Atmospheric CO2 absorption - actual quantification?

  • Thread starter cmb
  • Start date

cmb

735
9
I have read oodles of hand wavy qualitative text about 'the greenhouse effect' and it is clearly dominated by the absorption characteristics of H2O vapour.

What I struggle to understand is the part of CO2 in radiative absorption, in relative quantified terms.

CO2 has absorption peaks at 2.7um, 4.3um and 15um, which relate to black body radiation temperatures of 1074K, 674K and 193K, by Wien's law.

The text I have read simply says something like 'there is no simple equation to describe radiative forcing with CO2'. My question is why not, and 'roughly' what is it, not looking for a precise answer, just an empirical plot and justification?
 
1
0
I have a difficulty with this too. What is the value used by the climate models for the absorption cross section of the carbon dioxide molecule for 15 micron infrared photons?
 

jim mcnamara

Mentor
3,525
1,736
Could you please cite at least one of your hand-wavy references? It would help a lot. 'I read' somewhere does not help us at all.
 

cmb

735
9
If you insist I can recap, but I am not sure it would help. I am just looking for a relationship showing IR absorption by CO2 versus H2O, and I am saying I have not found anything that appears ready to state that.

We live on a black body radiator that peaks at 10um and CO2 doesn't have any absorption peaks anywhere near that. I am just looking for a dumbo's explanation for why CO2 has a bigger impact than H2O when H2O has a huge wide absorption band right there at 10um.
 

jim mcnamara

Mentor
3,525
1,736
Okay - that's clear. Let's try General Physics, a better fit.

Moved to General Physics
 
1,889
192
We live on a black body radiator that peaks at 10um and CO2 doesn't have any absorption peaks anywhere near that. I am just looking for a dumbo's explanation for why CO2 has a bigger impact than H2O when H2O has a huge wide absorption band right there at 10um.
I think you're reading the graphs wrong. H20 has a fairly wide band where is does not absorb much, from about 7 to 20um, with a minimum at around 10um. The absorption peak of CO2 at 15 um does absorb a significant amount of additional radiation, that would go through this gap in the absorbtion by H20.

See figure 2 at https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/wea.2072 or look at , the graph under https://en.wikipedia.org/wiki/Greenhouse_effect Greenhouse Gases.
Or make your own graph with http://www.spectralcalc.com/spectral_browser/db_intensity.php
 
9,111
2,028
Climate change is problematical for this site in that it is a very contentious issue. We would normally prefer a peer reviewed paper to discuss. Also none of the mentors or science advisers here, at least the ones I am aware of, are experts in the field. This again makes it hard. So please keep comments here on topic and not let politics come into it. The big issue here, politically, is you go and ask people if something should be done about climate change and you get - of course it should. But then you ask are you happy with 3 times the electricity prices (that happened here in Australia), the loss of jobs such as coal mining, rural areas depending on these things going under, ie all the stuff associated with doing something about it, and you get an entirely different answer as was demonstrated by recent election results here in Australia. Because we stick to the science here, we do not want to get involved in the above conundrum. So please stick to the science.

Scientifically I do not know the answer to that one either. Water vapor is clearly also a green-house gas just like CO2 and I have no idea why CO2 is the one that gets the most attention. It would also seem the most abundant of the lot. Its the same with methane from belching and flatulence of agricultural animals - that clearly is as well - yet I have not seen a lot on it - some but not a lot. But according to Professor Barry Brook, climate scientist from Australia (see attached) : 'I’ve done some work recently with a couple of colleagues showing that in fact Australia’s contribution to global warming is more to do with methane, at least in the short term, than it is with anything else. And even more than that, it’s more to do with methane produced by ruminant animals, cattle and sheep and goats for instance. As part of their natural digestion.' So it maybe, at least in Australia, the usual suspects of coal mining etc may not be the main cause of greenhouse emissions. Imagine the political and economic consequences of trying to do something about that.

I am no expert and others more knowledgeable, preferably with peer reviewed literature as a reference, need to comment. But please, please, with my mentors hat on, do not let it slip into politics as that will result in it being shut down. It does not matter what side of the climate change debate you are on knowing the facts is important so that would be a pity.

Thanks
Bill
 

Attachments

Last edited:
15
0
I am not an expert but have studied it a bit. The actual physics of absorption of radiative energy and then converting that absorbed energy to temperature through molecular collision is quite complicated in the overall. Water vapor absorbs much more radiation than CO2 because it has significantly more absorption bands, although they are more concentrated at the larger wavelengths (lower energy). There is also a lot more water vapor, in the 0.1% to 3-4% range, than CO2, in the 300ppm range. But the difference for the concern between H2O and CO2 in general is simple. CO2 increases its concentration in the atmosphere while H2O does not on average. Evaporation rate stays relatively equal to the rainfall while many processes keep adding to the CO2 concentration. Further, we can do something about the CO2 concentration but nothing about water vapor.

One interesting aspect is the unknown CO2 absorption rate as the concentration comtinues to increase. The science simply assumes that the energy absorption stays in ratio to the concentration no matter how large the concentration. However, there is some thought that there comes a point where CO2 is absorbing all of the radiation that the earth emits, and adding more CO2 will not have any effect. This is called saturation. However science hasn't 't figure that out, so it makes the only assumption it can.
 

Lord Jestocost

Gold Member
2018 Award
396
258
However, there is some thought that there comes a point where CO2 is absorbing all of the radiation that the earth emits, and adding more CO2 will not have any effect.
A physical misconception! A lot of misconceptions regarding the "greenhouse effect" result merely from the unphysical thinking of the atmosphere as a single slab. I recommend to read the following post by Spencer Weart:
 
Last edited:

phyzguy

Science Advisor
4,207
1,192
If you insist I can recap, but I am not sure it would help. I am just looking for a relationship showing IR absorption by CO2 versus H2O, and I am saying I have not found anything that appears ready to state that.

We live on a black body radiator that peaks at 10um and CO2 doesn't have any absorption peaks anywhere near that. I am just looking for a dumbo's explanation for why CO2 has a bigger impact than H2O when H2O has a huge wide absorption band right there at 10um.
I don't think anyone is saying that CO2 has a bigger impact than H2O. But it's the CO2 content of the atmosphere that is changing. It's the change that is the issue, not the absolute quantity. In physics terms, a 2 degrees C change in surface temperature isn't much, but it has a major impact on human life.
 

cmb

735
9
I am not an expert but have studied it a bit. The actual physics of absorption of radiative energy and then converting that absorbed energy to temperature through molecular collision is quite complicated in the overall. Water vapor absorbs much more radiation than CO2 because it has significantly more absorption bands, although they are more concentrated at the larger wavelengths (lower energy). There is also a lot more water vapor, in the 0.1% to 3-4% range, than CO2, in the 300ppm range. But the difference for the concern between H2O and CO2 in general is simple. CO2 increases its concentration in the atmosphere while H2O does not on average. Evaporation rate stays relatively equal to the rainfall while many processes keep adding to the CO2 concentration. Further, we can do something about the CO2 concentration but nothing about water vapor.

One interesting aspect is the unknown CO2 absorption rate as the concentration comtinues to increase. The science simply assumes that the energy absorption stays in ratio to the concentration no matter how large the concentration. However, there is some thought that there comes a point where CO2 is absorbing all of the radiation that the earth emits, and adding more CO2 will not have any effect. This is called saturation. However science hasn't 't figure that out, so it makes the only assumption it can.
Thanks for your thoughts, but;
a) I don't believe water vapour levels have remained constant. I say that not because of any particular opinion about any particular research or hypothesis, but because the planet's atmosphere just isn't that stable. If I saw a graph with a horizontal straight line from left to right set at one given water vapour concentration for the whole of the 20th century, I wouldn't believe it was correct. Would you?
b) There are tons of things we humans might be doing to alter the levels of water vapour in the atmosphere; dam-ing and diverting rivers, industrial irrigation, deforestation, directly from fossil fuel emissions, etc..

Please show me this horizontal measurement of water vapour concentration for the 20th century that humans cannot have any influence on?
 
9,111
2,028
I don't think anyone is saying that CO2 has a bigger impact than H2O. But it's the CO2 content of the atmosphere that is changing. It's the change that is the issue, not the absolute quantity. In physics terms, a 2 degrees C change in surface temperature isn't much, but it has a major impact on human life.
The science is detailed on a number of reputable sites like the Chemical Society eg:

The issue is what it says at the end:
'Thus the possible positive and negative feedbacks associated with increased water vapor and cloud formation can cancel one another out and complicate matters. The actual balance between them is an active area of climate science research. '

I will not argue one way or the other, but simply reiterate its an area of investigation. In practical terms ie what we can or should do about it, its a political issue based on simple psychology. I have mentioned it many times before but will say it again. Ask someone if we should do something about climate change and they will say yes. But put it another way - are you willing to pay a lot more for your power, perhaps loose your job etc and you often get an entirely different answer. Often in democratic societies the second observation wins. I am not saying anyone is right or wrong - simply pointing out the consequence of a democracy. It was demonstrated here in Australia where the party that said this was an election on climate change was supposedly a shoe in. But the second view won out and the supposedly unloseable election was lost. This is just an observation - we do not discus politics here. However sometimes science becomes entangled with politics and the politics becomes the dominant factor.

Thanks
Bill
 

cmb

735
9
I want to specifically avoid considering political aspects of this.

I understand why the forum wants to avoid AGW threads, because the desire to head to opinions is very strong.

I am not interested in that here, and would expect a desire by moderators to lock the thread if people pull it in that direction, so please don't.

I just want some data on what CO2 and H2O do in regards absorbing black body IR passing through them, by concentration.

I haven't even got a starting point of an order of magnitude of how H2O or CO2 compare. I mean, at atmospheric concentrations, are we talking about 10%, 1%, 0.0000001%? Where are we on this log scale, to begin with?
 

Lord Jestocost

Gold Member
2018 Award
396
258
This might be a starting point:
 
We salute moderators/editors their concern to try to keep 'politics' out of FORUMS discussions (an evident semantic self-contradicton). Minimise it if possible, or at least avoid the current emotional side-taking, demonizing, marginalising. Even the naive sometimes zero in on Truths missed ( ?avoided?) by us 'cognoscenti'. It is comforting that Earth and Universe percolate ever-onwards without us, in spite of our New-Comer vanities-of-the-day. It is to laugh. Or to cry? ......
 
15
0
A physical misconception! A lot of misconceptions regarding the "greenhouse effect" result merely from the unphysical thinking of the atmosphere as a single slab. I recommend to read the following post by Spencer Weart.
I understand the science and rationale for assuming that saturation has no limits, but it has not been scientifically proven, certainly not by crude experiments in the 19th and early 20th centuries, or even more thorough analysis in the 40s. Lack of saturation is still conjecture but it is the only assumption scientists can make given their degree of knowledge.

Weart refuted a contention that I did not make or believe (although in his defense others have and do). H2O cannot steal CO2's radiation. Secondly I did not contend nor believe that we are already at CO2's saturation limit. In my opinion I believe their is a saturation limit. It might be soon or it might be a long long way off; it might save us from global wrming, or it might not.
 
9,111
2,028
I haven't even got a starting point of an order of magnitude of how H2O or CO2 compare. I mean, at atmospheric concentrations, are we talking about 10%, 1%, 0.0000001%? Where are we on this log scale, to begin with?
I appreciate participants desire to keep politics out of this. This is not an interest of mine, other than knowing the basics, so I am not familiar with the details. I do know water vapor is the main cause and the other gasses cause climate change mostly by moderating water vapor as explained in the Chemical Society link I gave,

But as for more specific details then I would normally suggest Google is your friend. The Chemical Society, being the professional body of Chemists like the Physical Society is the professional body of Physicists it meets our reliable reference standards. However after doing a google search on this issue I am flabbergasted at the quality, misconceptions etc that comeback. This seems a very difficult area to get reliable information. Just another issue with the climate science debate.

I will leave the thread open and keep an eye on it, but we really have to be careful about the source of any information posted. Just an example of how bad it is I found a site meant I suspect for High School students that did not even mention about water vapor.

It really is a difficult issue for this site.

Thanks
Bill
 
Last edited:

cmb

735
9
...However after doing a google search on this issue I am flabbergasted at the quality, misconceptions etc that comeback. This seems a very difficult area to get reliable information. Just another issue with the climate science debate.
....
^^^ This, exactly. It's what I found and why I am asking.
 
19,164
3,782
Here is a reference from the open literature showing how the effect is typically quantified, including references to the sources of data on the absorption spectrum of the various greenhouse gases: Owens, A.J., Hales, C.H., Filkin, D.L., Miller, C., Steed, J.M., and Jesson, J.P., A Coupled One-Dimensional Radiative-Convective, Chemistry-Transport Model of the Atmosphere, 1. Model Structure and Steady State Perturbation Calculations, J. Geophys. Res., 90, D1, 2283-2311, (1985)

The models have gotten more sophisticated since this.
 
257
66
I'm not sure there's a difficulty vis-a-vis conversation. No politics, nor esoteric physics required :

With the exception of a certain spot in Antarctica, CO2 never freezes, never undergoes phase changes of any kind. Put more CO2 into the atmosphere, and there it stays (more or less).

Meanwhile you could dump massive amounts of water into the atmosphere (which we do, and it isn't a problem) because it all ends up miles deep in Antarctica and Greenland, on mountaintops and in oceans, where it's effectively inert. Or used to be inert, anyways.

But hey, when the temperature on Earth is high enough that water never condenses, then we'll count it as a greenhouse gas.
 
Last edited:

sophiecentaur

Science Advisor
Gold Member
23,275
3,790
But hey, when the temperature on Earth is high enough that water never condenses, then we'll count it as a greenhouse gas.
The cloud cover and the ice / snow cover has a massive effect and certainly the ice cover is reducing so it has to be a relevant factor. How that all fits into the OP is a moot point.
 

phyzguy

Science Advisor
4,207
1,192
I'm not sure there's a difficulty vis-a-vis conversation. No politics, nor esoteric physics required :

With the exception of a certain spot in Antarctica, CO2 never freezes, never undergoes phase changes of any kind. Put more CO2 into the atmosphere, and there it stays (more or less).

Meanwhile you could dump massive amounts of water into the atmosphere (which we do, and it isn't a problem) because it all ends up miles deep in Antarctica and Greenland, on mountaintops and in oceans, where it's effectively inert. Or used to be inert, anyways.

But hey, when the temperature on Earth is high enough that water never condenses, then we'll count it as a greenhouse gas.
You seem to be saying that, since there is liquid and solid water on the Earth, that this means there is no water vapor in the atmosphere. This is not correct, as anyone living in Florida can tell you. The amount of water vapor in the atmosphere of course varies, but can be as high as 4% by weight, much larger than the 400 ppm which is CO2. Water vapor is actually the dominant greenhouse gas in the atmosphere.
 

Bandersnatch

Science Advisor
2,821
1,662
You seem to be saying that, since there is liquid and solid water on the Earth, that this means there is no water vapor in the atmosphere.
No, they're saying any added water vapour rapidly condenses out of atmosphere, so the overall content stays constant for a given temperature. The content is already in equilibrium, as if the atmosphere could hold more vapour, it would (there's plenty liquid water to evaporate).
That's why something else needs to first drive the temperature change, with water acting as a positive feedback.
 

Vanadium 50

Staff Emeritus
Science Advisor
Education Advisor
22,972
5,239
Boy, this thread has meandered.

First, CO2 saturates and there is no question about this. What this means is that there is enough CO2 in the atmosphere that adding more does not change the absorption at the CO2 peak. However, the peak is a set of measure zero - i.e. the amount of energy at that single exact frequency is zero. What matters is the CO2 line shape (more exactly, the convolution of the line shape with the incident spectrum)

It turns out that typical lab measurements are uniquely unsuited for this. The reason spectral lines have a Breit-Wigner shape is because that is the Fourier transform of an exponential, and you get exponentials when the probability of interaction is independent of time. For low pressure gas, that's a good approximation. The problem is that measuring the shape takes a good amount of CO2 (after all, there's a couple miles of atmosphere) and that means you either need to change the composition or the pressure, both of which distort the results.

I believe the best measures involve satellites shooting lasers through miles of atmosphere. There one measures the actual thing of interest rather than something one needs to correct for.
 

Lord Jestocost

Gold Member
2018 Award
396
258
Spencer Weart in http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument/:

"What happens to infrared radiation emitted by the Earth’s surface? As it moves up layer by layer through the atmosphere, some is stopped in each layer. To be specific: a molecule of carbon dioxide, water vapor or some other greenhouse gas absorbs a bit of energy from the radiation. The molecule may radiate the energy back out again in a random direction. Or it may transfer the energy into velocity in collisions with other air molecules, so that the layer of air where it sits gets warmer. The layer of air radiates some of the energy it has absorbed back toward the ground, and some upwards to higher layers. As you go higher, the atmosphere gets thinner and colder. Eventually the energy reaches a layer so thin that radiation can escape into space.

What happens if we add more carbon dioxide? In the layers so high and thin that much of the heat radiation from lower down slips through, adding more greenhouse gas molecules means the layer will absorb more of the rays. So the place from which most of the heat energy finally leaves the Earth will shift to higher layers. Those are colder layers, so they do not radiate heat as well. The planet as a whole is now taking in more energy than it radiates (which is in fact our current situation). As the higher levels radiate some of the excess downwards, all the lower levels down to the surface warm up. The imbalance must continue until the high levels get hot enough to radiate as much energy back out as the planet is receiving.

Any saturation at lower levels would not change this, since it is the layers from which radiation does escape that determine the planet’s heat balance."
 

Want to reply to this thread?

"Atmospheric CO2 absorption - actual quantification?" You must log in or register to reply here.

Related Threads for: Atmospheric CO2 absorption - actual quantification?

Replies
8
Views
11K
Replies
1
Views
2K
  • Posted
Replies
4
Views
6K
  • Posted
Replies
3
Views
8K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top