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The plant-methane link again

This week in Nature we have a news story on an attempt to follow up Frank Keppler’s work on methane produced aerobically by green plants which we published early last year (news story | paper). The Keppler piece, which suggested that methane emissions from green plants were a significant but previously unappreciated factor in global methane emissions, caused quit a lot of fuss, understandably, in the media — since methane is a greenhouse gas which, over short time horizons, is about 75 times more powerful than carbon dioxide — and quite a lot of befuddlement among plant scientists. If it were true, it would have significant implications for the way that people model methane production, and the levels of production that one might predict in a warming world. The debate rumbled on last year (another news report, this time by my colleague Quirin).

The new work that Tom Dueck and colleagues have published in New Phytologist (paper), though , finds no methane emissions from plants at all.

Obviously, not necessarily the last word. As Mike Hopkin reports:

Both groups have criticized the other’s choice of experimental method. Dueck says that Keppler’s group kept plants in sealed plastic containers instead of flow chambers, and exposed them to sources of stress such as bright sunlight and high temperature, which could have produced methane as an artefact. Keppler retorts that the use of 13C is an artificial piece of chemical trickery with unknown effects on plant metabolism, and also argues that methane production can vary by up to three orders of magnitude between species.

Keppler says other teams will be publishing results that back him up on the methane; but Mike reports that at least one other team is siding strongly with Dueck.

What Mike doesn’t mention, because an evil news editor (me) wouldn’t give him the space, is that various people in the community have pointed to an interesting contrast between the way plant scientists responded to the discovery of isoprene emissions and the Keppler work. With isoprene people said oh that’s interesting, replicated, and got on with it. This work has had a far frostier welcome.

On isoprene, this is as good a place as any to mention an interesting perspective by Manuel Lerdau in Science a few weeks ago on a possible isoprene-ozone positive feedback (paper). Isoprene within leaves protects the plants that produce it against ozone. But when isoprene gets out into the air, as it will, it can react with nitrogen oxides to make ozone. Only some species produce isoprene, and so these isoprene-producing plants both protect themselves against ozone and, in Nox-rich environments, increase the ozone stress on their non-isoprene-producing neighbours.

If this effect is real, it might have significant effects on forest composition over the next century.

One last thing to note on the Keppler story: it led to Carl Zimmer saying something nice about us, and that is always a good thing. As of course is Carl.

Comments

  1. Report this comment

    John Fleck said:

    The Keppler paper, and the response to it by those in my profession (the news media) is, I think, a classic example of what I’ve come to call “the Nature problem.” As you all who work within the scientific enterprise know, science out at the most interesting part of the cutting edge – like the Keppler paper – is highly contingent. It depends on further work that either supports or refutes fascinating new bits of science. The problem at the news media interface is that we journalists often do not treat it that way, and the public generally does not understand it that way, and much is lost in the translation from the science to the public mind. A “paper published today in the prestigious journal Nature” carries a stamp of authority in the public mind that the contingent nature of the underlying science does not merit. But folks don’t get this.

    Thus we have, for example, the “Europe headed for deep freeze” headlines that accompanied the Bryden et al paper suggesting that the thermohaline circulation is slowing, and the “trees cause global warming” reaction to the Keppler paper.

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    bubba said:

    “The problem at the news media interface is that we journalists often do not treat it that way, and the public generally does not understand it that way, and much is lost in the translation from the science to the public mind. A “paper published today in the prestigious journal Nature” carries a stamp of authority in the public mind that the contingent nature of the underlying science does not merit. But folks don’t get this."

    Mr. Fleck, this is one reason I visit your web-site several times a week. You do seem to get that aspect of science reporting.

    Well, the biking stories have always been good as well.

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    Ike Solem said:

    Look – you are writing for one of the most widely recognized science journals, and yet there is almost no discussion of the science. Where is a basic discussion of the % contribution of isoprenes to ozone? Where is the justification for the phrase “a significant effect”?

  4. Report this comment

    Mark Bahner said:

    “Only some species produce isoprene, and so these isoprene-producing plants both protect themselves against ozone and, in NOx-rich environments, increase the ozone stress on their non-isoprene-producing neighbours.

    If this effect is real, it might have significant effects on forest composition over the next century."

    It’s not likely to have a significant effect on forests in the United States in the 21st century.

    Forests in the United States are obviously concentrated in rural areas, rather than urban areas. Tropospheric ozone (“smog”) levels are lower in rural than urban areas. Further, even urban areas in the United States should see tropospheric ozone levels approach pre-industrial levels (i.e. “zero,” not including tropospheric ozone produced entirely by natural causes) in the next 2-3 decades.

    For example, even assuming gasoline usage increases by 2 percent per year from 2004 to 2024, NOx emissions from automobiles and light trucks are expected to decrease by 85% from 2004 to 2024. See Figure 12 of this document.

    Air pollution will continue to decline in U.S.

    Due to these transportation reductions, and reductions from electrical generation, U.S. NOx emissions will probably decline by more than 70 percent from 2007 to 2027.

  5. Report this comment

    ic said:

    Cut down the trees before they do more harm.

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    Oliver said:

    Ike: I’m not, in this instance, writing for Nature — I’m writing on a blog that Nature hosts. Different people blog for different reasons,and a given person can blog for different reasons at different times. On this occasion I was just pointing to the isoprene paper in Science because it came to mind and I think the positive feedback is an intriguing possibility that maybe people aren’t tracking. I was not analysing it in depth — hence the lack of quant.

    Mark: I must say that the AEI aren’t the first people I would necessarily go to for ozone figures, but then I wouldn’t necessarily limit myself to US forests either. A quick glance at Hauglustaine and Brasseur in GRL 106 pp32,337-36360 (PDF) suggests that Central America and parts of Asia may expect higher ozone for some time.

    Thanks for the comments.

  7. Report this comment

    Fergus Brown said:

    Oliver: can you point me to any material which shows methane as ‘75 times more powerful’ than CO2 over short time horizons? I have read several papers which say that methane is 23 times more powerful a GHG, over the 8-10 years in which it stays in the atmosphere, than the longer-lived CO2, but have not come acroos your number before.

    Given that methane release from permafrost melting is likely to far exceed any changes in vegetation dynamics, I am not sure how important this factor is likely to prove, anyway. It needs some more specific quantification.

    Regards,

  8. Report this comment

    Oliver said:

    Fergus, there are doubtless readers, not to mention contributors, who could explain this better than I, but the key figure is the Global Warming Potential. The GWP tells you how much radiative forcing a molecule of a given gas will provide compared to the warming provided by a molecule of carbon dioxide. The currently accepted GWPs are in Table 2.14 of the Working Group 1 AR4, which is on page 212.

    The complexity comes from the fact that to say how much warming a molecule does you have to go on to say over how long: GWP has to be calculated for a given span of time. This matters because different gases have different lifetimes. In the case of methane, the lifetime is 12 years — after that it oxidises into carbon dioxide.

    If you ask about GWP over just 20 years you get an answer, according to AR4, of 72 (up from 62 in the third assessment). But if you look at the methane molecule’s GWP over 100 years, that high figure is “diluted” by the fact that for 88 of those years the molecule will in fact be carbon dioxide, with a GWP of one. If you look at the 100 year GWP in the table in AR4 you will see that it is put at 25, compared to 21 in the second assessment and 23 in the third. (There are probably other issues at play too, but as I understand it, that’s the main one.)

    The IPCC typically cites the 100 year GWP, and so that’s the figure normally given for the “strength” of methane, and it’s fair enough if you’re looking at a century timeframe. But if you’re looking at a 20 year timeframe you need a figure >3x higher, because over the next 20 years your methane molecule is going to do that much more warming.

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