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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methane is produced in huge quantities within the earth due to plate tectonics. fossilized CO2 (limestone) along with water is reduced by iron and heat within the earth to form methane.
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in this regard, methane is a fossil fuel. however, it is not made from dinosaurs. it is made from limestone, which is the fossilized remains of CO2.
For Warmenistas, the question isn’t relevance but rather usefulness in selling their product, Alarmism. Methane is useful as a red herring distraction, taking some of the heat off CO2, and also in amping up the “guilt” factor, since we humans (supposedly) produce via our “human activities” some 60% of the methane. Now, they’ve even ramped up methane’s GWP (global warming potential), measured over a 100-year timespan, formerly estimated to be 21 to now thought to be 34 times what CO2’s is. So, it’s even worse than we thought, of course. It’s amazing, with all that “extra” warming power, though, how little effect it actually has on climate. I guess without man’s influence, we should now be back to ice age conditions, by their calculations.
MikeB claims that an opaqueness exists at certain wavebands due to the presence of CO2.
Apparently adding CO2 will make no difference at current levels, although simple logic would indicate that reducing it will make a difference, at some point. And what about absorption levels?
More clarity needed for this post, along with some references.
I’ve decided that global wamists 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.
DirkH says:
April 11, 2014 at 6:15 am
The right word is “cannibalization”. When two gases compete for the same photon only one of them can get it. It doesn’t “negate” the effect, it neutralizes it, obviously.
ne·gate
to nullify or cause to be ineffective: Progress on the study has been negated by the lack of funds.
But the real question is whether any amount of water in the atmosphere will negate the effect of any amount of methane in the atmosphere. Some have suggested that this is happening at an altitude which contains little H2O. Is the absorption at a lower level not important?
What’s next, if history is any guide, is some revolutionary new theory that shows methane
to be absolutely essential for life on Earth, and needs to be maintained at a certain critical minimum level. Oh, wait, I’m confusing methane with that other GHG, whose name I can never remember. Oh yes – it’s carbon dioxide. Does anyone worry that the warmist crowd might actually succeed in drastically reducing CO2 levels?
swood100 says:
April 11, 2014 at 6:58 am
“Some have suggested that this is happening at an altitude which contains little H2O. Is the absorption at a lower level not important?”
The level is not important, the mean free path length for a given frequency is. Whether a photon is intercepted 1 time or a thousand times on its absorption-reemission-voyage through the atmosphere does not matter. One absorption-re-emission changes its direction arbitrarily and suffices. If the mean free path length is on the order of the thickness of the atmosphere it escapes directly to space; otherwise it becomes part of the “greenhouse effect”.
(Ignoring the tiny blackbodies in the sky here)
Col Mosby says:
April 11, 2014 at 7:02 am
“What’s next, if history is any guide, is some revolutionary new theory that shows methane
to be absolutely essential for life on Earth, and needs to be maintained at a certain critical minimum level. ”
http://en.wikipedia.org/wiki/Methanotroph
BTW, Methane is not oxidized, so it naturally tends to undergo chemical reactions in an oxygen atmosphere. (UV, radicals, VOCs, Ozone) Will never become significant.
The last I heard was that methane is broken down in the atmosphere within 10 years
I also thought that methane is broken down in the upper atmosphere due to ultraviolet light? This then means that methane is not durable, unlike water vapor and CO2, which are both very durable molecules in the atmosphere. If we were to cease methane emissions, methane would eventually drop to zero. Whereas co2 and h20 will remain forever (ignoring biological processes)
This article is inaccurate. The net temperature rise is about the delay in release of energy from the lower atmosphere to space. Energy absorbed by water is released by water, often at the same wavelengths it absorbs. More CH4, increases the delay in that energy reaching space, trapping heat and increasing the warming effect.
None of my argument means that CH4 IS relevant, just that the article doesn’t support the conclusion. It is simply not correct to say water has already absorbed that wavelength so CH4 is irrelevant.
Remember people can’t be far behind!
blackadderthe4th says:
April 11, 2014 at 5:11 am
“…70 years ago the view that co2 could affect the global climate was held by only a tiny minority of climate scientists, many assumed there would be a self regulating mechanism that would put things back into balance.”
There IS such a mechanism. Rainwater removes CO2 from the atmosphere and puts it in the crust, and volcanic action releases CO2 from the crust and puts it back in the atmosphere. When there’s more CO2 in the atmosphere, it’s a little warmer, we get more evaporation of water and more rain to remove CO2 from the atmosphere at a greater rate.
I haven’t seen it brought up here, but it seems to me that an intercepted IR photon at a particular wavelength, when absorbed and re-emitted, is re-emitted at a lower energy state, ie. a different wavelength than when first intercepted. The magically infinite emission, absorption and re-emission does not exist as the photon loses energy each interaction.
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.
I should have added what I see others have; it’s a catch and release process , and the more molecules fishing in the pond of photons, the more catches (and cool down delays) there will be.
But I still think methane is overblown.
The oxygen that methane combines with to form CO2 and H2O is ozone. The concern with methane is not it’s GHG properties but the ozone depletion aspect. This goes back to the scare before Global Warming of “We’re all going to die of skin cancer”.
what is more worrisome is that guys lay awake at night worrying about this stuff. They should worry more about the regulations and taxes that result in this needless worrying.
You know when the government says we created so many jobs this year? They create jobs for these over educated buffoons and all the administration that goes with it. Jobs shouldn’t need to be created, they should be the result of demand and growth. And government gets bigger and taxes increase.
fredburple,
Several of your comments above suggest that you are a bit confused about how an infrared absorbing gas reduces the rate of heat loss to space. Your argument that GHG’s always cool the atmosphere is simply mistaken. GHG’s both absorb and emit photons at specific wavelengths. When a photon is absorbed, the GHG molecule is ‘promoted’ to an activated state, from which it can re-emit a photon of the same wavelength (in any random direction). But collisions with other molecules are very frequent, so following absorption of a photon the activated molecule is almost always immediately ‘quenched’ to the ‘ground state’ via collisions, before re-emission takes place, and the energy of the absorbed photon is therefore almost always converted to heat (‘thermalized’). The entire population of GHG molecules can, due to random collisions, be at any time briefly promoted to the ‘activated state’, and while in that state, may emit a photon in a random direction. There is always a very small population of GHG molecules in the activated state which are capable of radiating photons in random directions. At higher temperature, the population of molecules in an activated state (capable of emission) increases due to more vigorous collisions; at lower temperature the population decreases. So warming will always lead to greater emission by GHG’s in all directions, and cooling will always lead to less emission.
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…. near maximum radiative cooling as described by the Stefan-Boltzman law. With GHG’s in the atmosphere, there is always random emission of photons in all directions from GHG molecules, and those re-emitted in the direction of Earth represent ‘back-radiation’ at the Earth’s surface. Which means less net loss of heat via radiation to space.
There are lots of very good reasons to doubt catastrophic projections of warming (the IPCC’s projections are consistently much too high, probably because their estimates of ‘feed-backs’ from clouds are much too high), but the basic physics of GHG warming in the atmosphere is well known and not seriously disputed.
It’s my understanding that the re-emission of IR by a molecule occurs in the same band as the absorption. Doesn’t that tell us that all the energy of the opaque bands is being converted to kinetic energy via conduction, to mostly O2 and N2 molecules?
The energy is free to move in the other direction, too, of course. O2 and N2 molecules collide with GHG molecules and energize them. But because they have much less mass, less energy is imparted to the larger molecule via impact. And exciting a larger molecule to the point where it will emit IR takes multiple such collisions that apparently do not happen with enough regularity to cause emission to space in that band. But what about the bands which are only partially opaque? Don’t they tell us that all their energy comes completely from re-emissions, and those re-emissions must come from both conduction and absorption? Is it not apparent that all the energy absorbed in the opaque bands must eventually leave to space in partially opaque bands?
So-called greenhouse gases are effective heaters of our atmosphere, not only because of the width of their absorption bands, but because they provide a 3-dimensional vector for the inefficient process of conduction. This vector is more or less unaffected by changes in the amount of GHGs, and argues for very low climate response due to any increases or decreases.
Are there experiments that can test whether or not a much warmer atmosphere will increase the emissions in partially opaque bands, or even shrink (at the margins) the opaque bands, due to increased conduction?
george e. smith : April 11, 2014 at 7:48 am , sums it up nicely.
In fact all this transmission stuff is based MODTRAN / HITRAN which is military atmospheric transmission software. I’d guess that this is probably the most though and accurate part of the whole modelling storey.
JudyW says:
April 11, 2014 at 8:14 am
The oxygen that methane combines with to form CO2 and H2O is ozone. The concern with methane is not it’s GHG properties but the ozone depletion aspect. This goes back to the scare before Global Warming of “We’re all going to die of skin cancer”.
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While that basis of argument has been deployed, the current tactic used by the scaremongers is that methane will cause warming, lots of warming. The actual reason that the methane scare is being promoted from on high is to raise the price of protein available to a hungry humanity.
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?
Amazing, as I was composing my last comment, Steve Fitzpatrick wrote his on exactly the same basic issues. Thanks, Steve. I had one more thought / question. When we measure the fully transparent IR bands from space, is there any systematic measurement of the emissions in those bands right next to the ground? Do we know that those bands energies are not actually enhanced by conduction in the atmosphere?
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.
http://www.earthonlinemedia.com/ebooks/tpe_3e/energy/solar_and_earth_radiation.gif