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

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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.


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Comments 501 to 509 out of 509:

  1. Thanks all. Continues to be really informative, and I think I now have a good handle on the different possible ways in which increasing co2 could increase troposphere temperatures. What isn't so clear is how significant those effects are at marginal increases from 400 ppmv, but to br honest I could probsbly spend the next 6 months studying the topic in detail and not be that much more certain given the myriad complexities.

    MA: The Zhong and Haigh paper is really interesting, but I'm not quote following figures 5a and b, top row, which seem to show minimal change in radiative flux (when averaged across the spectrum) even up to 32x co2. I'm not convinced they have taken into account all those phenomena when working out thr logarithmic relationship - they say

    "our calculations assume no change in the surface or atmosphere, do not consider the climate response to the RF, or any issues related to climate sensitivity"

    But perhaps there's something intrinsic to the models that goes without saying. My point was more that it be *more* of a factor going forward, as the proportion of time when thr altitude of emission is in a scenario where increasing altitude does not mean decreasing temperatures is presumably increasing with increasing co2.

    Given the point has been made to me repeatedly that the science is apparently so settled no one's even really looking at it anymore, i thought it might be useful to go back and look at past predictions to see how they measure up against present day. This might not be the right page to discuss such matters, but I came across this analysis of a Hansen 1988 paper, in which apparently actual temperatures are matching up in line with his 'zero co2 increase after 2000' scenario Thoughts?

  2. @eclectic: it appears that is the case. the point wasn't about a single altitide; rather it's about where the altitude of emission is in an area where increasing altitude no longer leads to decreasing temperatures

  3. LTO @501 ,

    A/  Please note that you have duplicated your post #501as #502 and #503. Do not be too embarrassed ~ a Moderator will correct that reduplication.   Likewise with your other duplication !

    B/  On #504 [soon to be #502, I expect] , you will need to explain what you mean by your first and second sentences.   What is the case?   What is the misunderstood point?   There seems to be considerable confusion of communication here.

    Take a look at the atmospheric temperature versus altitude graphs.   For most of the troposphere, the temperature reduces with height.   Above that, the temperature holds steady for a short distance ~ and above that, temperature increases with altitude through the stratosphere.   You need to integrate that information with the decreasing air density ~ because both factors are important in comprehending the (15um) radiative loss to space.   The low density in stratosphere is the reason the tropospheric (15um) loss is vastly more important (and why the weighted average "emission height" is generally in the troposphere, affected by the lapse rate there).

    Things get more complex, if we consider other radiative output from CO2 ~ and also other radiative properties of H2O and all molecules of 3 or more atoms.   But for our mutual purposes, it is enough to consider the 15um band, here in this thread.

  4. LTO , you have mentioned the "realclimatescience" website.

    Perhaps you are not yet aware ~ but that is not a science website, it is a propaganda website.   "Fake News".   As a pointer, one should always be on the alert for disinformation, on any site prominently quoting Feynman, Popper, or Galileo.   [ Feynman, Popper and Galileo are of course very worthy gentlemen in their own right ~ but their philosophies are seriously abused by propagandists hoping to drape themselves with reflected glory . . . propagandists hoping to mislead the uninformed/unthinking reader. ]

    As to your question on Hansen etcetera ~ the propagandists are diverting your attention onto some old predictions/projections of 30+ years ago, in the early days of such assessments.   Worse by far, they are deceiving you by comparing to more recent high-altitude data, not the planetary surface temperatures.   That is classic bait-and-switch deception.

    Also, note that site's reference to "the 52% consensus" ~ based on some very unrigorous survey of members of some meteorological society.  No detailed explanation.   Quite shameless propaganda.

    LTO , please get your information from an honest scientific website.   For instance : RealClimate.  (You can see how the anti-science propagandists are trying to piggy-back, by using a similar sounding name such as realclimatescience, to mislead the careless into their own site.)

  5. Erp . . . I have committed an unfortunate ambiguity, in the second post above this one.   "the temperature reduces with height" should read "the temperature reduces with increased height" ~ which is much clearer !   My apologies for that initial statement, which might well pass in colloquial conversation, but which was very open to misinterpretation (in written form).

  6. LTO:

    A scientific discussion of Hansen's 1988 paper is here Realclimate discussion .  Note that this is the website the denier site "realclimatescience" is attempting to hijack. 

    Hansen's projections were skillful but the release of greenhouse gasses was not what he forecast. While he forecast lower CO2 emissions, he forecast higher cloroflurocarbon emissions.  Simplistic evaluations of only CO2, like the one at realclimatescience, ignore these important greenhouse gasses. 

    The writers at realclimatescience have had the opportunity to read the correct science at Realclimate and continue to push their incorrect ideas.  In my book that is a deliberate lie.  I recommend you stop wasting your time (and ours looking up the correct analysis) reading denier web sites.

    There is an easy way to find out how increasing CO2 affects temperature: read the IPCC summary and figure they are correct!!  It is a waste of time to attempt to calcualte or completely understand the atmosphere yourself, it is too complex. 

  7. LTO @501: "i thought it might be useful to go back and look at past predictions to see how they measure up against present day."

    You can easily do that right here on SkS. Scroll up to the "thermometer" in the left-side banner. Under that are some rectangular "buttons". Click on the "Lessons from Predictions" button. That will take you to a page listing blog posts dealing with past predictions/projections and how they have measured up.

  8. LTO @501,

    Concerning Zhong & Haigh (2013), you ask about the meaning of "our calculations assume no change in the surface or atmosphere, do not consider the climate response to the RF, or any issues related to climate sensitivity." This is simply saying that they calculate the climate forcing due to these changes in CO2 levels. Such forcings would increase global temperature and the resulting changes to other GHG levels, cloud, surface albedo, etc, which would result from that forcing are not being considered. This is solely about the direct effect of the CO2 and not any feedbacks.

    And the top row of their Fig 5a/b is simply the traces within all the other rows plotted together. The one rather confusing part of this Fig 5 is that Fig5a plots the zero CO2 alongside all the other CO2 levels while Fig5b plots the difference between each different level and a current level (as was) of 389ppm. Thus the second row of Fig5a shows both zero (lt blue) and 1.5ppm (green) but the difference between 389ppm & 1.5ppm (green) appears down in the fourth row of Fig5b.

    Note I still intend to tap out a screed as I promised @492.

  9. LTO - Responding to your earlier questions about energy exchange via collisions, and the relative energies of emitting GHGs and the rest of the atmosphere (sorry this is weeks later, just got back to this thread after a while):

    At sea level each molecule in the atmosphere collides with others at about 10^9 times per second. The time for an excited CO2 molecule to emit IR is about 10^-6 seconds. Therefore (comparing relative numbers) each CO2 molecule collides ~1000 times before emitting. Thus the CO2 molecules emit at a rate tied directly to the gas temperature. 

    The tropopause (where the break-even point of CO2 emission lies, ending convection) ranges from about 7 to 20 km, poles to equator. Air pressure at 15km is roughly 100mb, 1/10 sea level, meaning that at that altitude each CO2 molecule collides 100 times with other atmospheric molecules before emitting. So again, CO2 molecules emit at rates tied directly to atmospheric temperature. 

    One side point, since I've seen this a particular error occur repeatedly - emitting CO2 molecules are by no means tied to absorbing CO2 molecules, and neither are the absorption energies directly tied to emission energies on a photon by photon basis. The gases warm due to convection and IR absorption, that temperature gets shared, and the emissions from any particular gas volume are statistical expressions of the GHG emission spectra from GHGs at that temperature. There's no immediate one-to-one connection. 

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