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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Kevin Roche,
“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?”
Well, because that means the surface has to become warmer as well. Most (~70%) visible and near infrared solar energy (that is, less than a couple of microns wavelength) is absorbed by the surface, either ocean or land. That visible and near infrared energy is converted to heat and that heat must be lost to space at the same rate as it is gained (on average) or the surface temperature rises. There are different ways heat leaves the surface (infrared radiation, convection, latent heat). Each of these heat losses depends mainly on a difference in temperature between the surface and some higher altitude in the atmosphere… only a modest amount of heat is lost to space directly from radiation from the surface (see the “atmospheric window” in Wikipedia). So when the temperature aloft is warmer, the driving force to transport heat from the surface is less….. unless the surface warms as well, restoring the required level of driving force to maintain energy balance. That is the primary way GHG forcing warms the surface.
None of this is very controversial. Direct GHG forcing would lead to ~1.1C to ~1.2C of warming for a doubling of CO2, which is clearly not enough to worry about. The real disagreement is in “amplification” of this warming. There are two types of amplification, one more doubtful than the other. The first is that warming at the surface (and throughout the troposphere) will increase the concentration of water vapor. Higher water vapor acts as a greenhouse gas, and so “amplifies” the direct warming by further restricting the loss of heat to space. That is, GHG’s cause a bit of warming, that warming causes a bit of increase in water vapor; the higher water vapor further restricts loss of heat to space, leading to more warming. The exact extent of water vapor amplification is somewhat uncertain, but a reasonable range is a 40% to 70% increase over the ‘bare’ GHG warming, or maybe 1.5C per doubling of CO2 to 2C per doubling of CO2. The upper end of this range is of modest long term concern, but would never be an “existential threat” under any plausible circumstances.
The second type of amplification is what I consider ‘wildly speculative’, including net warming from clouds, change in snow/ice albedo, and others…. all of which are extremely uncertain. The best empirical data is that these other amplifications are minimal, or perhaps even negative. Yet most climate models are based on assumed (‘parameterized’) amplification from clouds and other factors that is very large…. bringing net warming to ~3.2C per doubling of CO2 on average for the models, and in some models, reaching more than 4.5C per doubling of CO2. Here is where climate models diverge sharply from reality… empirical data says they are simply wrong about the second type of amplification. Which is why most climate models are, IMO, little better than rubbish when it comes to making useful predictions of warming. The only rational way to deal with current model projections of warming is to ignore them.
Hope that helps.
Steve Case says:
“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?”
The climate sensitivity of CH4 is about (_0.11)C° per doubling in the atmosphere.
The climate sensitivity of CO2 you gave of 1.2 C per doubling is the no-feedback response. The IPCC models give an average feedback multiplier of +2.75. The CERES data suggests the feedback multiplier is 0.4.
The IPCC WII TS page 52 says CH4 increased from 722 ppb in 1750 to 1803 ppb in 2011. The report claims this results in a direct forcing of 0.48 W/m2. Therefore, a doubling of CH4 would cause a forcing of 0.36 W/m2, calculated by 0.48 x ln(2)/ln(1803/722). The Planck response is 3.2 C/W/m2. Therefore, the no-feedback climate sensitivity of CH4 is 0.11 C, (0.363 W/m2 /3.2W/m2/C). This assumes you wait a thousand years of oceans to reach equilibrium. The transient climate response based on AR5 Table 9.5 would be 56% of that, or just 0.062 C. However, at the current CH4 growth rate of 0.2% per year, you would have to wait 347 years to double methane.
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.”
Long-term, the radiation out of the earth equals the radiation in, so the radiation temperature is fixed, and the higher up the effective altitude of that outward radiation, the greater, for a given lapse rate, must be the surface temperature. Greater greenhouse-gas concentration means higher effective radiation altitude means a greater distance over which the lapse rate raises the surface temperature.
This is over-simplified, of course, since the optical distances are different at different wavelengths and different latitudes, but I believe that’s the general idea.
Steve Case,
“The climate sensitivity of CH4 is about (___)C° per doubling in the atmosphere”
My SWAG is about 0.45C per doubling of methane….. but anything close to doubling methane is extremely unlikely; more likely is that methane’s trend can be safely ignored.
scarletmacaw says: “Why 1750? What’s so special about that particular date?”
1750 is the date the IPCC has chosen for the start of the industrial revolution. Their table of radiative forcings Figure SPM.5 is relative to that date. The CDIAC table of CO2 emissions starts in 1751.
http://cdiac.ornl.gov/ftp/ndp030/global.1751_2010.ems
Funny that the methane producing cows could also be the solution…. If used properly…
Steve Fitzpatrick says:
April 11, 2014 at 2:36 pm
“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.”
You mean I’m incorrigible? Now that’s just TOO BAD.
WHY do you not bring ANY argument against what I said? Maybe because you have none? 🙂
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.
If I were wrong the warmists would have their tropospheric hotspot but they don’t.
“If I were wrong the warmists would have their tropospheric hotspot but they don’t.”
The tropical hotspot has nothing to do with radiative physics. The theory is that a warmer atmosphere will be moister, and that will reduce the moist adiabatic lapse rate. A shallower lapse rate means that altitudes below the average emission-to-space altitude warm less and altitudes above it warm more. So a moister atmosphere partially cancels the greenhouse warming near the surface (i.e. lapse rate feedback is negative), while reinforcing it at higher altitude.
The lack of a tropical hotspot may be due to one of two things – either the surface temperature measurements are biased upwards, so we’re looking for a bigger effect than is actually there, or the atmosphere is not getting moister at the rate the models expect. This of course would imply that the water vapour feedback, that multiplies a 1.2 C/2xCO2 sensitivity to a scary 3.5 C/2xCO2 sensitivity, is likely not correct.
But it’s got nothing to do with radiative physics.
Leonard Weinstein says:
April 11, 2014 at 5:58 am
+ 1
Steven Mosher on same issue
+1
I skimmed most of the rest of the comments. If water vapour was well mixed throughout the atmosphere, this article would be accurate. But it isn’t well mixed, it precipitates out at high altitude and high latitude while other gasses, CO2, CH4, O3 do not. Hence the the overlap in absorption spectra with H2O is immaterial for the majority of the atmospheric air column.
Earlier I asked:
The climate sensitivity of CH4 is about (___)C° per doubling in the atmosphere
Can anyone fill in the blank?
Ken Gregory said 0.11C°
stevefitzpatrick said 0.45C°
Some time ago I searched the internet with the question, “Why is CH3 reported to be 20 times more powerful as a greenhouse gas than CO2?” And the answer I came up with is, it’s because it’s at such a low volume in the air it doesn’t take much to double, quadruple etc. So whatever its sensitivity is, it’s easily multiplied into significant increases in temperature with relatively small absolute quantities of CH3.
So thank you Greg & Steve for your response.
leftturnandre says:
April 11, 2014 at 4:17 am
The mega methane mania finds its roots in the Greenland ice cores where it was observed that large fluctuations of methane coincided with fluctuation in stable hydrogen and Oxygen isotopes (erroneously considered a paleothermometer). Especially around the erroneously considered cold Younger Dryas. To the cherry pickers it looked like the methane spikes had caused sudden large temperature swings, like the ‘more-than-ten-degree-within-a-decade’stuff of Richard Alley and friends.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Mosher posted a video of Richard Alley the other day at the AGU in 2009 I think. I watched the whole thing as it got really interesting when Alley said climate changes in the past were tied to orbital changes of the earth and changes in the perihelion and axis tilt; and that CO2 followed warming. It appeared that his message was that “things are different this time” because MAN is introducing CO2 into the atmosphere – but then he talks about mass extinctions due to natural warming due to orbit variation AND huge vulcanism that caused the oceans to be seeded with nutrients. He talks of warming of the Atlantic ocean to 30 some degrees Celsius and then massive acidification due to biological masses. And then in the end he talks of the effect of AGW in a time frame of hundreds of years or perhaps a millennial scale.
So, I am supposed to worry about something that might happen a few hundred or a thousand years from now? Plus Alley himself pointed out the huge natural variations in the past. Is it not likely that those variations could cause temperatures to move one way or another in spite of human activities? And given those time scales, I would think humans will have moved on to new sources of energy besides fossil fuel.
So, being an old fossil myself, I am having trouble getting really excited about a little CO2. I do worry about long lasting chemical pollutants and energy for my grandchildren.
In the meantime, I just bought a new pair of powder skis on sale today so off to the mountains to try them out.
Have a great weekend and enjoy life.
A couple of years ago I tried to postulate that the logarithmic nature of radiative forcing due to greenhouse gasses was independent of the species of gasses involved. All I got was grief. No one could understand what in the world I was talking about. I even tried to explain it by describing a simple experiment and stating the likely outcome. And I got grief over that. Everyone just assumed I was just some redneck without any scientific understanding. That assumption appears to have blinded them from actually reading what I was trying to say. This incident really soured me on the WUWT community. So now I tend to keep my comments to anti-Marxist snark.
“All I got was grief. No one could understand what in the world I was talking about. I even tried to explain it by describing a simple experiment and stating the likely outcome. And I got grief over that.”
We’re sceptics. We’re sceptical of everybody and everything. And we all have our own competing theories, of which we are each utterly convinced, and not all of us can be right.
The idea is that science is like a gladiatorial arena. You put your pet theory into the ring, and everybody attacks it simultaneously from all sides. If your theory survives, it gains in credibility. The more vicious the attack it can survive, the stronger it is shown to be. This is a good way to think of it, it makes it easier to accept the criticism. But nobody likes to see their beloved pet theory being mauled.
Regarding that logarithmic relationship, what happens to it as the concentration approaches zero? What is the logarithm of zero? Can that be right?
So millions of beef cows are causing the climate to change…
Just over a century ago millions of buffalo were roaming the western plains of North America. Is it possible that their methane (flatulence) caused the current warming cycle?
Just asking…
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
Sorry, I don’t know how to add images to blog comments
Dr. Tom, a most interesting post. I never considered that.
Of course, being a man who checks everyone, I went to MODTRAN to check your assertions. I found the following increases in longwave absorption if we double the methane concentration. Here’s how absorption increases when you double the methane.
Clear Sky Tropics +0.75 W/m2 from doubling of methane
Clear Sky US Standard Atmosphere +0.69 W/m2
Clear Sky Subarctic Winter +0.34 W/m2
Note that these are reduced somewhat if there are clouds. On a global average, then, it seems that a doubling of methane would lead to an increase in absorption of somewhere around half a W/m2 … color me totally unimpressed.
So your claim is upheld by MODTRAN, my congratulations … always more for me to learn, thanks for schooling me on methane.
w.
Cross sections for CO2 in cm^2 per molecule
http://vpl.astro.washington.edu/spectra/co2pnnlimagesmicrons.htm
Cross sections for CH4 in cm^2 per molecule
http://vpl.astro.washington.edu/spectra/ch4pnnlimagesmicrons.htm
Cross sections for H2) in cm^2 per molecule
http://vpl.astro.washington.edu/spectra/h2opnnlimagesmicrons.htm
For well mixed atmospheric gas constituents, the fraction of beam absorption per meter of gas column by each type of molecule at a given wavelength will be proportional to the molecule’s cross-section at that wavelength and also proportional to the number of molecules of that type. These have to be summed over the earth surface thermal emission bands in order to see which molecules absorb the most energy. Water vapor, because of its great numbers of molecules is dominant, but there is significant absorption by both CO2. The low numbers of CH4 result in much less energy absorbed by it. It has a fair sized cross-section but very low numbers.
Only ruminants burb and methane, and methane doesn’t stay around as long as carbon dioxide.
We do too, and kangaroos are not ruminants. That’s why Ross Garnaut suggested farmers turn to farming kangaroos instead of sheep and cattle. Very hard as they are marsupials, and don’t taste as nice as lamb and beef.
Sorry I am tired, it should have read …’only ruminants burp and fart methane…”
Nullius in Verba says:
April 11, 2014 at 8:08 pm
####
That is not what happened. No one came close to even trying to understand what I was saying. As for the relevance of their comments, they might as well have been grammar nazis. For instance, when I tried to explain a laboratory experiment, no one caught on to that fact. “But the atmosphere is not well mixed”. Well, duh. That’s NOT what I was talking about, and was not relevant to a discussion on the nature of gasses.
BTW:
ln| 1 + q|, q = 0 results in 0
sheesh
Nullius in Verba says:
April 11, 2014 at 8:08 pm
####
Another thing, its not concentration (parts gas a / total parts atmosphere) that is impotent, but density ( parts gas a / volume).
bushbunny says:
April 11, 2014 at 9:45 pm
###
Vegans produce more CO2 then people with a normal diet.