2010-09-11 14:38:21Which rebuttal applies to the "parts per million" argument?
Jim Meador

jimm58@gmail...
68.164.185.81
There was that guy in the "Stephen Schneider vs 52 skeptics" video using the "CO2 is a trace gas"
 argument, and I've heard it before. Surely there is an SkS rebuttal already for this, but I didn't find it yet.
2010-09-11 16:30:57There is no rebuttal to this yet
John Cook

john@skepticalscience...
121.222.93.62
However, I've just added it to the list of rebuttals - argument #127. it's one I've been meaning to hit and even started to write once but never completed. Someone touched on this in the basic rebuttal to "co2 effect is weak" which has some overlap with this argument. Anyone is free to write a rebuttal to this one, at any level. I'll track down my notes and post them here, if that helps.
2010-09-11 16:42:57Notes on this
John Cook

john@skepticalscience...
121.222.93.62

I collected a few random thoughts on this but haven't had the time to assemble it into a single, coherent rebuttal. Note: for more information, I recommend the wonderful CO2 an insignificant trace gas series - parts one, two, etc. Here's the thoughts I'd collected so far, anyone is welcome to run with it and take full credit for the rebuttal :-)


The level of atmospheric carbon dioxide just ticked over 392 parts per million (CO2Now.org). Now a skeptic might say that value sounds a lot less impressive when you express it as a percentage - CO2 comprises 0.039% of our atmosphere. This is known as a trace gas - it only comprises the tiny amount of our atmosphere. How can a gas that takes up less than half of a tenth of a percent of our atmosphere have such a strong impact on climate?

The first point to realise is that over 99% percent of our atmosphere isn't a greenhouse gas. Oxygen, O2, constitutes 21% of the atmophere and nitrogen, N2, constitutes 78%. Neither gas absorbs a significant amount of infrared radiation.

As far as greenhouse effect goes, CO2 has a potent effect because its absorption wavelength is right in the middle of Earth's infrared spectrum. [MORE DETAIL ON THIS...]

Of course, we can measure the CO2 effect directly by measuring infrared radiation returning back to the Earth's surface. If there was no greenhouse effect, we would find no infrared radiation coming back down (and the Earth's temperature would be -18°C). Instead, what we observe is downward infrared radiation at exactly the wavelengths that CO2 absorbs energy. [CITE EVANS, PHILIPONA, HARRIES, ETC]

Perhaps mention that CO2 absorbs radiation at higher altitudes where water vapor doesn't?

Interesting logical argument that uses the small amount of CO2 to flip the skeptic argument on it's head: It is because there is so little carbon dioxide in the atmosphere that it is a problem. This is because it has a potent effect and it doesn't take much for human activity to have a significant impact on the quantity.

2010-09-11 16:55:08I think it should be #31, "The CO2 effect is weak"
nealjking

nealjking@gmail...
84.151.46.1

(Note: I wrote this note about the same time that John was writing the note above; but I think these still apply.)

The current articles mostly focus on quantifying the impact of CO2 (3.7 W/m2 for a doubling), but don't give the qualitative reasons for why this trace amount of CO2 can "compete" against the H20 vapor. Other issues I would think should be mentioned:

- CO2 is inherently more effective for some infrared frequency bands (4 and 15 micron) than H20

- CO2 is present at altitudes well above the level at which H20 quits, so this gives a more direct enhanced greenhouse effect. 

 

These factors give CO2 a "greenhouse" advantage over water vapor.

 

2010-09-11 20:45:40It's not just about co2 vs water vapor
John Cook

john@skepticalscience...
121.222.93.62
Before getting too technical and specific, this rebuttal needs to answer a simple question, "how can a gas that comprises less than half of one percent of our atmosphere have so much effect?" Imagine if someone in a pub asked you this question. How would you answer in plain English?
2010-09-11 21:17:46
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.150.110
How would you answer in plain English?.......hmmmm.......something along the lines, "Yes CO2 is a tiny fraction of the Earths atmospheric gases, but it makes up a large fraction of the greenhouse gases that trap all of the Earth's heat." or variations thereof. 
2010-09-11 21:30:24Here's a pub-level argument
nealjking

nealjking@gmail...
84.151.46.1

- CO2 constitutes 0.04% of the atmosphere; other greenhouse gases (mostly H20) constitute about 1%. All other atmospheric gases are non-interacting with infrared radiation.

- Imagine an election in which 99% of the residents were NOT registered to vote. Whose opinion would be of interest in determining the results of the election? It would be the 1% that is registered.

- Now imagine that on the day of the election, of that 1%,  96% got the flu and stayed home in bed. That means that only 0.04% showed up to the polls on the day. Those are the only people that are going to affect the election.

- The enhanced greenhouse effect is similar to that: 99% of the atmosphere does not affect the radiation balance ("not registered to vote"); the water vapor (96% of the 1%) remains at altitudes lower than 10 km ("caught the flu"); so only 0.04% remains at high altitudes, which have the most influence on the EGE. Those are the votes that count!

 

2010-09-11 22:28:13Sweet metaphor!
John Cook

john@skepticalscience...
121.222.93.62

In my early purist days, I scorned metaphors. I've since matured in my old age (blogging years are like 7 human years) and see the strong need for metaphors when communicating to an average audience. Think Steve Schneider talking to a room of skeptics. The one argument that held any sway, that changed a mind, was the bathtub metaphor.

So yes, love the election metaphor. Dappledwater, your way of expressing it is good too - I could see that serving as a good introduction (and one-liner/tweet) followed by Neal's metaphor.

2010-09-11 23:02:36
nealjking

nealjking@gmail...
84.151.46.1

but Dappledwood's

"but it makes up a large fraction of the greenhouse gases that trap all of the Earth's heat"

should =>

"but it's at a good altitude for maximum effect on the enhanced greenhouse effect"

2010-09-11 23:27:56complicated
John Cook

john@skepticalscience...
121.222.93.62

I think Dapplewater is on the right track. Greenhouse gases comprise less than 1% of the atmosphere. CO2 is 0.38 of 1% so it is a large fraction of the greenhouse gases.

I would say that's the take-home point to this argument - when people say CO2 is less than 1% of the atmosphere, they also need to know that 99% of the atmosphere is not a greenhouse gas.

2010-09-11 23:58:44Careful ...
nealjking

nealjking@gmail...
84.151.46.1

John,

- I think greenhouse gases constitute about 1% - nearly all H20; "The percentage water vapor in surface air varies from a trace in desert regions to about 4% over oceans" (http://en.wikipedia.org/wiki/Water_vapor); and I saw an "average" value of about 1% for H20. Everything else is quite small:

Relevant to radiative forcing
Gas Current (1998)
Amount by volume
Increase
(absolute, ppm)
over pre-industrial (1750)
Increase
(relative, %)
over pre-industrial (1750)
Radiative
forcing
(W/m2)
Carbon dioxide  365 ppm
(383 ppm, 2007.01)
   87 ppm
(105 ppm, 2007.01)
31 %
(38 %, 2007.01)
1.46
(~1.53, 2007.01)
Methane 1745 ppb 1045 ppb 67 % 0.48
Nitrous oxide  314 ppb    44 ppb 16 % 0.15

 (http://en.wikipedia.org/wiki/Greenhouse_gas)

 

- But CO2 is 0.038% = about 4% of that 1%.

- So CO2 is also only a small part of the GHGs, as well.

2010-09-12 00:56:08arsenic
dana1981
Dana Nuccitelli
dana1981@yahoo...
71.137.148.215

My pub-style response is usually "then I guess you won't mind drinking a glass of water with just 0.039% cyanide in it" (the EPA Maximum Contaminant Level for cyanide in drinking water is 0.2 ppm).

Concentration is by itself insufficient information to determine the danger posed by a substance.  You also need to know things like total volume, toxicity, how carcinogenic it is, and global warming potential, in the case of greenhouse gases.

2010-09-12 01:39:39a bit more elaborate
Riccardo

riccardoreitano@tiscali...
93.147.82.171

I bit more elaborate response in line with Dappledwater's could be that, as for poisons or polluttants, it depends on the reaction of the system more than on the absolute concentration of the poison/polluttant. This is why we have a different limit for each substance.

Indeed, about just 1% of GHG makes life possible on earth and 180 ppm instead of 280 ppm would make life hard, if not impossible, in large parts of Europe and North America.

2010-09-12 04:31:42There is a difference
nealjking

nealjking@gmail...
84.151.46.1

between the significance of the relative concentration of CO2 with respect to all gases and the relative concentration with respect to all greenhouse gases:

a) For purposes of the greenhouse effect, the non-greenhouse gases don't matter at all. The infrared photons aren't absorbed by them, emitted by them or bounce off them: they do not have any effect on each other at all. So the ratio of GHG molecules to all gas molecules is completely unimportant. This means that the ratio of 380 ppm for CO2 is not directly important, by itself; more interesting is the absolute number density of CO2 molecules.

b) However, the relative abundances of the different types of GHG molecules among each other IS significant, provided there are frequencies of photons that they share. Also, of course, the absolute number of GHG molecules is important. So the fact that CO2 is only 4% by number compared to the H20 molecules means that CO2 would normally be unimportant compared to H20, for the IR bands that they share - except that the H20 is restricted to lower altitudes, but CO2 is not. Since all the action with the enhanced greenhouse effect takes place at the top of the atmosphere (specifically, at the photosphere for the relevant frequency of IR), CO2 has a front-row seat, but H20 is way out in the bleachers. So even though 4% is a relatively small number, CO2 punches "above its weight" with respect to the EGE.

2010-09-12 06:52:04Lots of great thouggts
John Cook

john@skepticalscience...
121.222.93.62
Just a matter of tying it together into a single narrative. If anyone would like to have a go, please claim the rebuttal on the rebuttal list (#127). I can already imagine Graham's Twitter headline, "the co2 effect: it's not size that matters, it's what you do with it that counts" :-)
2010-09-12 07:25:28
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.214.144

Neal, I don't know what pubs you drink at, but all the one's I've ever been in, no one would understand what the heck you're talking about. Not too many scientist rugby players where I live - actually none. I guess that's why I liked Carl Sagan so much when I was a young fella, he had a knack of clearly communicating complex subjects.

Anyway, how's this? "Yes, CO2 is only a tiny fraction of the Earth's atmosphere, but it's responsible for a large fraction of the Greenhouse Effect which traps all heat on Earth". 9-26% of the Greenhouse Effect is a sizable chunk, that laypeople would understand, how the figure is arrived at? - no.

Yeah, I've used the poison analogy plenty of times, but I normally reserve that for the absolute idiots.  

Err John......... , "It's not the size of the wand, it's the power of the magic".  I know skeptics/deniers would froth at the word "magic" though. Perhaps some analogy in society where there's only one boss, but he calls the shots? (guess you can eliminate the US President though, he doesn't seem to be in charge of much).

 

 

 

 

2010-09-12 07:47:57
nealjking

nealjking@gmail...
84.151.46.1

Dappledwater,

- What, the blokes at your pub don't vote?

- Of course, you can just say that the CO2 provides 26% of the greenhouse effect - but don't you invite the same issue? "How can 1 part in 2500 produce 26% of the effect?" It seems to me that, even at the Basic level, you have to provide some kind of justification. Otherwise, aren't you just answering, "Because."?

2010-09-12 08:20:07
Rob Painting
Rob
paintingskeri@vodafone.co...
118.93.214.144

Neal,  I was referring to your later post. Guess it wasn't part of the pub argument eh?. No, the idea was a lead in, to your voting analogy, that's why it's only one sentence. 


 

 

2010-09-13 04:00:27Conflict with the "saturation" argument
Jim Meador

jimm58@gmail...
67.101.223.178

One thing that will be worth noting is the fact that the "trace gas" argument is laughably in conflict with the "CO2 effect is saturated" argument....

"It is generally accepted that the concentration of carbon dioxide in the atmosphere is already high enough to absorb virtually all the infrared radiation in the main carbon dioxide absorption bands over a distance of less than one km.

2010-09-13 05:19:10Absorption coefficient vs concentration
Jim Meador

jimm58@gmail...
67.101.223.178

I have been trying to find a commonsense explanation as to why CO2 is such a powerful IR absorber even at low concentrations, without luck. I was hoping to find some kind of graspable calculation showing how often an IR photon would encounter a CO2 molecule at the surface in today's atmosphere. Basically this is kind of a numbers argument...if an outgoing IR photon passed by a million molecules of atmosphere every 'L' distance, then even low concentrations mean that you pass by a lot of trace molecules on your way out of the atmosphere...

 

 

2010-09-13 06:18:51CO2 Absorption
nealjking

nealjking@gmail...
84.151.43.165

Jim Meador,

- There is the concept of "optical path length", which is the integral of the absorption coefficient over distance. The absorption coefficient is proportional to the molecular number density, so the OPL is proportional to the number of molecules passed by. The probability of a photon passing over OPL value x, without being absorbed, is

Probability(x) = e(-OPL)

So once OPL has reached value 1, the photon has most likely gotten absorbed; but there is no firm distance. But if the photon travels through a distance X, you can calculate how large the OPL value is; and the number of photon/molecule collisions would be about equal to OPL.

One point to be seen from this discussion: The relative concentration (which is the ratio of the molecular number density to the number density for ALL kinds of molecules) is actually irrelevant. What matters is the absolute number density of molecules of the specific type that absorb photons of the specific frequency.

- Another point: There is no real saturation, because after a molecule absorbs a photon, it re-emits it shortly thereafter, in a random direction. It is not as though the photons were "eaten" and disappear: They are captured and then tossed out again.

- Basically, as long as the value of the OPL, measured from outer space down to the surface of the ground, exceeds the value 1, there will be a greenhouse effect. The point at which this OPL value reaches 1 is called "the photosphere". What gives CO2 an advantage over H20, for certain photons, is that there is enough CO2 even very high up into the troposphere to create the photosphere, even before the reaching the level where water vapor starts to show up.

Anyway, the story is complicated. It is not easy to explain, and it will never be a Basic-level explanation! 

 

2010-09-13 07:41:19co2 absorption
Riccardo

riccardoreitano@tiscali...
93.147.82.100

Jim,

i'm not aware of any easy way to explain the magnitude of the absorption coefficient, the lowest level explanation i know still requires a bit of physics: the probability of interaction between a photon (electromagnetic wave) and a molecule is proportional to the dipole moment of the molecule, i.e. the separation of positive and negative charges in the molecule.

2010-09-13 13:38:41385 ppm is still a lot when you talk about air
Jim Meador

jimm58@gmail...
67.101.214.230

I agree that explaining the details of the absorption would be too much for a basic argument. However putting some numbers on the absolute density of CO2 as neal suggests seems useful.

I found that one atmosphere pressure (at 0 C) is 2.69e25 molecules per cubic meter. Dividing the volume down until I get a number I can somewhat relate to, I find 26 billion molecules per cubic millimeter, so 385 ppm is 10 trillion CO2 molecules per cubic mm.

Looking at it a different way, on average each molecule of this air occupies a cube 33 angstroms on a side. (Simplifying assumption is that  they are all the same size) Moving linearly through this air one centimeter, you encounter on average 3 milliion molecules , out of which 1150 are CO2.

Someone should check these figures.

I think the usefulness of this approach is to appeal to the emotional power of the "parts per million" argument. Parts per million just emotionally seems insignificant. This view of it shows that in terms of the atmosphere, a given IR photon headed towards space will have many, many chances to be absorbed by a CO2 molecule, even at trace concentrations. It makes the absorptive power of CO2 seem more plausable.


2010-09-13 13:43:41Water vapor window
Jim Meador

jimm58@gmail...
67.101.214.230
Isn't the other thing about CO2 compared to water vapor that it absorbs in one of the window bands that water vapor passes? So adding CO2 closes down one of the IR radiation exit windows?
2010-09-13 17:45:10OK, I signed up for: "#127: COO2 is just a trace gas"
nealjking

nealjking@gmail...
91.33.124.246

I'm going to do this in 3 parts:

- The Basic explanation: will cover the fact that the big concentration ratio doesn't matter (none of the non-GHG matters one way or another), but that the surprise is that CO2 can dominate the much more abundant H20 vapor. It can do this because of the special properties of the CO2 molecule.

- The Intermediate explanation: will explain that the special properties of the CO2 molecule are: a) Its absorption bands, which are only partially shared with H2O; and b) its presence even at high altitudes (beyond the point at which H2O vapor exists). The highest altitudes are the "high leverage" positions for effecting the enhanced greenhouse phenomenon.

- The Advanced explanation:  will explain WHY the highest altitudes are better for the GHG role. This will require some explanation of the Optical Path Length, the photosphere, and the relationship between the altitude of the photosphere and the new steady-state temperature. Although I'll try to simplify it as much as I can, this explanation is going to be tough: tough to write, and probably tough to follow.

My expectation is that any interested reader will first be directed to Basic; at the end of Basic, there will be a pointer to Intermediate; and if s/he's still up for more, the Intermediate's pointer to Advanced will take the reader to as far as I'm able to explain. 

2010-09-13 17:59:39Jim's points
nealjking

nealjking@gmail...
91.33.124.246

Jim Meador,

My post above must have been written at about the same time as yours.

 

 a) "385 ppm is a lot": Yes, the point is that the amount of non-GHG molecules, whether it's large or small, it totally irrelevant: So the number could be 38.5%, 385 ppm or 3.85 ppm. What counts is 1) the absolute number of CO2 molecules, and 2) the fraction this is of GHG molecules (about 4%). The 4% number is highly significant, and would, by itself, suggest that CO2's role would be small compared to H2O's role. However, see point b) below.

b) Water-vapor window: CO2 has an absorption window that is not shared with H2O, and one that is partially shared. But probably even more important than that bit of atomic physics is that CO2 extends up to almost 100 km, whereas H20 only gets as far as 10 km. For the mechanism of the greenhouse effect, that means the CO2 molecules get "first shot" at controlling the emission of IR from these windows; I'll try to explain that in the Advanced explanation.

I will probably get into the spacing of CO2 molecules in the Basic or Intermediate explanation.