Seeing the forest for the methane
Plant-made methane has become a slush fund for balancing the global methane budget, but how big is the account?
In January 2006, researchers announced that plants exhale methane. Though the amount from each plant is tiny, it adds up globally to a sizable slug of the potent greenhouse gas. Researchers have been scrambling ever since to understand the finding.
More than a year later, scientists are closing in on the amount of methane potentially released globally by plants. Yet independent confirmation of the basic discovery and its mechanism remains elusive. If plants do indeed make methane, it is probably not enough to throw the entire methane budget out of whack, according to a new analysis published today on ES&T's Research ASAP website (DOI: 10.1021/es062404i).
The original discovery, published in Nature, estimated that plants spew 62–236 million metric tons per year (t/yr) of methane globally. That would rival major methane sources, such as wetlands, and would mean a major rethinking of the budget.
The new ES&T study is so far the "most comprehensive and detailed analysis" of factors expected to affect methane emissions from plants, says study coauthor Aslam Khalil of Portland State University. The study ratchets the plant source down to 20–60 million t/yr of methane, enough to be "still important" but "on par with the smaller emission sources", says Chris Butenhoff, a doctoral candidate in Khalil's lab and lead author of the paper.
By creating a global model that accounts for varying emission rates on the basis of temperature, light, leaf production, and changing seasons, Butenhoff and Khalil have a template into which they can drop future information about emission rates by different plant species across the globe.
But first, scientists will need to learn much more about this enigmatic process. The first report that living plants make methane came from a team led by chemist Frank Keppler in Germany. Most surprising, they concluded that plants make the gas in the presence of oxygen; this runs counter to any known biological mechanism. Only microbes were thought to have methane-making ability, and then only in oxygen-free conditions.
The discovery was met immediately with both skepticism and wild speculation. Because the total amount of methane in the atmosphere is well measured, the methane coming from plants would have to be subtracted out of other known pools to balance the budget. But what to cut, and by how much? Wetlands are the biggest natural source, with a large range (100–200 million t); some scientists thought perhaps they had been overestimated. More controversially, some in the natural-gas industry started to agitate for lowering estimates of human-caused methane sources.
A spate of papers followed, reanalyzing global estimates and chipping away at the higher end of the estimated plant source. Early on, some speculated that plants might actually cause global warming with their methane. Soon it became clear that the warming effect of plant methane would slice off at most a few percent from the cooling effect caused by the CO2 plants take up.
The new global estimates still scale up from the small number of emission rates that Keppler got from individual plants. And when it comes to independent lab and field studies confirming that plants do indeed make methane and explaining how, the research community is almost eerily silent.
Keppler says his own move from the Max Planck Institute (MPI) for Nuclear Physics to the MPI for Chemistry (Germany) slowed down research for a while. And Khalil's lab stalled out on confirmation work after a colleague had to quit the project. Keppler says it's "just a matter of time," though, before groups he's spoken to publish confirmatory results. He says his group is now "making progress" on a mechanism too, though he's not giving away any details.
As for why no one else has confirmed emissions, both groups cite the painstaking methods needed to measure subatmospheric levels of methane. Khalil's group saw methane levels rise in a preliminary test with a live plant, only to find that lab air was seeping back into the test chamber through a tiny leak. Khalil still thinks that the original work was "probably correct" in its basic finding, though, because it explains otherwise unaccounted for methane over the tropics and in ice cores.
The new study uses a "more accurate" approach that refines the initial "rough" estimate, Keppler says. He maintains that the new paper "confirms that plants might be an important source of atmospheric methane . . . [but] as long as we don't exactly know each source's strength, it's difficult to make any predictions for the future."
Meanwhile, scientists seem a bit unsure what to do with the global flux estimates. "People are using, or not using, these measurements from plants as it suits them right now," Butenhoff says. If a budget doesn't balance, "you can add plants," he says, "but then you can leave [the plant source] out if you don't need it, because it hasn't been proven." He adds that "you can model it and model it," but without more measurements directly from plants, "there's only so far you can go."
In any global budget, "there's always a bit of slush," Khalil says. For now, the plant source of methane has become a budgeting slush fund, and the account just got a little smaller.
