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 didn’t say this, the vegan diet tends to be very low in B.12. And they don’t produce any more CO2 than people on a balanced diet or vegetarians.
Many thanks to : Nullius in Verba (4:16 pm), Steve Fitzpatrick (4:24 pm), Ken Gregory (4:28 pm) and Joe Born (4:31 pm). A very clear and understandable summary of the GHG effect.
MikeB,
” The atmosphere is therefore opaque to radiation at 15 microns”
This is not true. Look at any wavelength by intensity graph and you will see light gets through:
http://geosciencebigpicture.com/2014/03/31/spectra-goddess-of-light/saturated/
You are correct in your assertion than most 15 micron outgoing is absorbed close to the surface, but that is precisely where water vapor is concentrated. Being better mixed CO2 would predominate at higher altitudes but it is saturated by Beer Lambert per Gavin Schmidt’s own graph:
http://wp.me/a1uHC3-hc
Everyone seems to forget that methane has a short half life in the atmosphere and degrades to CO2 and water, the real man’s greenhouse gas.
Must be a lot of methane in Africa. Elephants watch out.
Another poster spoke to this, but the point needs a little expansion. As wavelength is directly related to energy, I prefer to use the term energy as it is less esoteric to most folks than the word wavelength.
When a photon is absorbed by an atom or molecule (a/m), the a/m gains energy and is in an excited state. The a/m does not like to be in this state and it will eventually try to return to its natural ground state. Time frames vary widely for when the energy will be lost, and may depend on other environmental conditions. There are a number of ways the extra energy may be lost. Remembering that energy is always conserved, here are some of the ways the a/m might lose the extra energy to get back to its ground state:
1) Emission of a photon. The emitted photon(s) may be of the same energy as the received photon, or may be of any other lesser energy value. The resulting photons are free to interact with another a/m.
2) Direct transfer of the energy to a different a/m. All or some of the energy may be transferred. The new a/m now has excess energy it needs to deal with. Sidebar: A radiation counting technique, liquid scintillation counting, takes advantage of direct energy transfer.
3) Re-arrangement of a molecule. The extra energy can be the catalyst for molecular transformation, e.g. breakdown, or to make the molecule susceptible to a chemical reaction with another molecule.
The gist of this article is that the factors influencing energy absorption and subsequent deposition of that energy are:
1) the atom or molecule’s energy absorption bands. Every atom or molecule has specific absorption bands. The molecule is opaque to all other energy bands. (I’m ignoring other methods of radiation interaction, such as particulate radiation, that may deposit energy within an a/m).
2) The quantity of a/m’s available. The more atoms, the greater the chance for photon interaction. there are a lot more H2O molecules in the atmosphere than CO2 molecules.
3) The quantity photons available in the specific absorption range of the a/m. Because there are fewer CO2 molecules, in a large field of specific range photons, each gains extra energy which will want to go somewhere, e.g. H2O molecules, any other molecule such as your skin (what we perceive as warmth is energy transfer from excited molecules), and even emission into space especially if the excess energy is emitted at a wavelength that is not in CO2’s energy range or the range of other GHG’s.
Honestly, except for climate folks like Leif, Anthony and many other good climate scientists, I think the interaction of radiation with matter is poorly understood by some really poor climate scientists, of whom we are acutely aware. I see many commenters here amazed that they have not heard about the crux of this article’s focus. I am not amazed at this reaction. About the only place you will find the study of radiation interaction with matter is within radiation physics, which few get. The interaction of radiation with matter is critical to an understanding of GHG’s role in our climate.
For any physicists who may want to get into electron energy levels, photon wave and particle functions, and all the other cool micro world theories, let’s not go there such that we may give the many here a basic understanding. Once they grasp the basics, they can go to folks like Lubl and get a deeper understanding if they like. I am very pleased with Sheahen’s effort to keep it simple in this article. Well done.
The forcing from methane is not logarithmic like CO2. I did say earlier that its effect was linear but that is wrong too. Here is what the IPCC has to say…
So, the IPCC says the forcing effect from methane is proportional to the square root of its concentration. It is not logarithmic and so ‘the effect per doubling’ is not a valid concept since this will depend on the starting concentration.
I tried to check this on MODTRAN but the results were baffling. No time to sort it out.
Nullius in Verba April 11, 2014 at 4:16 pm —- Very Good.
bushbunny says:
April 11, 2014 at 10:37 pm
I didn’t say this, the vegan diet tends to be very low in B.12. And they don’t produce any more CO2 than people on a balanced diet or vegetarians.
####
I said this mostly to be funny, but the truth is that vegetarians do indeed produce significantly more gas, which contrary to popular belief, is not mostly CH4, but CO2. And for the humor impaired, this is funny because the anti-meat activists are promoting the cattle – greenhouse gas connection as a talking point to attack the cattle industry.
gymnosperm says:
April 11, 2014 at 11:15 pm
It is not getting through! What you are seeing in the graph you link to is radiation being re-emitted from the top of the CO2 fogbank (see Nullius in Verba April 11, 2014 at 4:16 pm ).
The atmosphere is absolutely opaque at 15 microns because of CO2.
Richard M at April 11, 2014 at 11:40 pointed out that cow farts just get recyled back into co2 and then back into the grass they eat, so they add nothing extra to the atmosphere. Good point. As for permafrost melting, the Russians who have most of it say that the permafrost is NOT melting.
A sigh is but a spout of wind coming Or the heart.
But should it take a downward trend, tis oft times called a Fart.
Now to fart tis but a pleasure, it gives the bowels ease.
It scents and warm the bed clothes and suffocates the fleas!
@ur momisugly Mike B – I’m having troubles finding your Nullius reference; could you post the URL for it?
Gymnosperm is referring to a broad spectrum of energies (wavelengths) such as visible light and is quite correct. For example visible light, generally defined in the 400-700nm range and which is not in the CO2 wavelength/energy absorption band, does get through CO2 – as does all the other wavelengths (including the other IR wavelengths) that CO2 does not absorb.
CO2 absorbs energy as photons which are in the IR range. IR has a longer wavelength, and therefore has less energy associated with the photon, than a visible light photon. It has a greater energy than a radio photon. (You may know this, but this is for others)
As a single CO2 molecule absorbs photon energy, it becomes “saturated”, meaning it will not absorb any more energy without releasing energy. It can and may hold the initially gained energy for awhile. However, the molecule does not want to stay in an excited stated of extra energy forever and depending on environmental conditions will eventually shed the energy by one of the routes I noted in my previous post. If the saturated molecule gets hit by another “correct” wavelength/energy, the highest probability is that the extra energy will be immediately emitted – in any direction – as a photon with the same energy in which it came.
I don’t know what you mean by a “fogbank” of CO2″. I’m assuming you mean a large area with lots of CO2 molecules in it, and only CO2 molecules, which are absorbing the right energy photons. Let’s say a box full. Let’s say IR light is coming from only one direction like (sort of, kind of) sun light does. As each CO2 molecule gets saturated, any further hits are transferred – in any direction. Other CO2 molecules might absorb this energy but then also become saturated. If all the molecules become saturated, any excess input energy will just depart from the box. There will be other energy losses such as other molecules getting excited via direct energy transfer or the molecules happen to absorb degraded (lesser energy) photons from the saturated CO2 molecules, which is why the box may feel hot as those molecules give up their energy. Even long before saturation of all the CO2 molecules, you will start seeing IR (in CO2’s absorptive range) above the fog, below the fog, and to the sides of the fog if an excess photon deflects and goes outside the CO2 fog area.
However, in the sense that all photons in CO2’s absorption range are highly likely to get absorbed, this is true, but not just because of CO2. Water will absorb in the same range. There are a LOT more molecules of water in the air and with a broader IR absorption range than CO2,
The concepts of radiative and energy transfer are fairly easy to grasp. However, it is what happens to the energy that starts boggling the mind. With even just one radiative transfer we can only imperfectly predict statistically as to where that energy will go, We cannot predict its actual endpoint with certainty. If we cannot do this with one interaction, how does anyone think we can do it in such a gigantic, complex system as our climate? CAGW from a physics point of view has to be the worst scientific hubris that mankind has seen.
Adding to
Joe Born says:
April 11, 2014 at 4:31 pm
Kevin Roche: “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.”
—————————-
off tangent onto co2,
I need to understand as well- where is there a point at which any warming higher up has an effect at or below lower clouds, surely there is such a river off energy flowing upwards that any co2 absorption ( at 15 microns) when re-emitted ( i assume that absorption and re -emitting takes microseconds) would simply join the upward flow of energy already flowing around co2. Even if there was a “supposed downwards flow of radiation” surely the lower co2 would just absorb that, why should it only absorb upwards flowing radiation at 15 microns, of course you would get the weird situation of perpetual motion as the energy flowed up and down between the co2.
Harvard –
“A low cloud, however, has a temperature close to that of the surface due to transport of heat by convection. As a result it radiates almost the same energy as the surface did before the cloud formed, and there is little greenhouse warming”
So-
Surely the limited co2 within this area would act in the same way.
I see the only way for co2 to cause any warming is for it to actually form a layer like cloud in the atmosphere, but even then, if at the level of lower cloud you would get the same result as lower cloud above.
If I look at the desert I see extreme heat cause by lack of moisture and at night extreme cold caused by lack of moisture.
It feels as though there are two processes at work here proved by the desert , a cooling in the day caused by higher moisture content, after all we do not need greenhouse gases to cause extreme heat in the daytime – the moon, and at night a slowing down of heat loss until Sun up.
So no extra warming up to the level of lower clouds for starters.
DirkH says:
April 11, 2014 at 5:17 pm
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.”
A distinction without a difference.
Which only goes to prove that you don’t have a clue about the subject!
aGrimm says:
April 12, 2014 at 3:23 am
The reference is to a previous comment in this thread, which you obviously haven’t followed. I gave the time so you should find it.
Gyosperm is not referring to a broad spectrum of wavelengths. Again, if you had followed the comments you would know this. We are referring specifically to radiation at 15 microns. Do have reading difficulties?
Of course visible light gets through. That’s how we see the Moon, the stars and the Sun.
Forgive me, it gets frustrating when I say something absolutely right and you chip in with a load of rubbish without reading the comment thread first.
“@ur momisugly Mike B – I’m having troubles finding your Nullius reference; could you post the URL for it?”
He means this one:
http://wattsupwiththat.com/2014/04/11/methane-the-irrelevant-greenhouse-gas/#comment-1611098
The ‘fogbank’ concept is just a simplified image to aid intuition. If you could see in infrared, the atmosphere would appear semi-transparent, like fog, so you could only see part way through it. The idea is that the visible ‘surface’ you see from above is what emits radiation to outer space, and it is this fuzzy ‘surface’ that settles at the -18 C average temperature needed for the Earth to emit as much energy as it absorbs.
Perhaps it would be clearer using Venus as an example. Even in the visible spectrum, you cannot see the solid surface of Venus. It is shrouded in high-altitude clouds that look like a fuzzy opaque fog. The energy absorbed by Venus from the sun is all emitted by these cloud tops, and so it is the cloud tops that have to achieve energy balance with the outside universe. The temperatures there are actually pretty similar to Earth. But because the visible cloud surface is around 50 km above the solid surface, and the adiabatic lapse rate on Venus is about 8 C/km, there is a 400 C temperature difference maintained between the solid surface and the average altitude of emission to space.
Carl Sagan wrote a short paper back in 1967 estimating the surface temperature of Venus by this method, before they were able to send probes there to measure it. ( http://adsabs.harvard.edu/full/1967ApJ…149..731S ) In a convective atmosphere, the surface temperature is the temperature needed to achieve energy balance plus the adiabatic lapse rate times the altitude of emission to space. Everything else going on below that point cancels out.
Or if that still doesn’t convince, try working out the magnitude of the greenhouse effect in liquid water. Since water absorbs all thermal IR within about 20 microns, it acts as a super-intense ‘greenhouse fluid’. Sunlight shines through and is absorbed deep down, but then on re-radiating it is immediately absorbed and re-radiated and absorbed again. It’s fairly easy to calculate that if you only consider energy transport by radiation the temperature ought to be hundreds of degrees less than a metre below the surface.
The reason the oceans don’t boil is that the difference between the level at which sunlight is absorbed and the emitting surface is only tens of metres, and the adiabatic lapse rate in water is only about 0.1 C/km, because water is incompressible. With a negligible near-zero adiabatic lapse rate, you get essentially no greenhouse warming, despite water being tens of thousands of times more intense a ‘greenhouse fluid’ than air. If the lapse rate was negative, then adding more greenhouse gases would result in cooling! (As happens in the stratosphere.) Try explaining that one using the other explanation!
Alex says:
April 11, 2014 at 8:34 pm
Looking at transmission graphs can very deceptive. They say nothing about the energy of those absorptions/emissions. To get a realistic view they should be incorporated in an energy/wavelength graph -blackbody curve. Then you could see quite clearly how much energy is transmitted. Area under a curve. Transmission graphs don’t tell you if you are transmitting a creek or a mighty river of energy.
REPLY: go ahead and work that up then, and we’ll add it as an addendum – Anthony
Which is why I present the graphs as a plot of wavenumber which is proportional to the energy rather than wavelength which is inversely proportional to energy. See George’s post above too.
Harvard –
“A low cloud, however, has a temperature close to that of the surface due to transport of heat by convection. As a result it radiates almost the same energy as the surface did before the cloud formed, and there is little greenhouse warming”
We have established that with or without the effect of the lower cloud there is no extra warming, this is the sun’s maximum output and heat within the range of the lower cloud so why should any other gases in the atmosphere have any other effect within this range ( up to lower cloud) .
If there was an addition to the temp within this range from some kind of extra gas i want to know as i will fill my greenhouse with it and generate more energy from my thermal air pump.
“If there was an addition to the temp within this range from some kind of extra gas i want to know as i will fill my greenhouse with it and generate more energy from my thermal air pump.”
You don’t want to use a gas, you want to use a liquid. In fact, what you want is a solar pond.
As I noted above, in a convective fluid, the adiabatic lapse rate dominates. But if you can suppress convection, then the ‘conventional’ radiative explanation tends to carry more weight. One way to do this is to fill a pond with water, but very carefully layer it so that at the bottom it is salt water, and at the top it is fresh. The saltwater is denser and the density gradient suppresses convection. This means the heat cannot escape from the bottom convectively, and because radiation is blocked by water being extremely opaque to IR, only conduction is left, which is very slow in still water. Solar ponds can routinely achieve temperatures of 90 C at the bottom of a couple of metres of water, (before convection starts up again).
They work best in sunny countries, where they’re sometimes used as a variety of solar energy collector. As the efficiency of an energy source depends on the temperature difference, they’re pretty low grade. But it would indeed help with your thermal pump.
Nullius in Verba says:
April 12, 2014 at 5:45 am
if i was building a house from scratch and had the grounds to do it i would without doubt go this route,
Castle Howard in Yorkshire, UK , have had great results from this method.
MikeB says:
April 11, 2014 at 12:46 pm
“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.”
Still no tropospheric hotspot. No heat trapping gases. Why does your stuff not work?
Steve Fitzpatrick says, April 11, 2014 at 8:24 am:
“The reason GHG’s reduce loss of heat to space is that absent GHG’s the surface would emit infrared photons directly to space (since the non-GHG’s in the atmosphere are essentially transparent in the infrared wavelengths), while space, with an effective temperature just above absolute zero, would emit essentially no photons back toward Earth.”
Steve, you forget about one thing. The troposphere is primarily warmed through convective processes (conduction/convection/evaporation), not solely by radiation as you seem to think. Absent ‘GHGs’, the troposphere would still be warmed convectively. It could however NOT adequately cool to space. It would thus ‘trap’ the energy (heat) transferred to it from the surface.
The so-called ‘GHGs’ do not enable the atmosphere to warm. It starts warming convectively as soon as it’s placed on top of a solar-heated surface. No, they enable it to cool, by ultimately radiating the acquired surface energy effectively to space.
The post has it nearly correct for CH4 methane -its absorptivity in the atmosphere is so small to be unmeasurable see here http://cementafriend.wordpress.com/2011/10/14/methane-good-or-bad/. However, the article is wrong about CO2. The absorption of radiation by water vapor at all temperatures is much -much greater (at least 10 times) that of CO2 The author should look at Chapter 5 of Perry’s Chemical Engineering Handbook which has an equation to determine the absorptivity of mixed combustion gases. The equation was formulated from a vast amount of experimental data carried out in furnaces by Prof Hoyt Hottel of MIT. CO2 absorbs at wavelengths at 4.3 microns and at over a very small interval around 14.8micron. Water vapor has some absorption at all wavelengths but particularly from 5 to 8 micron and then again above 13 micron to well into the microwave range
Engineers use real data, physicist seem to base their data on what suits their thought bubbles.
Ron C. says, April 11, 2014 at 11:36 am:
“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.”
Any sane person would understand, if they only took the time to think it through, that the delay in energy OUT from earth’s surface relative to energy IN (from the sun) lies in the convective process, not in the radiative. The first one needs to physically move massive air against gravity. The second one needs no such thing.
“The troposphere is primarily warmed through convective processes (conduction/convection/evaporation), not solely by radiation as you seem to think.”
Convection can only redistribute energy. It’s not a source.
Sunlight warms the surface. The opacity of the atmosphere stops it radiating directly to space. Convection short-circuits the effect of the opacity, cancelling out its effect, but only up to the point where the adiabatic lapse rate is achieved. The ALR controls the surface temperature, but it isn’t the source of the energy.
“Absent ‘GHGs’, the troposphere would still be warmed convectively.”
Absent GHGs (but still with clouds), the effective altitude of emission to space would be zero, the surface would be at an average temperature of -18 C in order to radiate as much as was absorbed, and the troposphere would be even colder. The ALR would still cool it with altitude at the same rate, but starting from a lower base point.
Nullius in Verba says, April 12, 2014 at 6:31 am:
Nullius, you really wanna reopen this discussion …? With nonsensical statements like this: “The ALR controls the surface temperature (…)” you’re simply not one that can be taken seriously. It proves how you absolutely won’t understand the most basic rules of how the earth system works, how any surface/atmosphere system works. You are no skeptic, Nullius. You adhere to dogma. Try to think for yourself rather than just credulously echo all that the Masters of the Climate Religion enunciate to the world.
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?
Think, Nullius. Think.