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.