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How do we know CO2 is causing warming?

Posted on 8 October 2009 by John Cook

We've just perused the empirical evidence that humans are raising atmospheric CO2 levels. In earlier posts, we noted that tallying up the planet's heat content shows that our climate is accumulating heat, proof of global warming. But is there any evidence that links the two? Is there empirical data proving that increased CO2 contributes to the energy imbalance that causes global warming?

The greenhouse gas qualities of CO2 have been known for over a century. In 1861, John Tyndal published laboratory results identifying CO2 as a greenhouse gas that absorbed heat rays (longwave radiation). Since then, the absorptive qualities of CO2 have been more precisely measured and quantified by laboratory results and radiative physics theory (Herzberg 1953, Burch 1962, Burch 1970, etc).

Satellite measurements of the change in outgoing longwave radiation

So according to lab results and radiative physics, we expect that increasing atmospheric CO2 should absorb more longwave radiation as it escapes back out to space. Has this effect been observed? The paper Increases in greenhouse forcing inferred from the outgoing longwave radiation spectra of the Earth in 1970 and 1997 (Harries 2001) attempts to find out. In 1970, NASA launched the IRIS satellite that measured infrared spectra between 400 cm-1 to 1600 cm-1. In 1996, the Japanese Space Agency launched the IMG satellite which recorded similar observations. Harries 2001 compared both sets of data to discern any changes in outgoing radiation over the 26 year period. The resultant change in outgoing radiation was as follows:


Figure 1: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature (Harries 2001).

What they found was a drop in outgoing radiation at the wavelength bands that greenhouse gases such as CO2 and methane (CH4) absorb energy. The change in outgoing radiation over CO2 bands was consistent with theoretical expectations. Thus the paper found "direct experimental evidence for a significant increase in the Earth's greenhouse effect".

This result has been confirmed by subsequent papers using the latest satellite data. Griggs 2004 compares the 1970 and 1997 spectra with additional satellite data from the NASA AIRS satellite launched in 2003. Chen 2007 extends this analysis to 2006 using data from the AURA satellite launched in 2004. Both papers found the observed differences in CO2 bands matched the expected changes based on rising CO2 levels. Thus we have empirical evidence that increased CO2 is preventing longwave radiation from escaping out to space.

Measurements of downward longwave radiation

What happens to longwave radiation that gets absorbed by greenhouse gases? The energy heats the atmosphere which in turn re-radiates longwave radiation. This re-radiated energy goes in all directions. Some of it makes its way back to the surface of the earth. Hence we expect to find increasing downward longwave radiation as CO2 levels increase.

Philipona 2004 finds that this is indeed the case - that downward longwave radiation is increasing due to an enhanced greenhouse effect. Evans 2006 takes this analysis further. By analysing high resolution spectral data, the increase in downward radiation can be quantitatively attributed to each of several anthropogenic gases. The results lead the authors to conclude that "this experimental data should effectively end the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming."

So we have multiple lines of empirical evidence for CO2 warming. Lab tests show CO2 absorbing longwave radiation. Satellite measurements confirm that less longwave radiation is escaping to space. Surface measurements detect increased longwave radiation returning back to Earth at wavelengths matching increased CO2 warming. And of course the result of this energy imbalance is the accumulation of heat over the last 40 years.

Acknowledgements: A big thanks must go to AGW Observer. Their lists of papers on changes in outgoing longwave radiation, changes in downward longwave radiation and laboratory measurements of CO2 absorption properties made this post possible.

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Comments 201 to 235 out of 235:

  1. To put it another way; The measured heat put off by a barbecue grill in use is much greater than the heat generated by sunlight hitting the grill when it is not in use. By your logic of taking amplitude of heating regardless of area or duration we would therefor have to conclude that the negligible heat generated by sunlight is not responsible for warming the planet, but rather the planet is kept warm by people barbecuing in their back yards. Further empirical evidence of this fact can be seen in that backyard barbecues are most common in the Summer, the warmest part of the year, and the temperature plummets in Winter when barbecuing is traditionally not performed.
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  2. Unfortunately, CBDunkerson, I think Tim's misunderstanding is at a much more fundamental level, a failure to understand the point of Minnett's experiment.
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  3. @TTTM: Okay, I'll give this one more try, but it's really starting to sound as if you *don't* want to understand. "I have read it and there is no analysis of how increased downward LW radiation from CO2 effects the amount of energy the ocean radiates. For that matter there isn't even an analysis of increased LW radiation from clouds and what effect that has on decreased radiation from the ocean." That's because LW from clouds and LW from CO2 are identical in nature, but that's not important because that wasn't the point of the article. "The only relevence to intensity of radiation is as relates to the fact the experiment measures SSTs relating to 100W downward LW radiation whereas CO2 only supplies 3W. The experiment therefore exagerates the effect." That's because the experiment isn't trying to measure the effect of CO2, but simply trying to show the mechanism by which IR (i.e. LW) radiation heats the upper ocean, even if IR radiation is stopped by the skin layer. Let me repeat so you can understand: the goal of the experiment was NOT to determined the amplitude of IR warming (from CO2 or any other source), but the mechanism by which the actual infrared radiation manages to heat the upper ocean under the skin layer. The fact that clouds send down more IR than CO2 only makes it easier to identify the mechanism. That's it, that's all. Anything else you read into this paper comes from you and you alone. So, no, I don't see a problem with the article, and you have failed to convincingly demonstrate that there is one. You still haven't said why you have no doubt your argument fails to convince me. Please elaborate.
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  4. You're both missing the point. If you look at the graph reposted by Doug you can see an amount of warming of the skin dependent on the amount of IR. It varies between about 0.1C and 0.3C. This is what they measured using 100W of downward LW radiation from clouds. If the amount of IR is reduced to 1/33 as is the effect of CO2, then the amount of warming will likely be be similarly reduced. It is the amount of warming of the skin that directly relates to any decrease in ocean heat loss decrease due to altered heat flux which is their theory. And you haven't even considered the fact that this heat loss hasn't been quantitatively considered and that is the point of the theory.
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  5. @TTTM: "You're both missing the point." No, you are missing the point. The article isn't about the amount of warming, but about the regulatory mechanism the skin layer has for IR radiation transfer to the water below it. "If the amount of IR is reduced to 1/33 as is the effect of CO2, then the amount of warming will likely be be similarly reduced." Indeed, but - as we keep telling you - that is not the point of the article. "It is the amount of warming of the skin that directly relates to any decrease in ocean heat loss decrease due to altered heat flux which is their theory." That sentence is nearly incomprehensible. You should try to clarify your own mind before trying to formulate your arguments. "And you haven't even considered the fact that this heat loss hasn't been quantitatively considered and that is the point of the theory." No, the point of the article is to show the mechanisms that allow LW radiation to warm the ocean despite the fact that infrared radiation is stopped by the skin layer. At this point, it's clear you do *not* want to understand. As for me, I will not respond until you answer this simple question: why you say you had no doubt your argument would fail to convince me. Please elaborate.
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  6. You know, when everyone agrees on something, and you don't, it's usually the sign you're either a genius or you're wrong - and you're not a genius.
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  7. "Can anybody else think of a way to explain this to Tim? " I agree with you Doug but you're missing the point entirely. The point is that nobody has investigated whether the change in fux can account for the ocean heat loss observed recently and blamed on CO2. These are the numbers needed to validate AGW theory as far as CO2 heating the oceans goes. The point about cloud LW vs CO2 LW is only to point out that the effect they've measured is as a result of a large effect and the effect of CO2 will be much smaller.
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  8. Tim, I suggest you read the Science of Doom post Does Back Radiation Heat the Ocean? Part 1". But it looks like you'll have to wait for Part 2 to get the full answer you're looking for.
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  9. TimTheToolMan at 09:07 AM, that effect is very much smaller indeed, the slope of the relationship was determined as being 0.002ºK (W/m2)-1. When such small values are being put forward, then I believe that it is incumbent upon those conducting the study to similarly explore and account for all other processes that occur simultaneously that might contribute similarly small values, or perhaps larger values either positively or negatively to the nett result, one such example being evaporation which I believe would be very relevant to the study under discussion.
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  10. TTTM OK, let's try another analogy. You say clouds are a strong influence and CO2 is small, and thát's a problem. Seeing as we're talking about heating, let's look at cooking. We could knock up some soup in a matter of minutes on the top of the stove. Or we chuck everything in a slow cooker, go off for a day's work and come back 8 or 10 hours later, job done. In both cases, the food is cooked and at serving temperature. All the paper tells us is that if you apply heat to the ocean it will warm. Just as an appliance salesperson can tell us that applying heat to food will cook it. You seem to be wanting the salesperson to become a chef telling us all the different methods of cooking various foods.
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  11. I agree with you Doug but you're missing the point entirely. You're right in that the experiment only shows the mechanism. It absolutely does. But what we need now is to take that further and show that the mechanism actually works in terms of its ability to heat the ocean at the rate observed. More specifically does the change in heat flux at the skin as a result of Anthropogenic CO2 account for sufficient decrease in ocean heat to account for the observed ocean warming rates? AFAIK, This question cannot be answered by science at the moment.
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  12. TimTheToolMan at 09:44 AM, the experiment actually shows an effect. What is there to show that that effect is the result of one particular mechanism to the exclusion of all others?
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  13. The effect is that of changed skin temperature which they have shown. The theory then goes... the result of that change is decreased heat loss from the ocean. It a two step thing. Simplistically, the skin warms and then the ocean heat cant "cross it" as effectively. What is now missing is the magnitude of the reduction of heat loss as a result of that skin heating. I only bring up the comparison between the LW radiation from the clouds vs LW radiation from CO2 to demonstrate that the effect they measured is very much larger than the effect of CO2. In practice this means that LW radiation from clouds changed the skin temperature by 0.2C whereas the effect of CO2 will be very much smaller than that, perhaps around 0.2/33 = 0.01C. Is that enough of a flux change to account for the ocean heating. Why does everyone think I'm trying to differentiate between different heating mechanisms of CO2 and clouds? Its only relevent to the magnitude of the effect.
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  14. Sorry about the wierd posting. For some reason my broweser has refused to refresh until now and suddenly I've got all manner of reples not seen today. I'll go through each as I can.
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  15. TimTheToolMan at 13:37 PM, I understand what you are seeking in wanting the effect quantified. However I also see the relevance of the clouds being used in this particular experiment to simulate the effects of CO2 is that the clouds could alter the evaporation rate in that the lower loss of heat due to less evaporation could manifest itself as warming of the skin layer.
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  16. Thanks for the reference Tom. Perhaps you're right about "part two". In part one, however, He doesn't get around to addressing the question until the very last section where he briefly starts on conduction as a mechanism of heat transfer and at that point has totally ignored the fact the ocean has a cold skin so his conduction theory is trying to move energy from a colder place to a warmer one.
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  17. @johnd Yes, the experiment has some value to demonstrate the effect but you're right in that its not a definitive answer without understanding all the energies at the surface at the time of increased vs decreased LW radiation. Evaporation has the possibility to actually turn the result on its head and its simply not being measured in that experiment.
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  18. TimTheToolMan at 13:43 PM, try running an anti-virus scan and see if it finds any unwanted cookies.
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  19. @TTTM, the point is that an increase in the temperature of the top of the skin layer will decrease the difference between it and the bottom of the skin layer, which reduces the heat flow from the ocean to the air above. In other words, the oceans will retains more The fact that the impact of CO2 seems small is deceptive, as the forcing is *added* to that of clouds. The small imbalance means that, on average, the oceans are retaining slightly more heat.
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  20. TTTM, this comment on the RealClimate thread (#28, from J.A. Smith) might help you understand this a little better: "As an oceanographer working on air/sea interaction and mixed layer dynamics, I hope I can clarify this issue somewhat (in fact, I’m at sea right now on the R/P FLIP, gathering data to study wave and mixed layer dynamics, but this is off the point). I think a major aspect of the balance has been glossed over: the ocean is heated mainly by the visible part of the spectrum, the energetic part of the sun’s glare. This penetrates several meters (blue-green can penetrate several 10’s of meters, particularly in the clear water found away from coasts). In contrast, the only paths for heat LOSS from the ocean are infrared (blackbody) radiation and latent heat (evaporation). The sun heats the uppermost few meters; this has to find its way to the actual very thin surface layer to be lost. In equilibrium, then, there is a significan flux toward the surface a few cm under, and the sense of flux from infrared alone has to be significantly upward. Given this, it is quite clear that any reduction in the efficiency of upward radiation (by, say, reflecting it right back down again), will have to be compensated for by increasing the air/sea (skin) temperature difference, hence having a warmer subsurface temperature. This still leaves aside the latent heat flux, which in general accounts for something like half the upward heat flux. The balance is NOT, as portrayed here, between up and down infrared; rather it is downward “visible” (including ultraviolet, even), versus upward NET infrared and latent heat fluxes. Once trapped in the mixed layer, any excess heat makes its way down into the interior via much larger scale processes, including lateral advection and mixed-layer deepening due to wind and wave induced motions. This large-scale vertical redistribution takes a while- decades to hundreds of years- before equilibrium is re-established. The fact that we can already see this is quite remarkable."
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  21. @adelady, Actually your analogy is off. A soup on the stove heats by conduction and then convection whereas the ocean specifically doesn't heat by conduction/convection from the downward LW radiation. Thats why they did the experiment at all. There may be some amount of mixing down of the slightly warmer (but still colder than the bulk) skin due to the LW but according to the RC article, essentially the "warming effect" is one of reduced cooling rather than actual "warming".
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  22. The article "Does Back Radiation Heat the Ocean" referenced by Tom Dayton at 09:18 AM provides a couple of interesting charts. The first one shows the rate at which solar radiation is absorbed as it penetrates below the surface. About 13% of the total energy has been absorbed in the top 1mm, or nearly 25% by the 5cm depth at which the bulk temperature was measured in the experiment under discussion. The second one shows the absorption of the DLR which is totally absorbed in the first 10μm in the skin layer. The skin layer is also where the evaporation process takes place extracting heat in the process. One wonders whilst all theses measurements seem able to be clearly defined on paper just how well it works out in practice with a surface that is generally anything but a millpond. ....
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  23. johnd in practice they work much better at sea than in a millpond, given that oceans mix much more than millponds.
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  24. OK TTTM. The analogy wasn't intended to demonstrate anything other than the problem of trying to get comparisons or measurements from a paper that intended to do neither. Sorry that wasn't clear.
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  25. @adelady My bad on understanding your analogy then. Having said that, the paper did get values for the skin temperature with varied downward LW radiation though. I offer the following chef analogy. A chef is asked whether he can cook for large groups and he says sure and cooks a single plate to "prove" it. The point of this is that he has shown he can cook the meal so its almost certain he could cook 50 of them...but can he do it fast enough to be successful?
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  26. The real way to run a CO2 warming experiment is to use just one bottle or jug, have a little water in it, use air for the first run of temps and then pump in the CO2 to 760 ppm, then run a 2d set of temps, in both instances using the same probe and warming lamp. Hmmm! I think I will try it myself.
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  27. henry justice @226 Problem is, the bottle would need to be as tall as the atmosphere, in order to mimic the pressure gradient. And be perfectly insulated. And be wide enough to allow convection without wall effects. And be connected to a series of other bottles to mimic heat transport through the atmosphere to different latitudes. etc. Let us know how you get on with that one.
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  28. VTG - to demostrate the basic physics of the greenhouse effect, no it doesnt. For the real atmosphere, you can just measure the effect directly but most people dont have access to satellites or appropriate instruments. Henry - it can be harder than you think to get this right. What wavelength really is that warming lamp emitting cf to the spectral sensitivities that you want? Water will mask the CO2 effect. Better to use two bottles and do simultaneously (otherwise you might be just measuring a difference in conduction heating between your two times). Have bottoms of bottle black and irradiate with normal sunlight or strong lamp. Need to think very carefully about your setup to eliminate conductive effects.
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  29. scaddenp the basic physics is the laws thermodynamics plus the absorpotion spectrum of CO2. All very well understood already. The point I'm trying to make is that you can't experimentally measure greenhouse heating - you need the water vapour and temperature profile of the relatmosphere to do that, and that's experimentally impossible. Hence those demonised computer models.
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  30. VTG - I disagree based on the papers listed in the articles. Taking say the Philipona/Evans approach, you MEASURE the DLR amplitude and spectrum. To compute what it "should" be (model result), then calculate from radiative transfer equations (Ramanathan and Coatley 1978 for the maths). This is a very different calculation from the demonised GCMs - no dynamic component and insensitive to minor variations so you dont need minutely accurate picture of atmosphere for clear-sky calculation. Not only does the calculation match measurement for amplitude and spectrum, but it is very hard to see how you could obtain that measurement without the CO2 greenhouse effect. Can do essentially the same at the TOA. This doesnt tell you all about surface heating but it does tell what the radiative heating due to CO2 is. To argue about AGW, you need to look at other factors and feedbacks but I'd need a convincing explanation of those measurements from another theory to doubt the radiative component of CO2.
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  31. The number 2 comment by WAG exhibits a classical mistake of logic.  The comment points to a correlation between CO2 and temperature.  Then the comment asserts that causation is present, unless somebody proves otherwise.

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  32. nodeniro - See the link that John Cook gave in his response to that very comment by WAG, pointing to Empirical evidence that humans are causing global warming. We start from the science that indicates CO2 is a powerful greenhouse gas (spectroscopy, atmospheric structure, thermodynamics), and from there correlations are useful for determining exactly how much of an influence CO2 is relative to other forcings, such as solar changes, aerosols, land use, etc. Correlation without causation (common in climate change denier arguments searching for alternatives) is an empty assertion, but correlation with causation is a valid tool - when, as here, it's backed by physics. 

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  33. nodeniro.

    Since WAG is probaly long since moved on - it was 7 years ago, perhaps you would like to open a conversation on a current issue?

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  34. Is there any measured evidence that the heat energy retention by CO2 and methane has had any measureable effect on the  temperatures shown on the official graphs of yearly global air temperature?

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  35. billev - read the main article? If you want a really direct measurement, then this article explains a 2015 method. However, you have to be a disbeliever in Planck's Law if dont think that increasing the radiative flux on a surface will not increase its temperature. Doubling CO2 directly increases surface temperature by about 1.1C - the calculation is pretty straightforward. The difficulty with climate sensitivity determination however is that increasing temperature causes other feedbacks to cut in as well notably decreasing albedo and water vapour. As to teasing out of the various influences on surface temperature, then this is known as attribution studies. The IPCC AR5 summaries the published science on this in Chpter 10.

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