It Took Me 6 Months, But I Finally Understand Methane Lifetimes
How to arrive at a number you can trust
Last week, I talked about how you can never trust a number. And yet if we’re going to make good decisions about climate, numbers are absolutely central. All the decisions are tradeoffs, and tradeoffs are all about numbers.
To avoid trouble with numbers, it’s important to have a solid understanding of the system in question, so that you can spot mistakes. I’ve had a frustratingly hard time building that understanding for climate change. “Popular” sources gloss over too many details, and “serious” sources assume you understand too much already.
One seemingly-simple question that I’ve had a heck of a time wrapping my head around: how long does methane last in the atmosphere? As I’ll explain in a later post, this turns out to be important to avoid a temperature spike as we phase out fossil fuels. But the more I read, the more confused I got, to the point where my core identity as a math geek felt threatened. I finally broke down and asked for help. Erika Reinhardt and Christian Haakonsen responded with helpful pointers and explanations, and I’m finally at the point where I can read relevant sections of the IPCC reports and understand them.
You Are In A Maze Of Twisty Numbers, All Different
Shouldn’t methane lifetime be something you can just Google?
Great, question answered: 12 to 15 years. Except when I click the link to Google’s source for that information, the first thing that jumps out is a table claiming it’s two years:
This has to be a typo, but it’s yet another reminder that – say it with me – you can never trust a number. I’d better check other sources. The following quotes are taken from the first page of Google results, including Wikipedia, the EPA, noaa.gov, sealevel.info, and the IPCC. You don’t need to read these, just note the variety of answers I’ve bolded:
Methane has a large effect but for a relatively brief period, having an estimated mean half-life of 9.1 years in the atmosphere.
This reaction in the troposphere gives a methane mean lifetime of 9.6 years. Two more minor sinks are soil sinks (160-year mean lifetime) and stratospheric loss by reaction with ·OH, ·Cl and ·O1D in the stratosphere (120-year mean lifetime), giving a net mean lifetime of 8.4 years.
Lifetime in Atmosphere: 12 years
The atmospheric residence time of methane is approximately 9 years. Residence time is the average time it takes for a molecule to be removed from the atmosphere. In this case, every molecule of methane that goes into the atmosphere remains there for 8 years [wait, I thought they just said 9 years?] until it is removed by oxidization into carbon dioxide (CO2) and water (H2O).
Various sources give the half-life of CH4 in the atmosphere as 6 to 8 years, which would make the average lifetime 1.4427 times that (because oxidation is an exponential process, rather than linear), yielding an average lifetime for a molecule of CH4 in the atmosphere of 8.7 to 11.5 years. The AMS gives a figure of 9.1 years (from Pranther et al 2012, which actually reports a figure of 9.1±0.9 years). However, page 11 of this presentation by Prof. Lyatt Jaeglé gives the directly-calculated atmospheric lifetime of CH4 as ≈8 years, but identifies a feedback mechanism which she says effectively increases the atmospheric lifetime of additional CH4 to ≈12 years.
Lifetime (yr): 8.4 / 12ᶜ
That last is from the IPCC report, and the “c” leads to a less-than-helpful footnote:
Species with chemical feedbacks that change the duration of the atmospheric response; global mean atmospheric lifetime (LT) is given first followed by perturbation lifetime (PT).
Confused yet? I sure was! Even excluding the obviously-wrong “2 years”, the numbers range from 6 to 15. I suspect some of this is due to out-of-date estimates continuing to get passed around. But the main problem is that the term “lifetime”, which sounds simple enough, actually turns out to be ambiguous.
Half-Life: Not Just For Radioactive Waste
Some of the sources use the term “half-life”. Methane lifetimes are not like, say, horse lifetimes. Google tells me that horses have a lifespan of “25 – 30 years”, and of course those are numbers so we can’t trust them, but we’ll use them for purposes of discussion. Some horses might die young, some might live unusually long, but most will make it to 25 or 30. If I buy a herd of newborn horses and take good care of them, I’ll expect most to gallop off to that great farm in the sky within a few years of one another.
A lot of processes in nature don’t work like that. If I release some methane into the atmosphere, it’s not going to hang out for 10 years and then all decay at once. It turns out that the main process through which methane is removed is reaction with an OH molecule. This happens at random, so one methane molecule might break down immediately, while another could bounce around for 30 years. You lose the most methane right at the start; over time, there are fewer remaining methane molecules to stumble into OH, so the rate at which you lose methane slows down. This is called “exponential decay”, and it looks something like this:
You can summarize a process like this by specifying the “half-life”: the time it takes for half of the substance to vanish. After around 5.8 years, half the methane is gone. After 11.6 years, we’re down to a quarter; after 17.4 years, an eighth remains; and so forth.
Fortunately, for our purposes, we need to understand just two things about exponential decay:
It’s is rapid at the start, and then trails off indefinitely.
Because some methane lingers for a long time, the mean (average) lifetime is 1.4427 times longer than the “half-life1".
This helps us start to make sense of the conflicting values for “methane lifetime”: if a source says that methane has a half-life of 6-8 years, we can multiply by 1.4427 and see that’s equivalent to a lifetime of 8.7 to 11.5 years.
Four Long Lifetimes Makes One Short One?
From page 6-23 of the latest IPCC report on the Physical Science Basis of Climate Change – you don’t need to make sense of this:
…The methane lifetime due to tropospheric OH, the primary sink of methane, was assessed to be 11.2 ± 1.3 years constrained by surface observations of methyl chloroform (MCF), and lifetimes due to stratospheric loss, tropospheric halogen loss and soil uptake were assessed to be 150 ± 50 years, 200 ± 100 years, 120 ± 24 years, respectively (Myhre et al., 2013b). Considering the full range of individual lifetimes, the total methane lifetime was assessed in AR5 to be 9.25 ± 0.6 years.
I’d mentioned that reaction with OH is the primary way methane is removed from the atmosphere. The IPCC report lists three other mechanisms, such as “soil uptake”, but these are much slower than the OH reaction. What this paragraph is saying is that if you were to consider any one of those four mechanisms in isolation, you’d get lifetimes of 11.2 years, 150 years, 200 years, or 120 years; but putting them all together, on average a methane molecule lasts for about 9.25 years. (Note that this is the latest report, AR6, recapping numbers from the previous report, AR5. Thus, the numbers are somewhat out of date.)
We can ignore all this, except to remember not to confuse “lifetime due to tropospheric OH” with overall lifetime.
“Perturbation Time”, What Fresh Hell Is This?
As I quoted earlier, the latest IPCC report specifies that methane’s “atmospheric lifetime” is 8.4 years, but the “perturbation lifetime” is 12 years. What the hell is a perturbation lifetime?!?
This is where I really got stuck. As it turns out, the idea is something like this:
In 2022, I release a molecule of methane into the atmosphere. (To be polite, I cover my mouth while doing so.)
In 2030, that molecule bumps into an OH and breaks down.
Some other methane molecule, which was otherwise destined to bump into the same OH, instead survives.
In 2034, the second methane molecule finally breaks down.
So, “my” methane molecule lasted 8 years, but the overall impact on the atmosphere ran 12 years. “Atmospheric lifetime” tells us how long (on average) a particular set of methane molecules survive; “perturbation lifetime” tells us how long it takes for overall atmospheric methane levels to return to normal, which is what we actually care about. Well actually, the perturbation never quite ends, it just trails off gradually, but the perturbation lifetime is sort of the average time that methane levels are elevated.
This is highly oversimplified; you may have noticed that I glossed over how the second methane molecule eventually broke down. The full explanation involves a vastly complex web of interactions between methane, OH, the reactions that cause OH to be created (ozone is important here), other reactions that consume OH, etc. Fortunately, the upshot is fairly straightforward: to a reasonable approximation, we can pretend that methane lasts for 12 years, more or less2. If we increase or decrease methane emissions today, the effect will play out over about 12 years3. Half-lives, OH reaction vs. soil uptake, perturbation lifetimes, can all be rolled up into that one figure.
Mo Methane Mo Problems
Remember, the chief process by which methane is removed from the atmosphere involves reaction with OH molecules. If methane emissions continue to increase, there won’t be enough OH to go around, and methane lifetime will also rise. This 2018 paper examines the relationship in detail. The part we care about boils down to a single graph:
The orange line is the important one. It shows how changes in methane concentration affect the perturbation lifetime of methane, and the point is simply that it goes up: higher concentrations cause longer lifetimes.
In pre-industrial times (1750), methane lifetime was around 25% lower than today (determined through the scientific process of eyeballing the graph while holding a piece of paper up to my monitor to serve as a translucent right angle). If methane levels double again, methane lifetime would increase another 35%. In other words, as we emit more methane, the impact gets disproportionately worse; but if we can reduce emissions, the impact gets disproportionately better. Cutting emissions in half would eliminate more than half of the warming impact.
Success At Last
With the timely help from Erika and Christian, 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 levels.
Now, if you should never trust a number, why do I trust these? Problems with a number usually boil down to either “the number is incorrect” or “you misunderstand what the number means”. The most important number here (12 years) comes from the latest IPCC report, which is very carefully vetted, so I’m comfortable that it’s not flat out wrong. And as summarized in this post, I’ve put in enough work to be confident that I understand exactly what the IPCC report means by “perturbation methane lifetime”. As a bonus, I now have a mental model that I can use to evaluate numbers I encounter in the future.
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As I have learned at some cost to my sanity, the average lifetime is also called the “e-fold time”, because it is equal to the time it takes for the quantity of substance remaining to decrease by a factor of the mathematical constant “e”, about 2.718.
Actually “11.8 ± 1.8 years”, meaning scientists can’t quite pin it down but as of IPCC AR6 they’ve determined the correct figure is very likely between 10 and 13.6 years.
The 11.8 year figure comes from the following paragraph, on page 6-24 of the latest IPCC report. I think I now understand all of the words here, give or take an acronym or two:
The methane perturbation lifetime (τpert) is defined as the e-folding time it takes for methane burden to decay back to its initial value after being perturbed by a change in methane emissions. Perturbation lifetime is longer than the total atmospheric lifetime of methane as an increase in methane emissions decreases tropospheric OH which in turn decreases the lifetime and therefore the methane burden (Prather, 1994; Fuglestvedt et al., 1996; Holmes et al., 2013; Holmes, 2018). Since perturbation lifetime relates changes in emissions to changes in burden, it is used to determine the emissions metrics assessed in Chapter 7 Section 7.6. The perturbation lifetime is related to the atmospheric lifetime as τpert = f * τtotal where f is the feedback factor and is calculated as f = 1/(1-s), where s = - δ (ln τtotal)/ δ (ln[CH4]) (Prather et al., 2001). Since there are no observational constraints for either τpert or f, these quantities are derived from CCMs or ESMs. AR5 used f = 1.34 ± 0.06 based on a combination of multimodel (mostly CTMs and a few CCMs) estimates (Holmes et al., 2013). A recent model study explored new aspects of methane feedbacks finding that the strength of the feedback, typically treated as a constant, varies in space and time but will in all likelihood remain within 10% over the 21st century (Holmes, 2018). For this assessment, the value of f is assessed to be 1.30±0.07 based on a six member ensemble of AerChemMIP ESMs (Thornhill et al., 2021b). This f value is slightly smaller but within the range of the AR5 value. This results in an overall perturbation methane lifetime of 11.8 ± 1.8 years, within the range of the AR5 value of 12.4 ± 1.4 years. The methane perturbation 33 lifetime assessed here is used in the calculation of emission metrics in Section 7.6.