Xin Lan PhD: The Most Important Molecule You’ve Never Heard Of
September 2, 2022
The largest “sink” for methane in the atmosphere is the destruction of methane by hydroxyl radicals created continuously by sunlight striking water vapor. The radicals only last for a brief moment, but are critical to life as we know it. (see animated explainer below)
Among their many functions – destroying methane molecules and helping to maintain the earth’s radiative balance.
More in a continuing series on methane and its pathways in the atmosphere. Still trying to get my arms around all this, stay tuned.
Climate Denial Crock of the Week 
September 7, 2022 at 2:06 pm
Good luck researching methane. I still can’t find a reliable source that definitively gives what the lifespan of methane is in the atmosphere. It is either a 10 – 15 year lifespan, or a 10 – 15 year half-life.
And you can’t evaluate what the forcing of methane is without knowing what the lifespan of its breakdown metabolite (CO2) is. Best I can find is several hundred thousand years for CO2.
Good luck finding out what the relative forcing of methane is compared to CO2. Every research paper simply uses the GWP-100, which looks only at the first 100 years of a methane pulse. Considering that methane seemingly has a lifespan much shorter than 100 years, and CO2 has a lifespan at least 1000 times longer, the GWP-100 is guaranteed to be incredibly inaccurate. In fact, it was invented NOT to do accurate calculations, but merely as a convenient short-hand.
And no research paper that I have ever seen even takes into account that some methane is different from other methane. Fossil methane’s forcing, if one wants to do it accurately, must include both the short-term methane forcing and the long-term CO2 forcing. But some biological methane sources – like ruminants – do not have a long-term CO2 component, because their methane is brand-new and is made from CO2 just scrubbed from the air.
Additionally, methane is also part of the carbon cycle AND it is a very short-term molecule. This means that it does not constantly increase in concentration in the atmosphere, like CO2 does. So, a constant methane source does NOT result in increased warming – the forcing is constant, not growing. So, a long-term methane source that is decreasing is actually producing net cooling – very different from CO2.
All of this gives me the distinct feeling that actual climate scientists, who are not particularly concerned about methane, are correct. CO2 is the molecule we need to be concerned about. Are there papers published which propose that methane is a real problem? Yes there are, but I have never seen one that uses anything but the over-simplistic GWP-100 calculations. Maybe I am off-base and I would love someone to show me where I am wrong, but it seems to me we can not take such research seriously because they don’t use accurate, valid calculations.
September 10, 2022 at 12:51 am
I’ve only seen references to methane forcing based on its presence as methane in the atmosphere, without reference to the properties of its breakdown products.
A “greenhouse gas” by definition blocks (reflects back at different angles) certain wavelength windows of radiation reflected from a planet.
Water is—as denialists love to point out—a very powerful GHG. And, for a given stable chemical mix in the atmosphere, using my magic spaceship to instantly double the proportion of H2O in the atmosphere would very quickly trap more radiation in a large part of the spectrum but overall the extra water would precipitate out of the atmosphere and reach its former equilibrium within a couple of weeks. The equilibrium amount of water the atmosphere will hold depends on the temperature (heat content) of the atmosphere.
Carbon dioxide molecules have rather middling GHG power, but there’s no effective limit on how much CO2 the atmosphere can increase. Once there is a certain amount of CO2 in the atmosphere trapping radiation their effectiveness overlaps (one molecule higher up might not block anything if there are a lot of molecules lower down catching the heat). This is why the amount of radiation blocked is calculated as a log of the amount of CO2 molecules in the atmosphere. As more and more CO2 is added, it’s less effective at blocking its parts of the spectrum. Beyond the total planetary respiration of plants (the wiggles in the Keeling Curve), the processes to remove CO2 from the atmosphere are relatively slow (ocean acidification, burial of vegetation, and chemical weathering of rock).
Methane, rather than precipitating out like H2O, just breaks down over the course of a decade, but orders of magnitude faster than CO2 level is reduced. Paradoxically, that is why a smaller increase of CH4 (now ~1.9ppm) can have such a big effect: That part of the radiation spectrum is open and every molecule of CH4 added has a greater effect than the teeming crowds of CO2 molecules.
From https://www.e-education.psu.edu/meteo3/l2_p7.html
