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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MikeB – Which side are you on? Yes, CO2 is at saturation. That means that increasing it makes no difference. That’s an argument *against* CAGW.
Leonard Weinstein – Yes that’s a very plausible theory. I’d take it more seriously if I’d heard it anywhere before Tom Sheahen posted this article. We’ve all seen this so many times; something threatens the original theory, so someone comes up with a totally different theory predicting the same result. Science is supposed to be about explanations, not predictions.
Tom Sheahen – Like many here, I’m not totally convinced, but it does show up one thing; that the phrase “greenhouse gas” is a gross simplification, and that the situation is a lot more complicated than is popularily supposed.
MikeB,
How do we know the provenance of a photon? Emission can only occur at the given temperature for the wavelength. A quantum of energy from the surface could be absorbed and remitted many times on its way out, or it could pass straight through. It makes no difference. We are talking about the speed of light.
Uncle Gus,
Whose side are you on? It’s not a matter of taking sides I hope. It isn’t a football match. It would be nice to get the truth. In climate science there are things which are known (and which have been known for centuries) and it seems very stupid to contest those as some people here do.
Having said that I think that the level of scientific debate on this site has improved recently. We still get the same stupid people making the same stupid comments of course but there seem to be more commentators who actually know something. Still in a minority, but growing.
The sceptic cause is not helped by backing crackpot theories. There is no need for that. The science as we know it does not lead catastrophe, but you cannot argue that case if ignorance is your only weapon.
Increasing CO2 will certainly make a difference or, as Professor Richard Lindzen (do you know who he is?) puts it, it should! The only question is how much?
“How would the solar-heated surface of the earth be -18C with an atmosphere on top of it that could be heated through conduction/convection (with the energy (and thus the temperature profile) distributed upwards along the ALR up to the tropopause) but NOT adequately cool to space via radiation?”
That’s easy. While the atmosphere cannot radiate to space, the surface can. It would warm or cool until the outgoing energy matched the incoming, which would be at -18 C.
The air in contact with it would rise at the equator and cool adiabatically with altitude, but when it came down again at higher latitudes it would warm again at the same rate to its original temperature, transfer its heat to the surface by conduction, and the surface would radiate it to space. All the energy that entered the system would leave it, just as it does now, but it would all leave from ground level. The atmosphere would help spread it around horizontally, but its net effect on surface temperature would be zero.
All of this I’ve independently figured out and checked for myself. But much of it was also previously developed in the 1960s, and constitutes the ‘mainstream’ mechanism used in the models, albeit with a different emphasis and slant to the usual ‘official’ explanations seen in universities. It’s perfectly orthodox, but I’m not accepting it just because it’s orthodox, but because on this occasion it happens to be correct, too.
The issue with CAGW theory has never been to do with the GHE mechanism itself, but with the feedbacks. Getting the basic GHE physics wrong just makes it easier for the warmists to discredit us. I wouldn’t ever suggest that you give up on expressing alternatives that you think are correct simply because it sets back the cause of scepticism – we should always argue these things out. But it can be frustrating sometimes when the discussion keeps on going round in endless circles.
I agree, we really don’t want to open up this discussion again. It annoys our host. So I’m happy to clarify things for people who are interested and want to understand something about it, but I’m not going to argue any further. I’ve said what I’ve said. It should be enough.
“Leonard Weinstein – Yes that’s a very plausible theory. I’d take it more seriously if I’d heard it anywhere before Tom Sheahen posted this article.”
You can find it in Soden and Held 2000 http://www.clidyn.ethz.ch/ese101/Papers/held00a.pdf See the discussion just above figure 1 on p446-447.
You can also find it in the Sagan paper I linked above. And I explained it previously on Judith Curry’s site – see http://judithcurry.com/2010/12/02/best-of-the-greenhouse/ and scroll down to ‘radiative-convective perspective’. I know Leonard has engaged in extensive discussions about it over at ScienceOfDoom – I’m sure he can point you to some of them.
Nullius in Verba
Alarmism has worked in one way, the 30 year endless repetition that co2 causes warming.
This has to be countered again and again until the knuckles bleed, the following- below, but in some way that is easy to digest for the average person- me! so easy that newspapers can print it, in some visual form, there are some papers that would use it.
http://acmg.seas.harvard.edu/p…
In order to maintain a constant terrestrial blackbody emission integrated over all wavelengths, it would be necessary to increase the emission flux in other regions of the spectrum and thus warm the Earth. Contrast this situation to a greenhouse gas absorbing solely at 15 mm, in the CO2 absorption band ( Figure 7-8 ). At that wavelength the atmospheric column is already opaque ( Figure 7-13 ), and injecting an additional atmospheric absorber has NO significant greenhouse effect.
“Alarmism has worked in one way, the 30 year endless repetition that co2 causes warming.”
Alarmism works by setting up a decoy position for you to waste your time attacking. The question is not, and never has been, whether CO2 causes warming. It has always been whether it causes enough warming to be a problem.
Alarmists like to pretend that once you accept the basic physics of the GHE, that all the disaster predictions follow as an inevitable consequence. It would be like people predicting a catastrophic asteroid impact follows from the law of gravity – there are rocks in the sky, they’re heavy, they fall down. You can’t win that argument by trying to claim that gravity doesn’t exist!
Don’t fall into the trap of assuming that just because they say that “CO2 causes warming therefore we must panic”, that this is so. Yes, CO2 causes warming. No, we don’t need to panic. If you waste your time in a fruitless attempt to prove CO2 doesn’t cause warming, and burn your own credibility in the process, they’ll be able to get away with the other, flawed part of their argument scot free.
MikeB says:
April 12, 2014 at 7:55 am
“Having said that I think that the level of scientific debate on this site has improved recently. We still get the same stupid people making the same stupid comments of course but there seem to be more commentators who actually know something. Still in a minority, but growing.”
MikeB and Phil.; it looks like you are right and I was wrong. BUT now I have a question. It’s a serious one. Really.
How can there be talk of downwelling IR in the CO2 absorption spectrum IF the mean free path length at surface pressure is about 30m or so AND the probability of photon absorption and thermalization is WAY higher than the probability of dethermalization and photon emission? All these photons should vanish after a few dozen meters.
Personally i think you have to put a cap on how much warming according to physics.
“Contrast this situation to a greenhouse gas absorbing solely at 15 mm, in the CO2 absorption band ( Figure 7-8 ). At that wavelength the atmospheric column is already opaque ( Figure 7-13 ), and injecting an additional atmospheric absorber has NO significant greenhouse effect”
To continue with this –
“It has always been whether it causes enough warming to be a problem”
and
“Yes, CO2 causes warming” is just a festering wound and will never be answered and the alarmism will continue for ever. The temps just have to rise again one day and just wait for the barrage of MSM on that one, it will be incessant.
Nullius in Verba said at 8:59 AM
Alarmism works by setting up a decoy position for you to waste your time attacking. The question is not, and never has been, whether CO2 causes warming. It has always been whether it causes enough warming to be a problem.
Alarmists like to pretend that once you accept the basic physics of the GHE, that all the disaster predictions follow as an inevitable consequence. It would be like people predicting a catastrophic asteroid impact follows from the law of gravity – there are rocks in the sky, they’re heavy, they fall down. You can’t win that argument by trying to claim that gravity doesn’t exist!
Don’t fall into the trap of assuming that just because they say that “CO2 causes warming therefore we must panic”, that this is so. Yes, CO2 causes warming. No, we don’t need to panic. If you waste your time in a fruitless attempt to prove CO2 doesn’t cause warming, and burn your own credibility in the process, they’ll be able to get away with the other, flawed part of their argument scot free.
B I N G O !
Nullius in Verba says, April 12, 2014 at 7:58 am:
“While the atmosphere cannot radiate to space, the surface can. It would warm or cool until the outgoing energy matched the incoming, which would be at -18 C.
The air in contact with it would rise at the equator and cool adiabatically with altitude, but when it came down again at higher latitudes it would warm again at the same rate to its original temperature, transfer its heat to the surface by conduction, and the surface would radiate it to space.”
Er, no, Nullius. The energy conducted/convected from the surface to the atmosphere would never be able to just as easily conduct/convect back down. Are you being serious?! This is the same nonsense that Stephen Wilde is promulgating on the ‘Correcting Trenberth et al.’ thread. What kind of bubble world are you living in?
The atmosphere (and hence the surface) would of course be much hotter WITHOUT radiatively active gases there to cool the system to space than WITH. The surface can’t cool adequately to space through radiation with an atmosphere present on top of it, because too much of the incoming energy is lost through conduction/convection to the atmosphere: If 2 parts of energy comes in from the sun to the surface and 1 part goes out to the atmosphere via convective processes, how many parts of energy do the surface have left to radiate out to space? The surface (and hence the system as a whole) could of course never reach proper radiative balance between incoming and outgoing with an atmosphere without the ability to adequately cool to space, because the atmosphere could receive heat from the surface, but not efficiently transfer it out and away.
The so-called ‘GHGs’ do not enable the atmosphere to warm, Nullius. It would’ve with or without them. They enable it to cool efficiently to space. That’s what they do.
Nullius in Verba says, April 12, 2014 at 7:58 am:
<em"The issue with CAGW theory has never been to do with the GHE mechanism itself, but with the feedbacks."
Exactly. And that’s precisely why they can perpetuate their myth that CO2 warms the earth system with you ‘CAGW skeptics’ as their useful idiots. The proposed ‘GHE mechanism’ is pseudoscientific nonsense, but as long as you defend it with your life, we will never get rid of the festering boil that is the CAGW hype. The hypothesis of CAGW builds upon the hypothesis of AGW that in turn builds upon the hypothesis of the GHE. The latter one is what engendered the warped physics that created the basis for the other two. The two former ones actually by themselves have more going for them, they’re more based on real physics than the former one. The only reason they won’t work is that they build upon the self-construed physics of the apparently sacred ‘GHE hypothesis’.
Nullius in Verba,
One little nitpick: Over much of the ocean there is thermally driven convection only near the surface (in the well mixed layer) because slow continuous upwelling of cold deep water keeps the sunlight which is absorbed below the WML from warming the deeper water to above the surface temperature. Absent the transport of cold water from high latitudes to low near the ocean floor, at low latitudes the ocean would become pretty uniform in temperature to great depth.
You are exactly correct (April 12, 2014 at 8:59 am); claiming higher concentrations of GHG’s do not cause warming just means you instantly lose the argument about catastrophic warming, because people will ignore everything you say after that. Arguing higher GHG concentrations do not cause warming only makes most technically trained people think you are either uneducated, stupid, or crazy. The real technical argument has never been about GHG driven warming, it has always been about the accuracy of ‘amplification’ of GHG driven warming to catastrophic levels. Winning the technical argument that catastrophic warming is extremely unlikely is relatively easy…. you have the data (AKA reality) on your side, while your opponents have mainly climate models…. which are both demonstrably wrong and so full of kludges and doubtful assumptions that their temperature projections are risible. If the technical argument is about the plausibility of catastrophic temperature increases, then the Mathusian-eco-loons (many of whom are, sadly, ‘outspoken climate scientists’) will lose the public policy argument to force drastic immediate reductions in fossil fuel use. And that is only argument which really matters.
One suggestion: There are some people who will forever refuse to be convinced that GHG’s can possibly cause warming. There are some people who will forever refuse to accept that burning fossil fuels causes atmospheric CO2 to increase. There are some people who will forever think radiative physics is fundamentally wrong… and others who will forever hold many other factually incorrect (even bizarre!) beliefs. IMO, once this situation becomes clear, it is a waste of time to engage any of these folks.
“One little nitpick: Over much of the ocean there is thermally driven convection only near the surface (in the well mixed layer) because slow continuous upwelling of cold deep water…”
🙂 One little nitpick back – what causes that slow continuous upwelling? Isn’t it the pole-to-equator global convection cycle?
It’s actually a cute example – I was going to mention it earlier and then decided it was getting too complicated. The oceans are actually a pretty neat analogy for what would happen with a non-GHG atmosphere, only inverted – with the roles of heating and cooling, rising and falling, equator and poles swapped around. The deep water formation near the poles is analogous to the heating of a non-GHG atmosphere near the equator. The water descends into the deep like the air rises into the troposphere. With no way to gain or lose heat, the temperature then varies only adiabatically until it cycles round to the other end, where the water warms at the equator or the air cools at the poles, and flows across the surface to begin the cycle anew.
But given the difficulty I was having explaining the straightforward physics, I thought this inverted analogy would just cause people’s brains to explode. Nevertheless, and with a health warning, I offer it up now to anyone who wants to think a little more deeply about this stuff. If you didn’t follow what I said earlier, don’t bother with it. It’s not important.
“The atmosphere (and hence the surface) would of course be much hotter WITHOUT radiatively active gases there to cool the system to space than WITH”
I know in the Desert in the daytime it leads to hotter, but at night?
richard says:
April 12, 2014 at 1:56 pm
At night in the desert, you can see how rapidly CO2 (no H2O) is able to cool things down.
This was informative because of the problems with the post, leaving lots of room for comments to provide more accurate information. I don’t know how the whole thing works though. I know that theory says the poles should have a much stronger warming effect from GHG’s since water vapour is hardly present there. This should mean that there is a strong masking effect where water vapour dominates the effect of anything else on its absorption bands. Yet the description of each molecule having its own slowing effect on heat cannot jive with that. I’m assuming that the masking effect is at least mostly valid, if not there would be no reason for anyone to have thought that the tropics are less affected. If that’s just wrong it’s pretty stunning, although it would help out warmers (a little) with their south pole problem.
Rob, I was just having a bit of fun with Richard.
The real deal is that after the desert broils in the sun all day, the accumulated heat is rapidly convected away. CO2 in the absence of H2O is utterly powerless to stop it.
The Moral: In the lower troposphere, convection rules. In the upper troposphere, radiation matters.
Dr. Tom Sheahen
Check the optical length and gas concentration used in your chart. Let’s assume it uses a standard 1 m length. The bandwidth of methane is from 2.5 to 8.5 um with peaks at 3.5 and 7.5 um. Note the low absorption values are not zero. It means methane is absorbing IR in the entire bandwidth. Let’s say the low value is 1% and the peak is 90%. That means after IR passes 1 meter of a gas with X% methane, 90% of 3.5 and 7.5 um IR will be absorbed and 1% for the rest of the bandwidth. Of course after passing 100 m the entire bandwidth will be 100% absorbed. And the troposphere is 10,000 m thick.
There is no such thing as saturated bandwidth. This will only happen if you ran out of photons to absorb and this will not happen because the absorbing gas molecules also emit the photons at the same wavelengths. The photons do not disappear in the atmosphere. You can check in TOA satellite data if 7.5 um IR is missing hence it is already “saturated.” It’s like saying bucket A can no longer scoop water in the well because bucket B is already full. Ten buckets can be full and you can still scoop water as long as there’s still water in the well.
Steve Fitzpatrick says:
April 12, 2014 at 10:30 am
One suggestion: There are some people who will forever refuse to be convinced that GHG’s can possibly cause warming. There are some people who will forever refuse to accept that burning fossil fuels causes atmospheric CO2 to increase.
=====
Just as there are people that use GHG’s and CO2 as if they are the same
….it is a waste of time to engage any of these folks.
Is there any meaning/accuracy to a claim I came across several years ago, based on the fact that there are only a few sun energy bandwidths available for co2 to “absorb” and at 20ppmv it had already absorbed 50% of all the energy available to it. Of course, now at 400 ppmv it would seem there’s little left. This would explain why when co2 was at higher levels it seemed to have no impact on global temperature. Is that claim accurate? meaningful?
Is that claim accurate? meaningful?
==
http://wattsupwiththat.files.wordpress.com/2010/03/co2_modtrans_img1.png
Desertvoke, explanation accepted. However, folks, methane is created by waste dumps, decomposing vegetable matter, and people use it to drive electricity in bush areas, using pooh.
Thermal solar are also exploring using methane to generate electricity at night.
Do you think that is why Parchari said once, if we became vegan or vegetarian this would help change the climate because of less methane? Sheep and goats also produce methane, all ruminants. I don’t think elephants are ruminants, but they deposit lots of manure.
Peat bogs create some gas, not sure if it is methane.
Why deserts get very cold at night is because there is no cloud cover.
They are grasping at straws, rehashing their science. Forget it, and enjoy your beef and lamb, goat, preferably free range not from feed lots. If feed lot animals are not fed the right balanced dry food diet, they are high in Omega 6 and not Omega 3.
“bushbunny says:
April 12, 2014 at 7:07 pm”
Largest sources of methane include wetlands (That would include peat bogs and swamps etc, hense the term “swamp gas”), rice fields, plants including healthy forests and termites to name several others outside the alarmist claims that we humans contribute the most. The current concentartion of CH4 at ~1.8ppm/v would make it near impossible to separate the human contribution. CH4 at waste dumps is usually burnt off to form CO2. I know that back in the 1980’s I worked at IBM, Havant Plant, near Portsmouth, England. Next door was a sewage treatment plant. IBM made a deal with the local council to have methane pupmed in to provide fuel for heating. Saved IBM millions of pounds on heating costs.
Thank you Patrick, I didn’t put references down to prove my point, because I know it is common knowledge. Of course methane also collects in mines too. That’s why miners, kept canaries in cages, and if they fell off their perches, they knew some gas was around. Methane is with nitrous oxide a trace gas in the air we breathe. Our university was given a grant to find out how to remove nitrous oxide from soils. I was studying my diploma in organic agricultural production at the time, and our lecturer said, “I can tell them how to get rid of it easily, add gypsum” I do think that sometimes you ask the farmers. From what I can remember, nitrous oxide is also laughing gas, and can be toxic to plants and is created by too much vegetable matter and decay in wet soils. Gypsum breaks up clay and aerates the soil. “If they would give me 250,000 dollars I could tell them that, it is common knowledge” said our lecturer.