Methane Question

[Anton Alferness]

I've read, seen and heard a fair number of numbers thrown around over the years stating methane is xx worse of a greenhouse gas than CO2. I've read 20x, 26x, 30x, 40x, 60x, 80x and yes I've heard 100x. 

My question is can someone describe for me how methane is worse, how is it measured (its  severity in terms of comparison to CO2) and why are there all these competing or conflicting contrast numbers? 

And if you have any scientific papers on this that you could send me I would appreciate it. 

Thanks-

-Anton 

[Paul Belanger]

Yes – Methane is a more potent green house gas (GHG).

Maybe you should look at it simply – it’s a GHG as are others. Concentration of any GHG is important and the molecule’s stability and residence time.

Methane oxidizes like other organic matter to CO2 and H20.

Someone might answer more clearly – but as a pure gas methane is often quoted as 30 times more potent than CO2 – but it’s orders of magnitude less than CO2 and has a shorter residence time.

 

Study up:

• https://www.epa.gov/ghgemissions/overview-greenhouse-gases
• https://en.wikipedia.org/wiki/Greenhouse_gas

`      excerpt

When ranked by their direct contribution to the greenhouse effect, the most important are:

Compound	Formula	Concentration in atmosphere[30] (ppm)	Contribution (%)
Water vapor and clouds	H 2O	10–50,000(A)	36–72%
Carbon dioxide	CO 2	~400	9–26%
Methane	CH 4	~1.8	4–9%
Ozone	O 3	2–8(B)	3–7%
notes: (A) Water vapor strongly varies locally[31] (B) The concentration in stratosphere. About 90% of the ozone in Earth's atmosphere is contained in the stratosphere.

 

 

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[Dan Miller]

Compared to CO2, methane traps about ~100X (I’ve heard as high as 200X) more heat while it is in the atmosphere. However, while CO2 lasts in the atmosphere for hundreds to thousands of years (after about half is absorbed by the oceans, plants, and soils), methane has a half life of about a decade.  So, over a 20-year period, methane has a Global Warming Potential (GWP) of about 86, and over 100 years, it’s about 28~37 (different groups calculate it different ways).

This means that while a steady emission of CO2 causes an ever increasing temperature, a steady stream of methane causes an increasing temperature over about 20 years and then stops (because the amount going into the atmosphere is balanced with the amount going out). Note when methane “leaves” the atmosphere, it is actually being converted to CO2, so some long-term warming remains (~1%).

This means that CO2 is the long-term culprit we need to stop.  Though reducing methane is also really important and that will result in short-term cooling (though it is offset by reduced manmade aerosols that result in warming).

Note that one reason that methane is so effective at trapping heat is that there is so much less of it in the atmosphere (~2 ppm vs. 420 ppm for CO2) so its absorption bands are not as saturated.

Dan

[Robert Cormia]

Hello Anton,

I'm a chemist and will give you an answer with two plus parts to it. One is easier to understand than the others. The first is that methane is a symmetric molecule with four light hydrogen atoms attached to it. Chemical bonds can be thought of as springs that can absorb energy and vibrate, which occurs at a specific vibrational energy in the electromagnetic spectrum. Methane happens to absorb at a couple frequencies that help it absorb more energy (and in different parts of the energy spectrum), per molecule, than carbon dioxide, which has a linear and bending mode for two double bonded oxygen atoms. Chemical bonds vibrate in different modes, but that is deeper into the weeds. Molecules like halocarbons and SF6 absorb even more vibrational energy, as they have more (and heavier) atoms bonded to them, also in a symmetric structure. The second part of the answer is a little harder to understand as it's less precise, which is why there are so many GWPs for methane. Methane degrades in the atmosphere principally through reactions with hydroxyl ions, and lasts, on average, for about 10 years, this is known as a "half-life" if the chemistry of the atmosphere changes, or there is a "burst" of methane that swamps the hydroxyl reservoir, it lasts longer, and hence will absorb more energy, cumulatively, per molecule. The half-life of methane varies, as does the concentration of N2O, another GHG, that affects the half-life of hydroxyl ions. This is even deeper into the weeds. The last piece (or "plus") is that the spectral vibration (absorption) of methane overlaps parts of the absorption spectrum for water, allowing the two molecules to "couple" like tuning forks near each other. This is why CH4 and CO2 are so critical as GHGs, they provide a "thermal foundation" for water vapor, that is a short lived (but potent) GHG. Hansen and Lacis did this work in 2010-11 (see the last reference).There are many, many, technical references, I'd start with Wikipedia, that link to them. These aren't perfect answers, and I'm not an expert, but wanted to give you a head start, other members can no doubt help explain this better.

-rdc

Chemistry faculty

Foothill College

https://en.wikipedia.org/wiki/Global_warming_potential

https://cdiac.ess-dive.lbl.gov/pns/current_ghg.html

https://www.tandfonline.com/doi/full/10.3402/tellusb.v65i0.19734 (Hansen and Lacis)

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[Peter Fiekowsky]

Anton- A quick answer:

1) ignoring last summer's IPCC report for now, and using the clear data in the AR5 report from 2013: The instantaneous ratio between methane and CO2 is 120X, by weight. 

2) IPCC uses weight, because economists want to know the optimal carbon price per ton of methane, compared to the price per ton of CO2.

3) Chemists and physicists care about "per volume" or "per molecule" warming--they're essentially the same. The 120X (by weight) equals 44X by volume--because CO2 is heavy (molecular weight 44) and methane (CH4, molecular weight 16) weighs less per unit volume-roughly 1/3 as much.

4)  In climate reports we always use ppmv (ppm volume). This means that the 2 ppmv of methane produces the same warming as 2 x 44 = 88 ppmv of CO2. That means that the CO2equivalent now is 420 (CO2) + 88 (methane) = 508 (plus a bit for other gasses like NOx)

5) The complex list of 20x, 26x, 30x, 40x, etc--they're used by economists to compute the social cost of methane emitted over some economically relevant period--10 years, 20, 50, 100 years. We're not discussing carbon taxes or pricing, so I ignore all those.

6) last year's IPCC AR6 report increased the estimate of warming from methane by 50% because they include the effect of ozone coming from methane oxidation, so you could say it's 180X by weight, or 44X by volume. However, no one has shown me the new calculations yet, so I'm sticking with the "tried and true" 44X.  I recommend staying with AR5 for now, since the climate community is familiar with it.

Peter

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[Govindasamy Bala]

On a per molecule basis, methane absorbs ~30 times more thermal energy than CO2. However, the amount of CH4 is too small (~ 2ppm) compared to CO2 (400 ppm) in the atmosphere. In the industrial era, CO2 has increased by ~120 ppm and CH4 has increased by 1 ppm. Therefore, as a very crude estimate, the forcing by methane should be approximately 30 * (1/120) = 1/4 of the CO2 forcing. Indeed, the CO2 forcing in the industrial era is ~2 Wm-2 and methane forcing is ~0.5 Wm-2. 

Further, Methane average lifetime is 10 yr while CO2 lifetime is about 300 years. 

Methane is important but its effect is only 1/4th of CO2 or less (if one considers the lifetime)

Best wishes,

Bala

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With Best Wishes,

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G. Bala
Professor
Center for Atmospheric and Oceanic Sciences
Indian Institute of Science
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India

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[Anton Alferness]

Thank you all for your thoughtful responses. I very much appreciate it!