Beyond methane and climate change, this is a lovely step through of simply grappling with a new concept to understand it, one site/paper at a time. I wish I could teach my parents how to do this, they never get beyond the first page
I finally understand methane lifetimes
11–20 of 86 posts
Re: I finally understand methane lifetimes
#12Additional source describing the difference between CH4 and CO2 in the atmosphere:
> "Methane makes up just 0.00018 percent of the atmosphere, compared to 0.039 percent for carbon dioxide. (CO2 is roughly 200 times more abundant.) Yet scientists attribute about one-sixth of recent global warming to methane emissions; what methane lacks in volume it makes up for in potency. Over a 20-year period, one ton of methane has a global warming potential that is 84 to 87 times greater than carbon dioxide. Over a century, that warming potential is 28 to 36 times greater. The difference occurs because methane is mostly scrubbed out of the air by chemical reactions within about ten years, while carbon dioxide persists in the atmosphere for much longer than a century."
https://earthobservatory.nasa.gov/features/MethaneMatters
A big uncertainty is the future of permafrost and shallow marine sediments, which store carbon in various forms. Some scenarios are, well, not good:
https://climatemodeling.science.energy.gov/presentations/imp...
> "Our simulations include a plausible release from clathrates in the Arctic that increases global methane emissions by 22%, as well as a scenario with 10 times those clathrate emissions. The CESM model includes a fully interactive physical ocean... The results indicate that such Arctic clathrate emissions (1) increase global methane concentrations by an average of 38%, non-uniformly; (2) increase surface ozone concentrations by around 10% globally, and even more in polluted regions; (3) increase methane lifetime by 13% ..."
It's kind of like defrosting a freezer full of 25,000 year old fish guts...
Re: I finally understand methane lifetimes
#13tl;dr > I finally have some confidence that I understand how methane lifetimes work, and that for my purposes it can be summarized as: Methane emissions decay gradually, with an average lifetime of about 12 years (“perturbation lifetime”, which is what matters for climate purposes). This will increase by roughly 35% if methane concentrations double, or decrease roughly 25% if concentrations return to pre-industrial l…
(Physicist/Engineer of sorts here. Zero atmosphere knowledge) That half life depends on concentration is not surprising to me; ethanol's half life in the blood also depends on it's concentration and the reason is rather straight forward: the liver has limited amounts of enzymes needed to process booze. What is surprising to me, though, is that there is a mechanism that has such a massive effect at the extremely low c…
FWIW, and this is a rather minor and pedantic point, these are hydroxyl radicals, not hydroxide ions, so it's:
H2O + γ -> OH• + H•
Rather than:
H2O + γ -> OH- + H+
Oh, and whichever way, H2O splits into OH and H, not OH and O, but i assume that was a typo!
Re: I finally understand methane lifetimes
#14Very interesting, and a great description of methane self-feedback.
The perturbation lifetime analysis - the idea that the time constant of the atmospheric response to methane is longer than the time constant of the reaction that removes methane - comes from a beautiful paper by Michael Prather that may be of interest to anyone with some linear algebra. See https://unfccc.int/files/meetings/workshops/other_meetings/a....
Re: I finally understand methane lifetimes
#15So, hydroxide radicals (OH) break down methane (CH4) into carbon dioxide (CO2). This is a good thing for the climate, as a molecule of methane has a much bigger warming effect than CO2. (Although it is odd that the CO2 produced by methane is not counted toward methane’s overall climate impact). There is limited OH in the atmosphere. As a result, more methane “uses up” the OH. That means that increased methane in the…
> the CO2 produced by methane is not counted toward methane’s overall climate impact It definitely is in some contexts. It's often cited in CO2 equivalent over time. Methane start off to be something like 80 times more potent as a greenhouse gas than CO2. Then as it decays to CO2 its impact is that of CO2 but time has to be accounted for. So for every ton of methane, you can estimate an equivalent CO2 tonnage over th…
Here is a source explaining why the CO2 produced by methane isn’t counted in estimates of global warming potential. It seems to come down to the different calculations required for anthropogenic sources.
https://ghginstitute.org/2010/07/13/what-is-different-about-...
Re: I finally understand methane lifetimes
#16It's this level of understanding that can't be pushed out to the general population. It is far too complicated for people to grasp. Add on to these complexities is that if we are trying to price carbon - the impacts change with the concentrations in the atmosphere.
The market needs to be dumbed down to simplistic values (which thankfully they have) so that we have a sense on where to target and incentivize change for policy makers otherwise it is far to easy for mud-rakers to try and undo meaningful work/change in our policy/business arenas. Abstract away the complications.
Great piece.
Re: I finally understand methane lifetimes
#17This is very enlightening!
Key fact is how much methane is already there when your gout entered matters a great deal, because its rate of clearance is limited by how much hydroxyl radicals it can muster.
Re: I finally understand methane lifetimes
#18Another question I had: what does methane become once it breaks down? It becomes CO2 (a relatively minor greenhouse gas per unit weight, but long-lived) and H2O, right? H2O is no big problem in the lower troposphere. It just rains out. But in the stratosphere… it can stick around for much longer and impact the climate.
How much does this matter for methane? I would imagine methane floats up pretty high into the upper reaches of the atmosphere. Does it keep going? What stops it from getting to the thermosphere and (ultimately) exosphere?
Re: I finally understand methane lifetimes
#19Another question I had: what does methane become once it breaks down? It becomes CO2 (a relatively minor greenhouse gas per unit weight, but long-lived) and H2O, right? H2O is no big problem in the lower troposphere. It just rains out. But in the stratosphere… it can stick around for much longer and impact the climate. How much does this matter for methane? I would imagine methane floats up pretty high into the upper…
> It becomes CO2 (a relatively minor greenhouse gas per unit weight, but long-lived)
Although it should be noted that 1 tonne methane decays into around 2.5 tonnes of CO2 (I don't remember the exact number but it's around that) and GWP is measured by weight.
So even after it's decayed, methane has a higher GWP than CO2. Which is why its GWP remains much higher than CO2 even over extremely long periods: methane has a GWP of ~80 over 20 years, ~30 over 100 years, but is still around 7 over 500 years, despite a lifetime of only 12 years.
Re: I finally understand methane lifetimes
#20It's this level of understanding that can't be pushed out to the general population. It is far too complicated for people to grasp. Add on to these complexities is that if we are trying to price carbon - the impacts change with the concentrations in the atmosphere. The market needs to be dumbed down to simplistic values (which thankfully they have) so that we have a sense on where to target and incentivize change for…
I think it is the dumbing down that makes it easier for mud-rakers. People know it's complicated anyway, and the average person can grasp complicated things as well as you.