What is Happening under a Cloud of Methane?

[TerryM]

When a cloud passes between us and the sun the immediate chilling is felt. When a mist hangs over a valley it acts like a blanket trapping heat beneath it in an inversion layer. What I'm interested in is the effect that a "cloud" of CH4 will have as it hovers over a particular location.

What I'm looking for is something similar to the albedo ratio that would indicate what fraction of  long wave radiation should be expected to be bounced back to earth when there is an additional 100 ppb of CH4 over an area.

Lodger pointed out a graph showing CH4 to be 130 times as powerful a greenhouse gas as an equal amount of CO2 over a 10 year period. I followed his lead and have been asking the AMEG if they might have an answer, but no reply as of yet.

I think this could be an important metric when trying to understand how or if Arctic ice reacts to abrupt releases of CH4 as opposed to the long term effect that the release has on the climate.

While the effects of CH4 over 20 or 100 year periods is probably more important overall, a cloud of CH4 that persisted over an area for even part of the melt season might cause a hot spot that would lead to rapid ice melt & albedo change causing a localized positive feedback affecting a much larger area.

Terry

[OldLeatherneck]

Terry,

Thanks for staring this thread.  One of my concerns/issues has been that in the dark winter months over the arctic, any methane released will remain at it's full radiative forcing potential because there is no sunlight to create Hydroxyl, which is necessary to begin breaking down the CH4. 

I would hope that in time this forum will attract the attention of some atmospheric chemists, who can better explain what is happening with methane relative to radiative forcing.

[Artful Dodger]

What I'm looking for is something similar to the albedo ratio that would indicate what fraction of  long wave radiation should be expected to be bounced back to earth when there is an additional 100 ppb of CH4 over an area.

Hi Terry,

To answer this, there are two concepts you need to understand. The first is the absorption spectrum of greenhouse gas, and the second is black body radiation.

Start here for a quick primer on the absorption spectra of various GHGs:
http://www.eumetrain.org/data/3/36/navmenu.php?page=2.3.1

The first chart below (in blue) shows the relative absorption for each component GHG over the range of the EM spectrum from UV on the far left to IR on the far right, with Visible light in between.

The second chart below (in red) shows the composite absorption spectra of Earth's atmosphere for 20th Century concentrations of GHGs.

[Artful Dodger]

Part 2:

Liquid ocean is very nearly a perfect black body in terms of emitted radiation (~96% IIRC), so it is very important, and very useful, to make the connection between the sea surface temperature (SST) and the frequency of the long wave energy released by the ocean.

Black body radiation has a characteristic, continuous frequency spectrum that forms a curve with a peak with at a characteristic frequency. Let's look at an example right now: (after Wikipedia topic "Black-body radiation")

[Artful Dodger]

Part 3:

So what determines the frequency peak for Black-body radiation? Well, as mentioned above, it the skin temperature of the black body. If fact, we can rescale (or dual scale, if desired) the emissions graph with a second scale, one that relates EM emission frequency to skin temperature.
SPACE FILLER - MORE TO COME 

[Artful Dodger]

Part 4:

We can assign a temperature to the emissions spectra observed based on Planck's Law.  The radiance observed at a given wavelength is converted to the temperature at which a black-body would emit the same radiance at the same wavelength.

So how do we put it all together? This comes down to the concept of the 'atmospheric window'. We saw in Part 1 of this discussion that Methane ABSORBS longwave radiation most strongly at around 3.0 and 7.0 microns. Planck's law tells us that a black-body EMITS longwave EM at a peak of 3.0 and 7.0 microns when that black body has a skin temperature of 693 C and 141 C, respectively.

See the handy "Calctool: Black body emission max calculator":
http://www.calctool.org/CALC/phys/p_thermo/wien

So, methane is probably a very important GHG for the runaway greenhouse effect as experienced on Venus, but hopefully will not reach that peak here on Earth! 

Now, not all the energy is emitted at the peak wavelength. As stated above, it is a curve with a PEAK at the characteristic wavelength. Attached below is a chart with two sample curves, one for -10C and one for +10C.

At near -1.6 C (the freezing point of sea water), very little energy is emitted in the 3 micron band. However, about 1.9 Watts per square meter is emitted around the 7 micron band (between 6.5 - 7.5 microns). It's an open question as to what fraction of this energy would be absorbed by CH4, one I will have to leave for another day. However, it puts an upper bound on the radiative forcing due to CH4 during the Summer melt season: about 2 W/m^2

As temps rise to near 16.85 C (approx Global mean SST), black body emissions around the 7 micron band rise to about 3 W/m^2 or 50% more energy available for capture by CH4. It's getting hotter in here now...
CAVEAT:
THIS IS A WORK IN PROGRESS. MY OPINIONS ARE STILL IN FLUX!

SPACE FILLER - MORE TO COME 

[TerryM]

Lodger

Let me see if I'm following.

The lower spike of CH4 is so low that it's not really a factor at any temperature?

As SST increases from -1.6c through 10c toward 16.85c, the long wave radiation peak moves toward the left side of the graph, or toward the wavelength of the upper spike of CH4.

CH4 therefor has a greater effect when it's over an area with warmer SSTs & a negligible effect at lower temperatures?

Terry

[Artful Dodger]

Lodger

Let me see if I'm following.

The lower spike of CH4 is so low that it's not really a factor at any temperature?

Yes and no: Yes, it isn't really a factor at today's temperatures. But no, methane would be a huge factor on a runaway greenhouse Earth. Do we really want a world with 141 C surface temps? Because that's the level supported by the 7.0 micron absorption peak of methane. And the 3.0 micron absorption peak keeps the Earth nice and toasty at 693 C. 

As SST increases from -1.6c through 10c toward 16.85c, the long wave radiation peak moves toward the left side of the graph, or toward the wavelength of the upper spike of CH4.

CH4 therefor has a greater effect when it's over an area with warmer SSTs & a negligible effect at lower temperatures?

Terry

Yes, the greenhouse potential of methane increases as SSTs get warmer because we're sliding left toward the peak of that CH4 absorption curve. We can see this by using the Spectral Calculator:

http://www.spectralcalc.com/blackbody_calculator/blackbody.php

Let's express the Band Radiance between 7 and 8 microns as a percentage of total Radiance at each of four temperatures: (this is the fraction on long-wave energy emitted by the earth at wavelengths absorbed by methane)

2.98% @ -30 C
4.45% @ 0.0 C
5.20% @ 15.85 C
5.28% @ 16.85 C
5.75% @ 30.0 C

More methane, more forcing, up to the limit of the fraction above. And the warmer it gets, the faster it gets warmer. Not good for the future Earth (and to be avoided if we are wise)

[Apocalypse4Real]

Lodger,

What you just postulated above was new to me, and thanks for it. The main problem, with that is that so much of the methane in the Atlantic, Barents, and Kara has been over waters with higher SST's.

Something more sobering to think about.

[Artful Dodger]

Lodger,

What you just postulated above was new to me, and thanks for it. The main problem, with that is that so much of the methane in the Atlantic, Barents, and Kara has been over waters with higher SST's.

Something more sobering to think about.

Hi Apocalypse4Real,

Keep in mind, this is me trying to understand the science, starting from 1st principles. I have probably made many mistakes, not because the science is wrong, but because i don't understand. Feel free to point out any mistakes you see as I work through it.

I have actually been working toward a new hypothesis since Nov 20, 2012. I hinted about it over at the 'old country', but nobody seemed to want to bite.

http://neven1.typepad.com/blog/2012/11/arctic-methane-why-the-sea-ice-matters-/comments/page/1/#comment-6a0133f03a1e37970b017d3dfd6526970c

Fundamentally, we either speed up the carbon cycle or slow down fossil fuel use. Or a some combination which restores equilibrium in the carbon cycle.

The new equilibrium temperature may well be higher, but still within a livable range. I propose we set a new goal to stabilize world temp at 16.62 C

Hint: 16.62 C represents the open 'atmospheric window' due to GHGs, and is may be our last best chance to stabilize Earth's temperature.

Black body emission max: (set 'Emission peak b' to 10 nm and click 'Calculate!')
http://www.calctool.org/CALC/phys/p_thermo/wien

I'll continue to post more charts and numbers as I work through this.

[TerryM]

I'd asked my question to one of the people at NOAA & I'll post his response. I've removed his name as he did say that he'd be unable to make the time to answer further questions when he agreed to let me post his reply. I do think we're fortunate to have someone on the payroll who would answer a question at such length .

Dear Mr. M****,

Your question is a good one.  It is also simple to ask, but unfortunately the answer is much more complicated than the question, and I can only give you an approximate answer.

The total amount of additional infrared radiation retained by long-lived greenhouse gases ("LLGHGs") today compared to pre-industrial times is 2.8 Watt/m2 averaged over the entire earth, day and night. This equals 1.2 % of all solar radiation absorbed by the earth that fuels our climate system. Thus, it is somewhat comparable to the sun having become brighter by 1.2%. The contribution of methane to 2.8 W/m2 is 0.51 W/m2. The warming caused by any LLGHG depends partially on the cloud cover that is present because clouds both absorb and emit infrared radiation. Everything in the universe emits radiation, and warmer surfaces always emit more radiation than the same surface at a colder temperature. In a cloudless atmosphere the effect of LLGHGs is largest because the radiation that is prevented from going to space comes from a Earth's surface which tends be warmer than the air, and in addition air gets colder at higher altitudes when it expands under lower pressure. If the radiation that is prevented from going to space comes from a cloud top, less energy is retained because clouds, at higher altitudes, tend to be colder than the Earth's surface (except under strong inversion conditions). The number 0.51 W/m2 thus averages over tropical and high latitudes, and over high (cold) and low (relatively warm) clouds. If 100 ppb is locally added to 1800 ppb of globally averaged CH4, the additional local retention of infrared radiation in the atmospheric column would be ~0.033 W/m2 on average, but could be higher or lower depending on latitude, cloud amount and cloud height etc. If the retained heat could stay in place, it would warm the entire atmospheric column in 24 hours by ~0.0007 deg F.  Very little. In a year that would be 0.14 deg C or 0.25 deg F, which is substantial.  However, in a year most of the extra heat would have been radiated to space in parts of the spectrum where methane does not absorb (because the atmosphere has warmed) and a fraction would have entered the oceans.

Best regards,

***** *****

Terry

[Artful Dodger]

I'd asked my question to one of the people at NOAA & ... he did say that he'd be unable to make the time to answer further questions

Argh. We know the basic physics now, having already found the Introductory course online (see upthread). But although these numbers are tantalizing, it's still not simple to recreate the calculated values because we still have at least three unknowns: temp, absorption band for methane, and emissivity in that band.

Too bad your contact couldn't provide at least one reference to a scholarly paper or textbook showing in the calculations. 

In the mean time, let's continue our discussion of the atmospheric window  around 10,000 microns, or peak emissions from black bodies at 16.63º C (~290º K). Here are 2 charts showing the absorption of the current composite atmosphere:

[TerryM]

Lodger

The last chart you posted has me wondering about another factor. Since the upper CH4 band and the water vapor band overlap, is it possible that CH4 will have a much greater effect in drier areas?

Arctic summers have been getting more humid with increased open water and higher temperatures,but I suspect they're still drier than global averages. Arctic winters I assume are still comparable to deserts.

With CH4 having about 130 x the effect of CO2 in a 10 year span would it be reasonable to assume some multiple of this in Arctic conditions?

Terry

[Artful Dodger]

Lodger

The last chart you posted has me wondering about another factor. Since the upper CH4 band and the water vapor band overlap, is it possible that CH4 will have a much greater effect in drier areas?

Hi Terry,
Yes, water overlaps CH4, but keep in mind not all water in the atmosphere is water vapour. Cloud is either water droplets, or crystals of ice or snow. These phases have different absorption characteristics than water vapour.

Also, keep in mind that CH4 is a long lived gas, and decomposes to CO2 and H²0, so this subject really needs empirical research to answer correctly.

Arctic summers have been getting more humid with increased open water and higher temperatures,but I suspect they're still drier than global averages. Arctic winters I assume are still comparable to deserts.

Satellite soundings provide good info on water vapour, IE: JAXA's "SHIZUKU" (GCOM-W1), or NASA's bird AQUA. So that data is out there if you look.

I find the loss of Orbiting Carbon Observatory (OCO) in a Feb 2009 launch mishap more troubling from a data collection perspective. The 'carbon-copy' OCO-2 mission will not fly before July 2014, leaving an important data gap.

With CH4 having about 130 x the effect of CO2 in a 10 year span would it be reasonable to assume some multiple of this in Arctic conditions?

Terry

As we've seen, the warmer it gets the more CH4 plays a role. Have you noticed how the absorption band-gap for C02 makes it especially important as a GHG at low Arctic Winter temps? And, how climate science focuses on 'Arctic Amplification'? And how we've seen how Arctic Winter's are warming faster than Summer? This is the fingerprint of increased CO² forcing.

[Apocalypse4Real]

Artful Dodger,

There is CO2 data/imagery available, see METOP 2 IASI mixing ratio of CO2. Unfortunately it is only imagery and there is only a 3 day archive. See below for a sample.

http://www.osdpd.noaa.gov/IASI/

[Artful Dodger]

Artful Dodger,

There is CO2 data/imagery available, see METOP 2 IASI mixing ratio of CO2.

Thank-you, Apocalypse4Real

That's an impressive list of capabilities from the ESA MetOp-2 bird. And, thank goodness for the International Joint Polar-Orbiting Operational Satellite System (IJPS) agreement. 

The Orbiting Carbon Observatory (OCO) mission was intended to make the first global, space-based measurements of atmospheric carbon dioxide (CO2) with the precision, resolution, and coverage needed to characterize CO2 sources and sinks on regional scales. Perhaps OCO-2 will be operational by 2015, if there are no further U.S. budget cutbacks.

Lot's more on the promise of OCO in papers like this one from 2004:

Crisp, David, et al. "The orbiting carbon observatory (OCO) mission." Advances in Space Research 34.4 (2004): 700-709.

[Apocalypse4Real]

Lodger,

Here are a couple of Google Earth views of the same CO2 image:

The legend is attached:

As you can see, the CO2 readings are fairly high across much of the Arctic - over 400 ppm.

[ritter]

As you can see, the CO2 readings are fairly high across much of the Arctic - over 400 ppm.

What, if anything, does this have to do with the breakdown of methane (as portrayed in your other thread) to CO2?

[Apocalypse4Real]

Ritter,

One of the breakdown products of methane is CO2. Thus the quick look at how and where the readings might correspond.

[crandles]

Methane concentration is under 2ppm. With a lifetime of 10 years that is 0.2ppm per year of CO2 addition and any variation in this is going to be smaller than that. CO2 level varies by over 6ppm during a year. So I think it is quite obvious that the methane break up to CO2 cannot play much of a role in the CO2 variation.

[Agres]

While average concentrations are ~2ppm, local concentrations at the sources are above 5%. That means on a local basis there are enough hydrocarbons for the atmospheric chemistry that we think of as "urban smog."  Locally, concentrations of ethane, propane, formaldehyde, NOx, and PAN will be detectable. This will broaden and smear the absorption curves. This is all part of a "methane plume" in the real world. We live in the real world.

http://www.sciencedaily.com/releases/2008/03/080320150032.htm

One can reasonably say that NOx does not form under natural conditions, e.g. that NOx is produced in auto engines and steam boilers.  In that same reasonable, natural world, the Arctic Ocean is covered with ice and huge plumes of methane do not billow out of it. 

The volume of methane released is a function of the pressure/temperature at the formations holding the clathrates.  When a formation gets to the decomposition point of clathrates, the clathrates in the formation are going to decompose.  Under some conditions, the process can go rather quickly. Claathrate decomposition causes oil well blow outs such as Deep Horizon. The next time somebody shows you a picture of a piece of clathrate sitting on a lab bench gently burning, remember Deep Horizon.  Three hundred million dollars of equipment and some of the smartest oil men on Earth could not control the decomposition of some clathrates trapped in a drill pipe.  The clathrates in the Arctic sea floor are not trapped in anything.  We are not likely to control them. 

Urban smog is ephemeral and will be gone shortly after dark. 

[Wipneus]

One can reasonably say that NOx does not form under natural conditions,

NO_x is formed by electrical discharges (lightning), you can smell it after a thunderstorm. IIRC the amounts are a significant  natural source of N for plant fertilization.

[Pmt111500]

"As you can see, the CO2 readings are fairly high across much of the Arctic - over 400 ppm."

at the risk of stating the obvious:
Yes, plants do not work in winter in the absence of light. Animals and decomposers and  specially humans do work even in winters so the concentration is highest  during winters on polar regions. CO2 is a well mixed gas on yearly basis but it takes some months to get around so we have local high values like that, but it's still belongs in normal yearly variation.

On methane, AFAIK, the darkness prevents much of the reactive species formation during winters so any methane released during winter stays a bit longer afloat. Normal chemistry. And one reason for the Arctic amplification.

But ok, it's been while since I've read atmospheric chemistry.

Took a look on the southern hemisphere and the Polar amplification is almost non-notable:

[Apocalypse4Real]

I have updated the METOP 2 IASI CH4 imagery from March 5-12. Almost each 12 hour report has recorded concentration levels about 2100 PPBv, in some parts of the imagery.

There has been a significant amount of release activity, in the Norwegian, Barents and Kara Seas. As fracturing has occurred in the Laptev, East Siberian and Chukchi Seas, there are spikes of methane as that trapped under the ice leaks through.

In the last few days, areas with thawing also reveal higher levels of methane for short periods of time.

Finally, Antarctica has a cloud of higher concentration of methane (above 1890 PPBv) that has been spreading over a larger areas.

I have posted some Google Earth images in the Arctic Methane Release thread.

The website is: https://sites.google.com/site/a4r2013metop2iasich4co2/home/2011-airs-ch4-359-hpa-vs-iasi-ch4-970-600-mb

[Paul Beckwith]

Hello Apocalypse4Real,
Are you generating the Google Earth views of CO2 concentrations yourself or is there a link to this?  Do you have these views for CH4 near the surface?
Thanks, Paul

[Apocalypse4Real]

Hi Paul,

I self-generate the Google Earth views. Each layer of CH4 or CO2 (there are 100 produced), including all the ones I do not post, from  0.016 mb down to 1028 mb, are available at http://www.osdpd.noaa.gov/IASI/html/M2/RP/mrm_t1.html

There is no long term save, the images are available for three days.

A4R

[Nightvid Cole]

Lodger

Let me see if I'm following.

The lower spike of CH4 is so low that it's not really a factor at any temperature?

Yes and no: Yes, it isn't really a factor at today's temperatures. But no, methane would be a huge factor on a runaway greenhouse Earth. Do we really want a world with 141 C surface temps? Because that's the level supported by the 7.0 micron absorption peak of methane. And the 3.0 micron absorption peak keeps the Earth nice and toasty at 693 C. 

As SST increases from -1.6c through 10c toward 16.85c, the long wave radiation peak moves toward the left side of the graph, or toward the wavelength of the upper spike of CH4.

CH4 therefor has a greater effect when it's over an area with warmer SSTs & a negligible effect at lower temperatures?

Terry

Yes, the greenhouse potential of methane increases as SSTs get warmer because we're sliding left toward the peak of that CH4 absorption curve. We can see this by using the Spectral Calculator:

http://www.spectralcalc.com/blackbody_calculator/blackbody.php

Let's express the Band Radiance between 7 and 8 microns as a percentage of total Radiance at each of four temperatures: (this is the fraction on long-wave energy emitted by the earth at wavelengths absorbed by methane)

2.98% @ -30 C
4.45% @ 0.0 C
5.20% @ 15.85 C
5.28% @ 16.85 C
5.75% @ 30.0 C

More methane, more forcing, up to the limit of the fraction above. And the warmer it gets, the faster it gets warmer. Not good for the future Earth (and to be avoided if we are wise)

Not so sure. If you go high enough, I think the entire infrared spectrum will be blocked by water, so methane will be irrelevant except at very high altitudes, at which the temperature would be much lower. High temperature both causes more water to evaporate and broadens the absorption lines.

[F.Tnioli]

...
So, methane is probably a very important GHG for the runaway greenhouse effect as experienced on Venus, but hopefully will not reach that peak here on Earth! 
...

"Hopefully"? I can't see how can it reach that peak here on Earth, because Earth does not get enough solar radiation to get that hot. Amount of heat radiated out by any warm body is proportional to FOURTH POWER of temperature difference ( http://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law ), and last time i calculated maximum possible theoretical increase of surface temperature (global average) on Earth, i got something not far from +32C (iirc) as a maximum possible warming above pre-industrial; this was done assuming nearby space being 3K (kelvins), and Earth albedo being 0% (completely black body).

In other words, i don't see how Earth could ever get to 50C average global temperature, even, - unless sun's output would be dramatically increased (Nova?), or unless something will move Earth's orbit much closer to the sun (some 50+ millions kilometers closer).

EDIT
I gave it some more thought, and it seems i was wrong just above, considering the context of this discussion (namely, how much forcing methane would do locally). For land surfaces, at least.

Land surface is not transparent (in a usual sense of the word), for most if not all wavelengths of sunlight, right? Thin enough layer of a typical land surface matherial (rock, sand, soil) - would be transparent, though. A micron-thin layer or so, rudely speaking. Point is, most photons of sunlight do not go deeper - they are either reflected (smaller fraction right?), or absorbed.

So reality should be - a very thin layer of atoms at very surface (on land) getting "hotter" as they absorb sunlight. Then, some of those "excited" atoms (hotter ones) - emit another photon, likely infrared spectre, out. Random direction. If it goes "down" - it is "spent" to warm up deeper layer. If it goes "up" - good chances are it leaves the surface, and is on its way back to space (unless absorbed by some GHG gas, of course).

But then, what exactly would be the mean temperature of this very thin layer near surface? How hot it is, actually?

I know, - as many of us do, - that sand gets very hot (in most usual sense of the word) during a sunny day, with clear skies. So hot one can't walk on it - one's feet can't take it, so hot it is. So, quite a layer of sand - probably few centimeters at least, - seems to be at some 70C or higher temperature (for skin to not be able to withstand the sensation, right?).

If few centimeters of sand is some 70C+, then how hot the micron-thin upper layer is? Few hundreds degrees? 300? 500? No idea, but it gotta be quite hot.

This layer will emit on corresponding wavelengths, i guess. Making above considerations about W/m^2 affected, i guess. So methane would be not some +2...+3W/m^2 max possible forcing, but much more than that. During sunny days over land, that is. Of course, night is different, and then there are cloudy days, too.

And then there oceans, and water is quite transparent. Most of sunlight travels many centimeters through the water column without being absorbed, i guess? Still, it's not clear to me how "hotter" atoms (mainly H and O, since most of ocean is H2O) will get upon absorbtion of a single photon. Do they "instantly" warm up hundred degrees, and remain so until they emit some IR back? My physics are totally insufficient to have any clue. But at least i can see how transparency of water would prevent formation of this "very hot, thin" surface layer like it happens on land. Thus there must be massive difference about methane forcing above land and above open ocean.

Snow (which covers most of ice, anywhere) - is something in-between, and it's also colder, in general, than average land surface tempeature.

Edit2

Obviously, presence and density of vegetation will also be a key factor to this. Tried to find any solid data on the effect. It seems "LST", a.k.a. "Earth skin temperature", is only a rude measurement, barely accounting for the "micron-thin hotter layer" effect i suspected just above. LSTs are measured some 0.4 centimeters deep into the surface, - nothing like micron-scale i am thinking about. Still, in hotter places of the world, this "official" Earth-skin-temperature - does get near or sometimes above 70 degrees celcius, every year, which is higher than ever-recorded near-surface air temperatures, of course.

The consideration about major role of vegetation - is indirectly confirmed by this nice picture which demonstrates amount of infrared radiation which Earth emits (2003-2011 - average, i guess):

http://en.wikipedia.org/wiki/File:AIRS_OLR.png

Desert areas are definitely far ahead of areas with much vegetation (even at same latitude), much more than a difference of just ~dozen degrees would cause, imho. Sadly, i am so far unable to find any solid experimental data about measuring land surface temperature in terms of micron-scale upper layer...

Edit 3

An  interesting detail about water. Water's transparency seems to drop dramatically for most (all?) infra-red wavelengths. Just seen a statement that on some infrared wavelengths, 30-micron-thin layer of water absorbs 90% of the radiation.

[jai mitchell]

I find it a little strange to talk about "clouds" of a well mixed gas. 

If a significant regional source (like arctic permafrost and oil exploration!) is in a region then there will be a higher concentration regionally, but this gas is as heavy as O2 when it enters the atmosphere it speeds up immediately to the speed of sound and disperses very rapidly.

In addition, it is moving into high altitudes where there are considerable winds for dispersion.

Maybe I have a wrong concept here?

In any event, there are no "clouds" of methane, (at least, not on THIS planet 

[F.Tnioli]

If it's pure CH4 - sure, it should mix well.

There are some people who state that methane which gets into the athmosphere from water - is initially not free (not pure), but in some chemical way still bound to water. For example, those internets say that professor Ryskin published a paper in 2003, which had this in it, quote:

"Whereas pure methane is lighter than air, methane loaded with water droplets is much heavier, and thus spreads over the land, mixing with air in the process (and losing water as rain). The air-methane mixture is explosive at methane concentrations between 5% and 15%; as such mixtures form in different locations near the ground and are ignited by lightning, explosions and conflagrations destroy most of the terrestrial life"

I certainly have difficulties imagining "methane loaded with water droplets", but this phraze could possibly be an example of incorrect translation (if the paper was originally in russian, which i guess it was). I'm not saying this quote is all correct; all i am saying is that perhaps there are several possible "modes" for athmospheric methane to exist, at least shortly after being emitted?

[TerryM]

I find it a little strange to talk about "clouds" of a well mixed gas. 

If a significant regional source (like arctic permafrost and oil exploration!) is in a region then there will be a higher concentration regionally, but this gas is as heavy as O2 when it enters the atmosphere it speeds up immediately to the speed of sound and disperses very rapidly.

In addition, it is moving into high altitudes where there are considerable winds for dispersion.

Maybe I have a wrong concept here?

In any event, there are no "clouds" of methane, (at least, not on THIS planet 

Jai
Your understanding of how CO2 reacts in the atmosphere is very different than my own though I'm certainly out of my comfort zone in trying to explain my take on things. I don't understand what would cause acceleration to the speed of sound & I recall that the HIPPO measurements had run into "clouds" of higher concentrations, some quite far from known emitting sources. I'm afraid I never bookmarked the HIPPO reports when they were being made and haven't been able to google them at present.
Any information you can provide would be appreciated.
Terry

[F.Tnioli]

Acceleration to the speed of sound is an easy part.

The answer to it - is that RMS speed ("root-mean-square speed") of air molecules - which, very roughly, we can consider to be "an average speed of any air moleculae", - this RMS speed is not much different from speed of sound; those internets say, RMS speed for O2 (oxygen) is some 461 m/c, for N2 (nitrogen) - is some 493 m/s. Those two gases form most of our athmosphere, and said speeds - is how fast those gases would have their molecules moving at 273°K (i.e. zero degrees celsius), should the athmosphere be pure 100% oxygen or 100% pure nitrogen.

Speed of sound is so much known and important because once any thick body exceeds it, - increasingly many molecules in the air are unable to "get away" fast enough, because they don't have enough speed to do so; so air start to "pile up" in a sort of shockwave around the "nose" of the moving object - one can see it around a nose of super-sonic fighter which breaks sound barrier.

It creates massive additional resistance, hence sound _barrier_ name - takes much additional energy to overcome this extra air resistance in order to further increase speed.

There is, then, clear dependance on molecular weight of the gas, - and its RMS speed. The heavier moleculas are, the lower their RMS speed. CO2 RMS speed is some 393 m/s (at 0°C), since the molecula is much heavier than both O2 or N2 - CO2 has three relatively heavy atoms per molecula, while both O2 and N2 have only two.

The opposite is also true. CH4 is much lighter moleculae than all those 3 gases mentioned above. Atomic weight of carbom is 12 (iirc), of hydrogen - is 1; so 1 moleculae of CH4 has atomic weight of 16 - exactly HALF of oxygen (O2 moleculae). Thus, methane's RMS speed must be much higher than all above - possibly it's well above 500 m/s for 0C? Thus, methane moleculaes (unless chemically tied to something else), - i.e., "pure" methane, - indeed tend to accelerate to speed of sound (and even well above it) very quickly, once they are free and in the air.

Alas, i'm not sure if "accelerate" is the most proper term; may be "decelerate" is? Because speed of sound in the water - is much greater than in the air. And, methane may well be dissolved right before being emitted into the athmosphere. But, since deceleration is a sort of acceleration - it just has negative increment, but principle is the same, - i guess either term would do anyways.

[icefest]

Jai, I'm not sure if you've seen this thread: http://forum.arctic-sea-ice.net/index.php/topic,479.msg11284.html#msg11284


"Cloud of methane" merely refers to the variances in concentration that occur due to local release.
In the first post for instance, there is a range of 1542-2241 ppb of methane across the globe.

As many people here have said, methane mixes fast; it's just that there is so much atmosphere for it to mix with that it takes a while to balance, during which more is released, perpetuating the 'cloud'.

As a comparison, do you remember the variations of average CO2 concentrations around the globe? The effect with CO2 is more pronounced as it takes longer to mix, but as most of CO2 is released in the NH, there are 'clouds' of CO2 there too.

If it's the semantics that's the problem then I understand. What do you think of 'locus of increased concentration' instead?

[jai mitchell]

Icefest,

I think that the locus of increased concentration is a very confusing term, clouds sounds more easy to understand.  "regionally higher temporary concentrations" is also pretty good. 

I think though that in this situation, the regional effect of a fluctuating methane concentration, at varying altitudes, over very short periods of time will average out to a value only very slightly above the forcing value for the global average.  So much so that it is unmeasurable with any accuracy. 

But that is just my sense of it.  I haven't really looked at it.

[Apocalypse4Real]

New IASI data available. Global mean methane hits 1807 ppb - 7 ppb higher than last year for April 5th! Same level not reached until July 15 in 2013!

[crandles]

I think speed of sound is about right for air molecules. However AIUI note that the distance travelled in any direction is very small before changing direction. So although some fraction of a millimetre is travelled at approx speed of sound, to disperse 10 times further will typically take 100 times longer and to disperse 100 times further takes 10000 times longer. According to this square root rule for typical dispersion distance, to disperse a long way takes a long time despite the high speed of air molecules. There are other modes of mixing besides this though.

[icefest]

I think you're looking for Fick's law of diffusion / diffusion equations.

I do not know enough physics/engineering to explain it. It links temperature, viscosity, solute size and works out the diffusion speed of Molecule X in Solute Y.

IIRC
Brownian motion usually occurs in a normal distribution with the peak changing with temperature, individual molecules move at close to the speed of sound in that medium. But as they keep bumping around, they don't diffuse at this speed.

[Apocalypse4Real]

Global mean methane hits 1810 ppb on April 27, 2014 which is 12 ppb higher than the same date in 2013. The same level was not reached until July 31, 2013, amidst the major Siberian forest fire outbreaks.

For more, see http://a4rglobalmethanetracking.blogspot.com/