GH gas warming dont forget the poles are supposed to warm the most, but in fact only one of them is warming, so it seems the planet earth also has a problem with the theory. I am however prepared ot accept that I am in deed missing somehting, hence asking these quesitons.
  11. Sorry, final quesuiton, you say CO2 can release energy as a photon or by direct warming of other molecules. Do you have any figures for the rough percentage of energy released by each meathod? Given CO2s absorbiton band is only 8% of the total IR spectrum 92% of any release as a black body would not be re-absorbed by another CO2 molecule and thus exit the atmosphere directly.
  12. matt sykes, #3 the effect of CO2 is mainly on the band at about 700 cm^-1 or about 15 microns while the graph you show ends at 3 microns. Also, cosider that the absorption band is relatively narrow, it's really a tiny fraction. It'd be good if you calculate it yourself approximating the incoming radiation with the Plank formula, it will probably be more convincing than my words. #4 i did not say that "it takes more than 12 hours for a photon on average to exit the atmosphere to space.". Would it be so long you couldn't have significant day/night temperature variation; just the opposite is true. The warming of the poles depends on a lot of things, not least on atmospheric and oceanic circulation. Antarctica is "isolated" both by the Antarctic Circumpolar Current and by the strong westerly winds blowing in the Southern Oceans. No one expect the same warming as in the Arctic. I do not any rough figure on the ratio between collisional and radiative de-excitation. It quite complicated and it also depends on density and temperature. You can estimate the overall effect in the atmosphere from the ratio of the energy leaving the atmosphere over energy emitted by the surface. You last claim is definitely not true as can be easily seen in the absorption spectra of CO2 from space. Indeed, at the surface level the absorption lenth is pretty short, no way to escape directly to space.
  13. #3 2, 4.3 and 15 microns in fact. The first of which is well inside the graph I linked to. This one shows even more clearly how cooler bodies release less IR than hotter ones. Mind you, this isnt surprising. A piece of metal at 30 `C will be warmer to the hand than one at 20`C becaue it is producing more IR. As for calculating the IR of a particular frequency emitted by an object at a particular frequency, I dont know how to, perhaps you could direct me to the relevant formula althogh I think we have establisged that hotter bodies do produce more IR than cooler ones. #4. You said "You clearly still get cooling at night but not all of the energy absorbed during the day" I understodd this to mean that the energy absorbed during the day cane be entirely lost at night. If you meant something else then I appologise for misunderstanding you although my understanding seems logical still. #5 And there is clearly something else at play in the Arctic too since it is only as warm as it was in the 1930's. But, thats the complexity of climate! #6 But you cant fell from space what is happening to the energy absorbed by CO2. It is either re-emmitted at the same frequency and thus bounces around the atmosphere fomr mollecule to mollecule or it is emmitted as broad band radiation in which case all of it except the 8% absorbed by CO2 will end up released to space. So from a sensor looking down from space you will never see the CO2 absorbed energy, ie those banmds will be missing from the spectrum, regardless of the re-emmission mathod. However, if the re-emission is of broad band, ie black body radiation, than CO2 effectively converts narrow band to broad band radiation. This will increase the levels of non absorbed energy transmitted to space, which is what one of the other respondants above stated had occured in the later sattelite measurement.
  14. @Ricardo. Just found an online Plank law calculator. For wavelenghts betweem 3.9 and 4.1 microns the sun produces 340,000 times as much energy as the earth. This is close to my orevious estimate of 400,000. So, what does all this SOlar IR energy do in the atmosphere in comparison to terrestrial IR? Is it blocked in the uper atmosphere, does it saturate the CO2? Is it absorbed and re-emmitted into space in tha same way as terrestrial IR is absorbed and re-emmitted to the surface? If so, and given that it is 400,000 times stronger the effect of CO2 is to actually reduce the IR at the surface, not increase it.
  15. matt sykes, from about 2 to 8 microns IR from the sun is absorbed by water vapour, CO2 has no influence. Also, you still integrate over the whole IR, not just the CO2 bands. At 15 microns, instead, the atmosphere is transparent and the CO2 band is centered at the peak of the thermal emission, the overall effect is then larger. Go back to your Plank calculator but this time plugin in the right numbers for a meaningfull comparison. The visible is 0.4-0.8 microns, the CO2 absorbtion band is centered at 15 micron and with a width of 2 microns (14-16 microns, in reality it is much narrower than this). You end up with a ratio of the energy coming from the sun in the CO2 related absorption band and the visible of the order of 10^-4. Negligible. #6 In this very same post you can see absorption from CO2, you must be wrong. Indeed, the radiation abosrbed is re-emitted isotropically and part of it will be converted directly into heat in the atmosphere. At the top of the atmosphere you will see less radiation. I'd suggest to read how an idealized model works.
  16. matt sykes, i forgot to add that in any case absorption of sunlight is included in the radiative transfer codes.
  17. @Ricardo At 15 microns the sun produces 180 times as much energy as the earth. It is irrelevant how much energy the sun produces in the visible, it is the energy emmitted by the earth and absorbed by CO2 which is key. Your Siki link states: "Thus heat is easily let in, but is partially trapped by these gasses as it tries to leave. " This isnt true. Visible energy is let in, not heat. The heat of the sun is bloocked by the same GH gasses as block the heat going out. The difference is that thr sun produces far more heat than the earth. The net effect of GH ghasses is therefore to reduce the maximum temperaturs that would be otherwise acchieved, Consider the moon,. Its daytime temperature is 105`C. It gets this hot because it hasnt got an atmosphere.
  18. The net effect of GH ghasses is therefore to reduce the maximum temperaturs that would be otherwise acchieved, Consider the moon,. Its daytime temperature is 105`C. It gets this hot because it hasnt got an atmosphere. Er, sorry, but no. The moon's daytime temperature may average +105C, but its nighttime temperature is around -150C. Thus, the mean temperature of the moon is around -20C. Now, the earth's albedo is higher than that of the moon, so if the atmosphere had no effect (or a cooling effect, as you claim) then the earth should be cooler than the moon. Fortunately for us, water vapor and CO2 in the atmosphere raise the earth's mean temperature via a phenomenon known as the greenhouse effect.
  19. Matt, I think I see where your mistaken understanding is coming from. I think you're calculating the spectral radiance from Planck's law (if not, please explain where you get your figure of 180). I get about 150 by my calculations, but that's close enough to demonstrate the problem. The thing is that spectral radiance has a rather complicated definition: energy per unit time per unit surface area per unit solid angle per unit frequency The surface area of the sun is different to the earth - it's about 12000 times greater. 150 times 12000 is actually about 1.8 million. Hence the sun produces about 1.8 million times as much energy at 15 microns. It's hotter and larger, so this shouldn't come as a great surprise. But consider the fact that the sun is radiating this energy out in all directions, and that the earth captures only a tiny proportion of that energy because of our small size. Even Jupiter only appears as a tiny dot in the sky without a telescope. We can work out exactly how much we capture by dividing the area of a circle the size of the earth by the surface area of a sphere at the radius of the earth's orbit: pi*(6400^2)/(4*pi*(150 million)^2) = 0.00000000046 Taking this, and the 1.8 million value found before into account, the earth would (in the absence of atmospheric absorption) radiate out about 1200 times as much energy at 15 microns as we receive from the sun. This isnt true. Visible energy is let in, not heat. The heat of the sun is bloocked by the same GH gasses as block the heat going out. Visible energy IS heat. The heat we get from the sun is mostly within the visible and the near infrared, because the sun is hot and has a blackbody curve centred in the visible. When we absorb it, we radiate it back into space according to a much cooler blackbody spectrum, deeper into the infrared. This is all fairly basic greenhouse theory, and I don't think you've quite grasped the science behind it.
  20. Matt, the implication of what Stuart just explained in his next-to-last paragraph is that the energy coming from the Sun to the Earth is in the wrong wavelengths to be much absorbed by CO2, but the energy being emitted by the Earth is very much in the right wavelengths to be absorbed by CO2.
  21. Sorry, I meant Chen (2007) Here is the working link [Source]
    Response: Thanks for pointing that out - I've updated the link.
    [RH] Hotlinked paper in order to fix broken page formatting.
  22. A related question about the basics of the greenhouse effect: I have been sent a "study" by a guy who claims that CO2 cannot be a greenhouse gas because any warming would cause an instant increase in outward radiation due to the increased temperature difference between the atmosphere and open space, which would immediately diminish the warming. I tried to discuss some sense into him, but in classic denier fashion, he remained stubborn that nobody so far could give him a "convincing" reason why his objection cannot be true. I tried the earth system's heat capacity, speed of propagation of temperature changes in a kilometer-thick atmosphere, dynamic equilibrium, I even quoted the paragraphs from "A History of Global Warming" at that say that the actual greenhouse effect is caused by a greenhouse gas concentration change at the tropopause, effectively shifting it into higher, cooler layers of the atmosphere, which radiate heat less effectively than warmer layers, which forces the whole of the temperature gradient in the atmosphere to do something like a parallel shift in order to achieve a high enough temperature at the tropopause to force enough radiation out into space to re-establish the equilibrium - all to no avail. What would you experts here tell him (assuming he might still be convinced)? Any reply very much appreciated! Cheers, babelsguy
  23. Babelsguy, your friend apparently has not so much a quibble with C02 as a GHG so much as he does with the concept of an atmosphere being able to trap heat. After all, what he believes applies to any GHG. How does he explain why Earth's climate is not the same as that of a planet with no atmosphere?
  24. doug_bostrom, well, that is the interesting thing: He claims that his "study" would avoid all discussion of other issues but just show that CO2 (and the other GHGs) *cannot* be the culprit due to what he calls the energy balance question...
  25. Well I have now worked out on my own why the bigger temperature difference between atmosphere and space does not cause an energy loss that lets global warming collapse again: There is no bigger temperature difference. Due to the absorption length of GHGs being much shorter than the height of the atmosphere, the only place that matters for outward radiation is the upmost layer of the atmosphere that radiates at all - minus the absorption length to any reasonably small non-absorbed residue. Because the GHG concentration change effectively shifts this outward radiating layer upward into colder heights, and the atmosphere below has an increasing temperature gradient towards the ground, the ground has to heat up "a lot" to let the boundary layer also heat up sufficiently *until is as warm as before*, so it can radiate enough to re-establish equilibrium! Q.E.D. So the guy's conclusion is wrong because his whole presupposition is wrong. Garbage in - garbage out.

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