Climate Science Glossary

Term Lookup

Enter a term in the search box to find its definition.


Use the controls in the far right panel to increase or decrease the number of terms automatically displayed (or to completely turn that feature off).

Term Lookup


All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Home Arguments Software Resources Comments The Consensus Project Translations About Donate

Twitter Facebook YouTube Pinterest

RSS Posts RSS Comments Email Subscribe

Climate's changed before
It's the sun
It's not bad
There is no consensus
It's cooling
Models are unreliable
Temp record is unreliable
Animals and plants can adapt
It hasn't warmed since 1998
Antarctica is gaining ice
View All Arguments...

Keep me logged in
New? Register here
Forgot your password?

Latest Posts


Climate Hustle

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)

The mistaken idea that the Greenhouse Effect is 'saturated', that adding more CO2 will have virtually no effect, is based on a simple misunderstanding of how the Greenhouse Effect works.

The myth goes something like this:

  • CO2 absorbs nearly all the Infrared (heat) radiation leaving the Earth's surface that it can absorb. True!
  • Therefore adding more CO2 won't absorb much more IR radiation at the surface. True!
  • Therefore adding more CO2 can't cause more warming. FALSE!!!

Here's why; it ignores the very simplest arithmetic.

If the air is only absorbing heat from the surface then the air should just keep getting hotter and hotter. By now the Earth should be a cinder from all that absorbed heat. But not too surprisingly, it isn't! What are we missing?

The air doesn't just absorb heat, it also loses it as well! The atmosphere isn't just absorbing IR Radiation (heat) from the surface. It is also radiating IR Radiation (heat) to Space. If these two heat flows are in balance, the atmosphere doesn't warm or cool - it stays the same.

Lets think about a simple analogy:

We have a water tank. A pump is adding water to the tank at, perhaps, 100 litres per minute. And an outlet pipe is letting water drain out of the tank at 100 litres per minute. What is happening to the water level in the tank? It is remaining steady because the flows into and out of the tank are the same. In our analogy the pump adding water is the absorption of heat by the atmosphere; the water flowing from the outlet pipe is the heat being radiated out to space. And the volume of water inside the tank is the amount of heat in the atmosphere.

What might we do to increase the water level in the tank?

We might increase the speed of the pump that is adding water to the tank. That would raise the water level. But if the pump is already running at nearly its top speed, I can't add water any faster. That would fit the 'It's Saturated' claim: the pump can't run much faster just as the atmosphere can't absorb the Sun's heat any faster

But what if we restricted the outlet, so that it was harder for water to get out of the tank? The same amount of water is flowing in but less is flowing out. So the water level in the tank will rise. We can change the water level in our tank without changing how much water is flowing in, by changing how much water is flowing out.

water tank

Similarly we can change how much heat there is in the atmosphere by restricting how much heat leaves the atmosphere rather than by increasing how much is being absorbed by the atmosphere.

This is how the Greenhouse Effect works. The Greenhouse gases such as carbon dioxide and water vapour absorb most of the heat radiation leaving the Earth's surface. Then their concentration determines how much heat escapes from the top of the atmosphere to space. It is the change in what happens at the top of the atmosphere that matters, not what happens down here near the surface.

So how does changing the concentration of a Greenhouse gas change how much heat escapes from the upper atmosphere? As we climb higher in the atmosphere the air gets thinner. There is less of all gases, including the greenhouse gases. Eventually the air becomes thin enough that any heat radiated by the air can escape all the way to Space. How much heat escapes to space from this altitude then depends on how cold the air is at that height. The colder the air, the less heat it radiates.

(OK, I'm Australian so this image appeals to me)

So if we add more greenhouse gases the air needs to be thinner before heat radiation is able to escape to space. So this can only happen higher in the atmosphere. Where it is colder. So the amount of heat escaping is reduced.

By adding greenhouse gases, we force the radiation to space to come from higher, colder air, reducing the flow of radiation to space. And there is still a lot of scope for more greenhouse gases to push 'the action' higher and higher, into colder and colder air, restricting the rate of radiation to space even further.

The Greenhouse Effect isn't even remotely Saturated. Myth Busted!

Basic rebuttal written by dana1981

Update July 2015:

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


Last updated on 7 July 2015 by pattimer. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Related Arguments

Further reading

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


Prev  1  2  3  4  5  6  7  8  9  10  

Comments 451 to 461 out of 461:

  1. Hope this is the right place to point out that Figs 1 & 2 have disappeared from the 'Advanced' article, apparently after Altervista suspended their hosting. They're still available in the PDF and via the Wayback Machine.


    [DB]  Updated.  Thanks for the heads-up!

  2. I fail to find convincing evidence as to how CO2 can be the cause of global warming with a occurence of only 400 ppm. This would entail that 1 CO2 molecule would need to heat up 2,500 other molecules in the atmosphere to cause any increase in overall temperature. How is this possibly explainable! It is impossible.

  3. Barcino:

    You are making an argument from incredulity. An equivalent counterargument would be for me to say "I can't believe you don't understand how this works". Very easy to say, but carries no weight.

    The place you want to look is on the "CO2 is a trace gas" page:

    Please read it before you comment again, and place comments on that thread, not this one.


    [PS] I am reasonably sure that nothing said by anyone will convince a person that doesnt want to be convinced but lets see. Barcino's statement suggests he hasnt actually read a proper explanation of how the greenhouse effect really works.

  4. @Barcino2017

    Are you trolling? Or can you genuinely not come up with an idea of how molecules of CO2 can heat up other molecules in the atmosphere? This idea around 2500 molecules seems to be quite common in denier circles and reminds me of a "discussion" I had with someone who was pushing this idea:

    Denier: There are 2500 molecules for each molecule of CO2. For an increase of 1C the CO2 molecule would have to be heated to 2500C.

    Me: Why would the CO2 molecule have to be heated to 2500C?

    Denier: Because er, 2500, you dishonest green rent grant seeker.

    Me: What? Why?

    Denier: Your smears and lack of empirical data and science are obvious. You are a dishonest liar. I win!!!!!!!!!!?

    Now, you don't want to be that person, do you? Your idea is slightly less extreme than his as you do not propose a CO2 molecule has to be heated all in one flash and heat 2500 other molecules at the same time. But you are suggesting a CO2 molecule cannot heat more than one molecule. Ever. Like, once it has heated another molecule it can never do it again? A one-off never to be repeated event?

    Perhaps you need to rethink that misconception.

  5. The facts are obscured by the above analysis. The facts are that the effects of CO2 on the atmospheric absorption of IR by the atmosphere diminish rapidly and logartithmically with increasing concentration, so most of the so called greenhouse effect, that absorbs and re radiates IR, which isn't how greenhouses work, occurs in concentrations below 150ppm, which is so low trees and other vegetation die. Lower than the lowest and ice age minimums.

    The effect is approximately the same for each doubling, so has expoentially LESS effect per ppm, so that at 200ppm another 200ppm, to get to where we are now, only produces the same effect as 20-40ppm.

    So the statement that its never satuarted is deceitfully true, because it is partial by omitting the dominant factor, that while it's never quire saturated, it may as well be for all the effect it has, versus the natural variation in the 300 times greater effect of Water vapour, for example.

    Reference? There is a useful course on this by the University of Chicago you can take for free w/o a certificate - that costs - which gives this initial fact in its promo video.  These are the facts on IR absorption of CO2 in the atmosphere. Not as advertised. The reverse of a tipping point in fact. We need to be looking elsewhere than CO2 for the rue cause of the tiny variation we are currently observing with the interglacial peak tempertaure range of an ice age interglacial. Soon be over, but at a rate humans cannot detect in a lifetime, as a quick study of the data shows anyone numerate. Real planetary climate hange of an interglacial is significant, but again any effect takes seberal lifetimes to be on bservable, even at the geologically rapid end of the current ice ages, over only 1,000 years...................


    [JH] url link activated. Please learn how to do this using the edit function provided.

    For future reference, you may want to spell-check your draft text prior to posting a comment.

    [TD] SkepticalScience is organized into a large number of narrow topics. You have commented on a topic that is only slightly relevant to your point about the direct IR effect of CO2 increase being logarithmic. Please click the View All Arguments link at the bottom of the list of Most Used Climate Myths, in the left margin, for more relevant posts, such as How Do We Know More CO2 Is Causing Warming?, and CO2 Is Main Driver of Climate Change. Note that many posts have Basic, Intermediate, and Advanced tabbed panes. The log relationship of IR absorption to CO2 level has been known for over 150 years, and always has been explicitly accounted for in all calculations of temperature effects of increased CO2.

    [TD] To find posts specifically addressing the logarithmic relationship, enter the word logarithmic in the Search field at the top right of the page.

  6. Brian Catt @455 , from what I've seen of the Uni. Chicago lecture, it does not support the thrust of the statements you have made.  Please explain better, what you are intending to say — since your comments are coming across as "confused about the science".

    Then there are the simple errors in your statements, such as (A) the current "tiny variation" [unquote] in surface temperature.  [Alas, not so.  The temperature is shooting upwards like a rocket.  There's been nothing like this in the 200,000+ years of human history.  And scientifically, the cause is obvious and undisputed. ]

    .... And (B) water vapour causing "300 time greater effect" [unquote] in warming compared with CO2.  Is that what you think?  Or did you mean to state 3 times greater?  Because that is around the upper limit of the multiplying "feedback" response by H2O to the driving force of CO2.

  7. BC embarrassed himself, and forever established himself as a denier, on this NOAA thread on Facebook. 

    Put your coffee down before reading.

  8. Daniel, "This page isn't available

    The link you followed may be broken, or the page may have been removed."

  9. As a brief background to this comment, I had an encounter with a denialist last week who stated that Arrhenius' observations regarding CO2 as a greenhouse gas had been debunked. In hindsight,  I should have asked precisely what he meant by that. Instead, I retorted that this was news to me, at which he remarked that my kind was unlikely to look up the facts (I have a masters degree in Physics, and found this an interesting example of projection rhetoric...). Anyway, here I am.

    I'm trying to follow the argument here, so that I can express it in layman's terms. It seems the idea of greenhouse warming isn't as straightforward as I thought. I get Angstrom's counter-argument to Arrhenius about current CO2 levels absorbing all IR radiation long before it has a chance to escape the atmosphere and that, therefore, adding more CO2 won't alter things. I'm having a problem grasping the explanation in response the Angstrom being put forward here. If I may paraphrase what I understand of it:

    1. IR photon from surface is repeatedly absorbed and re-emitted (by CO2) in random directions (losing a little energy each time). Check.

    2. Photon is finally able to escape into space at a high enough altitude. Check.

    3. Adding more CO2 increases the density, and hence the altitude this  escape occurs. Check.

    4. Being higher means lower temperature, and hence less energy is emitted. Errm... as a general consequence of a blackbody spectrum, sure. As a discrete energy photon emission, of what significance is the temperature?

    I'm not saying the explanation's wrong, but it is very hard to follow, and is even counterintuitive (we're talking about rising temperatures, and then lowered temperatures? It's hard enough for me to follow, let alone someone without a scientific background)

    My own quick explanation would be that increasing CO2 concentrations reduces the mean free path of IR photons, and that increases the number of scattering (heating) events before the photons can escape. It seems a lot more concise and intuitive than the explanation put here, but is it right? What am I missing?

  10. Arf @459 , the usual explanations follow the course of IR photons (of the bandwidth absorbed/emitted by CO2 molecules) as they are radiated out from the warm surface of the land/ocean.   Layer after layer of atmosphere absorbs and re-emits IR photons in all directions (as you are already aware, of course).

    Rising through air, as the layers become less dense, the individual photon "journeys" (between CO2 atoms) become longer — yet each same-depth layer is still emitting the same previous total of upwards and downwards amount of IR radiation (of course).   Only in the most tenuous uppermost layers, does this "stacking" of upward/downward emissions begin to break down, as an increasing percentage of upward IR photons evade reabsorption and make a straightline escape to outer space.  In effect, we can think of the upper atmosphere as producing only back-radiation (at this particular bandwidth we are interested in) as far as the Earth is concerned.

    I am sure I am telling you nothing new, in all this.   I will point out that at an individual level, each IR photon maintains its same energy level, as it is "reincarnated" — the individual CO2 molecule recipient of photonic energy "cools itself" by imparting kinetic energy to a neighbouring N2 or O2 . . . and at a later time regains energy kinetically from a neighbouring N2 molecule, and "reincarnates" & emits an IR photon of the same energy level as previously received but in a random direction.   Of course, as intermolecular distances increase, and the kinetic temperature of N2 molecules reduces, then these "deaths & reincarnations" of IR photons (per second per cubic mm) must reduce.   But the final total product is back-radiation towards the Earth's surface plus upwards "lost" radiation (and of course the totality of all "lost" radiation over the whole spectrum must equal what's originally entered the planetary system from the sun, at equilibrium — or at least extremely close to that total while the system is in transition to equilibrium). 

    Myself, I find it easier to mentally picture these events if you rotate the Earth surface 90 degrees.   Instead of a horizontal surface emitting upwards, choose to picture the surface as the y-axis and the atmosphere layers stacked outwards along the x-axis.   The cool outermost layers of air are losing radiation outwards to space, and are emitting "back-radiation" inwards.   Through the bulk of the atmosphere, each layer is transmitting fractionally more energy inwards than outwards, and these fractional differences integrate to produce a gradient of temperature, highest at the surface and "sloping down" to the outermost air.   Hence the surface is warmer than the outermost air.

    When the atmospheric CO2 concentration becomes raised, the x-axis is extended further (so to speak) . . . and the same gradient produces a higher cumulative back-radiation at the planetary surface: in other words, the surface becomes warmer than under the previous conditions.

  11. Arf,

    My understanding is a little different from yours, just a little.  To review your points:

    1) I think the energy of photons is fixed so energy is not lost each time.  I am not sure what you mean by lost energy.  This point is not important.

    2) Check.

    3) Check.

    4) This is the important step for the greenhouse effect.  We agree that the escape altitude increases. 

    As you know the amount of energy radiated is related to the temperature.  The temperature at the escape altitude must be high enough for all the energy that comes from the sun to escape to conserve energy.  This fixes the temperature at the escape altitude.  When more greenhouse gas is added the temperature at the new altitude increases so that energy is conserved.  

    The temperature in the troposphere (the lower atmosphere) increases as you decrease in altitude.  The rate of increase is called the lapse rate.  The lapse rate is about 6.5C per kilometer.  The lapse rate is a physical property of the atmosphere and is fixed by basic physics and chemistry laws and properties.

    The temperature at the altitude of escape is fixed by the law of conservation of energy and the temperature varies in the atmosphere according to the lapse rate.  When the altitude of escape is increased that results in an increase in temperature at that altitude.  This is then passed down to the surface according to the lapse rate.

    It is interesting to note that an increase in the altitude of only 100 meters results in an increase in the surface temperature of 0.65C.  2C is only 300 meters increase in escape altitude.

    On Venus the CO2 concentration is much higher so the altitude of energy escape is much higher.  That results is the surface of Venus being about 462C.  This temperature can be calculated using the equations that describe the greenhouse effect on Earth.

    Your explaination seems OK to me but does not well describe the changes in the atmosphere.

Prev  1  2  3  4  5  6  7  8  9  10  

Post a Comment

Political, off-topic or ad hominem comments will be deleted. Comments Policy...

You need to be logged in to post a comment. Login via the left margin or if you're new, register here.

Link to this page

The Consensus Project Website


(free to republish)

Smartphone Apps


© Copyright 2018 John Cook
Home | Links | Translations | About Us | Contact Us