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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Doesn’t the IR radiation originate from the surface if the earth, as it is baked by the sun? Doesn’t it then proceed upwards through the “greenhouse” gases?
Would this not mean that a lot of the “blocking” occurs down low, where the gases are dense? And where water vapor is plentiful?
As you got up to where the air is thin and water vapor is more scarce, wouldn’t much of the “blocking” have already occurred? Is not much of the IR radiation escaping earth occurring at wavelengths in the spectrum where “blocking” does not occur?
Forgive me if I’m missing some obvious and major point, but everyone seems to be looking way, way too high. Once you get up where the air is too thin to breathe, so thin water boils at room temperature, the game is already over. That which was going to be blocked is blocked, and that which is escaping is heading off to Pluto.
DirkH says: “By definition TOA is where the greenhouse gases radiate to space, so when a greenhouse gas occurs higher up, TOA is higher. So what.”
I have no idea what you are asking. The word “higher” is a comparison, so are you saying TOA is now higher than what it was in 1750 AD, or what?
My comment refuted the author of the lead post conclusion that methane changes can have no effect on temperatures because “the effects of CH4 are completely masked by H2O.” Water vapor concentration is very low in the upper atmosphere, so H2O does not mask the effects of CH4. What did you not understand?
If this is true:
Then this is wrong:
. The cannot both be right.
DirkH says:
April 11, 2014 at 11:38 am
Steve Fitzpatrick says:
April 11, 2014 at 8:24 am
“Isn’t it the non-IR active gases, O2 and N2 that delay the cooling by capturing heat from the IR-active gases and then cannot radiate, but only rise and cool according to the lapse rate?”
No; under local thermodynamic equilibrium thermalization and dethermalization must happen to equal amounts (Kirchhoff’s Law); meaning – CO2 aborbs IR photons and gives energy to N2 and O2, but the reverse must happen equally often – N2 or O2 giving energy to CO2, making it re-radiate an IR photon.
Kirchoff’s law doesn’t say what you think it does! It just says that emissivity equals absorptivity.
Also IR excites vibrational and rotational modes in CO2, multiple collisions with surrounding gases (N2 & O2) cause exchanges of translational energy in small packages. The transfer of energy from the N2 and O2 ( a very small fraction of which have excess energy sufficient to excite the vibrational level- see Boltzmann) to CO2 is most likely to excite translational modes not Ro-vib. As a result the CO2 is not likely to be able to emit.
I was promised no scientific facts … now I have to actually read and understand … darn it …
Phil. says:
April 11, 2014 at 12:21 pm
Kirchoff’s law doesn’t say what you think it does! It just says that emissivity equals absorptivity.
Quite right Phil. But Dirk thinks it means that an object must emit as much radiation as it receives. He keeps saying this and no amount of correction will get him to think why that is obviously wrong.
No problem! Strap an inflatable bladder to the backs of cows and attach a hose with a
check valve. Now, shove the other end of the hose up the cows ass. When the bladders
are full, they can be taken to a power plant to generate electricity.
Are these people INFREAKINGSANE? CH4 .0001745%
1.745 parts per million, are you kidding me?
Re Dirk H “Oho! But did you consider the effect of gravity on the photon? It is redshifted when leaving Earth’s gravity well! So: Are you talking about the photons that go up or the ones that go down?”
I also didn’t mention pressure broadening, doppler broadening (your red/blue shift) or line wings growing as line center absorption saturates. Even considering all that, unless the H2O lines are coincident with the CH4 lines, CH4 is still a green house gas.
BTW detection H2O in stars was done a long time ago relying on the effect of pressure broadening, Hot H2O vapor in stars had broader spectral lines than cool H2O in earth’s atmosphere. So could subtract the contribution of terrestrial H2O from the spectra of stars to detect H2O in stars.
Kirchoff’s Law, and the common argument for it, are restricted to the case of thermodynamic equilibrium, presumably between the “EM radiation field” and the physical medium , which presumably has some defined Temperature.
Solar thermal collection efficiency often depends on the use of spectrally selective surfaces, or coatings, that rely on the fact that the “absorption” of solar energy, is taking place at solar spectrum wavelengths; which correspond roughly to a 6,000 K black body spectrum; while the LWIR thermal emission from the collection surface, which might be at 100-200 deg. C will be spectrally at a totally different wavelength range, for which the emissivity is quite different from the solar spectrum emissivity.
I think people trundle out Kirchoff’s law too often, and for no good reason; or good result either.
I get spooked by Phil’s wavenumber based spectra, since I am not a chemist. So I don’t expect to find the CO2 absorption band right at the peak of the surface LWIR, instead of on the falling tail end.
Readers need to appraised of an important distinction.
The usual wavelength based spectra, a la the solar spectrum, have units of Watts per meter squared PER MICRON OF WAVELENGTH INCREMENT.
The Chemist’s wave number based spectra, like the ones Phil linked to, have units of Watts per square meter PER WAVE NUMBER INCREMENT.
So what ??
Well on a wavelength based scale, the CO2 band is somewhat lower than the peak at 10.1 microns, for the 288 K LWIR spectrum, BUT there are a lot of MICRONS of band width, typically from around 13 microns up to around 17 microns..
On Phil’s wave number based spectrum, the CO2 band is sitting on the spectrum peak, but there are fewer wave numbers down at the low end of the spectrum, whereas the much higher frequency solar spectrum, is up where there are plenty of wave numbers to spare, so even if the Y scale value is lower the PER WAVE NUMBER INCREMENT can add up to a lot of energy.
So you have to pay attention to the scale units of these absorption spectra, so you are comparing apples to apples.
I live near a lot of cattle ranches. (The newborn calves in the fields these days are absolutely tiny!) One remark I often make to the city folk is that all you need to make a cow (and by extension, cow farts) is a field of grass and water. At the basics. Sure, you need the heifer and the bull, but think about it. A few cows and that field of grass and stream of water will create herd over time. So all those cow farts and everything else have come from the grass and the water. (Of course, let’s not forget the sunshine for the grass to grow).
Does it matter how much methane is produced? It is all renewable. Everything a cow has, or is, or does, comes from the simplicity of growing grass (and carbon dioxide in the air), water and sunshine.
To consider this a danger is to consider life itself a danger.
A lot of what is being said in this comment section is beyond me, with people picking holes in other peoples explanation..interpretations…guess work.
What does seem amazing clear though is that the earths climate system is massively complex and it has variables upon variables.
I just can’t see how any computer model even has the scope to account for all of the factors involved, and the biggest limiting factor is that people are inputing the data/variables of system’s that are not fully understood. (we can’t even find a plane that was designed and built by people and probably has some of the best experts in the world looking for it)
So does anybody really know what the hell is going on with the climate?.. the 97% of IPCC scientist? the people on here? (who seem to be in the minority, but do seem to come out with the most facts)..I have no idea, only time will tell I guess!
Steven Mosher says:
April 11, 2014 at 9:03 am
“Not even wrong.
The problem here of course is that the author doesnt understand the vertical structure of the atmosphere. yes, C02 and Ch4 overlap with water.”
We know LWIR is escaping the earth at a substantial rate. It must be coming from re-radiation well below the TOA. Surely if CH4 is so well mixed and it is 1.7ppm, there isn’t much of a “screen” for capturing this LW from below the TOA. What is wrong with my view of it.
I find all this absorption/wavelengths/concentrations rather complex, so I try to simplify the question and look for the simplest answer that I can be sure of.
So, in the question of do the so called GHG’s actually warm the atmosphere/planet. Rather than argue about the absorption/wavelengths/concentrations, surely there are data from detectors high up that directly measure the radiated energy (that CO2 & Methane would affect) leaving the planet?
Do they exist?
Are such data available?
Have they shown less energy reaching those heights as CO2 and Methane increased?
How would this explain the fact that the earth hasn’t warmed for over 17 years as CO2 & methane kept rising?
I’m thinking that the warming ‘blanket’ atmosphere is very very thick, adding another thin blanket (so to speak) will not have any effect.
Anyhow, the CO2 concentrations rise as the temperature rises as shown clearly by Salby & others.
So I’m guessing the radiating energy of wavelengths under question is already absorbed 100% by what’s there.
I think you could probably remove half or more of the existing blanket and the temperature/heat values in the atmosphere would stay the same.
Does anyone know if above said data have been collected?
Leonard Jones says: “Are these people INFREAKINGSANE? CH4 .0001745%
1.745 parts per million, are you kidding me?”
No Leonard, this low percentage does not imply that methane changes have an insignificant effect on climate. What is important is the radiative forcing caused by methane. Since 1750, the change in methane concentration cause a radiative forcing of about 0.48 W/m2.
However, the methane concentration in recent history has changed insignificantly, only 0.2%/year from 2005 to 2010. Also, climate is quite insensitive to changes in greenhouse gas forcing.
The CERES satellite with surface temperatures measure the changes in the greenhouse effect from 2003. Using HadCRUT4 and CERES outgoing longwave radiation data, a doubling of CO2 with a forcing of 3.71 W/m2 would cause a transient temperature response of only 0.74 +/- 0.54 C (95% confidence). The transient climate response is 0.20 C/W/m2. This is a staight forward calculation because during the CERES era, there was no net change in surface temperatures, so there were no net feedbacks. See:
http://www.friendsofscience.org/index.php?id=739
From 2000 to 2012, CO2 changes were 84.9%, and CH4 (methane) was 3.3% of the total greenhouse gas radiative forcing, so methane was insignificant.
DirkH says:
April 11, 2014 at 11:38 am
I do not know who you think you were quoting, but it surely was not me who wrote what you quoted. You are also very confused about Kirchoff’s law and what it means. In any case, based on your several comments up thread, I very much doubt I, or anyone else for that matter, can help you understand radiative heat transfer. A Deus.
Methane is buried by H2O, it’s effects should be minimal. Download the accompanying atmospheric absorbance poster created by the Air Force Research Lab and Spectral Sciences to see this.
https://dl.dropboxusercontent.com/u/3758743/AtmosphericTransmission.pdf
I’m not at all convinced that conduction and convection don’t swamp the minimal amount of heating by back radiation from CO2.
The climate sensitivity of CO2 is about 1.2C° per doubling in the atmosphere
The climate sensitivity of CH4 is about (___)C° per doubling in the atmosphere
Can anyone fill in the blank?
DayHay, April 11, 2014 at 11:00 am
There are no simple ways to answer your detailed questions. What is pretty clear is that methane for certain adds to total GHG forcing (as some have noted, a net of ~0.48 watt/M^2, but with some uncertainty). Would adding a bunch of methane to the atmosphere increase GHG forcing? For sure it would. Is there much chance forcing from methane is going to skyrocket in the coming decades? For sure there is not. Giant methane releases from methane clathrates, either from the oceans’ deep continental shelves, or from deep permafrost melting, is so far from reality as to be laughable…. which is why worrying about methane is nuts. This all falls in the “scary story” realm of climate pseudo-science which the late Stephen Schneider advocated: “So we have to offer up scary scenarios, make simplified, dramatic statements, and make little mention of any doubts we might have.”
this is a very informative comment stream. Would someone please explain, though, why we would care if the the upper atmosphere, which is where people seem to be saying the action of gas molecules like CO2 and Methane predominates, gets warmer? What does that have to do with the surface or near surface temperature. If what is absorbed and prevented from going out into space is then re-radiated in part downward, would it not randomly again be re-radiated in part upwards. Eventually everything goes out the top to space and the layer right next to space may be a little warmer but I am unclear by why other layers would be warmer. Just trying to understand how you possibly end up with a net warming at the surface if the “heat” capture is at the top of the atmosphere, which is a long way up.
Why 1750? What’s so special about that particular date?
Dr. Sheahen – 3 questions: 1) is it known whether there is any affinity associated with IR wavelength and each of the molecules in the overlapping bands? That is, because H2O, CO2, etc. are of different configurations and atomic weights, would a particular wavelength excite an H2O molecule “in preference” to a CO2 (or other) molecule? 2) has anyone quantified the IR energy being given off by Earth? That is, do we know how much energy the Earth is producing or reflecting back into the atmosphere at each wavelength? And 3) What happens to the excited molecule? I assume it almost instantaneously returns to its non-excited state and releases energy at an even lower wavelength.
I ask because, if there is an affinity and that affinity is stronger toward H2O, then other GGs are pretty much irrelevant. OR, if Earth is producing little or no energy in the wavelengths associated with CO2 or CH4 then, again, they are irrelevant.
I really fail to understand how so many people could gloss over the TRILLIONS of people already killed by that awful CAGW. This methane problem must be stopped immediately. I propose that all person eating baked beans along with all those involved in preparing them for consumption, canning them for preparation, selling, transporting, harvesting or growing them be arrested, terminated with extreme prejudice and then tried in absentia. After that, we should go after those responsible for the pockets of over-heated CO2 clustering around state and national capitals . . .
I was pulled over by a policeman wearing a green hat the otherday and asked to blow in his fartometer. Most embarassing. Fortunately I’d followed the government’s new guidelines on keeping fart gas below the permissible volumes.
The Warmista Politicians here in the UK are now seeking to control human farts in order to ‘save the Planet’.
http://www.dailymail.co.uk/news/article-2600681/Cut-eating-baked-beans-reduce-smelly-emissions-minister-suggests-battle-tackle-climate-change.html
“Would someone please explain, though, why we would care if the the upper atmosphere, which is where people seem to be saying the action of gas molecules like CO2 and Methane predominates, gets warmer? What does that have to do with the surface or near surface temperature.”
The greenhouse effect requires the understanding of two separate bits of physics: radiation of heat in a vacuum, and the adiabatic lapse rate.
First, the Earth absorbs energy from sunlight, and radiates energy to outer space as thermal IR. The Earth warms or cools until the energy radiated by the emitting ‘surface’ balances the energy absorbed, which happens at a temperature of about -18 C. However, the surface emitting to space isn’t the solid ground, because the atmosphere is partially opaque at IR wavelengths. The emitting ‘surface’ of the Earth appears to be about 5 km up on average. (You can think of it as a bit like fog, visually. Obviously I’m hand-waving over a lot of technical detail, here, to calculate/interpret that average height.) So it’s the layer 5 km up that settles at -18 C.
The second bit of physics is that gases tend to increase in temperature when they are compressed, and cool when they expand. And pressure decreases with altitude. So as convection drives air round convective cycles, it warms as it descends and cools as it rises. This means that convection turns on and off as the vertical gradient exceeds or falls below the rate of compressive warming, and this acts as a sort of thermostat that holds the *gradient* very close to a constant called the ‘adiabatic lapse rate’. Note that only the gradient is fixed, not the offset.
We modify this slightly to account for the condensation/evaporation of water, which has a very similar altitude effect. The result is called the ‘moist adiabatic lapse rate’, and is about 6.5 C/km of altitude, although it varies a bit depending on humidity.
So the surface temperature will be warmer than the emitting layer by an amount equal to the gradient times the height difference between the two. The gradient is about 6.5 C/km, the average height difference about 5 km, so the surface is 33 C warmer than the emitting layer, or +15 C.
When you add greenhouse gases, the atmosphere becomes more opaque, and the emitting layer appears to be higher. The increase in altitude makes the surface warmer.
This is the official, mainstream mechanism for the greenhouse effect, as used in the technical literature. (e.g. Soden and Held 2000, Manabe and Strickler 1964, Sagan 1967, etc.). Accept no substitutes!
As several people have noted, at the top of the atmosphere there’s not much H2O, so other gases have a much stronger effect on emission altitude. Methane has a significant effect (I think) between 1200-1400 cm^-1 at the upper tail of the thermal black body distribution, raising the altitude of emission in this band significantly. Overall, it’s not a big deal. But its not zero.