<p>Borrowing an explanation from one of my other <a href="https://earthscience.stackexchange.com/questions/7502/is-there-any-experiment-to-prove-that-co2-with-the-atmosphere-concentration-can/7522#7522">answers</a>, the basic mechanism of the greenhouse effect is roughly as follows (note this is also a simplified model)</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>Thus it is irrelevant that most IR emitted by the surface is absorbed by the atmosphere, it is the height from which IR is <strong>not</strong> absorbed that matters.  If it is colder than the surface, it will emit less IR than the surface is emitting and hence less energy is radiated into space.</p>

<p>For a more detailed explanation of why absorption at the surface is of little relevance, see this RealClimate <a href="http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument/" rel="noreferrer">article</a> by Spencer Weart.</p>

<p>Just to add a historical note, this explanation goes back at least as far as Ekholm's paper of 1901 "On The Variations Of The Climate Of The Geological And Historical Past And Their Causes":</p>

<blockquote>
  <p>The atmosphere plays a very important part of a double character as to the
  temperature at the earth’s surface, of which the one was
  first pointed out by Fourier, the other by Tyndall. Firstly, the
  atmosphere may act like the glass of a green-house, letting through
  the light rays of the sun relatively easily, and absorbing a great
  part of the dark rays emitted from the ground, and it thereby may
  raise the mean temperature of the earth’s surface. Secondly, the
  atmosphere acts as a heat store placed between the relatively warm
  ground and the cold space, and thereby lessens in a high degree the
  annual, diurnal, and local variations of the temperature.</p>
  
  <p>There are two qualities of the atmosphere that produce these effects. The 
  one is that the temperature of the atmosphere generally
  decreases with the height above the ground or the sea-level, owing
  partly to the dynamical heating of descending air currents and the
  dynamical cooling of ascending ones, as is explained in the mechanical
  theory of heat. The other is that the atmosphere, absorbing but little
  of the insolation and the most of the radiation from the ground,
  receives a considerable part of its heat store from the ground by
  means of radiation, contact, convection, and conduction, whereas the
  earth’s surface is heated principally by direct radiation from the sun
  through the transparent air.</p>
  
  <p>It follows from this that the radiation from the earth into space does 
  not go on directly from the ground, but on the average from a
  layer of the atmosphere having a considerable height above sea-level.
  The height of that layer depends on the thermal quality of the
  atmosphere, and will vary with that quality. The greater is the
  absorbing power of the air for heat rays emitted from the ground, the
  higher will that layer be, But the higher the layer, the lower is its
  temperature relatively to that of the ground ; and as the radiation
  from the layer into space is the less the lower its temperature is, it
  follows that the ground will be hotter the higher the radiating layer
  is.”</p>
</blockquote>

<p><a href="https://dx.doi.org/doi:10.1002/qj.49702711702" rel="noreferrer"> [Ekholm</a>, 1901, p19-20]</p>

<p>(h/t Steve Easterbrook's blog <a href="https://www.easterbrook.ca/steve/2015/08/who-first-coined-the-term-greenhouse-effect/" rel="noreferrer">article</a>)</p>

CC BY-SA 4.0
2020-01-17T17:56:20.260Z
This is very interesting, thank you! If I may ask, could you please clarify the statement "the more CO2 we put into the atmosphere, the higher this emitting layer becomes"? Why does an increased concentration of CO2 move the upper boundary of the layer higher up? And is the change in concentration of CO2 quantitatively consistent with a significant change in this layer's thickness? I'm asking because I was talking about this at work with some colleagues today, and one of them argued that there is no evidence that reducing CO2 emissions will improve anything, which baffled me a lot.
user6376297
CC BY-SA 4.0
2020-01-17T18:05:45.077Z
Thank you very much for your answer, @Dikran Marsupial. I am starting to get the picture, but I am still struggling to intuitively understand your statement: "If it is colder than the surface, it will emit less IR than the surface is emitting and hence less energy is radiated into space." Because if the heat is being transported up to a higher layer in the atmosphere before departing as radiation into space, wouldn't the heat just increase the temperature of that higher layer, until such time as its radiant output to space is in equilibrium with the heat energy input to that higher layer?
Ash90
CC BY-SA 4.0
2020-01-17T18:19:55.990Z
@user6376297 CO2 is "well mixed" in the atmosphere, so if we add more CO2 to the atmosphere, there will be more CO2 above the old effective radiating layer and it will absorb some of the IR emitted from that layer.  This means that on average, more of the IR that escapes will be from slightly higher in the atmosphere than before.  There isn't really an identifiable layer, in reality there is a distribution of heights from which IR escapes to space, but adding CO2 shifts the distribution a bit higher.
Dikran Marsupial
CC BY-SA 4.0
2020-01-17T18:26:13.060Z
@Hip89, yes, that is correct.  The top-of-the-atmosphere radiative imbalance caused by the increase in the height (and thus coolness) of the effective radiating layer only lasts until the planet accumulates enough energy for the new effective radiating to become as warm as the old one.  However, for that to happen the surface needs to be warmer than before (because of the lapse rate) in order to provide the necessary energy.
Dikran Marsupial
CC BY-SA 4.0
2020-01-17T18:29:03.770Z
Reducing CO2 emissions will help because the natural carbon cycle (oceans and terrestrial biota) is taking more CO2 out of the atmosphere than it emits into it (because we have pushed the system out of equilibrium, the carbon cycle feedbacks are opposing the perturbation).  So if we stop emissions then CO2 levels will begin falling and the planet will cool.  If we reduce emissions, we will at least reduce the rate of warming and limit the problems climate change is likely to cause.
Dikran Marsupial
CC BY-SA 4.0
2020-01-18T14:13:10.487Z
@Dikran Marsupial, you say: "the surface needs to be warmer than before ... in order to provide the necessary energy". That I can understand in the context of thermal conductivity: if you add an extra layer of thermal insulation in a house, then the internal temp will rise. So we need to understand how an effectively taller atmosphere acts analogously to increased insulation. I guess if the atmospheric heat transfer is mainly convection, then to drive warm air to a higher altitude requires a higher surface temp. Like a hot air balloon: to attain a higher attitude requires a higher balloon temp
Ash90
CC BY-SA 4.0
2020-01-18T15:06:54.943Z
The atmosphere isn't any taller, it is just the height at which IR radiation can escape into space without being absorbed by greenhouse gasses rises.  AIUI most of the heat transfer within the atmosphere is by convection rather than conduction or radiation, and it is that that sets up the lapse rate (the decrease in temperature with increasing height).  Because there is a lapse rate, if you want the upper atmosphere to warm, you need the surface to warm as well.
Dikran Marsupial
CC BY-SA 4.0
2020-01-18T18:23:17.943Z
FYI, [here](https://physics.stackexchange.com/questions/300046/explain-it-to-me-like-im-a-physics-grad-greenhouse-effect/300058#300058) is a post from SE\physics that seems related to what is being discussed here. I confess that I scrolled to the bottom, skipping all the equations... and unfortunately I couldn't understand the conclusion :(
user6376297
CC BY-SA 4.0
2020-05-25T02:58:05.560Z
@DikranMarsupial For the second question [here](https://earthscience.stackexchange.com/q/19763/6535), I guess it is because the effective bottom-of-the-atmosphere radiating layer is a few km up, where it is colder.
Keith McClary
CC BY-SA 4.0
2020-01-17T15:10:02.720Z
2020-01-17T16:19:01.647Z
2020-01-17T16:19:01.647Z
Dikran Marsupial
<p>I would like to address what appears to be an over-generalization of your premise.  You seem to be simplifying the radiative heat from the Earth.  Yes, if you take an average temperature and an average atmosphere, the present amount of CO2 is almost completely maxing out the sides of the absorption bands and an increase in CO2 gets you almost no additional absorption.  However, that is an average.  Keep in mind that the majority of warming that is currently happening is at high latitudes and high altitudes, where infrared from the Earth is far below average.  The increase in global CO2 has a larger affect in these colder regions because there is greater sensitivity to the increase in CO2, affecting those fringes of the absorption bands. </p>

CC BY-SA 4.0
2020-04-20T02:53:34.757Z
2020-04-20T02:53:34.757Z
f.thorpe
<p>A complimentary article to the one written by Spencer Weart and recommended by Dikran Marsupial is this other article from September 2019 written by Jason West, Professor of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill.</p>
<p><a href="https://theconversation.com/climate-explained-why-carbon-dioxide-has-such-outsized-influence-on-earths-climate-123064" rel="nofollow noreferrer">Climate explained: why carbon dioxide has such outsized influence on Earth’s climate</a></p>
<p>It underpins what has been said here that what matters most is the height that the carbon dioxide layer can reach at the upper atmosphere.</p>
<p>In the article is said that carbon dioxide is important for climate despite the fact that water vapor absorbs more infrared radiation and the two gases absorb at several of the same wavelengths.</p>
<blockquote>
<p>The reason is that Earth’s upper atmosphere controls the radiation that escapes to space. The upper atmosphere is much less dense and contains much less water vapor than near the ground, which means that adding more carbon dioxide significantly influences how much infrared radiation escapes to space.</p>
</blockquote>
<p>The article also re-states information given by by Spencer Weart about the role of carbon dioxide in climate change:</p>
<blockquote>
<p>The scientists who first identified carbon dioxide’s importance for climate in the 1850s were also surprised by its influence. Working separately, John Tyndall in England and Eunice Foote in the United States found that carbon dioxide, water vapor and methane all absorbed heat, while more abundant gases did not.</p>
<p>Tyndall and Foote showed that nitrogen and oxygen, which together account for 99% of the atmosphere, had essentially no influence on Earth’s temperature because they did not absorb heat. Rather, they found that gases present in much smaller concentrations were entirely responsible for maintaining temperatures that made the Earth habitable, by trapping heat to create a natural greenhouse effect.</p>
</blockquote>
<p>The reason why carbon dioxide has a much more importance in warming the planet while much more abundant gases like nitrogen and oxygen do not play an important role is related to its molecular structure:</p>
<blockquote>
<p>Carbon dioxide and other heat-trapping gases have molecular structures that enable them to absorb infrared radiation. Carbon dioxide and other heat-trapping gases have three or more atoms and frequencies that correspond to infrared radiation emitted by Earth. Oxygen and nitrogen, with just two atoms in their molecules, do not absorb infrared radiation.</p>
</blockquote>
<p>The article provides important pre-historic information about the concentration of carbon dioxide in the atmosphere and its correlation with the concentration of water vapor in the atmosphere and the consequences for the climate.</p>
<blockquote>
<p>The influence of carbon dioxide can be seen in past changes in climate. Ice cores from over the past million years have shown that carbon dioxide concentrations were high during warm periods – about 0.028%. During ice ages, when the Earth was roughly 7 to 13 F (4-7 C) cooler than in the 20th century, carbon dioxide made up only about 0.018% of the atmosphere.</p>
<p>Even though water vapor is more important for the natural greenhouse effect, changes in carbon dioxide have driven past temperature changes. In contrast, water vapor levels in the atmosphere respond to temperature. As Earth becomes warmer, its atmosphere can hold more water vapor, which amplifies the initial warming in a process called the “water vapor feedback.” Variations in carbon dioxide have therefore been the controlling influence on past climate changes.</p>
</blockquote>
<p>It finishes explaining why it is foolhardy not to diminish the carbon dioxide emissions to prevent big effect on Earths climate:</p>
<blockquote>
<p>Today the level of carbon dioxide is higher than at any time in human history. Scientists widely agree that Earth’s average surface temperature has already increased by about 2 F (1 C) since the 1880s, and that human-caused increases in carbon dioxide and other heat-trapping gases are extremely likely to be responsible.</p>
<p>Without action to control emissions, carbon dioxide might reach 0.1% of the atmosphere by 2100, more than triple the level before the Industrial Revolution. This would be a faster change than transitions in Earth’s past that had huge consequences. Without action, this little sliver of the atmosphere will cause big problems.</p>
</blockquote>
<p><a href="https://i.sstatic.net/FG0xN.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/FG0xN.png" alt="Carbon dioxide concentration in the atmosphere from 1958 to 2019" /></a></p>

CC BY-SA 4.0
2020-04-21T01:20:18.080Z
2020-04-21T01:20:18.080Z
2020-06-18T08:25:19.090Z
Philthy Phil
<p>CO<sub>2</sub> is dark in the long wave infrared wavelength so the higher the concentration the darker the atmosphere will be in this wavelength, and as you probably know darker colours absorb more heat.</p>

<p>CO<sub>2</sub> is not a very powerful greenhouse gas but the amount of it in our atmosphere make this effect the dominating source for the heating we can see today.</p>

<p>Lots of other gases have a larger heating potential than CO<sub>2</sub> but their concentration is low at the moment. The more complex a gas is the more heating potential it will have, this is due to the size of the individual gas molecules (CO<sub>2</sub> has 3 atoms and methane has 5, so it will look darker in the long wave infrared band and therefore have a stronger heating potential).</p>

CC BY-SA 4.0
2020-01-19T21:25:22.943Z
I don't think this answers the question.
Semidiurnal Simon
CC BY-SA 4.0
2024-07-02T19:34:36.470Z
You may want to mention the vibrational and rotational states of energy for the molecule, which causes the specific wavelengths being absorbed/emitted.
f.thorpe
CC BY-SA 4.0
2020-01-18T06:16:14.590Z
2020-04-20T14:15:06.897Z
2020-04-20T14:15:06.897Z
trond hansen
<p>The atmosphere has heated about .5 C since 1979. It takes 3 Zeta Joules of energy to accomplish that. That would be .075 ZJ per year if purely cumulative. Residual heat from human energy consumption amounts to .5 ZJ per year or about 20 ZJ since 1979. That is 7 times what would be needed to heat the atmosphere by .5 C. (We note that the oceans are accumulating thermal energy too). In addition to direct transfer of black body radiation to space, molecular collision induced IR generation in the upper atmosphere is where cooling of the global energy reservoir establishes equilibrium. Waste heat energy added to the climate system (with an overprint of natural fluctuation) is the primary cause of the slight increase in temperature of the atmosphere since 1979. But sceptics and alarmists alike tend to ignore waste heat as a serious contributor to climate change.</p>

CC BY-SA 4.0
2022-10-10T11:01:25.543Z
This answer would benefit from some sources.
Jean-Marie Prival
CC BY-SA 4.0
2022-10-10T12:09:38.720Z
Refering to this your numbers are way off.
https://earthscience.stackexchange.com/questions/3041/how-does-anthropogenic-heating-affect-global-warming
smichel
CC BY-SA 4.0
2022-10-10T22:09:37.780Z
Haven't looked at your calculation... but I'm guessing it suggests it would take 3 ZJ of energy in the Earth NOW to be .5 C warmer... in other words retaining 3 ZJ over that period.  Whereas energy leaves the system continually.  And warmer temperature means more energy is emitted.  Only by having a method to *retain* extra energy are you going to heat the Earth longterm.
JeopardyTempest
CC BY-SA 4.0
2022-10-11T03:27:28.770Z
Indeed your numbers show you're saying the Earth would have to continually retain 6 entire years worth of waste energy. Yet the temperature drop each winter shows just how well Earth sheds heat continually (including our added waste heat). The greenhouse effect, the topic of the question, is the way the Earth can retain some of its heat, including waste heat... but not 6 years worth! Instead, though, a tiny increase in greenhouse effect would wind up adding more heat, as the scale of total energy (i.e. solar insolation) is so giant compared to waste heat.
JeopardyTempest
CC BY-SA 4.0
2022-10-11T03:29:43.913Z
Using the exponential series sum equation $\sum_{n=0}^N r^n = \frac{r^{N+1} - 1}{r-1}
$, it looks like since 1979 Earth would have to be retaining [5/6](https://www.desmos.com/calculator/8bl6xr3d9i) of the waste energy annually (or a bit more since it's really a continual rather than annual decay) for it to now have 3 ZJ worth at this point? Or did I make a mistake somewhere??? (Always possible?) But if correct... it clearly doesn't retain anywhere near that amount of solar energy, or we'd have big problems, so waste heat doesn't seem the feasible culprit to me at least...
JeopardyTempest
CC BY-SA 4.0
2022-10-10T09:06:32.523Z
2022-10-10T09:06:32.523Z
Erwin Unger
<p>The greenhouse effect analogy of global warming is that atmospheric carbon dioxide CO<span class="math-container">$_2$</span> absorbs some of the infrared radiation emitted by the Earth, and redirects a portion of that radiation back down to the Earth's surface, thereby heating the surface more than it would have done if that radiation had been able to escape into space. </p>

<p>Global warming is then simplistically explained to the general public by the idea that as atmospheric CO<span class="math-container">$_2$</span> concentrations rise, more infrared radiation is absorbed by CO<span class="math-container">$_2$</span> and re-emitted back down to Earth, causing increased heating of the Earth. </p>

<p>However, this explanation is not technically correct, because at present atmospheric CO<span class="math-container">$_2$</span> concentrations, just one kilometer of atmosphere is sufficient to fully absorb all the infrared radiation emitted by the Earth, at the wavelengths at which CO<span class="math-container">$_2$</span> absorbs. </p>

<p>Carbon dioxide absorbs infrared at the wavelengths of 2.7, 4.3 and 15 µm, and the CO<span class="math-container">$_2$</span> in the first kilometer of atmosphere alone is able to completely absorb all infrared at these wavelengths.</p>

<p>So the infrared absorption process is already fully saturated, and thus further increases in atmospheric CO<span class="math-container">$_2$</span> will not lead to any additional absorption. This is why the simplistic explanation provided for the general public does not seem to be technically correct, even though it roughly outlines the idea.</p>

<p>I found one <a href="http://clivebest.com/blog/?p=1169" rel="nofollow noreferrer">blog article</a> by Clive Best that tries to explain the actual process behind CO<span class="math-container">$_2$</span>'s ability to cause global warming. Judging from that article, the actual process is more complex than the simple explanation provided for public consumption. However, I don't fully understand the explanation given in the article (and from what I did manage to understand, I am not sure if it is fully correct).</p>

<p>So I wonder if anyone here can provide an easy to understand explanation of the actual mechanism by which increased atmospheric CO<span class="math-container">$_2$</span> leads to global warming. Or perhaps if you know any good articles that explain it, please can you post the links.</p>

<p>I tried to find some info on the actual mechanism of global warming via Google, by using search terms such as "mechanism of greenhouse effect in global warming", but was surprised to find very little information available. </p>

<p>I also asked this question on physics.stackexchange.com <a href="https://physics.stackexchange.com/questions/525177/can-anyone-explain-the-real-reason-why-co-2-increases-global-temperatures-not">here</a>.</p>

CC BY-SA 4.0
2020-01-16T19:21:29.227Z
The explanation, while it may be simplified, is not "simplistic".  (You might consider entertaining the idea that you've made a mistake in your math.)  For a basic explanation, start here: https://history.aip.org/climate/index.htm  If you want to go deeper, consider a text on climate physics such as Ray Pierrehumbert's "Principles of Planetary Climate".
jamesqf
CC BY-SA 4.0
2020-01-16T20:11:16.023Z
@jamesqf, the explanation provided to the general public is simplistic: it's a reduced complexity explanation. Whereas there is nothing wrong with giving a simplified explanation: that's always desirable.
Ash90
CC BY-SA 4.0
2020-01-16T20:17:03.427Z
@jamesqf, thanks for that link; it gives a good explanation here: https://history.aip.org/climate/simple.htm . But I am struggling to understand this statement on increased CO2 levels: "So the place from which part of the heat energy finally leaves the Earth will shift to higher layers. Those are thinner and colder layers, so they do not radiate heat as well." I understand increased CO2 transports the departing IR radiation to higher layers. But if this radiation arrives at a higher layer, surely it will heat that layer, making it radiate better. That's the part I do not understand
Ash90
CC BY-SA 4.0
2020-01-17T02:19:48.797Z
It can't just heat the higher layer. The radiation must pass through the entire atmosphere, heating it proportionately (with multiple absorptions and emissions, of course, and convection plays a role).
Keith McClary
CC BY-SA 4.0
2020-01-17T02:25:42.387Z
Not sure it this will help but - The principle heat transport to the higher layers is not IR radiation, it is physical air movement - wind and convection. That rising air will cool at higher altitude as air pressure drops. Due to raised amounts of CO2, the altitude where IR can escape to space, (rather than be reabsorbed) is higher, where it is also cooler. The rate of radiation is slower at that higher altitude. The air will circulate back down a bit warmer than with atmosphere with lower CO2 concentration.
Ken Fabian
CC BY-SA 4.0
2020-01-17T06:30:06.807Z
@KenFabian Right, although [Raymond T. Pierrehumbert says: "At higher altitudes, heat transfer is dominated by radiative
transfer instead of fluid motions; the corresponding region is
the stratosphere."](https://geosci.uchicago.edu/~rtp1/papers/PhysTodayRT2011.pdf) But what I was trying to say is that more CO2 effectively creates an extra layer that energy has to get through, so the Earth's surface and the whole atmosphere below needs to be hotter to achieve the same radiation to space, in order to balance the incoming solar energy.
Keith McClary
CC BY-SA 4.0
2020-01-17T13:20:27.683Z
@ebv, surely you must realize that when complex physics is involved, that excludes most of the population from properly understanding it. I have degree in physics, so I am hoping to be able to understand the mechanism of global warming. But don't expect your average person in the street to understand.
Ash90
CC BY-SA 4.0
2020-01-17T15:04:36.337Z
There is an excellent blog article by Spencer Weart http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument/ that explains why the saturation at the surface argument is incorrect.  It is energy balance at the top of the atmosphere that determines whether the planet warms or cools, the absorption at the surface is largely irrelevant.
Dikran Marsupial
CC BY-SA 4.0
2020-01-17T15:15:17.130Z
BTW I would be wary of Clive Bests blog.  He is a very bright chap, but tends to be resistant to correction/criticism.  Being clever is not a substitute for experience and research, you'll just make the same mistakes that climatologists made 30 years ago all over again.  They were clever people as well! ;o)
Dikran Marsupial
CC BY-SA 4.0
2020-01-20T00:41:00.603Z
Not sure if it is correct to say "So the infrared absorption process is already fully saturated, and thus further increases in atmospheric CO2 will not lead to any additional absorption." What about the re-emitted IR after initial absorption, this can go on and on for a long time I guess?
y chung
<climate-change><carbon-cycle><greenhouse-gases>
Can anyone explain the reason why CO$_2$ increases global temperatures (not the simplistic greenhouse analogy provided for public consumption)?
Ash90
# Can anyone explain the reason why CO$_2$ increases global temperatures (not the simplistic greenhouse analogy provided for public consumption)?

(Asked by: username_0 on 2020-01-16)

The greenhouse effect analogy of global warming is that atmospheric carbon dioxide CO$\_2$ absorbs some of the infrared radiation emitted by the Earth, and redirects a portion of that radiation back down to the Earth's surface, thereby heating the surface more than it would have done if that radiation had been able to escape into space.

Global warming is then simplistically explained to the general public by the idea that as atmospheric CO$\_2$ concentrations rise, more infrared radiation is absorbed by CO$\_2$ and re-emitted back down to Earth, causing increased heating of the Earth.

However, this explanation is not technically correct, because at present atmospheric CO$\_2$ concentrations, just one kilometer of atmosphere is sufficient to fully absorb all the infrared radiation emitted by the Earth, at the wavelengths at which CO$\_2$ absorbs.

Carbon dioxide absorbs infrared at the wavelengths of 2.7, 4.3 and 15 µm, and the CO$\_2$ in the first kilometer of atmosphere alone is able to completely absorb all infrared at these wavelengths.

So the infrared absorption process is already fully saturated, and thus further increases in atmospheric CO$\_2$ will not lead to any additional absorption. This is why the simplistic explanation provided for the general public does not seem to be technically correct, even though it roughly outlines the idea.

I found one [blog article](http://clivebest.com/blog/?p=1169) by Clive Best that tries to explain the actual process behind CO$\_2$'s ability to cause global warming. Judging from that article, the actual process is more complex than the simple explanation provided for public consumption. However, I don't fully understand the explanation given in the article (and from what I did manage to understand, I am not sure if it is fully correct).

So I wonder if anyone here can provide an easy to understand explanation of the actual mechanism by which increased atmospheric CO$\_2$ leads to global warming. Or perhaps if you know any good articles that explain it, please can you post the links.

I tried to find some info on the actual mechanism of global warming via Google, by using search terms such as "mechanism of greenhouse effect in global warming", but was surprised to find very little information available.

I also asked this question on physics.stackexchange.com [here](https://physics.stackexchange.com/questions/525177/can-anyone-explain-the-real-reason-why-co-2-increases-global-temperatures-not).

**Tags:** climate-change, carbon-cycle, greenhouse-gases

> **Comments:**
> **username_1**: The explanation, while it may be simplified, is not "simplistic".  (You might consider entertaining the idea that you've made a mistake in your math.)  For a basic explanation, start here: https://history.aip.org/climate/index.htm  If you want to go deeper, consider a text on climate physics such as Ray Pierrehumbert's "Principles of Planetary Climate".  
> **username_0**: @username_1, the explanation provided to the general public is simplistic: it's a reduced complexity explanation. Whereas there is nothing wrong with giving a simplified explanation: that's always desirable.  
> **username_0**: @username_1, thanks for that link; it gives a good explanation here: https://history.aip.org/climate/simple.htm . But I am struggling to understand this statement on increased CO2 levels: "So the place from which part of the heat energy finally leaves the Earth will shift to higher layers. Those are thinner and colder layers, so they do not radiate heat as well." I understand increased CO2 transports the departing IR radiation to higher layers. But if this radiation arrives at a higher layer, surely it will heat that layer, making it radiate better. That's the part I do not understand  
> **username_2**: It can't just heat the higher layer. The radiation must pass through the entire atmosphere, heating it proportionately (with multiple absorptions and emissions, of course, and convection plays a role).  
> **username_3**: Not sure it this will help but - The principle heat transport to the higher layers is not IR radiation, it is physical air movement - wind and convection. That rising air will cool at higher altitude as air pressure drops. Due to raised amounts of CO2, the altitude where IR can escape to space, (rather than be reabsorbed) is higher, where it is also cooler. The rate of radiation is slower at that higher altitude. The air will circulate back down a bit warmer than with atmosphere with lower CO2 concentration.  
> **username_2**: @username_3 Right, although [Raymond T. Pierrehumbert says: "At higher altitudes, heat transfer is dominated by radiative transfer instead of fluid motions; the corresponding region is the stratosphere."](https://geosci.uchicago.edu/~rtp1/papers/PhysTodayRT2011.pdf) But what I was trying to say is that more CO2 effectively creates an extra layer that energy has to get through, so the Earth's surface and the whole atmosphere below needs to be hotter to achieve the same radiation to space, in order to balance the incoming solar energy.  
> **username_0**: @ebv, surely you must realize that when complex physics is involved, that excludes most of the population from properly understanding it. I have degree in physics, so I am hoping to be able to understand the mechanism of global warming. But don't expect your average person in the street to understand.  
> **username_4**: There is an excellent blog article by Spencer Weart http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument/ that explains why the saturation at the surface argument is incorrect.  It is energy balance at the top of the atmosphere that determines whether the planet warms or cools, the absorption at the surface is largely irrelevant.  
> **username_4**: BTW I would be wary of Clive Bests blog.  He is a very bright chap, but tends to be resistant to correction/criticism.  Being clever is not a substitute for experience and research, you'll just make the same mistakes that climatologists made 30 years ago all over again.  They were clever people as well! ;o)  
> **username_5**: Not sure if it is correct to say "So the infrared absorption process is already fully saturated, and thus further increases in atmospheric CO2 will not lead to any additional absorption." What about the re-emitted IR after initial absorption, this can go on and on for a long time I guess?  

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## 5 answers

### Answer by username_4 on 2020-01-17. Score: 13

Borrowing an explanation from one of my other [answers](https://earthscience.stackexchange.com/questions/7502/is-there-any-experiment-to-prove-that-co2-with-the-atmosphere-concentration-can/7522#7522), the basic mechanism of the greenhouse effect is roughly as follows (note this is also a simplified model)

> 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.

Thus it is irrelevant that most IR emitted by the surface is absorbed by the atmosphere, it is the height from which IR is **not** absorbed that matters. If it is colder than the surface, it will emit less IR than the surface is emitting and hence less energy is radiated into space.

For a more detailed explanation of why absorption at the surface is of little relevance, see this RealClimate [article](http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument/) by Spencer Weart.

Just to add a historical note, this explanation goes back at least as far as Ekholm's paper of 1901 "On The Variations Of The Climate Of The Geological And Historical Past And Their Causes":

> The atmosphere plays a very important part of a double character as to the
> temperature at the earth’s surface, of which the one was
> first pointed out by Fourier, the other by Tyndall. Firstly, the
> atmosphere may act like the glass of a green-house, letting through
> the light rays of the sun relatively easily, and absorbing a great
> part of the dark rays emitted from the ground, and it thereby may
> raise the mean temperature of the earth’s surface. Secondly, the
> atmosphere acts as a heat store placed between the relatively warm
> ground and the cold space, and thereby lessens in a high degree the
> annual, diurnal, and local variations of the temperature.
>
> There are two qualities of the atmosphere that produce these effects. The
> one is that the temperature of the atmosphere generally
> decreases with the height above the ground or the sea-level, owing
> partly to the dynamical heating of descending air currents and the
> dynamical cooling of ascending ones, as is explained in the mechanical
> theory of heat. The other is that the atmosphere, absorbing but little
> of the insolation and the most of the radiation from the ground,
> receives a considerable part of its heat store from the ground by
> means of radiation, contact, convection, and conduction, whereas the
> earth’s surface is heated principally by direct radiation from the sun
> through the transparent air.
>
> It follows from this that the radiation from the earth into space does
> not go on directly from the ground, but on the average from a
> layer of the atmosphere having a considerable height above sea-level.
> The height of that layer depends on the thermal quality of the
> atmosphere, and will vary with that quality. The greater is the
> absorbing power of the air for heat rays emitted from the ground, the
> higher will that layer be, But the higher the layer, the lower is its
> temperature relatively to that of the ground ; and as the radiation
> from the layer into space is the less the lower its temperature is, it
> follows that the ground will be hotter the higher the radiating layer
> is.”

[[Ekholm](https://dx.doi.org/doi:10.1002/qj.49702711702), 1901, p19-20]

(h/t Steve Easterbrook's blog [article](https://www.easterbrook.ca/steve/2015/08/who-first-coined-the-term-greenhouse-effect/))

> **Comments:**
> **username_6**: This is very interesting, thank you! If I may ask, could you please clarify the statement "the more CO2 we put into the atmosphere, the higher this emitting layer becomes"? Why does an increased concentration of CO2 move the upper boundary of the layer higher up? And is the change in concentration of CO2 quantitatively consistent with a significant change in this layer's thickness? I'm asking because I was talking about this at work with some colleagues today, and one of them argued that there is no evidence that reducing CO2 emissions will improve anything, which baffled me a lot.  
> **username_0**: Thank you very much for your answer, @username_4. I am starting to get the picture, but I am still struggling to intuitively understand your statement: "If it is colder than the surface, it will emit less IR than the surface is emitting and hence less energy is radiated into space." Because if the heat is being transported up to a higher layer in the atmosphere before departing as radiation into space, wouldn't the heat just increase the temperature of that higher layer, until such time as its radiant output to space is in equilibrium with the heat energy input to that higher layer?  
> **username_4**: @username_6 CO2 is "well mixed" in the atmosphere, so if we add more CO2 to the atmosphere, there will be more CO2 above the old effective radiating layer and it will absorb some of the IR emitted from that layer.  This means that on average, more of the IR that escapes will be from slightly higher in the atmosphere than before.  There isn't really an identifiable layer, in reality there is a distribution of heights from which IR escapes to space, but adding CO2 shifts the distribution a bit higher.  
> **username_4**: @Hip89, yes, that is correct.  The top-of-the-atmosphere radiative imbalance caused by the increase in the height (and thus coolness) of the effective radiating layer only lasts until the planet accumulates enough energy for the new effective radiating to become as warm as the old one.  However, for that to happen the surface needs to be warmer than before (because of the lapse rate) in order to provide the necessary energy.  
> **username_4**: Reducing CO2 emissions will help because the natural carbon cycle (oceans and terrestrial biota) is taking more CO2 out of the atmosphere than it emits into it (because we have pushed the system out of equilibrium, the carbon cycle feedbacks are opposing the perturbation).  So if we stop emissions then CO2 levels will begin falling and the planet will cool.  If we reduce emissions, we will at least reduce the rate of warming and limit the problems climate change is likely to cause.  
> **username_0**: @username_4, you say: "the surface needs to be warmer than before ... in order to provide the necessary energy". That I can understand in the context of thermal conductivity: if you add an extra layer of thermal insulation in a house, then the internal temp will rise. So we need to understand how an effectively taller atmosphere acts analogously to increased insulation. I guess if the atmospheric heat transfer is mainly convection, then to drive warm air to a higher altitude requires a higher surface temp. Like a hot air balloon: to attain a higher attitude requires a higher balloon temp  
> **username_4**: The atmosphere isn't any taller, it is just the height at which IR radiation can escape into space without being absorbed by greenhouse gasses rises.  AIUI most of the heat transfer within the atmosphere is by convection rather than conduction or radiation, and it is that that sets up the lapse rate (the decrease in temperature with increasing height).  Because there is a lapse rate, if you want the upper atmosphere to warm, you need the surface to warm as well.  
> **username_6**: FYI, [here](https://physics.stackexchange.com/questions/300046/explain-it-to-me-like-im-a-physics-grad-greenhouse-effect/300058#300058) is a post from SE\physics that seems related to what is being discussed here. I confess that I scrolled to the bottom, skipping all the equations... and unfortunately I couldn't understand the conclusion :(  
> **username_2**: @username_4 For the second question [here](https://earthscience.stackexchange.com/q/19763/6535), I guess it is because the effective bottom-of-the-atmosphere radiating layer is a few km up, where it is colder.  

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### Answer by username_7 on 2020-04-20. Score: 0

I would like to address what appears to be an over-generalization of your premise. You seem to be simplifying the radiative heat from the Earth. Yes, if you take an average temperature and an average atmosphere, the present amount of CO2 is almost completely maxing out the sides of the absorption bands and an increase in CO2 gets you almost no additional absorption. However, that is an average. Keep in mind that the majority of warming that is currently happening is at high latitudes and high altitudes, where infrared from the Earth is far below average. The increase in global CO2 has a larger affect in these colder regions because there is greater sensitivity to the increase in CO2, affecting those fringes of the absorption bands.


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### Answer by username_8 on 2020-04-21. Score: 0

A complimentary article to the one written by Spencer Weart and recommended by username_4 username_4 is this other article from September 2019 written by Jason West, Professor of Environmental Sciences and Engineering, University of North Carolina at Chapel Hill.

[Climate explained: why carbon dioxide has such outsized influence on Earth’s climate](https://theconversation.com/climate-explained-why-carbon-dioxide-has-such-outsized-influence-on-earths-climate-123064)

It underpins what has been said here that what matters most is the height that the carbon dioxide layer can reach at the upper atmosphere.

In the article is said that carbon dioxide is important for climate despite the fact that water vapor absorbs more infrared radiation and the two gases absorb at several of the same wavelengths.

> The reason is that Earth’s upper atmosphere controls the radiation that escapes to space. The upper atmosphere is much less dense and contains much less water vapor than near the ground, which means that adding more carbon dioxide significantly influences how much infrared radiation escapes to space.

The article also re-states information given by by Spencer Weart about the role of carbon dioxide in climate change:

> The scientists who first identified carbon dioxide’s importance for climate in the 1850s were also surprised by its influence. Working separately, John Tyndall in England and Eunice Foote in the United States found that carbon dioxide, water vapor and methane all absorbed heat, while more abundant gases did not.
>
> Tyndall and Foote showed that nitrogen and oxygen, which together account for 99% of the atmosphere, had essentially no influence on Earth’s temperature because they did not absorb heat. Rather, they found that gases present in much smaller concentrations were entirely responsible for maintaining temperatures that made the Earth habitable, by trapping heat to create a natural greenhouse effect.

The reason why carbon dioxide has a much more importance in warming the planet while much more abundant gases like nitrogen and oxygen do not play an important role is related to its molecular structure:

> Carbon dioxide and other heat-trapping gases have molecular structures that enable them to absorb infrared radiation. Carbon dioxide and other heat-trapping gases have three or more atoms and frequencies that correspond to infrared radiation emitted by Earth. Oxygen and nitrogen, with just two atoms in their molecules, do not absorb infrared radiation.

The article provides important pre-historic information about the concentration of carbon dioxide in the atmosphere and its correlation with the concentration of water vapor in the atmosphere and the consequences for the climate.

> The influence of carbon dioxide can be seen in past changes in climate. Ice cores from over the past million years have shown that carbon dioxide concentrations were high during warm periods – about 0.028%. During ice ages, when the Earth was roughly 7 to 13 F (4-7 C) cooler than in the 20th century, carbon dioxide made up only about 0.018% of the atmosphere.
>
> Even though water vapor is more important for the natural greenhouse effect, changes in carbon dioxide have driven past temperature changes. In contrast, water vapor levels in the atmosphere respond to temperature. As Earth becomes warmer, its atmosphere can hold more water vapor, which amplifies the initial warming in a process called the “water vapor feedback.” Variations in carbon dioxide have therefore been the controlling influence on past climate changes.

It finishes explaining why it is foolhardy not to diminish the carbon dioxide emissions to prevent big effect on Earths climate:

> Today the level of carbon dioxide is higher than at any time in human history. Scientists widely agree that Earth’s average surface temperature has already increased by about 2 F (1 C) since the 1880s, and that human-caused increases in carbon dioxide and other heat-trapping gases are extremely likely to be responsible.
>
> Without action to control emissions, carbon dioxide might reach 0.1% of the atmosphere by 2100, more than triple the level before the Industrial Revolution. This would be a faster change than transitions in Earth’s past that had huge consequences. Without action, this little sliver of the atmosphere will cause big problems.

[![Carbon dioxide concentration in the atmosphere from 1958 to 2019](https://i.sstatic.net/FG0xN.png)](https://i.sstatic.net/FG0xN.png)


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### Answer by username_9 on 2020-01-18. Score: 0

CO2 is dark in the long wave infrared wavelength so the higher the concentration the darker the atmosphere will be in this wavelength, and as you probably know darker colours absorb more heat.

CO2 is not a very powerful greenhouse gas but the amount of it in our atmosphere make this effect the dominating source for the heating we can see today.

Lots of other gases have a larger heating potential than CO2 but their concentration is low at the moment. The more complex a gas is the more heating potential it will have, this is due to the size of the individual gas molecules (CO2 has 3 atoms and methane has 5, so it will look darker in the long wave infrared band and therefore have a stronger heating potential).

> **Comments:**
> **username_10**: I don't think this answers the question.  
> **username_7**: You may want to mention the vibrational and rotational states of energy for the molecule, which causes the specific wavelengths being absorbed/emitted.  

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### Answer by username_11 on 2022-10-10. Score: -4

The atmosphere has heated about .5 C since 1979. It takes 3 Zeta Joules of energy to accomplish that. That would be .075 ZJ per year if purely cumulative. Residual heat from human energy consumption amounts to .5 ZJ per year or about 20 ZJ since 1979. That is 7 times what would be needed to heat the atmosphere by .5 C. (We note that the oceans are accumulating thermal energy too). In addition to direct transfer of black body radiation to space, molecular collision induced IR generation in the upper atmosphere is where cooling of the global energy reservoir establishes equilibrium. Waste heat energy added to the climate system (with an overprint of natural fluctuation) is the primary cause of the slight increase in temperature of the atmosphere since 1979. But sceptics and alarmists alike tend to ignore waste heat as a serious contributor to climate change.

> **Comments:**
> **username_12**: This answer would benefit from some sources.  
> **username_13**: Refering to this your numbers are way off. https://earthscience.stackexchange.com/questions/3041/how-does-anthropogenic-heating-affect-global-warming  
> **username_14**: Haven't looked at your calculation... but I'm guessing it suggests it would take 3 ZJ of energy in the Earth NOW to be .5 C warmer... in other words retaining 3 ZJ over that period.  Whereas energy leaves the system continually.  And warmer temperature means more energy is emitted.  Only by having a method to *retain* extra energy are you going to heat the Earth longterm.  
> **username_14**: Indeed your numbers show you're saying the Earth would have to continually retain 6 entire years worth of waste energy. Yet the temperature drop each winter shows just how well Earth sheds heat continually (including our added waste heat). The greenhouse effect, the topic of the question, is the way the Earth can retain some of its heat, including waste heat... but not 6 years worth! Instead, though, a tiny increase in greenhouse effect would wind up adding more heat, as the scale of total energy (i.e. solar insolation) is so giant compared to waste heat.  
> **username_14**: Using the exponential series sum equation $\sum_{n=0}^N r^n = \frac{r^{N+1} - 1}{r-1} $, it looks like since 1979 Earth would have to be retaining [5/6](https://www.desmos.com/calculator/8bl6xr3d9i) of the waste energy annually (or a bit more since it's really a continual rather than annual decay) for it to now have 3 ZJ worth at this point? Or did I make a mistake somewhere??? (Always possible?) But if correct... it clearly doesn't retain anywhere near that amount of solar energy, or we'd have big problems, so waste heat doesn't seem the feasible culprit to me at least...  

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