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Is the CO2 effect saturated?

What the science says...

Select a level... Basic Intermediate Advanced

The notion that the CO2 effect is 'saturated' is based on a misunderstanding of how the greenhouse effect works.

Climate Myth...

CO2 effect is saturated

"Each unit of CO2 you put into the atmosphere has less and less of a warming impact. Once the atmosphere reaches a saturation point, additional input of CO2 will not really have any major impact. It's like putting insulation in your attic. They give a recommended amount and after that you can stack the insulation up to the roof and it's going to have no impact." (Marc Morano, as quoted by Steve Eliot)

At-a-Glance

This myth relies on the use (or in fact misuse) of a particular word – 'saturated'. When someone comes in from a prolonged downpour, they may well exclaim that they are saturated. They cannot imagine being any wetter. That's casual usage, though.

In science, 'saturated' is a strictly-defined term. For example, in a saturated salt solution, no more salt will dissolve, period. But what's that got to do with heat transfer in Earth's atmosphere? Let's take a look.

Heat-trapping by CO2 in the atmosphere happens because it has the ability to absorb and pass on infra-red radiation – it is a 'greenhouse gas'. Infra-red is just one part of the electromagnetic spectrum, divided by physicists into a series of bands. From the low-frequency end of the spectrum upwards, the bands are as follows: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Gamma rays thus have a very high-frequency. They are the highest-energy form of radiation.

As our understanding of the electromagnetic spectrum developed, it was realised that the radiation consists of particles called 'photons', travelling in waves. The term was coined in 1926 by the celebrated physicist Gilbert Lewis (1875-1946). A photon's energy is related to its wavelength. The shorter the wavelength, the higher the energy, so that the very high-energy gamma-rays have the shortest wavelength of the lot.

Sunshine consists mostly of ultraviolet, visible light and infra-red photons. Objects warmed by the sun then re-emit energy photons at infra-red wavelengths. Like other greenhouse gases, CO2 has the ability to absorb infra-red photons. But CO2 is unlike a mop, which has to be wrung out regularly in order for it to continue working. CO2 molecules do not get filled up with infra-red photons. Not only do they emit their own infra-red photons, but also they are constantly colliding with neighbouring molecules in the air. The constant collisions are important. Every time they happen, energy is shared out between the colliding molecules.

Through those emissions and collisions, CO2 molecules constantly warm their surroundings. This goes on all the time and at all levels in the atmosphere. You cannot say, “CO2 is saturated because the surface-emitted IR is rapidly absorbed”, because you need to take into account the whole atmosphere and its constant, ongoing energy-exchange processes. That means taking into account all absorption, all re-emission, all collisions, all heating and cooling and all eventual loss to space, at all levels.

If the amount of radiation lost to space is equal to the amount coming in from the Sun, Earth is said to be in energy balance. But if the strength of the greenhouse effect is increased, the amount of energy escaping falls behind the amount that is incoming. Earth is then said to be in an energy imbalance and the climate heats up. Double the CO2 concentration and you get a few degrees of warming: double it again and you get a few more and on and on it goes. There is no room for complacency here. By the time just one doubling has occurred, the planet would already be unrecognisable. The insulation analogy in the myth is misleading because it over-simplifies what happens in the atmosphere.

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

This myth relies on the use of a word – saturated. When we think of saturated in everyday use, the term 'soggy' comes to mind. This is a good example of a word that has one meaning in common parlance but another very specific one when thinking about atmospheric physics. Other such words come to mind too. Absorb and emit are two good examples relevant to this topic and we’ll discuss how they relate to atmospheric processes below.

First things first. The effect of CO2 in the atmosphere is due to its influence on the transport of 'electromagnetic radiation' (EMR). EMR is energy that is moving as x-rays, ultraviolet (UV) light, visible light, infrared (IR) radiation and so on (fig. 1). Radiation is unusual in the sense that it contains energy but it is also always moving, at the speed of light, so it is also a form of transport. Radiation is also unusual in that it has properties of particles but also travels with the properties of waves, so we talk about its wavelength.

The particles making up radiation are known as photons. Each photon contains a specific amount of energy, and that is related to its wavelength. High energy photons have short wavelengths, and low energy photons have longer wavelengths. In climate, we are interested in two main radiation categories - firstly the visible light plus UV and minor IR that together make up sunshine, and secondly the IR from the earth-atmosphere system.

The Electromagnetic Spectrum

Fig. 1: diagram showing the full electromagnetic spectrum and its properties of the different bands. Image: CC BY-SA 3.0 from Wikimedia.

CO2 has the ability to absorb IR photons – it is a 'greenhouse gas'.So what does “absorb” mean, when talking about radiation? We are all familiar with using a sponge to mop up a water spill. The sponge will only absorb so much and will not absorb any more unless it's wrung out. In everyday language it may be described, without measurements, as 'saturated'. In this household example, 'absorb' basically means 'soak up' and 'saturated' simply means 'full to capacity'. Scientific terms are, in contrast, strictly defined.

Now let's look at the atmosphere. The greenhouse effect works like this: energy arrives from the sun in the form of visible light and ultraviolet radiation. A proportion reaches and warms Earth's surface. Earth then emits the energy in the form of photons of IR radiation.

Greenhouse gases in the atmosphere, such as CO2 molecules, absorb some of this IR radiation, then re-emit it in all directions - including back to Earth's surface. The CO2 molecule does not fill up with IR photons, running out of space for any more. Instead, the CO2 molecule absorbs the energy from the IR photon and the photon ceases to be. The CO2 molecule now contains more energy, but that is transient since the molecule emits its own IR photons. Not only that: it's constantly colliding with other molecules such as N2 and O2 in the surrounding air. In those collisions, that excess energy is shared with them. This energy-sharing causes the nearby air to heat up (fig. 2).

CO2 heat transfer

Fig. 2: The greenhouse effect in action, showing the interactions between molecules. The interactions happen at all levels of the atmosphere and are constantly ongoing. Graphic: jg.

The capacity for CO2 to absorb photons is almost limitless. The CO2 molecule can also receive energy from collisions with other molecules, and it can lose energy by emitting IR radiation. When a photon is emitted, we’re not bringing a photon out of storage - we are bringing energy out of storage and turning it into a photon, travelling away at the speed of light. So CO2 is constantly absorbing IR radiation, constantly emitting IR radiation and constantly sharing energy with the surrounding air molecules. To understand the role of CO2, we need to consider all these forms of energy storage and transport.

So, where does 'saturation' get used in climate change contrarianism? The most common way they try to frame things is to claim that IR emitted from the surface, in the wavelengths where CO2 absorbs, is all absorbed fairly close to the surface. Therefore, the story continues, adding more CO2 can’t make any more difference. This is inaccurate through omission, because either innocently or deliberately, it ignores the rest of the picture, where energy is constantly being exchanged with other molecules by collisions and CO2 is constantly emitting IR radiation. This means that there is always IR radiation being emitted upwards by CO2 at all levels in the atmosphere. It might not have originated from the surface, but IR radiation is still present in the wavelengths that CO2 absorbs and emits. When emitted in the upper atmosphere, it can and will be lost to space.

When you include all the energy transfers related to the CO2 absorption of IR radiation – the transfer to other molecules, the emission, and both the upward and downward energy fluxes at all altitudes - then we find that adding CO2 to our current atmosphere acts to inhibit the transfer of radiative energy throughout that atmosphere and, ultimately, into space. This will lead to additional warming until the amount of energy being lost to space matches what is being received. This is precisely what is happening.

The myth reproduced at the top – incorrectly stating an analogy with roof insulation in that each unit has less of an effect - is misleading. Doubling CO2 from 280 ppm to 560 ppm will cause a few degrees of warming. Doubling again (560 to 1130 ppm) will cause a similar amount of additional warming, and so on. Many doublings later there may be a point where adding more CO2 has little effect, but recent work has cast serious doubt on that (He et al. 2023). But we are a long, long way from reaching that point and in any case we do not want to go anywhere near it! One doubling will be serious enough.

Finally, directly observing the specific, global radiative forcing caused by well-mixed greenhouse gases has - to date - proven elusive. This is because of irregular, uncalibrated or limited areal measurements. But very recently, results have been published regarding the deep reinterrogation of years of data (2003-2021) from the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua Satellite (Raghuraman et al. 2023). The work may well have finally cracked the long-standing issue of how to make finely detailed, consistent wavelength-specific measurements of outgoing long-wave radiation from Earth into space. As such, it has opened the way to direct monitoring of the radiative impact (i.e. forcing + feedback) of greenhouse gas concentration changes, thereby complimenting the Keeling Curve - the longstanding dataset of measured CO2 concentrations, down at the planet's surface.

Note: Several people in addition to John Mason were involved with updating this basic level rebuttal, namely Bob LoblawKen Rice and John Garrett (jg).

Last updated on 31 December 2023 by John Mason. View Archives

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Further reading

V. Ramanthan has written a comprehensive article Trace-Gas Greenhouse Effect and Global Warming.

Further viewing

Video by Rosh Salgado on his "All about Climate" YouTube channel in which he debunks Will Happer's claim that the CO2 effect is saturated in the atmosphere:

Comments

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Comments 26 to 50 out of 627:

  1. A few of your links are not working, but I think I found them. Griggs 2004 http://rose.bris.ac.uk/dspace/bitstream/1983/999/1/paper.pdf Philipona 2004 http://www.agu.org/journals/ABS/2004/2003GL018765.shtml However, I could not find the Chen 2007 article. And thanks for the informative article. Blessings,
    Response: I've updated the links (and tracked down Chen 2007). Many thanks for the URLs - let's hope they stay active for as long as possible :-)
  2. This page makes the argument that the lower layers of the atmosphere are ruled by convection, and that it's the "temperature of this 'last' layer" that is important for radiation of energy. However, the majority of the warming projected by the IPCC is caused by water vapor feedback in the troposphere. The outer layers of atmosphere do not have water vapor. So the argument on this page seems counter to any argument of water vapor feedback.
  3. After thinking on this I think I understand the IPCC argument better. I think the idea is there are two areas of increased warming. One is the outer atmosphere which is caused by CO2 as this article talks about. The second is the area between the troposphere and stratosphere, which is caused by H2O feedback. This increased H2O would supposidly prevent radiation from the troposphere to the relatively still stratosphere. The problem with the H2O feedback idea is it brings us back to the "skeptic" arguments raised in this article. H2O can approach (but never reach) saturation just like CO2 can. So each additional amount of H2O does less warming. Except now we don't have the counterarguments that the H2O is getting farther apart or that the H2O that is radiating is getting cooler and therefore radiates less (because it's getting warmner). I tried to be brief so I hope that makes sense.
  4. Climate4you has graphs that may conflict with this statement. "So we have multiple lines of empirical evidence for an enhanced CO2 greenhouse effect. 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." Graph which indicates that longwave radiation is not following CO2 increase. Explanation given to the above graph: "For the equatorial region, the diagram above suggests a certain chain of events, indicating the existence of a mechanism regulating the surface temperature: Periods of surface warming appears initially to be associated with decreasing outgoing longwave radiation (OLR). After some surface warming, OLR then stops decreasing and instead begins to increase, and after a while, surface air temperature then begins to decrease, etc. This chain of events is clearly illustrated by, e.g., the time period around the 1998 El Niño event (diagram above). Part of the explanation of the above succession of events might be that tropical surface warming leads to enhanced atmospheric convectional transport of heat to high levels of the atmosphere above the Equator, resulting in enhanced longwave radiation at the top of the atmosphere. This, in turn, eventually leads to surface cooling, which results in reduced atmospheric convection, etc. Also the potential connection to variations in tropical sea surface temperatures and the tropical cloud cover is interesting, and should be considered in a more detailed analysis." This graph shows poor correlation of CO2 to Longwave raditation. This graph questions the conclusions of the Skeptical Science.
    Response: Your first link is broken.
  5. Still won't connect. I will try another graph from Climate4you that shows the same thing, cycles but no downward direction for the outgoing longwave radiation. Outgoing longwave radiation cycles but does not trend down.
  6. Norman, where exactly is that image on climate4you.com? The image itself does not give enough information about exactly what the graph represents (e.g., what wavelengths).
  7. Tom, I believe this is the graph and related text in question. However, it says nothing about wavelength. The suggestion seems to be that increasing CO2 should reduce ALL wavelengths of OLR... which doesn't seem an accurate statement of greenhouse gas behavior to me. In short, a straw man... the graph shows that total long wave radiation is not decreasing, but the claim that it should be has no basis in AGW theory.
  8. Norman, without knowing more about the image you linked to, it is difficult to respond. But I'm going to guess that your graph presents an average across a really wide range of longwave radiation. In contrast, Figure 1 at the top of this Skeptical Science page (the Intermediate tabbed pane) is presented to illustrate the reduction of outgoing radiation in the very particular wave numbers that are specific to greenhouse gases. Your graph does not contradict that. Energy trapped (delayed in escaping, really) by greenhouse gases raises the temperature of the atmosphere/land/water, which causes an increase in that system's attempted emission of radiation to space. Radiation outside of the greenhouse gases' absorption wavelengths will merrily escape to space, so total longwave radiation escaping will increase. The problem is that the escaping total cannot increase fast enough to prevent the temperature from increasing, because the increase in the escaping radiation is a response to the increasing temperature.
  9. Thanks CB! I didn't think to look in the "temperature" section for that graph. I skimmed there, and skimmed the result of clicking the "i" for more info, and though I found no listing of the wavelengths covered by "total OLR," I take that to mean you are correct that it covers a wide range.
  10. Norman - I think you are falling for a lot of denialist junk. At best this is amateurs making simplistic analyses about data they dont understand and at worst, the work of clever people practicing on the uninformed (and who dont want to be informed). A first cut filter for the rubbish is the simple question of "is it published?". There are glittering prizes for anyone who can disprove climate theory or come up with an improved theory. If the analysis is sound then why wouldnt you publish? The unscrupulous will say one thing to a naive audience (eg congress) but dont make such claims to their peers. However, this is a good site for finding out what is wrong with the some of wild stuff out there.
  11. Tom Dayton "Radiation outside of the greenhouse gases' absorption wavelengths will merrily escape to space, so total longwave radiation escaping will increase." The graph does not seem to show this increase. It does not show a trend in increase or decrease, just cyclic pattern. If the Earth is warming then the Outgoing longwave radiation should increase for all wavelengths not absorbed by GHGs. I do not see an increase or decrease.
  12. Re: Climate4you stuff Went to Norman's website source for his graph & poked around a bit. On this page I noted that: 1. All data is in absolute temps, not anomalies 2. They establish the post-industrial runup in the temperature trend and use that trend to de-trend the signal in the data. I.e., they "hide the incline" in the 20th Century temperature data. 3. They attribute 100% of CO2's effects on temperatures when comparing the CO2 rise to temps, showing that since temps don't rise in lockstep with CO2 levels it can't be the CO2 affecting temps 4. They use a paper by Dr. Syun-Ichi Akasofu as a basis to say that any warming since the LIA is just a reflection of the Earth returning to "normal" and that it's a natural cycle. Trenberth demolished Akasofu here. The whole site is a bait-trap for the unwary. The Yooper
  13. Norman, the only way energy leaves the earth is radiation, mostly LW. What it should do is match incoming energy at TOA (first law). If it doesnt, (it doesnt), then planet is accumulating heat but you dont expect to see a trend. The surface heats because of increased GHG effectively impede the efficiency with which the surface can radiate (LW is radiated back onto the surface). You would expect TOA outgoing LW to increase only if the cause of warming was more incoming energy from the sun.
  14. The problem posed by Norman has nothing to do with the saturation of CO2, any comparison with fig. 1 here makes no sense. The OLR is a balance between the increasing blackbody emission from the earth surface (some 250 W/m2 with a variability of some tens of W/m2) and anything that can block it, including CO2 absorption (of the order of 1 W/m2 over the full period of the figure). The former depends on temperature. Anything else being equal, if for some reason temperature does not rise fast enough the OLR decreases (of a fraction of a W/m2), and viceversa. If you add ENSO, clouds, GHG and all other sources of variability, it's a mess. Trying to draw any conclusions eyeballing a graph like the one shown by Norman is meaningless.
  15. Yooper - Thanks for the Climate4you evaluation! I had taken a quick look at it, saw that all the temp records appeared to not show 20th century temperature increases, but hadn't had time to dig far enough to find out why. Apparently the creator of the site is attempting to compare the various records GISS, UAH, RSS, etc.) against each other - apples/oranges, really, especially surface vs. satellite. Norman, detrending the data without saying you're doing it is rather deceptive (the real data is there, but at least one level deeper, and the detrended graphs are not labeled clearly as such). I would rate Climate4you as a junk site as well.
  16. Re: Climate4you I also went to a ring of sites linking to it (there are dozens) - all very similar, some with even more polish. Some of the data manipulation gets sophisticated - to the point that (coupled with the quality graphics) the "errors" become intentional (the knowledge needed to pull off what they're trying to do makes it obvious they should know better). That's why I called it a bait-pile (deer season in 6 days colors my thinking process a bit). The Yooper
  17. Figure 1: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature (Harries 2001). Well, well, well. Let's see. FIGURE 1. Examples of IRIS and IMG observed and simulated spectra for a three-month average (April–June) over selected regions. a, Observed IRIS and IMG clear sky brightness temperature spectra for the central Pacific (10°N–10°S, 130°W–180°W). b, Top, observed difference spectrum taken from a; middle, simulated central Pacific difference spectrum, displaced by -5 K; bottom, observed difference spectrum for 'near-global' case (60°N–60°S), displaced by –10 K. c, Component of simulated spectrum due to trace-gas changes only. 'Brightness temperature' on the ordinate indicates equivalent blackbody brightness temperature. "Component of simulated spectrum due to trace-gas changes only" is not the same as "Change in spectrum [...] due to trace gases". If you have references, you are expected to omit misquotations. Observed (as opposed to simulated) near-global difference spectrum (Fig. 1. b, bottom) does not show an overall energy imbalance. There is simply more outgoing longwave radiation in the atmospheric window, compensating for somewhat less radiation elsewhere. It is consistent with a slightly decreasing average upper tropospheric IR optical depth at water vapor absorption bands (especially in the water vapor continuum). I would also like to know if measurements were performed in the 400 cm-1 - 1600 cm-1 (6.25 μm - 25 μm) range, why Harries at al. only show the 710 cm-1 - 1400 cm-1 (7.14 μm - 14.08 μm) range? The so called arctic window is below 625 cm-1 (above 16 μm). At these frequencies all absorption/emission is from water vapor, therefore changes in brightness temperature spectrum here should be rather informative.
    Response: Harries explained that the spectra below 700 were too noisy to include. See the "Response" in the green box of this comment on the post about the American Thinker article.
  18. BP, those graphs you pasted were presented and discussed thoroughly in the comments section of the post Have American Thinker Disproven Global Warming?". Norman, the answer given to you by Riccardo is expanded in the comments of that other post. I suggest you start with comment 72 by Pierre-Normand, followed by my much less technical analogy in comment 71.
  19. BP and Norman, in my previous comment I should have also said to look at Pierre-Normand's followup comment 78 on that other post, in which he corrected a misstatement he made in his comment 72. Also note that a large portion of the many comments on that post (Have American Thinker Disproven Global Warming?) is in response to the American Thinker article's writer suffering a similar misunderstanding that Norman has--that total outgoing longwave radiation must change in lockstep with greenhouse gas levels. See further the comments 80 by me and 81 by Pierre-Normand. The entire set of comments on that post is very enlightening, by folks far more knowledgeable than me, so I suggest that BP and Norman read them all.
  20. The question of the total outgoing longwave amount is addressed by Harries in his responses to John Cook's questions, in the green "Response" box of this comment on the American Thinker post.
  21. #43 Tom Dayton at 10:58 AM on 10 November, 2010 BP, those graphs you pasted were presented and discussed thoroughly in the comments section of the post Have American Thinker Disproven Global Warming? Not thoroughly enough. Near-global difference spectrum of brightness temperature between 750 cm-1 and 900 cm-1, where the atmospheric window is really transparent (except for the water vapor continuum), is about +2 K. No layer has warmed nearly that much between 1970 and 1996, therefore the upper troposphere must have got more transparent at this IR frequency band, making lower (warmer) layers "visible" to satellites. It means a negative water vapor feedback. Worth mentioning that effective warming (by lowering) of photosphere in this band should have been even greater, because brightness temperatures are calculated for black body equivalent temperatures, while actual emissivity is always lower than that. It is also consistent with balloon radiosonde measured decreasing humidity trends above the 700 mbar level.
  22. #40 KR "Norman, detrending the data without saying you're doing it is rather deceptive (the real data is there, but at least one level deeper, and the detrended graphs are not labeled clearly as such)." Not sure which page in Climate4you you looked at. You must have done a very quick look to make the conclusion it is a a "junk" site. He does label his graphs as detrended and he explains the purpose (sometimes it pay to take time to read what the analysis is about). The claim was made that the warming from 1981 to present was unique. The warming slope from 1908 to 1940 is very similar to the present one. Have another look and see what the person is attempting with his analysis.
  23. Berényi Péter, Please clarify. Do you believe that the CO2 effect is saturated?
  24. Berényi Péter update your citation database, apparently NCEP is biased.
  25. Norman, can you please reference exactly where "The claim was made that the warming from 1981 to present was unique". I'd like to see the exact text of this supposed claim. The errors already pointed out to you dont ring any alarm bells for you?

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