Friday, January 13, 2006

Methane emissions from plants

Time to raise the tone with a return to scientific matters, in particular this recent paper has cause quite a stir: Methane emissions from terrestrial plants under aerobic conditions : Nature. Believe it or not, our institutional subs have lapsed entirely, because our admin department is not capable of anticipating the end of the year, so I've only seen bits of it. Nevertheless, the message seems clear: plants seem to generate significant quantities of methane which will, unless this research is overturned, require some adjustment to our understanding of the methane balance in the atmosphere (in particular, some increases to the natural sinks will need to be found in order to explain the observed/pre-industrial equilibrium). It's a significant effect but won't turn the climate modelling world upside down, as methane is only a modest contributor to the overall picture.

RC says it is surprising, and shies away from commenting on implications, other than pointing out that such a consensus-busting paper is proof that the scientific method is alive and well. Stoat tries to put an alarmist spin on it [update ok, I'll add half a smiley before I get lynched - see comments ;-)]: since deforestation means that this natural production has shrunk recently, we need to increase the estimated anthropogenic emissions to compensate, thus implying a greater anthropogenic effect (but...anthro effects include the deforestation anyway, so that argument is meaningless a priori..). Anyway, while this might make a very small difference to the details of historical attribution, I'd rather point to way in which this new research might help to explain why the measured methane concentration in recent years has spectacularly failed to increase as all SRES scenarios and model results predicted (projected). As I've pointed out before (eg here1, here2 and here3) this is a growing discrepancy that IMO requires some re-evaluation of the scenarios. Now that there is a fundamentally "scientific" excuse to look again at methane (not just the embarassing fact that the scenarios fail to match observed reality) I hope we can soon expect to see some improved predictions.


Anonymous said...

Stoat alarmist? I don't get it.

Actually the one thing that seemed really significant about this paper was the suggestion that further warming would result in even more plant methane emissions (although I'm sure it's not at all clear that this effect will end up as a major concern).

William M. Connolley said...

Hmmm... do I really have to defend myself against charges of alarmism? James, you're as bad as the Grauniad (which I attacked for its alarmism): puffing things up to make a story... If it was a joke you need to put in smileys.

My point was that the natural reaction from many (inc the Grauniad) was that this means that natural sources of methane are bigger than believed (if it gets bourne out). BUT by the same token (and the deforestation) that seems to imply that the anthro contribution to *increases* in methane is larger than previously believed. Since the deforestation CO2 is already taken into account, this could affect the attribution balance slightly (so that argument is *not* meaningless a priori) if any of these changes are above noise level.

James Annan said...

You said:

if plants are a net source that means we've been losing plant sources which means (given that we know the absolute levels) that anthro methane sources have grown more than previously believed...

But the deforestation is itself anthropogenic in the first place! So it cancels out your hypothesised extra growth in the (other) anthro sources anyway. As you said, the atmospheric level is measured, the anthropogenic component is what drove the change (basically). So the total is known even if the components are not.

coby said...

Nothing alarmist at Stoat. It is a reasonable point that IF plants are a bigger chunk of the methane budget AND plants have decreased as a whole THEN the observed increase in methane concentrations is likely more direct anthropogenic emissions than thought.

This of course has no bearing on the past, we have observations for that. But it may impact projections of the future. It seems to me this impact could be more or less growth in methane depending on the details.

As Stoat did not indicate this means a bleaker picture for the future, you might consider withdrawing your "alarmist" characterization. Who knows what this man of many names is capable of! :)

James Annan said...


The decrease due to deforestation is itself part of the anthropogenic effect! We know a priori that essentially all of the net increase in observed methane is due to the net anthropogenic effect, so this research only changes the relative magnitudes of different components of that net effect. Therefore, I don't see how this provides any support for his claim that anthro methane sources have grown more than previously believed. They have still grown just enough to explain the observed methane record, no less and no more (although in fact the "previously" believed" estimates actually don't seem to stand up to the observations over the last few years, giving a significant overprediction instead).

William M. Connolley said...

Aha... now I understand. By "anthro", I meant "anthro-except-forests" of course. The idea I was trying to put forward was exactly what we agree on: that the observed methane record is good, hence if one source has changed then another must also.

However, I've now noticed that the emissions are supposed to be temperature dependent, so its possible that a reduced area might be producing more, given higher T's.

James Annan said...


When you said "perhaps the attribution will change" I first expected you to be talking about the standard natural vs anthropogenic attribution problem. I did also consider that you could have been talking about the CO2 v CH4 v ... attribution problem. It did not, however, initially occur to me that you were talking about adjusting the relative magnitude of different components of the anthropogenic CH4 changing...which we can probably agree is a racing certainty, unless this paper is quickly falsified.

Anyway, I added at least a winking smiley. Maybe I'd better find a septic to disinfect, to maintain my street-cred. Trouble is, it's the alarmists who are getting the air-time right now!