I’ve not found much of interest to write about tonight, and story submissions have been a dry hole lately

Not me you silly - that's a quote from WUWT. And as if in answer to his desperation, along comes The effectiveness of CO2 as a greenhouse gas becomes ever more marginal with greater concentration, a deeply stupid post.

It starts with a nod towards pretending to have a clue:

According to well understood physical parameters, the effectiveness of CO2 as a greenhouse gas diminishes logarithmically with increasing concentration... This inconvenient [sic] fact is well understood in the climate science community...

...because not even WUWT readers are going to fall for the idea that it's a surprise that CO2-effectiveness-is-log. He does feel obliged to pretend IPCC has "hidden" the obvious, although I'm not sure why - a token gesture to stir up the unwashed masses, or a feeling that since its a post at WUWT it really needs to have some obvious lies in it? But its unconvincing, as a commentator writes:

The sentence in chapter 2 AR4 “Note that for CO2, RF [radiative forcing] increases logarithmically with mixing ratio” is so very well hidden. You would never know it was there if you didn’t read it.

But, that's not the main reason I want to take the piss out of this particular article. That is reserved for:

From the present concentration of atmospheric CO2 at approaching 400 ppmv, only ~5% of the effectiveness of CO2 as a Greenhouse Gas remains. This can only give rise to a maximum of a further of ~+0.21°C. Thereafter beyond 1000+ ppmv the effect of increasing levels of CO2 can only ever be absolutely minimal even if CO2 concentrations were to increase indefinitely.

Yes, really. He thinks the log function is bounded above. Truely this is the level of sophistication expected at WUWT. I can only echo commentator "Janice Moore" who enthuses:


However, this stuff is so obviously drivel that not even all the WUWT readers fall for it. The fourth comment points out its all wrong. A little lower down, "Phil." tries to rescue WUWT commentators well deserved reputation for stupidity with a high-scoring "But at higher concentrations it will transition to a square root dependence which will give a higher sensitivity" (its the other way round: CO2 goes linear - squareroot - log, though I forget at what levels [Update: my commenters seem to be telling me I'm wrong about that. Ah well. I don't believe it leads to higher sens, though]).


* CO2 – An Insignificant Trace Gas? Part Seven – The Boring Numbers - the best on-line ref for "its log" that I could find. Understanding Atmospheric Radiation and the “Greenhouse” Effect – Part Twelve – Curve of Growth looks to be good for linear (which is obvious) and square root (I didn't trouble my pretty head with the details). Calling Eli...

More like this

I'm sure I've heard tales that at extremely high concentrations (Venus-like conditions) CO2 direct radiative effect returns to something more like a square root or linear relationship. Something to do with an emergent threshold caused by CO2 molecules interacting with each other in the atmosphere, I think. Could be rubbish though, and not relevant to Earth in any case.

[I hope you're wrong. I'll look stupid otherwise. Perhaps Eli can set us straight -W]

The saturation of CO2 was discussed in an article in last month's "Weather", the Royal Meteorological Society's monthly magazine. The article, by Wenyi Zhong & Prof. Joanna Haigh, was entitled "The greenhouse effect and carbon dioxide". It is behind a firewall but the first page can be seen here. They show that although the 15 micron band is saturated, there is a 10 micron band which increases linearly at higher concentrations of CO2.

They conclude "that as the concentration of CO2 in the Earth's atmosphere continues to rise there will be no saturation in its absorption of radiation and thus there can be no complacency with regards to its potential to further warm the climate."

[SOD has quite a ncie article about saturation - http://scienceofdoom.com/2010/05/12/co2-an-insignificant-trace-gas-part… . Is there anything that missed that the Weather piece has? -W]

By Alastair McDonald (not verified) on 09 May 2013 #permalink

From a quick search it appears Ray Pierrehumbert commented on changing response at extreme CO2 concentrations at Real Climate several years ago (response to comment #10). Sounds like Principles of Planetary Climate might have some further material.

At sufficiently high concentrations (say, when you start to get around 10% or 20% of CO2 in the atmosphere) the absorption starts to be dominated by weak bands that have a different probability distribution than the bands that dominate in the present climate; this again starts to lead to an increase in sensitivity.

Nothing there to do with CO2 molecules interacting with each other so maybe that was just crossed wires.

For cluelessness per unit length, the comment by johnmarshall (May 9, 2013 at 2:39 am) is hard to beat. Well worth a read.

By American Idiot (not verified) on 09 May 2013 #permalink

"Truely this is the level of sophistication excepted at WUWT"

"excepted"??? - a cross between accepted and expected?

[Grr. Fixed -W]

I wasn't impressed by the SOD article. I found it rather confusing :-( Is CO2 saturated or not?

[Well, it depends what you mean by "saturated". With the conventional meaning of "if you add more it makes no difference" the answer is No, and that's what SOD is saying -W]

Z&H (2013) accept that CO2 is saturated and that its forcing increases logarithmically, but point out that when CO2 reaches levels 32 times greater than today, then the very weak band at 10 um in the centre of the IR window will cause the forcing to increase linearly.

[Ah, interesting, perhaps. We don't need to worry about 32x though -W]

By Alastair McDonald (not verified) on 09 May 2013 #permalink

The same thing happens with water vapor, google water vapor continuum, but you would need a much higher pressure of CO2 because it is non polar, while water vapor is polar, eg, the distribution of charge sets up a dipole moment in water vapor and the interactions of the dipoles with each other are much stronger than the interactions of the non-polar CO2 molecules would be.

That's the zeroth order explanation. It turns out that the physical mechanism is in some dispute.

In short, we are all long gone before that effect becomes important for CO2.

By Eli Rabett (not verified) on 09 May 2013 #permalink

Eureka! That's why water vapour has a continuum - because it is polar. The polar molecules bounce off each other creating a continuum EMF.

Do other polar molecules exhibit a continuum?

By Alastair McDonald (not verified) on 09 May 2013 #permalink

Don;t know if it has ever been asked that way, but certainly the same rules apply. For example NO2 forms the dimer N2O4, which is the form you find it in at more than a couple of torr and there is no reason to think that the wings of the lines are not collisionally broadened. The only caveat is that H2O is really really polar as these things go.

By Eli Rabett (not verified) on 09 May 2013 #permalink

Alistair's point is well taken since the usaturated 10 micron band is the one overlapping the Earth's blackbody peak.


What I am suggesting is that the H2O continuum is not caused by either of the current theories (viz lines from dimers, and the overlap of existing lines) being investigated by the CAVIAR Project.

I am proposing that it is the result of Bremsstrahlung type radiation resulting from the collisions of polarised molecules. Bremsstrahlung radiation is a continuum type rather than a line type and so would seem a better candidate to produce the H2O continuum than some combination of molecular lines.

Presumable H2S is also polar but I cannot find its infra-red spectrum. OTOH, CO2, O2 and N2 all have weak continuum spectra but they are not polar. However, where they contain atoms of different isotopes they will be polar, so if the collision of two polar molecules does produce an emission that might be the cause of the continuum radiation.

By Alastair McDonald (not verified) on 10 May 2013 #permalink

Anyway, as a convenient meme to increase smug denial, it works like a charm. You could watch it spread over the last couple of months - how will we tell when the denialosphere reaches saturation?


Z&H begin their paper with the sentence:

"It is well known that carbon dioxide plays an important role in the natural greenhouse warming of the Earth's atmosphere but the extent to which increases in its concentration might enhance the warming has, over the years, been controversial."

I thought that since Platt (1952) had shown that changes in CO2 concentration would affect radiation in the upper atmosphere that the dispute was over. Are there any peer reviewed papers published in the last 50 years which claim that increasing CO2 would have little effect, or is it just the sceptic blogs which raise that issue?

By Alastair McDonald (not verified) on 10 May 2013 #permalink

Of course I meant Plass :-(

By Alastair McDonald (not verified) on 10 May 2013 #permalink

Bremstrahlung usually is taken as the interaction of charged particles, most often at relativistic speeds which allows for close encounters and stronger forces. Polar molecules interact with each other much more weakly. You could look at one as the weak sister of the other, with the line center surviving in the polar-polar case. Dispersion forces (interactions of non-polar molecules with each other) are the weakest of them all. CO2/CO2 would fall into that category.

By Eli Rabett (not verified) on 10 May 2013 #permalink

A quick place to find low resolution IR spectra is webbook.nist.gov

By Eli Rabett (not verified) on 10 May 2013 #permalink


Thanks for that link, I had a look at it but did not find any IR spectra :-(

I am suggesting that a CO16O18 molecule will be polar and that a collision between two of them could produce weak Bremsstrahlung radiation/absorption, weaker than H2O because such collisions would be so much rarer.

But this is really a quantum mechanics problem, an area where angels fear to tread, so I won't rush in. In fact I will just leave continuum spectra to those who understand those things :-).

By Alastair McDonald (not verified) on 11 May 2013 #permalink

For H2S >try this

As to the isomers being polar, not very much thank you. You can get some idea by using spectralcalc and looking at the absorption of CO2 in the 1300 - 1400 cm-1 range where the symmetric stretch falls. That absorption is forbidden for 16-12-16 CO2, but slightly allowed for 16-12-18, but about four orders of magnitude down from the bending vibrations in absorption strength.

Sorry you are pushing a chimera.

By Eli Rabett (not verified) on 11 May 2013 #permalink

"CO2 is a trace gas, that at such low concentrations cannot possibly have any affect on climate."

"CO2 is at saturation levels and any increase will have no effect."

Nothing like a bit of consistency in the denialist position is there?

Eli, isn't water vapour not much more interesting than CO2??

For a spectroscopist CO2 is boring, but water vapor, ah water vapor, there are careers to be had there.


By Eli Rabett (not verified) on 14 May 2013 #permalink

Actually the Curve of Growth does go: lin-log-sqrt as I stated, the reference from SoD agrees with me and the other citations that I gave. Needless to say there were no responses to my post.

William, do you ever grow weary of continually having to shore up the alarmist propaganda ? You might find yourself enlightened if you chose to change your ways. And it would improve your public image immensely. Unless of course you would rather be seen forever more as the [dull PA redacted -W]

By Georgie LeBonk (not verified) on 14 May 2013 #permalink

In astrophysics we call this the curve of growth. Basically, you have two ways to broaden a spectral line: the thermal width and the natural one given by quantum mechanics. Initially you have a weak line and the strength is proportional to the abundance. Then you have a saturated line with a logarithmic response. After that you start to have a lot of absorbers far from the center of the line and the wings start to grow, this is a root (N) process.

well, Eli misled me into reading some Richard Tol, which led me to one of the places his stuff gets listed, which featured other stuff, some even wackier, including this warning: http://www.economicsclimatechange.com/2011/10/natalist-bias-of-pollutio…

Appears if you make fossil fuel more expensive, people will just lie around, have sex, and make more babies, thereby passing the cost of controlling climate on to larger future generations. Or something. If I'm reading it right.

I hope I misread it.

By Hank Roberts (not verified) on 26 May 2013 #permalink