Posting in this very interesting thread is a major challenge due to the high scientific level of discussion. First of all, thank you Ned W for taking the time and the head-banging to clearly explain the concept of radiative forcing and its calculations, as opposed to the GWP concept.
I find myself thinking that, based on your numbers:
A. Current methane contribution to RF relative to 1750 is NOT insignificant, compared to CO2. Had there been no change in CH4 concentration at all since 1750, current RF would be much lower. You provided the numbers for this.
B. Current annual change of CH4 concentration is such that its RF annual change is small, while CO2 RF annual change is much larger. You provided the numbers for this.
Those are both correct, and very nicely and clearly stated.
One minor wrinkle is that as the methane concentration increases, an
additional increment of the same size has less warming effect. For example:
Adding 100 ppb from 800-900 ppb warms the Earth by 0.067 W/m2 (for a given value of N2O)
But adding 100 ppb from 1800-1900 warms the Earth by only 0.044 W/m2, one-third less than before.
CO2 has a similar pattern, but methane has already increased much more (it's nearly tripled while CO2 has not even doubled yet).
This is part of the reason why the forcing from methane emissions today is relatively small -- it takes a larger amount of emissions to produce the same degree of warming.
Of course if it turns out that future methane feedbacks are very large, that effect will be swamped.
C. This does NOT mean however that current annual anthropogenic emissions of CH4 are not important. Due to the short residence time of CH4 in the atmosphere, new emissions replace the old emissions and give the feeling that there is little annual change. But if we were to stop all anthropogenic emissions of CH4, its concentration would go down relatively quickly. So the RF delta value of such a policy would be significant. (Of course we do not know the amount of natural CH4 emissions, so this RF value would is not easily calculable.) OTOH, if we were to stop all CO2 emissions, we would save the annual delta of RF that is due to increasing concentration, but only a very small RF delta value thanks to naturally dropping concentration. (Of course we should stop both CH4 and CO2 but I am trying to understand the relative policy value).
As A and B are points you made yourself (hope I understood them correctly), I would like to get your opinion of C.
Well. What you say makes sense intuitively, but I generally distrust my intuition on topics like that where I don't have much expertise. All else being equal you're probably right, it's just not clear to me whether all else is in fact equal.
If our goal is to prevent future hypothetical methane feedbacks from kicking in, then we want to quickly stop warming, regardless of the source of that warming. What you've expressed is a good argument for why reducing methane emissions might be expected to be a particularly effective way of doing that. But all that really matters is how much "bang you can get for your buck" -- i.e., how best to spend X dollars (or, more realistically, euros) to maximize the reduction in total forcing (without causing further harm to the environment, obviously).
If reducing direct anthropogenic CH4 emissions (from fossil fuel production or whatever) is the most cost-effective way to reduce total radiative forcing, I'm in favor of that. If something else is, I'd be in favor of that instead.
As I keep telling people, I have no particular background in this (economics/policy/mitigation issues). Not that I think it's unimportant, I just don't want to speculate about something outside my area of comfort.
All that said ... I will reiterate that I believe the following two things are true:
* Continued warming will cause an increased release of methane from natural sources (feedback)
* But CO2 remains the biggest problem and a certain fraction of the ASIF populace has unrealistically apocalyptic ideas about the likelihood of extremely large and sudden methane fluxes. In a nutshell, I generally follow Gavin Schmidt's line on this.
People can feel free to yell and fling things at me now for the second point there; I'm very busy for the next few days and probably won't do much to defend myself.
Maybe before the yelling starts, though, people can pause and re-read what I said 4 paragraphs up (starting with "If reducing..."). We're all on the same side here, folks, even if some of you think I'm secretly a denier.