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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I’m not very impressed with this article. It leaves too many questions unanswered. Why is the range shown for CH4 so short? Is this the only range where CH4 absorbes IR? There is no source for the graph. The range in the video @5:11am shows half the escaping radiation occurring outside the range of the graph in this article.
It does explain masking at the two CH4 spikes, but water vapor is not constant over the globe. It’s going to be less in colder areas.
All of that translates to a persistence of ~11 years. There has been continuing increase in methane release from 1970 — but at a continually decreasing rate of increase.
If China stops showering coal dust all over the Eastern Arctic (which they will do long before cutting CO2), that will take off a lot of the warming pressure and there will be less CH4 emitted than is currently feared.
CO2 has great persistence, and when an anthropogenic CO2 molecule is absorbed, it bumps aside a natural CO2, so it looks as if anthropogenic CO2 is going out of the system, but it’s just shifting around. The alarmists are right about that much. Because of the relatively long persistence of CO2, we need to keep a reasonably sharp eye on things. But without pushing the panic button six times before breakfast. And an eye favoring empirical evidence.
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. The important thing is what is the atmosphere composed of where the radiation escapes.
The definitive measurements of this were made in the 50s by the US Airforce.
Here is what you need to know
1) You still get an increase in warming even if the atmosphere is saturated because its absorption in the thin upper atmosphere that counts. The Air Force determined that the upper
atmosphere was dry. The point is simple. you have to look at the concentrations at the point
of escape: the ERL and above.
2. Water vapor doesnt overwhelm either C02 or Ch4 because there is little or no water vapor
in the cold high layer of the atmosphere where radiation escapes to space. The low pressure at that altitude makes what little water is there relatively ineffective.
Basically, methane (CH4) breaks down into H2O and CO2 (a minuscule amount). So, unlike CO2, when methane is gone, it’s GONE.
“Unfortunately, this numerical reality is overlooked by most people.” Well yeah, because you never pointed it out to us until now. Only a couple weeks ago Obama said he wants to attack milk to stop global warming. http://articles.latimes.com/2014/mar/28/business/la-fi-methane-emissions-20140329 Stop the cows? What about stopping irrigating golf courses in Arizona?
Mickey Reno,
There are spectral measurements looking up from the ground and looking down from space. The influence of GHG absorption bands is clear. (eg http://www.sundogpublishing.com/shop/a-first-course-in-atmospheric-radiation-2nd-ed/) If you don’t want to spend money, there is: http://people.su.se/~rcaba/teaching/PhysMetLectNotes.pdf.
Leonard Weinstein, Jeff Id, and stevefitzpatrick, thanks for the clarification.
However, the comments weren’t very quantitative. Over how much of the infrared spectrum of interest is the stratosphere’s optical depth (if that’s the right term) significant? (I can’t blame you for not having all this at the tips of your fingers, but it never hurts to ask.)
arthur4563 says:
April 11, 2014 at 6:54 am
I’ve decided that global wa[r]mists are climate chauvinists and a bunch of Dr Panglosses, all believing that this climate is the best of all possible climates, despite having only experienced
this one.
Isn’t that a bit too Candide for the warmists?
I’ll get my coat.
scarletmacaw says:
April 11, 2014 at 8:45 am
Tom, you should also add a graph depicting the LW radiation spectrum, since absorption outside the bulk of the LW spectrum contributes nothing to the ‘GH effect.’ The figure I found on a quick Google search isn’t great, but does give an idea. Maybe you can find a better one.
Also such a low resolution spectrum such as the one in the OP is totally misleading, since as George points out it gives the illusion of overlap where there is none!
Here’s high resolution spectrum showing the two gases, the top one being water, the much sparser lines of which give very little overlap with CO2.
http://i302.photobucket.com/albums/nn107/Sprintstar400/H2OCO2.gif
Here’s a spectrum through the atmosphere with the GHGs progressively remove starting with water, which is at a very low concentration higher in the atmosphere where the radiation to space actually takes place.
http://i302.photobucket.com/albums/nn107/Sprintstar400/Atmos.gif
By the way methane doesn’t react with O2 in the atmosphere it reacts with OH which is a scavenger molecule which is in short supply which limits the breakdown rate of methane.
Methane when it reaches the stratosphere acts as a source of H20 there which otherwise has difficulty passing through the temperature minimum at the tropopause.
Ron C. says:
April 11, 2014 at 8:39 am
N2 and O2 do indeed ‘quench’ GHG molecules which have been promoted to an active state via absorption of a photon (a higher quantum energy state), and so do “take heat away from” the GHG molecules. But it works both ways: random collisions with N2 and O2 molecules can also promote a small number of GHG molecules to a higher quantum energy state, from which those GHG molecules can emit radiation. The difference in energy level between the two quantum energy states is equal to the energy content of the photon which is absorbed or emitted (energy is always conserved), so GHG’s have emission and absorption only at very specific wavelengths which correspond to the different “allowed quantum energy states” which the molecules can have. This is why GHG’s often have several distinct ‘bands’ of absorption and emission, with each band corresponding to a transition between two allowed quantum energy states.
Phew! That’s a relief in more ways than one. Just a few days ago a UK peer was talking a load of hot air.
http://chemistry.about.com/od/medicalhealth/f/What-Is-The-Chemical-Composition-Of-Farts.htm
Sorry folks, but this is very incorrect. Changes in methane concentration do have a significant effect on climate. The figure of longwave radiative absorption by greenhouse gases shows the total absorption of the total atmosphere. In the real atmosphere, heat is transported upward by thermals and evapo-transpiration to above much of the water vapor. The water vapor concentration is low in the upper atmosphere, so this is where much of the heat energy is radiated out to space. (Several other commenters have also made this point.) The forcing effect of methane must be determined by line-by-line radiative code computer programs which take into account the declining water vapor with altitude.
The direct radiative forcing due to the methane concentration increase in the industrial era after 1750 AD is 0.48 W/m2 (Forster et al., 2007). The IPCC WGI SPM page 13 correctly shows the direct concentration-based estimate of methane radiative forcing from 1750 is 0.48 [0.38 to 0.58] W m−2. (CO2 forcing was 1.68 W/m2)
The methane concentrations increased from 722 ppb around 1750 AD to 1800 ppb. Most of the increase occurred prior to 1990 and has since leveled off. The actual CH4 concentration increased at 0.2%/year from 2005 to 2010. CH4 concentrations are forecast by the IPCC to increases at 1.34%/year by 2050 in the RCP8.5 scenario. The RCP8.5 methane growth rate by 2050 is 6.7 times the recent historical growth rate. The IPCC describes the RCP8.5 scenario as a “high emissions” scenario, but numerous reports incorrectly describe it as a “business-as-usual” scenario. The RCP8.5 is actually an extreme and unrealistic scenario as both CO2 and CH4 increases much faster than the historical changes. (CO2 concentrations increase at 0.54%/year from 2005 to 2013. The RCP8.5 CO2 concentrations increase at 1.00%/year by 2050, and at 1.16%/year by 2070, which is more than double the historical growth rate.)
Methane is not likely to cause a significant temperature rise because the methane concentration is not increasing, not because the methane radiative absorption is “completely masked by H2O.”
Here are the four IPCC forecasts of methane concentrations.
http://www.friendsofscience.org/assets/documents/FOS%20Essay/IPCC_RCP_CH4.jpg
dumb question: if co absorbs less heat than co2, why not eliminate catalytic converters from cars and take that part of co2 emissions out of the atmosphere?
O2 and N2 are non-radiative gases, they get warmed mainly by convection from ground and water. They can transfer heat by mixing with other gas components. They never realease heat by radiation. Why there is always talking about radiation or radiation balance? It’s the most ineffective way of heat transfer.
stevefitzpatrick says:
April 11, 2014 at 9:20 am
If you don’t want to spend money, there is: http://people.su.se/~rcaba/teaching/PhysMetLectNotes.pdf
Ok, skimmed the paper by Rodrigo Caballero. He seems to know several metric tons about the atmosphere and the various interactions. What does HE think about methane? This is the problem with the blogosphere, where we cannot seem to drive the stake of truth anywhere and convey what that actually means. Information gets published like this article, there seems to be some very good comments on why this is a simplistic analysis and not quite right, but again, what is right? Are spectral absorption lines so narrow as to preclude what looks like overlap? I guess I just need to be spoon fed the proof. After reading this thread I still have little idea if methane is the Mothra to the Godzilla of C02, and before I engage with the CAGW crowd, even though their arguments are every bit as vague, I (and it looks like we all) need more info.
Glass should be – by all this GHG nonsense reasoning – termed a greenhouse solid. It is not! But it should. It – just like the so called GHG’s – is transparent to the visible, and blocks IR. And what about N2 and O2 – how can they trap IR heat and yet not be GHG’s? Just because they don’t show up with an instrument we all think they’re not GHG’s. It is the instrument that is special: N2 and O2 (98% of the atmosphere) are IR inactive to thermopile sensors – they are like stealth aircraft, they’re there, and loaded with energy (they have a heat capacity), but don’t show up. Read the instructions of any IR temperature instrument!! And end the Tyndall magic trick. All gases are ‘GHG’s.
Your graph shows molecular “bands” not individual molecular “lines” Unless H2O “lines” fall exactly coincident with CH4 “lines”, then H2O does not absorb the same photons as CH4. The photons that sneak between the H2O lines are the ones susceptible to absorption by CH4 lines (and vice versa). So yes, CH4 does matter as a greenhouse gas.
Blair M says:
April 11, 2014 at 11:23 am
“All gases are ‘GHG’s.”
No. Diatomic gases like N2 have no absorption bands, only a few harmonic needle peeks. They lack the many vibration modes of a triatomic molecule.
lgp says:
April 11, 2014 at 11:24 am
“Your graph shows molecular “bands” not individual molecular “lines” Unless H2O “lines” fall exactly coincident with CH4 “lines”, then H2O does not absorb the same photons as CH4.”
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?
Ken Gregory says:
April 11, 2014 at 9:55 am
“thermals and evapo-transpiration to above much of the water vapor. The water vapor concentration is low in the upper atmosphere, so this is where much of the heat energy is radiated out to space. ”
By definition TOA is where the greenhouse gases radiate to space, so when a greenhouse gas occurs higher up, TOA is higher. So what.
Steven Mosher says:
April 11, 2014 at 9:03 am
“1) You still get an increase in warming even if the atmosphere is saturated because its absorption in the thin upper atmosphere that counts.”
I thought even warmists knew about Kirchhoff’s Law? so why do you talk about absorption only and not about re-emission? Is it ignorance or manipulative use of language?
Summarizing what people have said upthread: There are 3 ways that UWIR can pass from the surface to space.
1) Some of the UWIR leaves directly, eg. All gases in our air are transparent to IR of 10-14 microns (sometimes called the “atmospheric window.”) This pathway moves at the speed of light, so no delay of cooling.
2) Some UWIR is absorbed and re-emitted by IR active gases up to the tropopause. Calculations of the free mean path for CO2 show that energy passes from surface to tropopause in less than 5 milliseconds. This is almost speed of light, so delay is negligible.
3) Bulk gases of the atmosphere, O2 and N2, are warmed by conduction and convection from the surface. They also gain energy by collisions with IR active gases, some of that IR coming from the surface, and some aborbed directly from the sun. Latent heat from water is also added to the bulk gases. O2 and N2 are slow to shed all this heat, and indeed must pass it back to IR active gases at the top of the troposphere for radiation into space.
This third pathway has a significant delay of cooling, and is the reason why our surface has the temperature it does.
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.
That’s why the term “heat-trapping gases” is a propaganda lie by the warmists.
The methane releases by animals is unimportant. The carbon in the methane came from the food the animals ate. The carbon from the food came from CO2 in the atmosphere. The methane eventually breaks down within a decade. Hence, the carbon is simply being recycled. It is only the release of fossilized methane that might be a problem.