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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Something that may help readers assess the criticisms that this post will inevitably attract is to provide tables of absorption (in percent/meter per mole/liter?) as a function of wavelength for the three compounds of interest.
My understanding is that its residence in the atmosphere is relatively short and the methane clathrates found in Arctic that alarmists claim will melt are more stable and less likely to melt/be affected by warming than previously believed. There were a few papers that came out in the last year or so on this topic. I hadn’t known about the comparative absorption spectra so unless there is a mechanism that will increase methane by 10,000% no reason to worry about CH4
By the way, that chart might be misleading. The water spectrum certainly does not end where that line ends in the chart 🙂
[img]http://upload.wikimedia.org/wikipedia/commons/9/97/Water_infrared_absorption_coefficient_large.gif[/img]
And the other side of the coin is that all green house gases also emits energy in the same wavelength bands.
Since when is sanity a pre-requisite for government regulations? lol
kramer says:
April 11, 2014 at 4:45 am
Wouldn’t methane be a factor in dry climates?
Furthermore, does any amount of water in the atmosphere negate the effect of any amount of methane in the atmosphere?
Chuck L says:
April 11, 2014 at 5:32 am
“My understanding is that its residence in the atmosphere is relatively short and the methane clathrates found in Arctic..”
They are found in more than the arctic – along continental margins all over the world – off the coast of southern British Columbia and the Bermuda triangle are a couple of examples.
http://geology.com/articles/methane-hydrates/
Scroll down for map.
I agree that methane is a small player, but not for the reasons you stated. The first thing you need to understand is that gases that nearly saturate absorption in a short distance result in the last absorption and emission (to space) to occur at very high altitude. It is the effective average altitude of emission to space that sets the energy balance and thus temperature at that (average) effective altitude, and also via the lapse rate, govern the greenhouse effect. However, as the atmosphere cools with increasing altitude (from the lapse rate), almost all of the water vapor condenses out, first as clouds at moderate altitudes, and most of the remainder as ice crystals at higher altitudes. The CO2 and methane are not condensed out at the temperatures encountered. Thus the relative fraction of water vapor to CO2 and methane decreases sharply as you approach the TOA, where the action is very important. This is the reason that CO2 and methane have a very large effect at small concentrations. However, due to their small concentrations, the effect is not totally dominate.
OT:
I’m flying over the upper-Midwest right now, and it looks like it could be mid-January out there!
Perhaps I’m blind or simply not seeing things correctly, but at 7.5 microns water only seems to absorb about 40% leaving plenty of room for CH4 to absorb at that wavelength.
Once again, actual science defeats alarmist propaganda !!
Tom,
I think it not very useful to look at the entire atmosphere when you consider the relative importance of different GHG’s. It it true that water vapor dominates near the surface, especially in temperate and tropical regions. But far more important when considering heat loss to space is the composition of the atmosphere from the upper troposphere to space, where relative water vapor concentration is many times lower than near the surface. It is completely true that adding a bit of methane will not significantly change atmospheric absorption at low altitudes because of the dominance of water vapor (except in the polar winter), but methane is well mixed in the atmosphere, and its absorption does make a difference in heat loss from the upper troposphere to space. Increasing atmospheric methane for sure reduces net heat loss to space. The concentration of methane in the atmosphere has not been rising very much (IPCC projections for methane were completely wrong!), and methane’s contribution to total GHG warming is not increasing very much….. so it really does not appear to be much of a cause for worry, but not because it does not act as a GHG; it does.
I think of the history of the world and vast herds of grass eating animals and I can’t help but snicker at people scared by cow farts.
Leonard Weinstein says:
April 11, 2014 at 5:58 am
“However, as the atmosphere cools with increasing altitude (from the lapse rate), almost all of the water vapor condenses out, first as clouds at moderate altitudes, and most of the remainder as ice crystals at higher altitudes. The CO2 and methane are not condensed out at the temperatures encountered. ”
Well, and what does water vapor do when it condenses? It forms tiny little blackbodies in the sky…
…which absorb what they don’t reflect with a Planck spectrum at their frigid temperatures… Icehouse effect not greenhouse effect… warm yourself up with the cosy IR blackbody radiation from a bunch of mineral water bottles right out of your fridge. Much warmer than absolute zero!
The atmosphere is therefore opaque to radiation at 15 microns, due to the small amount of CO2 present. Water vapour (if present) also absorbs at 15 microns but not at the 100% level of CO2.
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if CO2 is already absorbing at the 100% level, adding more Co2 cannot have any effect whatsoever. Or are we to believe that doubling CO2 will result in 200% absorption?
swood100 says:
April 11, 2014 at 5:54 am
“Furthermore, does any amount of water in the atmosphere negate the effect of any amount of methane in the atmosphere?”
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.
MikeB What is the provenance of this graph?
Similarly for methane. In addition, the only reason that methane is considered to be more potent than CO2 is because there is so little of it. Its absorption bands are not saturated and its affects are therefore linear and not logarithmic.
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And what is the provenance of your information?
What’s the free path of a photon at the 3.5 and 7.3 micron bands? And what’s the probability of it meeting an H20 before a CH4 molecule?
ferd berple says:
April 11, 2014 at 6:13 am
“if CO2 is already absorbing at the 100% level, adding more Co2 cannot have any effect whatsoever. Or are we to believe that doubling CO2 will result in 200% absorption?”
Enhanced GHE comes about through pressure broadening; only the frequencies at the edges of the band which are NOT saturated can become more saturated.
“Notice that sometimes CO2 bands overlap with H2O bands, and with vastly more H2O present, CO2 doesn’t matter in those bands.”
Even though the absorbtion bands for CO2 and H2O overlaps at som frequencies there must be some regional effects from higher CO2. For example dry air over large deserts or central Antarctica. Is’nt the ammount of water vapour there at times so low that the CO2 actually matters in bands that overlap?
However, as the atmosphere cools with increasing altitude (from the lapse rate)
———————–
GHG cools the atmosphere – otherwise it would be isothermal due to conduction. The lapse rate (DALR) is due to gravity and convection, limiting the cooling. Condensing water during convection reduces the actual lapse rate to less than the DALR.
EArlier was an article that calculated the percentage of methane released from the permafrost
that actually made it to the atmosphere. It was very small, almost trivial.
Increasing atmospheric methane for sure reduces net heat loss to space.
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don’t be so sure. methane absorbs energy from the surrounding air and radiates this to space. otherwise the atmosphere would be warmer than it is (since O2 and N2 do not radiate).
GHG cannot reduce the net heat loss to space because energy in and energy out must be equal when the earth is in equilibrium.
You will hear more and more about the evils of methane…..I promise. It is going to be the new Devil against which all good citizens must fight. There are a couple reasons:
1. The anti- CO2 campaign has pretty well run its course. People are beginning to get wise to the scam. Fighting CO2 came about because the anti-pollution campaign was pretty well at an end. However, the organizations who had fought that battle were not ready to close shop and go home; they needed a new enemy to battle. No organization ever willingly dies. There are jobs at stake. (This shifting of a crusade to a new cause to keep the army alive is common -I give you the continuing existence “The March of Dimes” an organization instituted to fight Polio…which was defeated more than 50 years ago.) The idea of attacking CO2 was ingenious – a pollutant to fight which can never be defeated, unlike noxious chemicals. Every breath you take feeds the demon!
The organization need never worry about going under again they thought….except the damned climate wouldn’t cooperate. So… A new enemy.
2. The other and more important reason for the growing attacks on methane is because of the discovery of practical means (only in testing so far) of extracting Methane Hydrate. This worries the Greens because “…methane hydrate deposits are believed to be a larger hydrocarbon resource than all of the world’s oil, natural gas and coal resources combined.” Further, the stuff is -everywhere-.
http://geology.com/articles/methane-hydrates/
Unless a good reason to stop its development, in 50 years cheap hydro-carbon energy is going to be bountiful…….
Why the new urgency in the campaign?
Here’s a link that says that Japan hopes to commercialize methane hydrate “within 10 years”.
http://www.platts.com/latest-news/natural-gas/tokyo/platts-feature-japan-makes-big-strides-in-methane-27779018
the percentage of methane released from the permafrost that actually made it to the atmosphere. It was very small, almost trivial.
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methane is an energy source. Life rapidly converts it to CO2 and H2O when oxygen is present. which is why there is very little methane in the atmosphere. long ago bacteria learned how to turn methane into food.
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. this bubbles up through the oceans to the surface. occasionally it is trapped beneath structures in the earth, where it can be harvested economically to run human industry.
” Kill off all the big time f*rters”
Well, I think what the Earth Firster-types have in mind is killing off the DOMESTIC f*rters.
That they will be replaced by NATURAL f*rters such as similar-sized herds of bison, for instance, is one of those “logic gaps” that doesn’t fit in with the emotion of hugging a tree.