Thanks, Willis. Though the premise of the article was incorrect, and the original paper’s contribution “incremental”, the topic is a useful reminder of the complex chemical processes going on in our atmosphere.

In other words, “It’s more complicated than I thought!” :-)

I remember from organic chemistry that UV light can break the C-H bond to form free radicals which are very reactive. Solar UV output has been going up. I wonder if that could account for the decrease in methane?

Solar UV

http://www.john-daly.com/hockey/solar.gif

Willis, will you edit the original article to reflect the subsequent discussion? I sometimes refer back to older articles on WUWT, and I assume others do, too. It saves a lot of time if discussion-based corrections are folded back into the original story.

I’m just a guest contributor, I can’t edit it. I’ll ask Anthony to pull it, as the original premise has been shown to be false.

By the way, to keep things clear, you should be careful with methane ‘concentrations’ and ‘emissions’. I see when you point out the concentrations above, you said emissions. After all, the Shindell paper underscores the importance of keeping the two straight.

Yes, if we assume (as the IPCC and Shindell et. al do) that methane will start to rise again at some assumed value, it could make a difference. If their model is correct. And if methane levels rise. And if GHG forcing is the secret global thermostat.”

Shindell’s paper does not actually make any assumptions about the future path of methane growth. The way he calculates a GWP is to assume that we are at an equilibrium methane concentration, and then calculates the radiative forcing (not global temperature) perturbation resulting from emission of an additional ton of methane. This is also how the IPCC calculates GWPs. GWPs are the measure of radiative forcing of a substance relative to that of CO2. The key quote from the paper, as it relates to methane: “The 100-year GWP for methane is ~10% greater (~20 to 40%, including AIE) than earlier estimates (5)”.

I grant you that the IPCC scenarios almost entirely consist of rising methane concentrations. That is a separate discussion, and one that is likely to be revisited in AR5. I will note that the uncertainty there comes less from uncertainty about the chemistry and more from the uncertainty about trying to measure area sources of methane (rice paddies, swamps, etc.) and therefore get an accurate inventory of anthropogenic vs. natural emissions. I will also note that there has actually been a significant amount of methane emission reductions in the U.S., Europe, and Russia, through some combination of landfill capping, reduced gas infrastructure leakage, and better flaring of fugitive emissions. Therefore, to some extent the lower-than-expected methane growth may be, in part, a result of actual environmental policy. (to some extent it is understood natural variation – rainfall and wildfires and such interacting with methane sources, and to some extent it really is unexplained)

By the way, “Marcus (16:49:53) : I also apologize for spelling Zimmermann’s name wrong.” No sweat. Having lived a year in the USA, I know how confusing the German spelling is. The really amazing thing is, what can happen to my first name. :)

If you’re looking for significant change in the numbers, it’s probably in the updated GWP of methane. I’m too tired to chase down exactly how those have changed.

“Yes, if we assume (as the IPCC and Shindell et. al do) that methane will start to rise again at some assumed value,”

They do? The IPCC FAR report says, “The reasons for the decrease in the atmospheric CH4 growth rate and the implications for future changes in its atmospheric burden are not understood.” It goes on to give some discussion. Methane sources and sinks are not so well understood.

The IPCC gives forty different scenarios for the future changes in GHGs. Of these, 36 of them give an estimate of future methane emissions.

In each and every one of these 36 scenarios, methane is shown as increasing starting now. In some of them the emissions continue to rise through 2100, in some of them the rise peaks somewhere between 2040 and 2080.

But in all of them it is shown as rising. So yes, the IPCC does assume that methane will start to rise again, as I stated.

w.

“Yes, if we assume (as the IPCC and Shindell et. al do) that methane will start to rise again at some assumed value,”

They do? The IPCC FAR report says, “The reasons for the decrease in the atmospheric CH4 growth rate and the implications for future changes in its
atmospheric burden are not understood.” It goes on to give some discussion. Methane sources and sinks are not so well understood.

There is some danger of more methane release with melting permafrost.

“In particular, methane emissions have a larger impact than that used in current carbon-trading schemes or in the Kyoto Protocol.”

That statement isn’t talking about the historical forcing, but rather the future forcings and therefore GWPs. _Those_ are changed more significantly by the paper (as I said in my first post, the methane chemistry was only included in a very rough fashion in the IPCC GWP estimation of “25″)

After rising for a number of years, the atmospheric methane emissions have levelled off during the last decade. Nobody knows why this has happened. Heck, nobody knows why they were rising prior to levelling off. Nobody knows which way they will go from here.

As a result, any honest scientist would have to say that we don’t have a clue what methane levels will be a century from now.

Yes, if we assume (as the IPCC and Shindell et. al do) that methane will start to rise again at some assumed value, it could make a difference. If their model is correct. And if methane levels rise. And if GHG forcing is the secret global thermostat.

Since we have absolutely no independent confirmation of the accuracy of the model, and no idea what future methane concentrations will be, and no data or observations saying that temperatures are ruled by GHGs, that’s a big stack of “ifs” to be calling science.

Thank you again, Magnus, for your contribution. Rather than merely state your ideas, you backed them up with clear citations to the literature. My hat is off to you.

w.

Willis, most science is incremental. The end numerical result of the paper might not seem impressive to you, but getting more aerosol chemistry into the models is one more thing they’ve got in there (how well they’ve gotten it there is hard to tell from this paper). Interactions like those between aerosols and clouds are another thing to improve, so aerosols really are of interest. Sadly, it’s just a short note in Science, so hopefully the details of the chemistry will be described elsewhere.

That’s why I was surprised (and misled) by them saying:

We found that gas-aerosol interactions substantially alter the relative importance of the various emissions.

Their result barely makes the “incremental” threshold, it definitely does not “substantially alter the relative importance”.

In particular, I’m not impressed by results of this type from a model. This is less than incremental, it is totally unverifiable. Yes, their model says that the methane forcing increases, but we have no data with which to compare that. How can we conceivably put error bounds on that tiny number? How do we even know that the sign of the change is correct? But I digress …

Thank you for your contributions to the thread. Our discussions here are another incremental step down the scientific path. My appreciation for your fighting my ignorance on this subject, and for your gracious and convivial tone.

w.

That statement isn’t talking about the historical forcing, but rather the future forcings and therefore GWPs. _Those_ are changed more significantly by the paper (as I said in my first post, the methane chemistry was only included in a very rough fashion in the IPCC GWP estimation of “25″)

* Preliminary Order of Magnitude Approximation