Water vapor has already absorbed the very same infrared radiation that Methane might have absorbed.
Guest essay by Dr. Tom Sheahen
Q: I read that methane is an even worse greenhouse gas than carbon dioxide, and cattle are a big source of methane emissions. How are they going to regulate that? Not just cattle, but dairy cows as well! That doubles the worry.
Fortunately, there is really nothing to worry about, scientifically. The main thing to worry about is over-reacting politicians and another layer of unnecessary government regulations.
To understand methane’s role in the atmosphere, first it’s necessary to understand what absorption means. When light passes through a gas (sunlight through air, for example), some molecules in the gas might absorb a photon of light and jump up to an excited state. Every molecule is capable of absorbing some particular wavelengths of light, and no molecule absorbs all the light that comes along. This holds true across the entire electromagnetic spectrum – microwave, infrared, visible, and ultraviolet.
The process of absorption has been studied in great detail. In a laboratory set-up, a long tube is filled with a particular gas, and then a standard light is set up at one end; at the other end of the tube is a spectrometer, which measures how much light of each wavelength makes it through the tube without being absorbed. (Mirrors are placed so as to bounce the light back and forth several times, making the effective travel path much longer; this improves the precision of the data.) From such measurements, the probability of radiation being captured by a molecule is determined as a function of wavelength; the numerical expression of that is termed the absorption cross-section.
If you carried out such an experiment using ordinary air, you’d wind up with a mixture of results, since air is a mixture of various gases. It’s better to measure one pure gas at a time. After two centuries of careful laboratory measurements, we know which molecules can absorb which wavelengths of light, and how likely they are to do so.
All that data is contained in charts and tables of cross-sections. Formerly that meant a trip to the library, but nowadays it’s routinely downloaded from the internet. Once all the cross-sections are known, they can be put into a computer program and the total absorption by any gas mixture (real or imaginary) can be calculated.
The many different molecules absorb in different wavelength regions, known as bands. The principal components of air, nitrogen and oxygen, absorb mainly ultraviolet light. Nothing absorbs in the visible wavelength range, but there are several gases that have absorption bands in the infrared region. These are collectively known as the GreenHouse Gases (GHG), because absorbing infrared energy warms up the air – given the name greenhouse effect.
The adjacent figure shows how six different gases absorb radiation across the infrared range of wavelengths, from 1 to 16 microns (mm). The vertical scale is upside-down: 100% absorption is low, and 0% absorption (i.e., transparency) is high.
It’s important to realize that these are shown on a “per molecule” basis. Because water vapor (bottom bar of the figure) is much more plentiful in the atmosphere than any of the others, H2O absorbs vastly more energy and is by far the most important greenhouse gas. On any given day, H2O is a percent or two of the atmosphere; we call that humidity.
The second most important greenhouse gas is carbon dioxide (CO2), which (on a per-molecule basis) is six times as effective an absorber as H2O. However, CO2 is only about 0.04% of the atmosphere (400 parts per million), so it’s much less important than water vapor.
Now it’s necessary to scrutinize the figure very carefully. Looking across the wavelength scale at the bottom, H2O absorbs strongly in the 3-micron region, and again between 5 and 7 microns; then it absorbs to some degree beyond about 12 microns. CO2 has absorption bands centered around 2.5 microns, 4.3 microns, and has a broad band out beyond 13 microns. Consequently, CO2 adds a small contribution to the greenhouse effect. Notice that sometimes CO2 bands overlap with H2O bands, and with vastly more H2O present, CO2 doesn’t matter in those bands.
Looking at the second graph in the figure, methane (CH4) has narrow absorption bands at 3.3 microns and 7.5 microns (the red lines). CH4 is 20 times more effective an absorber than CO2 – in those bands. However, CH4 is only 0.00017% (1.7 parts per million) of the atmosphere. Moreover, both of its bands occur at wavelengths where H2O is already absorbing substantially. Hence, any radiation that CH4 might absorb has already been absorbed by H2O. The ratio of the percentages of water to methane is such that the effects of CH4 are completely masked by H2O. The amount of CH4 must increase 100-fold to make it comparable to H2O.
Because of that, methane is irrelevant as a greenhouse gas. The high per-molecule absorption cross section of CH4 makes no difference at all in our real atmosphere.
Unfortunately, this numerical reality is overlooked by most people. There is a lot of misinformation floating around, causing needless worry. The tiny increases in methane associated with cows may elicit a few giggles, but it absolutely cannot be the basis for sane regulations or national policy.
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The mega methane mania finds its roots in the Greenland ice cores where it was observed that large fluctuations of methane coincided with fluctuation in stable hydrogen and Oxygen isotopes (erroneously considered a paleothermometer). Especially around the erroneously considered cold Younger Dryas. To the cherry pickers it looked like the methane spikes had caused sudden large temperature swings, like the ‘more-than-ten-degree-within-a-decade’stuff of Richard Alley and friends.
However this could have been challeneged immediately comparing the Greenland ice core records with Antartica, nothing adds up considering global temperatures. The Antarctic CO2 record has no corrolation with the Greenland isotope ‘temperatures’ while the Greenland CH4 records don’t compare with the Antarctic isotope ‘temperatures’. So something had to be wrong there. But that methane would have nothing to do with global temperature is obviously a too unconvenient truth to alarmists.
Thank you for that. It neatly describes one of the most important issues to be remembered when discussing problems with cagw theory.
Amazing really when the end game is considered. Kill off all the big time f*rters – meat and milk disappears. Eat cats, dogs, birds…ok for China/Europe? So we get veggie with leaves and stuff that needs CO2 for growth…oh no, thats gone as well.
Good article…thanks
CH4 does have a negative effect on the atmosphere in that its breakdown by chemical reactions in the atmosphere use a lot of oxygen. Thankfully the CO2 and H2O products are used by plants to produce O2. So a neat closed circuit. Cow farts are actually good for the planet.
The term ”Greenhouse Gas” is a misnomer since the action of gasses in the atmosphere in no way acts like a greenhouse. Greenhouses get warm because they stop convection of the hot air inside. The atmosphere has no lid so convection is free to act.
Great article, the key to the whole CAGW scam I believe.
CO2 and possibly methane could have a played a major role in getting Earth out of ice ages and snowball Earth conditions, i.e. when there was much less water vapour in the atmosphere. But once the Earth is warm and humid (as it is now) water vapour dominates.
Water vapour even dominates the “consensus” CAGW scare, supposedly doubling CO2 causes a small rise in temperature, which then causes more water vapour, which is the thing that dominates the eventual rise (allegedly).
Reblogged this on Maley's Energy Blog and commented:
A concise and thoroughly understandable explanation of the greenhouse effect, and why a little methane in the mix is irrelevant.
Why haven’t I heard about this before?
I have an ongoing conversation with a friend who’s always trying to convince me of AGW. Recently he emailed this link with regard to CO2 causing warming:
“Permafrost thawing could accelerate global warming, researchers say
Science Recorder – 40 minutes ago
On Monday (April 7), researchers from Florida State University announced that they have discovered new evidence that permafrost thawing is releasing copious amounts of greenhouse gases into the atmosphere by way of plants.”
In my research I was looking for articles referencing why the permafrost is melting. Would your article be sufficient to convince that even with melting permafrost, the effect is negligible?
“The second most important greenhouse gas is carbon dioxide (CO2), which (on a per-molecule basis) is six times as effective an absorber as H2O” – H2O is polar. CO2 is not, so it has non radiative modes. H2O is non linear. CO2 is linear, more symmetric (meaning: degenerate modes). For those reasons, the CO2 infrared spectrum is not so rich as the H2O spectrum.
Wouldn’t methane be a factor in dry climates?
And if sane people ran the governments, it would not be an issue. However, given past actions, the sanity of government officials is always in question.
Harken to me! The day will come when a man will be burned at the stake for daring to eat a can of baked beans.
“Somebody says:
April 11, 2014 at 4:44 am”
Exactly! I was talking to a colleague at work today and I asked what his opinion was with regards to CO2 concentration in air, assuming well mixed etc. He said about 50%. Jaw dropping! So I pointed out that in, well mixed air, ~78% is N2, ~21% is O2 and CO2 is ~.03%. Stunned looks abound on his part!
Wait, but the H20 also absorbs bands which overlap CO2. And the absorption of Methane is much greater in it’s bands than H20. This doesn’t explicitly discount anything in the article, but there are more complex interactions than just the one listed here.
Excellent post! I’ve been blogging about AGW for 10 years, but haven’t really had a grasp on the mechanism until reading this article.
Termites and healthy forests are the biggest source of CH4 on this rock. I recall “debates” in the past where methane (CH4) is discussed. One commenter said, and I quote “CH4 has for carbons”…I kid you not!
Excellent, easy to understand article discussing this important realm.
Excellent post. Just a couple of (simplistic) comments:
1. The energy of electromagnetic photons is related to their frequency (E = h x f), and inversely to the wavelength. If you get an absorption diagram similar to the one in the article, but with photon energy on the x-axis, you can see that water is THE massive ‘greenhouse gas’, and that the contribution of CO2 energy absorption is small in comparison.
2. The water vapour content of the atmosphere varies widely, usually between 0.5% and 5% in the lower atmosphere (NB. anything over 3.5% tends to mean it is raining heavily). Combustion systems I work on are noticeably more ‘sluggish’ to react on rainy days . This variation absolutely dwarfs the effect of the miniscule increase in CO2 content.
Tom, this is one of those posts where I receive a quantum addition to my education on this subject. Also, the presentation of it is understandable to every reader, regardless of background education and experience. Thanks. I was wondering if your graph above could be made more effective (or probably better make a second graph) by changing the ordinates from simple unit absorption to quantitative units that take the abundance of the gases in the atmosphere into consideration? This would be a more powerful illustration of your very thorough explanation. I hope some of the CO2 worriers join this discussion – Mosher for example, to argue their case. I have no doubt they understand the details of this subject.
‘Water vapor has already absorbed the very same infrared radiation that Methane might have absorbed’
On that basis, seen as this is not an effective part of the spectrum as CO2 the most effective GHG is still co2!
Water vapour and co2 as GHGs and heat vent blockers. [Therefore resulting in AGW!].
‘70 years ago the view that co2 could affect the global climate was held by only a tiny minority of climate scientists, many assumed there would be a self regulating mechanism that would put things back into balance. Then there was the scientifically valid view that water vapour also trapped radiation and warms up the Earth and it is more abundant in the atmosphere than co2. But research in the 1940s changed all that, Guy Stewart Callender, a British engineer showed that radiation absorption is not even. Water vapour absorbs is mainly in the 18-30 micro-meter band and allows most of the rest to escape into space, in effect these absorption gaps act like cooling vents , but co2 absorbs in a different range, 8-18 micro-metre so Callender concluded that co2 mops up this escaping radiation, effectively acting as a plug to these cooling vents’.
As you can see here!
TRG says:
April 11, 2014 at 4:40 am
Because it’s wrong.
Indeed, it would be truly amazing if everyone in the world hadn’t realised this except Tom Sheahen.
First it would be helpful to describe the graph as ‘transmission’ instead of ‘absorption’, since that is what is shown. It should also state the path length over which these transmission figures have been assessed. ‘A per molecule basis’ is very dubious ( it looks more like total transmission from ground to space). What is the provenance of this graph?
The failure here is that although CO2 is a very small proportion of the atmosphere, there is no sightline through the full thickness of the atmosphere which will not intersect a CO2 molecule in a very short distance. For example, 95% of radiation at 15 micron will be absorbed within a distance of 1 metre. The atmosphere is therefore opaque to radiation at 15 microns, due to the small amount of CO2 present. Water vapour (if present) also absorbs at 15 microns but not at the 100% level of CO2.
Similarly for methane. In addition, the only reason that methane is considered to be more potent than CO2 is because there is so little of it. Its absorption bands are not saturated and its affects are therefore linear and not logarithmic.
Very well-written–and, I’m embarrassed to say, informative; I had not previously reflected on the methane claims being bruited about.
“The main thing to worry about is over-reacting politicians and another layer of unnecessary government regulations. ”
Might soon come to a point where it becomes necessary to leave the West as it strangulates itself under unnecessary self-imposed shackles; and wait it out til the parasites have died off.
MikeB says:
April 11, 2014 at 5:15 am
“Water vapour (if present) also absorbs at 15 microns but not at the 100% level of CO2.”
So CO2 manages to saturate its absorption lines while water vapor doesn’t? Tell us more about it, Mike! Hint – you mentioned mean free path length yourself. Think about what that means…
Ex-expat Colin says:
April 11, 2014 at 4:31 am
“Amazing really when the end game is considered.”
_______________
Yes.
What we’ve seen from those people certainly fits with “the end game” which has been discussed here. What’s the end game? There is an idea, widely promoted from within certain circles, that too many human beings exist and that steps must be taken to reduce human populations. The people promoting that idea are the same people promoting the idea of man made global warming. When you are confronted by one of those idealogues, or their useful idiot followers, recommending a high building to put their beliefs into practice has no effect, as it is you that they envision being reduced, not themselves.