It's Not Really Important, But...

I first became interested in the global warming debate because I stumbled upon the hockey stick controversy when it was new, and I was amazed at how bad some of the most popular science in the global warming debate is. That's never changed. I've never labeled myself a "warmist," "skeptic," "lukewarmer," "denier" or anything else. I accept the greenhouse effect is real and everything that comes with that belief, but otherwise, I have very little interest in global warming as a whole.

That said, there are a few questions in the global warming debate I am truly interested in. They aren't the important ones. They're just the ones which struck me as interesting. I originally thought they'd be easy to answer. I figured global warming is such an important subject, the answers to my little questions would be easy to find. After ten years of waiting for my answers, I've started to think I was really naive.

You see, years back I was reading some random paper or article on Wikipedia when I saw it say atmospheric methane breaks down into (amongst other things) carbon dioxide. I thought that was kind of cool. It creates an interesting problem. Humans emit CO2 and methane, but if methane emissions are effectively result in CO2 emissions, how would we distinguish the effects? How would we tell what portion of the rise in CO2 levels is due to CO2 emissions as opposed to methane breaking down into CO2?

I didn't know the answer. I had never dealt with a problem like that. I figured other people had though, so I started looking. I was shocked to not be able to find an answer. I'm not going to go into all the history of my pursuit, but long story short, not even the IPCC had anything resembling an answer. It didn't even try to solve the problem. It just fudged its radiative forcing estimate for CO2 a bit to pretend to account for the issue (and even surreptitiously changed the AR4 report on the matter, something nobody else seems to have ever noticed).

The newest IPCC report is a bit better. It even cites a paper which addresses one of my main questions: What portion of atmospheric methane breaks down into CO2. Knowing methane can break down into CO2 is interesting, but without knowing what proportion actually does, it's impossible to know just how important the effect might be. That's why I was surprised to find the paper says:

We are unaware of any other published estimates of the rate of conversion of CH4 to CO2.

That explains why I was unable to find an answer to my question. The paper was published in 2009, but before that, there was apparently no estimate I could have found. The global warming issue has been studied for decades, but until a few years ago, no lay reader could possibly have hoped to guess how much methane converts into CO2 in our atmosphere.

I find that incredible. We spend billions of dollars on global warming each year, for decades, nobody has had an answer to such a simple question? And even now, our best estimate is very poor. No offense to the authors of the paper, who I respect for trying to look at the problem, but their best estimate is:

This gives a lower bound of 0.51 (= 0.50 × 88% + 1 × 7% + 0 × 5%) and an upper bound of 1.0 for the fraction of methane that is converted to carbon dioxide.

I don't understand how a range of 50% - 100% is considered acceptable. Those authors were limited in how much they could do, but why hasn't climate science as a whole nailed this question down? It shouldn't be that difficult.

But I'm not one to just whine. That estimate is accepted by the IPCC, so why not look at what it implies? To do so, I grabbed estimates for CO2 and methane levels over the last ~150 years. I then worked on figuring out how to estimate the effect of methane on CO2 levels. This was the first result:


Methane's atmospheric lifespan is ~10 years. That means methane molecules will break down ~10 years after they're emitted. To estimate that amount, I took a ten year rolling average of methane levels (subtracting a 800ppb baseline) and divided it by 10. I believe that should give an approximation of how much methane has broken down. When I tested it on synthetic data, it worked pretty well. It was within a couple percent of the right answer (usually giving a slight underestimation).

I'm sure there's a better way to approach this. I'd be happy to hear if anyone knows of one. In the meantime, that graph shows only the upper estimate according to that paper (every methane molecule converts into CO2). It could be the difference in that graph should be half the size it currently is. I don't know. There are other factors involved as well.

For instance, using a more accurate lifespan for methane would increase the difference, but I didn't account for the fact some of the CO2 methane converts into will cycle out (CO2's atmospheric lifespan is much longer than methane's, but it is still finite). I also didn't try to use a precise baseline for methane, and I'm still not sure how precise an answer my methodology could even provide.

Ultimately, I don't know how all those factors would affect the results. What I do know is, at a first pass, methane's contribution to CO2 levels seems to be growing at into a meaningful effect. Here is a graph showing the percent contribution of methane to the previous figure (using a baseline for CO2 of 280ppm):


The fairly constant slope in the first half is because we don't have direct measurements of atmospheric methane prior to about 1950. Additionally, methane levels leveled off for a while recently, so that causes the most recent portion to be pretty flat. Still, methane levels seem to have begun rising again, and this data only goes up to 2011. If methane levels start going up fast enough, methane might wind up being responsible for as much as 5% of the rise in atmospheric CO2. And who knows, in the future, the percent could rise even higher.

I know 5% isn't huge. It doesn't change the fact global warming is real. It doesn't change the fact CO2 emissions have been causing temperatures to rise for decades (though the rate of the rise seems to have shrunk a lot lately). It doesn't even change the fact anthropogenic CO2 emissions are causing atmospheric CO2 levels to rise.

But it does raise an interesting question about the importance of methane. Imagine if we started reducing CO2 emissions without reducing methane emissions as well. Could we see methane being responsible for 10% of the total rise in CO2 levels? 15%? 20%? I don't know. I wish I did. I wish I knew what this effect would be like in the future.

Heck, I wish I knew what this effect has actually been in the past. The crude estimate I've done here is nothing. I'd like to see some scientists who know more about the subject do it better. I wish they would have done so ten years ago. I'm still kind of shocked they haven't.

On a final note, atmospheric methane's lifespan depends on what molecules exist in the atmosphere. As the levels of those molecules drop due to methane consuming more of them, methane's lifespan could increase. I have no idea how one would incorporate that into an analysis like this. Maybe people who know more than me could give some insight.


  1. No answer to your question, but I have a question of my own. What causes the conversion of methane to CO2?

  2. Hey Gary, that's a good question. I had started to write about it when I wrote this post, but it was getting too long so I cut it out. There are just too many details I could cover.

    Long story short, methane's relatively short atmospheric lifespan is because it is fairly reactive. You can find a lot of details about the reaction chains online if you want, but basically, methane is CH4. When it reacts with things, the Hydrogen atom breaks off and becomes H2O, or water vapor. The Carbon atom then winds up bonding with an Oxygen atom to form carbon monoxide, and that in turn converts to carbon dioxide.

    There are a few different paths those reactions can take, but it basically comes down to reactions causing methane's Carbon molecule to bond with oxygen and become CO2 while its Hydrogen molecule turns into H20. The same is true for hydrocarbons in general. They (all?) oxidize into carbon monoxide then into carbon dioxide.

    One thing I need to look into is if I need to consider the "airborne fraction" of CO2 for this analysis. Natural sinks currently absorb something like half of the CO2 we emit (on net). I need to study up on the details of that to see how that would apply to the CO2 created by methane. It might just be that natural sinks will absorb this CO2 like all other, or it might be that because of where the conversion happens, less (or none?) gets absorbed in that way.

  3. Brandon, thanks. More questions: what's the energy source for decomposition, sunlight? What wavelengths? Catalysts? Does it occur near or in the ground where plants can readily absorb the CO2? The answers govern the importance of the process in regards to warming effects. I'm not demanding an answer so don't feel obligated; just curious. This may be an area open for research.

  4. Glad to help as much as I can Gary. Sunlight is the (predominant?) energy source which allows for the oxidation to happen. I don't know the wavelengths offhand though. I've seen discussion of them, but it's not something which stuck in my mind. I think I remember halocarbons mostly oxidize due to wavelengths on the 330-400 range, but I can't remember the unit type, and I might be remembering something else all together.

    As for where it happens, what I've been focusing on happens in the stratosphere. There are other reactions which happen in other areas (the paper I linked to in this post gives a broad examination of them), but I can't remember all the details offhand. The stratospheric reactions are the most important ones (as they're where most of the methane oxidizes), but some methane washes out in rainfall, and some methane has reactions closer to the ground.

    I'm not sure how active an area of research this is. There's a pretty good understanding of what reactions happen, but I'm not sure how well we can quantify each component. I haven't been able to find a lot of information on the nuances/details of the matter. That might be due to a failure of my research skills, the obnoxious paywall issue or a general lack of knowledge. I'm not sure, but I think it's basically - we understand what reactions which happen and where, but we don't know just how often they happen.

    If you'd like me to look up what is known on any specific issues, just let me know. I may not remember everything offhand, but I've collected a number of resources for this subject so I can look up things I might need to know.

    Just don't expect too much of an answer in the next 12 hours. I've gotten my second eBook published, and I've finally caught up on everything around the house I was behind on. I've been celebrating a bit. There might be some Fireball and lack of sleep involved 😀

  5. Russ R, there are actually something like 40 different greenhouse gases. Most of them get ignored because of how small an effect they have. Even the IPCC tends to just group dozens of them together. I don't think there's anything wrong with that, but it did surprise me. I wouldn't have guessed there are so many different greenhouse gases.

    There is a lot of stuff out there if you're interested in atmospheric chemistry. I don't know how important any of it is, but it's cool to read about. I just can't keep all of it in my head. Chemistry was never my strong suit because of how much memorization it requires.

  6. Chemistry was one of my favourite subjects. Yes it did require some degree of memorization (which I was good at), but beyond that it became highly logical and nearly intuitive. I found that once you had the basics down, the broader ideas and relationships somehow just made sense and fell into place.

    I'd offer the analogy of having to memorize the alphabet in order to unlock a world of understanding via words and language.

  7. It sounds like you might be better suited to this than I am. My problem with chemistry was my same problem with Spanish. I understood the rules of both, no prob. I just couldn't make myself care enough to memorize the "vocabulary" of either to do well in them.

    When it came to written assignments in my Spanish class, I had no problem with them. After two years, I was able to do fourth year translations (this was in high school, so that doesn't mean a lot). I couldn't speak the language worth anything though. I knew all the functional words, and I had no problem using them correctly, but I couldn't remember what 90% of the verbs/nouns were.

    In Chemistry class, I understood the reactions and could balance equations, but I sucked at remembering things. I pretty much always had to look up what chemical the string of letters/numbers I was looking at stood for.

    I think it might just be that I'm lazy.

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