All gas molecules have the capability to absorb radiation energy. $\rm{CO}_2$ has much less capability to absorb radiation energy, comparing with water vapor. In Earth's atmosphere currently $\rm{CO}_2$ constitutes only about 0.04% (400 parts per million) of the atmosphere. On average, about 2 to 3% of the molecules in the air are water vapor molecules. In the air the water content is about 50 times higher than $\rm{CO}_2$. So it seems that $\rm{CO}_2$ content increase in the air should not have any measurable contribution to global warming. Is there any experiment to prove that $\rm{CO}_2$ with the atmosphere concentration can have greenhouse effect?

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    $\begingroup$ " On average, about 2 to 3% of the molecules in the air are water vapor molecules. In the air the water content is about 50 times higher than CO2. So CO2 content increase in the air will not have any measurable contribution to global warming. " This is a very simplistic way to look at things that misses the point, is misleading and simply wrong. $\endgroup$ – Gimelist Feb 13 '16 at 22:05
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    $\begingroup$ @DavidHammen I think the question should stay open. It's a common misconception and deserves a solid answer on this site. To my mind, the main problem was that the phrasing was loaded: it baldly stated "CO2 will not have a measurable contribution", which is an (incorrect) answer rather than a question. I've edited the question to rephrase that bit. $\endgroup$ – Pont Feb 14 '16 at 7:54
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    $\begingroup$ The greenhouse effect depends on the height from which outbound IR is not absorbed by the greenhouse gasses above. The more GHGs in the atmosphere, the higher the layer becomes, the colder it is (due to the lapse rate), the less IR escapes. This creates an energy imbalance, and so the planet warms (starting at the surface) until the radiating layer warms sufficiently to restore balance. The upper atmosphere is cold and so very dry, so there isn't as much absorption by water vapour above the radiating layer realclimate.org/index.php/archives/2007/06/… $\endgroup$ – Dikran Marsupial Feb 17 '16 at 9:50
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    $\begingroup$ Given that you spent less than 12 minutes looking, I am not greatly surprised, it is set out on pages 19 and 20 of Ekholms paper. The wikipedia page for Gilbert Plass gives references to several of his papers, which would take more than 10 minutes to read. The mechanism is also explained in the realclimate link I gave earlier in the discussion. I'm surprised you managed to get through Principles of Planetary Climate in less than 12 minutes! I think your responses have demonstrated that you are not really interested in the answers to your questions. $\endgroup$ – Dikran Marsupial Feb 17 '16 at 14:41
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    $\begingroup$ It's really annoying that these questions always seem to be asked by deniers (e.g. people who are not actually interested in the answer, just interested in "proving the science is wrong" or what ever). It means that the whole page gets tied up in stupid arguments that are irrelevant to the question... It would be nice to have some canonical questions and answers on these topics that don't devolve into that mess.. $\endgroup$ – naught101 Feb 29 '16 at 7:41

Your question about water vapour is quite a common one among people learning about the greenhouse effect. Once you discover the relevant proportions of water vapour and CO2 in the atmosphere, it's perhaps natural to assume that the CO2 can't be playing a major role. In reality it doesn't work like this, for at least a couple of reasons. First, let's look at what the 98% actually means in practice.

It's not just about the concentration

The concentration of a gas in the atmosphere isn't the only thing that determines its warming potential -- otherwise nitrogen, at 78% concentration, would be the most important greenhouse gas. Each gas has a different radiation absorption spectrum, and a different warming potential. So despite its much greater prevalence in the atmosphere, water vapour only has about twice as much long-wave absorption potential as CO2.

At this point, you may be thinking ‘but twice as much is still a lot more, so the CO2 can't be significant!’. But there's another non-obvious factor here, which has to do with the difference between feedbacks and forcings.

Forcings versus feedbacks

In the climate system, CO2 concentration is a forcing, whereas the water vapour concentration is a feedback. To illustrate the difference, here's a crude analogy:

Suppose that I'm trying to lose weight, but I'm reluctant to reduce the 300 grams of delicious chocolate cake that I eat every day. Having read that the human body is around 80% water by weight, I conclude that the cake can't be a problem: after all, I drink 2 kg of water per day, so it would make more sense to reduce that! So I cut down to 1 kg of water per day and maintain my cake intake. Several weeks later, I'm surprised to find that I haven't lost any weight! What's happened? It turns out that the human body regulates its water content, so reducing my intake just reduced my output. Fat storage isn't regulated in the same way, so the cake keeps piling up.

It's a similar story with CO2 and water vapour: CO2 concentration in the atmosphere actually changes for a long time (decades to centures) when we release more gas. Water vapour is, in effect, self-regulating. If we could wave a magic wand and instantaneously remove all the water vapour from the atmosphere, here's what would happen:

water removal experiment

Within 50 days, the water vapour is back to within 1% of where it would have been without our intervention.

So, if the water vapour concentration isn't controlled by input, what is it controlled by? Temperature. The warmer the atmosphere is, the more water vapour it can hold. This means that when the temperature goes up due to increased CO2, the water vapour content also increases, which further intensifies the greenhouse effect. It's acting as a positive feedback.

Since you're not the first person to ask this question (by a long way), there are already some good resources online specifically addressing the different roles of water vapour and CO2 in global warming.

Laboratory measurements of CO2 absorption

You seem to be particularly interested in laboratory experiments on carbon dioxide absorption. As an excellent starting point, I can recommend the (currently) 26 publications in AGW Observer's list of papers on laboratory measurements of CO2 absorption properties. If you're really keen to see experimental confirmation that CO2 can still absorb radiation even at atmospheric concentrations, you could take a look at (for example) Taylor and Yates (1957), Yates and Taylor (1960), or Streete (1968), all of which clearly demonstrate that CO2 absorption bands are present in normal atmospheric air.

As an aside: personally I find find that the numerous spectroscopic observations of the whole atmospheric column -- from satellites or ground stations -- provide a more compelling demonstration of the greenhouse effect. After all, the atmosphere isn't a homogeneous bottle of gas that can be faithfully scaled down into a lab sample. But your question and subsequent comments indicate that you're not interested in measurements of the atmospheric column itself, so here I'm just concentrating on ground-level experiments which demonstrate the long-wave absorption properties of CO2.

Answers to comments

If there is a such positive feedback exist without other negative feed backs, the earth will get warmer without co2 contribution.

I think you might be a little confused about the definition of a feedback here. As I've tried to explain above, the term ‘feedback’ means that the water vapour content is determined by other factors within the climate system. It can amplify the effects of forcings, but it can't in itself ‘push’ the system one way or the other. So you can't have a positive feedback without some kind of forcing: the forcing is precisely the input which the feedback is ‘feeding back’ into the system, in this case via the global temperature increase.

To understand the difference, it might be helpful to think about another form of feedback familiar to a lot of people -- audio feedback through an electric guitar and amplifier system. As you can see in the linked video, there's no sound until the guitarist plays a chord. But once he does play a chord, the guitar pickup itself detects the amplified sound and feeds it back into the amplifier, creating a loop which sustains the sound indefinitely. You can think of the initial chord as analogous to the CO2 input, and the feedback loop as analogous to the water vapour effect: the audio feedback, like the water vapour feedback, doesn't do anything until it's got an initial input from some other source.

All co2 released from fossil fuel was in the atmosphere during the dinosaurs time. If co2 release from all fossil fuel, it will restore the dinosaurs time atmosphere status. ... you can not deny that those co2 was in the atmosphere in earth history and earth animals and plants flourished

This doesn't really have much to do with your original question! If you want to ask about this, please go ahead and post a new question; StackExchange isn't a discussion site, and we try to keep each question focused on a single topic.

With this in mind, I'll just give a very brief answer here. Your formulation is a little inaccurate: the age of the dinosaurs spans at least 135 million years, there was a lot of carbon burial before it started, and all the carbon currently in the ground wasn't in the atmosphere at the same time. However, it is true that the Earth has seen higher CO2 levels than at present, and that much of that excess carbon is now locked up in fossil fuels. So what would happen if we put a lot of that carbon back in the atmosphere and tried to recreate the high-CO2 atmosphere that, say, T. rex breathed in the Late Cretaceous? Back then, the Earth was a lot warmer, with little or no ice at the poles. One of the biggest effects of re-releasing all that carbon would be the melting of our current polar ice sheets, raising sea levels by some tens of metres. The problem is not that the Earth will become completely uninhabitable by any life form whatsoever (although many current species will go extinct). The problem is that a lot of species, including Homo sapiens, are poorly prepared for a climate change of this magnitude. Ten percent of the world's human population and 8% of its urban land area is in low-elevation coastal zones, and would be completely submerged by a 10-metre sea-level rise -- which is still far less than the sea levels seen during the age of the dinosaurs.

Sea-level rise is just one effect among many, but for now I'll leave it at this since (as I mentioned above) this isn't related to your original question.

If a person cut his water intake in half, he will get sick before he can get fatter.

This is a little peripheral to the question, I think :-). If you don't like the analogy you can ignore it: it's not essential to the explanation. However, just to reassure you: the EPA Exposure Factors Handbook (2011 edition) gives a mean value of 1,043 ml for the daily drinking water intake of an adult in the US. Thus, I don't think that restricting one's water intake to 1 kg per day, as in my thought experiment, would necessarily cause sickness.


  • Streete, J. L. (1968). Infrared measurements of atmospheric transmission at sea level. Applied optics, 7(8), 1545-1549.

  • Taylor, J. H., & Yates, H. W. (1957). Atmospheric transmission in the infrared. JOSA, 47(3), 223-226.

  • Yates, H. W., & Taylor, J. H. (1960). Infrared transmission of the atmosphere (No. NRL-5453). Naval Research Lab, Washington DC.

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    $\begingroup$ If a person cut his water intake in half, he will get sick before he can get fatter. $\endgroup$ – Charlie Jiang Feb 13 '16 at 19:42
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    $\begingroup$ @Charlie Jiang: if you're going to ignore the proof once it's presented, why even bother to ask? As for your claim re dinosaurs' time, 1) It's wrong - lots of fossil fuel deposits were laid down long before the dinosaurs; and 2) Life forms existing today have had at least 65 million years to evolve away from their ancestors that flourished in that atmosphere. $\endgroup$ – jamesqf Feb 14 '16 at 5:16
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    $\begingroup$ @Charlie Jiang: Certainly I can and do deny it. There is a geological carbon cycle, in which (over millions of years) CO2 is added from e.g. volcanos, and removed as fossil 'fuels' and carbonate rocks. $\endgroup$ – jamesqf Feb 16 '16 at 5:24
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    $\begingroup$ @CharlieJiang Do you want to understand or do you just want to argue? Attacking an analogy (drinking water; which is there to make the main explanation more understandable) is only 'trying to score a point'. This method of arguing is called a strawman fallacy. $\endgroup$ – Jan Doggen Feb 16 '16 at 15:18
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    $\begingroup$ @CharlieJiang So what? Who exactly is arguing that increased CO2 levels are bad for life? Even your question makes no such claim. The main issues people have with global warming is (1) it's a change (which is ridiculous - nature means change, just get over it), (2) there may be significant consequences we didn't predict, (3) a lot of humans will have to move. In any case, the root of your argument doesn't make any sense anyway - the pre-oxygen atmosphere allowed the anaerobic life of the time flourish too, but that doesn't mean we'd want an atmosphere without oxygen, does it? $\endgroup$ – Luaan Mar 9 '16 at 16:27

Experiments don't necessarily prove things, per se; and in particular, a single experiment tends not to prove anything - at the very least, replication of the experiment is required. Experiments provide contributory evidence. Confidence in a hypothesis can come about from a combination of theory, lab experiments and natural experiments.

Our knowledge about the greenhouse effect comes about from a combination of lab experiment and natural experiment, complemented by various theories that are testable and have tested successfully.

The first experimenters and theorists worked in the nineteenth and early-to-mid twentieth century: Joseph Fourier, John Tyndall, Svante Arrhenius, Guy Callendar. Perhaps most notably, it's John Tyndall's work with a thermopile, measuring the absorption spectrum of various gases, that were the first to most directly address your question. More recently, advances in spectrographic analysis mean that lab experiments can identify which parts of the electromagnetic spectrum are absorbed by $\ce{CO2}$, and by how much. Crucially, there are bands of very high absorption by $\ce{CO2}$, at frequencies where $\ce{H2O}$ has much lower absorption:

enter image description here

So, we have lab experiments that demonstrate that CO2 with atmospheric concentrations do contribute to the greenhouse effect. These experiments go back almost two hundred years, and the more research we do, the greater corroboration we find, from both lab experiments, natural experiments, and wider theories in physics and chemistry.

  • $\begingroup$ were there published papers during the Industrial revolution about 200 years ago that pointed out the dangers of global warming due to the use of fossil fuels ? $\endgroup$ – gansub Feb 15 '16 at 13:54
  • $\begingroup$ @gansub would you like to ask that as a question in its own right - it's too good to have it, and the answer, tucked away in comments $\endgroup$ – 410 gone Feb 15 '16 at 14:26
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    $\begingroup$ @CharlieJiang That has already been explained in a previous answer. $\endgroup$ – bon Feb 15 '16 at 16:57
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    $\begingroup$ @CharlieJiang you asked specifically about experiments showing CO2 absorption - which I've answered. Please don't ask more than one question at one time. This site works by asking one question at a time. If you want to ask whether and why CO2 has a greater contribution to the greenhouse effect than H2O does, then please post that as a separate question. But do bear in mind that you'll get the same answer as Pont has posted above. If you think you can overturn 200 years of science, then go ahead and write up your work: Nobel prizes and billions of dollars await you. $\endgroup$ – 410 gone Feb 16 '16 at 1:15
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    $\begingroup$ @CharlieJiang - EnergyNumbers did not say you cannot question his or any other answer here. He merely said you can only get one answer per question. You can ask another question on the answer he has written or any other question. $\endgroup$ – gansub Feb 16 '16 at 13:58

The short answer to your question really is just this: Yes, CO2 causes global warming.

There are many resources out there on the internet that explain this in about as much detail as you can tolerate, and whatever we could answer here does not come close to what others have already collected. I would suggest you start at the wikipedia page on global warming (and in particular the section on greenhouse gases, as well as the wikipedia page on greenhouse gases.

  • $\begingroup$ I just skimmed those pages, but have there been actual laboratory experiments demonstrating the greenhouse effect? $\endgroup$ – Barry Carter Feb 13 '16 at 3:21
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    $\begingroup$ They've been doing them for over 100 years. en.wikipedia.org/wiki/Svante_Arrhenius#Greenhouse_effect $\endgroup$ – farrenthorpe Feb 13 '16 at 4:36
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    $\begingroup$ farrenthorpe: The question is about experiment prove, not theory. I have read all kinds of theories out there. $\endgroup$ – Charlie Jiang Feb 13 '16 at 13:29
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    $\begingroup$ @CharlieJiang and Barry Carter: Lots and lots of experiments. Google "absorption spectrum co2" for just a few of those. $\endgroup$ – Wolfgang Bangerth Feb 14 '16 at 18:29
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    $\begingroup$ @Wolfgang Bangerth: Do you have difficulty to understand my question? I did not ask for people's opinion. I asked to show me the experiment proof of greenhouse effect. In simulated atmospheres, higher concentration of CO2 will cause temperature higher. $\endgroup$ – Charlie Jiang Feb 15 '16 at 11:33

The Faint Young Sun Paradox - how greenhouse gases can keep a planet warm:

When the Earth formed 4.5 billion years ago the Sun was around 30% less luminous than it is today and it has increased steadily since, based on well established models of solar evolution. Simple energy balance models of the Earth show that, with a similar atmosphere to today, the mean global temperature would have been well below freezing. However, there are sedimentary rocks from at least 3.8 billion years ago which show clear signs of being lain down in liquid water. Therefore, something must have kept the planet warm in its early history. The culprit is $\ce{CO_2}$, and probably $\ce{CH_4}$ as well, which may have been present in levels at least 100 times as high as today, and possible far higher. Another example of the power of the greenhouse effect of $\ce{CO2}$ is Venus, which has an atmosphere 50 times as dense as that on Earth and composed of 97% $\ce{CO2}$. Surface temperatures reach 500°C.

The Quaternary Record - non-linear responses to solar forcing:

This figure shows the $\delta ^{18}\text{O}$ record from benthic foraminfera preserved in deep sea sediment cores from the past 1.2 million years alongside the July 21st insolation at N65 (northern hemisphere summer) as calculated from celestial mechanics. $\delta ^{18}\text{O}$ from benthic forams is a good proxy for ice sheet volume (high $\delta ^{18}\text{O}$ = high ice volume, and vice versa) and so what we see is that ice sheet volume generally correlates well with solar forcing. enter image description here

However, look closer and there are problems. Although deglaciations correspond will with increases in summer insolation, the magnitude of the increase is not always well correlated to the scale of the deglaciation. In particular, the stage V termination shows a huge deglaciation (one of the largest in the Quaternary) in response to a very small solar forcing. Clearly some other factors must amplify the forcing signal to produce the changes observed. Secondly, we observe that in the later part of the Quaternary the major period of glacial-interglacial oscillation is about 100,000 years. The solar forcing consists of three components, eccentricity (100,000 and 400,000 year periods), obliquity (41,000 year period) and precession (21,000 year period). If you look at each of the forcings separately, the eccentricity forcing is by far the weakest and yet it is dominant in the $\delta ^{18}\text{O}$ record. Although it is not obvious from this chart (the earlier half of the Quaternary is not shown) there is a switch at around 1 million years ago from a dominant glaciation period of 40,000 years to 100,000 years and yet there is no change in the forcing pattern. Additionally, note that in the present day insolation is low and yet the world is warming.

The next figure shows atmospheric $\mathrm{CO_2}$ and $\mathrm{CH_4}$ records as measured from Antarctic ice cores from the past 800,000 years, alongside $\delta \text{D}$ records which are a good proxy for temperature. There is a very good correlation between $\mathrm{CO_2}$ levels and temperature, and a good correlation with $\mathrm{CH_4}$ levels, particularly for large temperature rises.

enter image description here

But haven't we been here before?

The figure below shows the $\delta ^{18}\text{O}$ record from the last 65 million years. The temperature scale on the right only applies to an ice-free world (i.e. before ~30 million years ago [I won't go into the reasons why unless you are particularly interested]) but the general trend is still evident. There has been gradual cooling since the Eocene, with some variability along the way. $\ce{CO2}$ levels in the Eocene were at least 1000ppmv (compared to 280ppmv for pre-industrial levels and 400ppmv for the present day) and this is reflected in the temperatures and lack of ice. enter image description here

You might then say, what is the problem with putting all this $\ce{CO2}$ into the atmosphere? The key issue is not the magnitude of the change, it is the rate of the change. The changes in $\ce{CO2}$ concentration in the Eocene took place over millions of years, giving most life time to evolve to cope with the changing temperatures. However, we are putting large volumes of $\ce{CO2}$ into the atmosphere in the space of a few hundred years and this is having drastically different effects. The closest analogue we have to our current situation is the Palaeocene-Eocene Thermal Maximum. If you look closely at the graph you will see a spike in the $\delta ^{18}\text{O}$ record at 55 million years ago. This corresponds to a rapid warming of around 5-8°C and subsequent cooling back to previous temperatures over a period of around 200,000 years. Although it is not shown on this graph, there is a corresponding dip in the $\delta ^{13}\text{C}$ record, indicating a large injection of carbon into the atmosphere, in the form of $\ce{CO2}$ and $\ce{CH4}$, which caused a rapid rise in temperatures due to the greenhouse effect. This is very similar to what we are doing today, only we are doing it at least 20 or so times faster.


Solar forcing is a factor that affects global temperatures, particularly in the recent past, but the responses are highly non-linear, and the forcing alone is not enough the explain the observed changes. $\ce{CO2}$ and $\ce{CH4}$ levels in the atmosphere are closely correlated with global temperatures and abrupt changes in these levels has caused abrupt changes in temperature in the past. The amount of water vapour in the atmosphere is a function of temperature (refer to @Pont's excellent answer) and so is not a relevant forcing for increasing temperatures (indeed clouds may have a slight negative feedback effect because they have a very high albedo).

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    $\begingroup$ Actually we perhaps have an even better parallel: the Permian-Triassic Extinction event (AKA "The Great Dying"). One plausible theory (though not the only one) is that large volcanic eruptions under coal beds added large amounts of CO2 to the atmosphere. $\endgroup$ – jamesqf Feb 16 '16 at 5:29
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    $\begingroup$ @CharlieJiang I disagree. The Earth's climate system is far too complex to simulate in a lab experiment. It is not enough just to put the contents of the atmosphere in a flask and vary the $\ce{CO2}$ concentration. This is in no way representative of the Earth's atmosphere as it ignores the very strong coupling with other parts of the climate system, namely the hydrosphere, cryosphere, lithosphere and biosphere. $\endgroup$ – bon Feb 16 '16 at 12:30
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    $\begingroup$ @CharlieJiang Yet you have no evidence for your belief and you dismiss any evidence against it as poor quality or 'theoretical'. Your argument is purely based on opinion. $\endgroup$ – bon Feb 16 '16 at 12:34
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    $\begingroup$ @CharlieJiang You say "Only direct measurements from simulated atmospheres will convince me." And yet, in other comments, you confidently tell us about the CO2 levels in "the dinosaurs time atmosphere", which are reconstructed using sedimentary records just like the ones you reject here. So it seems that for you, palaeo records are sometimes reliable... but only when they happen to serve your argument at that particular moment. $\endgroup$ – Pont Feb 16 '16 at 15:41
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    $\begingroup$ @CharlieJiang In a simplified model, climate is a reflection of conditions in the troposphere but as I said before it is influenced strongly by processes in the oceans, on land, in the stratosphere, tectonic processes, etc. Unfortunately, we can't model climate perfectly yet (and never will be able to) but we are making steady progress through the use of computer modelling and interpretation of paleoclimatic data in order to test the models. $\endgroup$ – bon Feb 16 '16 at 21:45

To see why we can't perform an experiment in lab conditions to verify the greenhouse effect, we need to start by considering how the [rather badly named] greenhouse effect operates:

The Earth is in (to all intents and purposes) a vacuum, so it can only gain or lose heat via radiation. The sun emits most of its radiation at visible and UV wavelengths. The Earth's atmosphere is fairly transparent at these wavelengths and so the Sun's radiation mostly passes through it and hits the surface. Some of this radiation (determined by the Earth's albedo) is reflected from the surface back out into space, but the rest is absorbed by the surface, which causes the surface to be warm. The surface loses heat by radiating in infra-red wavelengths. Greenhouse gases absorb some of the IR radiation, which causes the atmosphere to warm up (the GHG molecules transfer some of this heat to non-greenhouse gasses by collisions, but heat is also transferred upwards by convection). The warm atmosphere re-radiates some of this energy both upwards into space and downwards back to the surface. The part that is radiated downwards is also known as "back-radiation" (and is directly observable). Now the important factor is not the amount of outbound IR radiated from the surface that is absorbed, but the altitude at which there are not enough greenhouse gases above to absorb the IR radiated upwards from that layer, so that it can escape out into space. The lapse rate means that the temperature of the atmosphere decreases with increasing height. This means that the more CO2 we put into the atmosphere, the higher this emitting layer becomes, and the colder it is. As the amount of IR radiated depends on the temperature of this layer, if this height increases then the amount of IR radiated from the planet falls, leading to an energy imbalance, with the planet absorbing more of the sun's radiation than it emits as IR, and so the planet warms up. This continues until the radiating layer warms up enough for the outbound IR to be in balance with the incoming radiation from the sun. So the more CO2, the warmer the mean surface temperature, all things being otherwise equal.

So in order to have a lab experiment that could replicate the mechanism of the greenhouse effect, we would need a vacuum chamber large enough to contain a vessel containing a column of air high enough to have a measurable lapse rate. This is clearly impractical. We can perform experiments in the lab to investigate the absorption of IR by greenhouse gases, and indeed Tyndall did this over a century ago, but we can't experimentally verify the greenhouse effect in laboratory conditions, just as we cannot experimentally demonstrate gravitational lensing in the laboratory.

We could of course commission the denizens of Magrathea to construct a replica of the Earth and experiment on that, but we would need a rather large laboratory.

This doesn't mean we have no evidence of the greenhouse effect. Of course we do, just as we do have evidence of gravitational lensing.

Update - brief description of the mechanism of the greenhouse effect from Spencer Weart's excellent book mentioned by @jamesqf:

What happens to infrared radiation emitted by the Earth's surface? As it moves up layer by layer through the atmosphere, some is stopped in each layer. (To be specific: a molecule of carbon dioxide, water vapor or some other greenhouse gas absorbs a bit of energy from the radiation. The molecule may radiate the energy back out again in a random direction. Or it may transfer the energy into velocity in collisions with other air molecules, so that the layer of air where it sits gets warmer.) The layer of air radiates some of the energy it has absorbed back toward the ground, and some upwards to higher layers. As you go higher, the atmosphere gets thinner and colder. Eventually the energy reaches a layer so thin that radiation can escape into space.

What happens if we add more carbon dioxide? In the layers so high and thin that much of the heat radiation from lower down slips through, adding more greenhouse gas means the layer will absorb more of the rays. So the place from which most of the heat energy finally leaves the Earth will shift to higher layers. Those are colder layers, so they do not radiate heat as well. The planet as a whole is now taking in more energy than it radiates (which is in fact our current situation). As the higher levels radiate some of the excess downwards, all the lower levels down to the surface warm up. The imbalance must continue until the high levels get warmer and radiate out more energy. As in Tyndall's analogy of a dam on a river, the barrier thrown across the outgoing radiation forces the level of temperature everywhere beneath it to rise until there is enough radiation pushing out to balance what the Sun sends in.

Some additional resources for Charlie:

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    $\begingroup$ @DikranMarsupial - Magrathea seems an interesting planet. Any idea when we will send some probes over there ? $\endgroup$ – gansub Feb 18 '16 at 1:16
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    $\begingroup$ @Charlie Jiang: If you don't want to wade through e.g. "Principles of Planetary Climate", there's a good, fairly non-technical, account of the history and science at "The Discovery of Global Warming", here: aip.org/history/climate/index.htm $\endgroup$ – jamesqf Feb 18 '16 at 6:20
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    $\begingroup$ @gansub, I believe the BBC ran a documentary series that went there in the 1980s (following a similar investigatory series on the wireless) ;o) $\endgroup$ – Dikran Marsupial Feb 18 '16 at 7:57
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    $\begingroup$ @CharlieJiang sorry, you are just digging yourself deeper, the realclimate link I gave is freely available, all you had to do was click the link and you would have found both an explanation of the greenhouse effect and an explanation of why water vapour doesn't nullify CO2 as a GHG, Alternatively you could read the IPCC reports (the first one is quite readable and available for free, as are they all). BTW the realclimate article was written by the same person that wrote the excellent (free online) book given in the link by jamesqf. $\endgroup$ – Dikran Marsupial Feb 18 '16 at 8:02
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    $\begingroup$ @CharlieJiang If you want to see what it takes to put numbers to the model, then read the papers by Gilbert Plass, which is where it was first done. You need to learn to walk before you can run, and learn a lot of basic physics your need to understand to make a useful numerical model. The best way to do this is to read a text book or two, such as the one by Ray Pierrehumbert to which I directed your attention earlier. $\endgroup$ – Dikran Marsupial Feb 18 '16 at 12:34

I think you can tell, because you were smart enough to ask the question in the first place, that the answers are not sufficient and ignore the big picture.

There is no observed or demonstrated greenhouse gas effect in the real world. The weight of the atmosphere alone gives an average global temperature at sea level of 288 degrees K, or 14.85 C. This is actually a little warmer than the true global average surface temperature because not all the surface is at sea level. There is no added heat from the greenhouse effect, and to know why you need to delve into thermodynamics and quantum mechanics.

In theory and shown by experimentation, a dipole molecule can absorb long wave radiation and cause it to vibrate and rotate, adding to its kinetic energy. However, there are limits on how much energy this long wave radiation can transfer to a molecule; if the molecule already has a certain level of translational kinetic energy, longwave radiation will not excite the molecule further. Just like you'd never see a 100 degree hot plate heat air above it greater than 100 degrees even as the hotplate continues to emit infrared radiation.

Most, err the vast majority, of the kinetic energy in air is derived from collisions of gas molecules with the solar-warmed surface, and from latent heat of water vapor. Most of this energy is translated throughout the lower atmosphere by molecular collisions and transferred to the upper troposphere by convection.

This is demonstrated by nature every night in arid regions of the planet. In deserts, such as the Sahara, the surface temperature quickly rises during the day. That's because no heat energy is being converted into latent heat by evaporating water, it's all going into kinetic energy which you can feel as temperature. At night, the surface and the air cool quickly as the heat is convected in air masses vertically and then easily radiate their energy into space with no measurable greenhouse gas warming. In fact, the Sahara Desert actually loses more energy into space than it receives from the sun on an annual basis. As counterintuitive as this sounds, it's been measured by satellites that have been operating for over a decade. The climate of Earth is dominated by water. The heat capacity and energy transfer mechanics of water in its three phases completely overwhelms any effect from longwave radiation donating electron volts of energy to trace gases in the atmosphere.


Or, you can believe conjecture, arm waving, and major assumptions that lead to claims like that Venus would be a mere -43 degrees F without the "greenhouse" effect.


  • $\begingroup$ These people are stuck with their erroneous conventional wisdom, hence, no actual rebuttal, just downvotes. There is no experiment to prove what you are looking for. The key point that they are missing is the quantum interactions of thermal radiation on molecules. Thermal radiation causes vibration of molecules, and vibrational modes of molecules is a quantized state. In other words, thermal radiation won't simply be absorbed by a molecule and make it vibrate more with each electron volt of added energy. The thermal radiation must make the molecule jump to the next quantized state. $\endgroup$ – RWT Sep 19 '17 at 15:58
  • $\begingroup$ If a molecule is already in a heightened quantized state, thermal radiation will simply transmit or reflect off the molecule instead of being absorbed. hyperphysics.phy-astr.gsu.edu/hbase/mod5.html $\endgroup$ – RWT Sep 19 '17 at 15:59
  • $\begingroup$ do you have any idea what you're talking about? Me and three others have downvoted this answer because the facts stated in it are nowhere close to true. $\endgroup$ – Eevee Mar 15 '18 at 16:45

The key part of this question is “Can CO2 with the ATMOSPHERE CONCENTRATION have a greenhouse effect?” Assuming this is asking “At current levels, can changing levels of the greenhouse gas CO2 be shown to cause changes in Earth’s temperature?” (Yes, it is a greenhouse gas, … BUT).

The answer is NO. In fact the reverse can be shown, and that “experiment” is practised by astronomers every time they do IR measurements. There’s complete absorption of CO2’s IR wavebands when measured from surface-based instruments for decades. Everywhere, not just on this blog, there’s been much discussion that this IR is re-radiated by CO2 molecules, in all directions, which is true.

However, none of this re-radiation can reach Earth because it encounters a greater concentration, ie higher absorption, in the lower levels. (Note, less than 50% can be directed back to Earth, and this % falls the greater the height of the re-radiation.)One can see this from surface-based instruments (mostly in telescopes) measuring zero transmission of CO2 IR spectra. This could not be zero if it did reach the surface.

[There’s a longer – too long for here – argument presented in Chapter One (plus importantly, its Addendum) at my site “Planet Earth Climate Topics” @ pjcarson2015.wordpress.com.]

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    $\begingroup$ "There’s complete absorption of CO2’s IR wavebands when measured from surface-based instruments for decades." Even if this were true, it is irrelevant as the greenhouse effect depends on the height from which IR is NOT absorbed, so even if all IR emitted from the surface were absorbed it would have no effect on the greenhouse effect. Note the IR backradiation that is absorbed on its way down will be redistributed by collisions with other air molecules, so the second part of the argument is also incorrect. $\endgroup$ – Dikran Marsupial Feb 29 '16 at 9:51
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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – gerrit Mar 1 '16 at 14:17

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