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.
Discover more from Watts Up With That?
Subscribe to get the latest posts sent to your email.
A number of posters up above reference Kirchoff’s law relating to absorption and emission of EM radiant energy.
It’s almost 60 years since I studied such things, but as I recall, the law applies to thermodynamic systems where an EM radiation field is in thermodynamic equilibrium with a physical medium, which is capable of exchanging energy between the medium, and the radiation field, at ANY possible frequency.
At thermal equilibrium, the spectral radiant emittance and the spectral radiant absorptance must be equal at each and every frequency present in the radiation field. It is not any integrated total energy exchange, it is a point by point match. And that implies that the medium is able to facilitate the movement of energy from one radiating frequency to another until they all are in balance.
That almost guarantees, that the radiation field must be a thermal radiation field, where the radiation spectrum, is a consequence of the equilibrium Temperature (which must be uniform throughout the system).
So it is highly unlikely, that Kirchoff’s law, can be applied to the case of absorption and emission of atomic or molecular resonant radiation frequencies, such as the GHG molecules. They are not capable of transmuting some photon energy into any arbitrary frequency present in some thermal radiation spectrum.
The “band” spectra of molecules and the line spectra of atoms, do not follow the same rules as the continuum spectra of thermal radiation such as black body spectra. In the case of atomic line spectra, some transitions from an excited state to a lower state are forbidden by selection rules, or are of much lower probability as a result. The excited state population inversions necessary for the operation of many laser systems, are a consequence of such restrictions.
Many sources of energy operate to warm the atmosphere; but it is what happens subsequently to that extra energy that is of interest for weather and the climate. In particular it can’t appreciably warm the ground; but additional sunlight energy that enters during the delayed exit of the LWIR, can. And the clouds will adjust to negate almost all of it.
“bushbunny says:
April 12, 2014 at 7:45 pm”
It maybe common knowledge to the likes of you and I and others here at WUWT but MOST people have no clue. During a heated debate about climate change one lunch time at work last week a workmate was prattling on about how GHG’s trap heat in the atmosphere. I then asked him, as I did my other friend a week or so ago, how much CO2 he thought was in the air. He thought about it for a short while and then replied “About 50%.” He had no idea that N2 was at ~78% an O2 was at ~21% of the air we breathe and that CO2 was actually ~0.0395% to which he found at odds with what the MSM was telling him.
Sadly, and I am sure you will agree, the Aussie MSM has done a spectacularly good job of spreading misinformation about “carbon pollution”.
Patrick, Yes, I had the same argument too, months ago with some troll, obviously young I felt. Enough carbon dioxide will kill you he reckoned. If one is shut up in a closed air tight room. Too little oxygen will kill you for sure, and carbon monoxide. But when I asked what were the proportions of gases that we normally breathe, he replied, ‘You tell me!” My son, a born again hippie and green, the youngest one, not the eldest, said the same. I replied, ‘Daarlink, 95% of Greenhouse gases is water vapor. We need it. They form those little fluffy things in the sky” If we didn’t have CO2 to breathe we’d be dead. Didn’t believe me of course.
I suspect it is easier to fool people than convince them they have been fooled. The thing is, nature has a way of balancing things, usually in her/him’s favor. On the one side we have the realists, and on the other – the political and naive greens who look at rainbows and think how lovely they are but don’t know what or how they are created.
Just on an end note, that scare about diminishing polar bears and ice. I suppose they don’t know, that male bears will kill and eat polar bear cubs, if they get a chance. And that they do hibernate like other bears too.
Ron C. says:
April 12, 2014 at 4:39 pm
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
————————————-
I don’t understand this , there is little moisture in the desert and yet the heat rapidly decreases.
“So, its clear that convection requires a medium (specifically, a non-solid medium). Unlike radiation, if there is no medium near the source, it cannot loose its heat simply using convection. (It can of course loose it via radiation.)
Convection or radiation
or is there enough moisture to rapidly convect away the heat.
but if convection requires a non solid medium – ie- moisture is the key for convection, you would assume the tropics on a cloudless night with more moisture content would cool rapidly.
Ron C. says:
April 12, 2014 at 4:39 pm
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.
————————————
Damn, I am none the wiser
http://www.erc.uct.ac.za/jesa/volume16/16-2jesa-dobson.pdf
1. Introduction
When a surface on the earth faces the night sky, it
loses heat by radiation to the sky, and gains heat
from the surrounding air by convection. If the surface is a relatively good emitter of radiation, it will
tend to radiate more heat to the sky at night than it
gains from the air. The net result is that the surface
temperature drops to below that of the air. This
phenomenon is termed night sky radiation cooling.
Richard, the air is the medium for convection. Block the air, as in a greenhouse, and convection stops, the heat stays put. Moisture is not needed, except for evaporative cooling.
Your link is to a paper on designing a radiator device that would cool even faster than convection.
Richard, the air is the medium for convection,
so where the air becomes thinner in the upper troposphere it changes to Radiation.
Fine with that,
so we find that lack of moisture heats the desert quickly in the daytime and without it the desert cools rapidly at night,
Well obviously apart from deserts near the sea where the higher moisture allows it to stay fairly hot at night.
Mike B –
I didn’t mean to get your goat. Like you, I’m not convinced by Sheahen’s argument. I really couldn’t make out if you were attacking it from a warmist of a sceptic perspective. You’re quite right, it shouldn’t make a difference, but it helps to know if someone is going to blast the top of your head off for doubting CAGW.
Of course, if I followed this blog more often I’d have remembered seeing you before. Mia culpa!
Sorry to have to tell you guys but Tom Sheahen’s knowledge of atmospheric physics is about 100 years out of date. Maybe he should take a refresher course.
TomP says:
April 13, 2014 at 2:55 pm
Sorry to have to tell you, but some random anonymous internet jumpup like yourself making extravagant claims about someone you’ve never met, without presenting a scrap of evidence to back up your big mouth, should take a refresher course in how to present and support an idea …
w.
Oh dear willis, we are not at university compiling a paper on this subject.
Tom’s paper is right, that’s what I learned during my studies for my diploma in organic agricultural
production. How can it be 100 years out of date, mate? Change ruminants digestive system, they won’t like that, they will die.
bushbunny says:
April 13, 2014 at 11:31 pm
I assume you are referring to something I said … but I have no idea what I said that has set you off. And I can’t be bothered to search it out, that’s a mug’s game.
Nor does anyone else know what you are talking about. Oh, they may think they know … but then they might or might not be right.
If you don’t quote what has your knickers in such a twist, how is anyone supposed to be clear about what you are going on about?
w.
Willis, the reason why Sheanen’s argument is wrong is that he fails to mention that the water vapor is limited to the lower troposphere by the temperature gradient. CH4 and CO2 are not. At the top of the troposphere the concentration of WV drops to the point where other GHGs are determining the escape of radiation. That’s why his claim that “the effects of CH4 are completely masked by H2O” is a straightforward flasehood. Either he doesn’t understand the physics involved, or he is being worse than disingenuous.
I’m glad to see that you know how to use MODTRAN. If you play around a bit more, you’ll find that doubling CH4 has about the same effect as increasing CO2 by around 50ppm. That is what is meant by the statement that CH4 is a potent greenhouse gas.
“TomP says:
April 14, 2014 at 3:23 am
If you play around a bit more, you’ll find that doubling CH4 has about the same effect as increasing CO2 by around 50ppm. That is what is meant by the statement that CH4 is a potent greenhouse gas.”
Equivalent to a 50ppm/v increase in CO2 on top of current concentrations? So, almost nothing in terms GHG “warming” via proven physics?
Dr. Tom:
Someone else may have pointed this out, but I don’t feel like looking through 219 responses to find it, so here goes. Looking at the absorption cross-section graph, it appears to me that 3.3-micron band for methane is not “narrow” at all, not compared to most of the other bands for other gases. Though the peak is not as “clean” as others, it appears that methane absorbs SOME infrared radiation over a range of from about 3.0 microns to about 3.8 microns. And in the upper end of that range, from about 3.6 to 3.8 microns, water vapor’s absorption is ZERO. Not only that, but none of the other GHGs absorb ANY infrared radiation between 3.6 and 3.8 microns. Granted, methane absorbs only 20-30% of IR in that portion of that band, and assuming (I highly doubt this is a valid assumption, but it’s all I have at this time) that the actual emissions of IR are evenly spread across the 1-16 micron range, then the .2-micron width of this methane-exclusive absorption range amounts to just 1.3% of all IR, and at 25%, methane, will absorb, at a maximum (full saturation) just 0.3% of all IR over and above what is or would have been absorbed by other GHGs. That’s just back-of-the-napkin calculations, and a more accurate calculation (including the actual distribution of IR emissions across the entire 1-16 micron spectrum) will likely result a different percentage. I’ll leave that calculation (and the analysis of whether that percentage, whatever it turns out to be, is “significant”) to people who know more about this than me. But if there is ANY IR emitted in the 3.6-3.8-micron range, then I don’t think it’s accurate to say that water vapor absorbs ALL the IR that methane might absorb.
george e. smith says:
April 11, 2014 at 7:48 am
Well expect a small nuclear explosion from Phil pretty soon.
While I tend to discount the threat of CH4, as overblown (it’s another nice fuel we can use) the picture is not as simple as Tom depicts.
But for starters, look at that CH4 band at 3.3 microns; also the CO2 band at 4.xx microns.
Those we can discard completely, as no threat; in fact they are beneficial from a MMGWCCC point of view, because the only source of any 4 micron or less radiation, is the sun itself, and any atmospheric absorption of sunlight is a global cooling effect.
But back to that 7.7 or so micron CH4 band overlaid by H2O.
Well Tom’s graphs are very low resolution transmission spectra, and those “bands” are actually groups of very many much narrower spectral “lines” and there is no assurance that some water line overlaps some methane line, so the fact that the bands overlap, is no assurance that the individual lines do.
The single molecule lines are in fact quite narrow, being resonances of a very precise structure. Those are then broadened as a result of inter-molecular collisions, giving Temperature broadening due to the Doppler effect, and the motion of the absorbing molecule; and also pressure broadening (actually density) due to the numbers of collisions.
Even so, the broadened lines at the modest atmospheric T&P are still quite narrow, and line overlaps are not going to be as Tom’s band transmissions assert.
But Phil, can do the honors. That ozone band at 9.6 microns is actually right in the middle of the so-called atmospheric window, so I’m not sure Tom’s water bands are correctly depicted.
Water is a great LWIR absorber though.
OK George, you are indeed correct that for the spectra to interfere the lines need to match. As you point out the low resolution spectra tell us nothing about that and the OP is misleading in this regard when he says:
“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.”
This statement is false, in fact the H2O lines are widely separated and are significantly outnumbered by the CH4 lines. For example between 7.4 microns and 7.41 microns there are approximately 300 CH4 lines and only about 10 H2O lines!
How much has the permafrost actually melted so far?
The permafrost bogeyman disappears in the light of the facts.
1) When there was warming in places like Alaska, atmospheric methane did not increase.
2) Permafrost depletion in the NH stopped since 2005.
3) When permafrost thaws, vegetation grows and removes more CO2 than is released by the melting. The region acts as a sink, not a source of CO2.
4) Past warm periods (Medieval and Holocene warmings) did not produce increases in methane.Runaway warming from permafrost thawing has not happened before, is not happening now, but we should believe it will happen if we don’t do something?
So the IPCC is stirring up alarm about thawing of Siberian permafrost. But there are scientists in Siberia monitoring the situation. What do they say?
“Indeed above at the surface it has gotten warmer, but that’s just part of a normal cycle. The permafrost is rock hard, And that is how it is going to stay. There’s no talk of thawing.” Michali Grigoryev
http://notrickszone.com/2012/11/19/russian-arctic-scientist-permafrost-changes-due-to-natural-factors-its-going-to-be-colder/
“It seems that the permafrost should be melting if the temperature is rising. However, many areas are witnessing the opposite. The average annual temperature is getting higher, but the permafrost remains and has even started to spread. Why? An important factor is the snow cover. Global warming reduces it, therefore making the heat insulator for the permafrost thinner. Then even weak frosts are enough to freeze the ground deeper below the surface.”
Nikolai Osokin is a glaciologist at the Institute of Geography, the Russian Academy of Sciences.
http://en.rian.ru/analysis/20070323/62485608.html
“The Russian Academy of Sciences has found that the annual temperature of soils (with seasonable variations) has been remaining stable despite the increased average annual air temperature caused by climate change. If anything, the depth of seasonal melting has decreased slightly.”
“This is just another scare story . . . This ecological structure is balanced and is not about to harm people with gas discharges.”
Vladimir Melnikov is the director of the world’s only Institute of the Earth’s Cryosphere. The Russian Academy of Sciences’ Institute is located in the Siberian city of Tyumen and investigates the ways in which ground water becomes ice and permafrost.
“The boundaries of the Russian permafrost zone remain virtually unchanged. At the same time, the permafrost is several hundred meters deep. For methane, other gases and hydrates to escape to the surface, it would have to melt at tremendous depths, which is impossible.”
Yuri Izrael, director of the Institute of Climatology and Ecology of the Russian Academy of Sciences.
http://en.rian.ru/analysis/20050822/41201605-print.html
Ron C. says:
April 14, 2014 at 8:12 am
thank you. That’s exactly the stuff I was looking for.
TomP says:
April 14, 2014 at 3:23 am
TomP, the moment you accuse a man you don’t know of being a liar, or either a liar or a fool, both of which you just did, I’m afraid you lose 97% of your credibility. In my experience, such accusations involving motive almost invariably come from a man who is very unsure of his science. If he were sure of his science, there’d be no need for accusations of malfeasance. So as a tactical matter, you’ve already lost the battle, even though you may be right … but you’re not. See below.
If increasing methane by 100% produces the same results as increasing CO2 by ~ 16%, I’d call it a weak GHG …
Next, while it is true that at the top of the troposphere there is little water vapor, there is also little gas of any kind. As you most condescendingly say, I’m glad to see that YOU know how to use Modtran. And if you “play around” with it a bit more, in your unpleasant terminology, you will find that indeed, as you say the H20 levels drop much faster with altitude than do the CH4 levels. Most of the water vapor is near the ground.
But what you obviously didn’t stick around to notice were the absolute levels. Lets take clear-sky tropics as the example. At ground level, the concentration of atmospheric water vapor is about 15,500 times that of methane.
But even at 17 km elevation, the approximate height of the tropopause, there is still almost twice as much water vapor as methane. So your claim above is simply incorrect. Water vapor is indeed pretty trivial at the tropopause … but methane is even more trivial.
So … should I now accuse you of “a straightforward falsehood”, and say you might be being “worse than disingenuous”? Heck, no. You just didn’t do your homework, happens all the time, to you, me and everyone.
But your accusations of the same thing against the author? Not only were they over the top, because the worst that you can really say is IF your accusations were true, Dr. Seagal didn’t do his homework … but in fact YOU were the one who hadn’t done your homework.
So in addition to being unpleasant, your accusations were simply untrue … and as a result, your own credibility has gone in the dumper.
w.
Trevor says:
April 14, 2014 at 6:30 am
Trevor, this is why I ask people to QUOTE what they object to. I don’t see anywhere that Dr. Seagal said that “water vapor absorbs ALL the IR that methane might absorb”, or anything like that. In fact, he specifically states that methane has an effect, but that the effect of methane is only about 1% of the effect of water vapor, viz:
So … you’re attacking something he didn’t say. We all know, including Dr. Seagal, that methane has some effect. The question is the amount.
w.