Global methane cycles in flux
Methane rise slows down, but questions remain as to whether such a leveling off will continue.
Although the concentration of methane in the atmosphere nearly doubled over the past century, fluxes of this greenhouse gas seemingly leveled off during the past 20 years. In a review published today on ES&T’s Research ASAP website (DOI: 10.1021/es061791t), researchers, led by Aslam Khalil of Portland State University, model the sources and sinks of methane. They also question whether human activities will have much of an impact on future methane levels.
Methane, which is 23 times more powerful than CO2 as a greenhouse gas, is emitted from numerous sources, including wetlands, cows, and thawing arctic permafrost. But the gas is also consumed by microbes and stored geologically, making its cycles hard to pinpoint, despite several decades of research. Recent reports have documented tropical-upland sources and reassessed emissions from arctic lakes, adding to previously known natural emissions. “People keep finding sources, but nobody’s finding the sinks,” says Patrick Crill, a biogeochemist at Stockholm University. “Understanding the root causes of the balance will be important, especially if CH4 begins to increase again.”
In their new assessment, Khalil and co-workers model recent atmospheric methane trends, and find a drop in methane fluxes from 25 parts per billion per year (ppb/yr) in 1980 to about 5 ppb/yr in 2004. Their models also show that methane emissions “disturbances”—changes in fluxes—originated at northern latitudes and rippled out to affect the rest of the globe. They hypothesize that possible fluctuations in hydroxyl radicals, which destroy methane, could have modulated the gas’s concentrations during the past 20 years.
Khalil and co-workers also postulate that the agricultural component of methane—which accounts for about 20% of the global budget of approximately 500 teragrams a year—will not change much in the near future. Organic fertilizer inputs, the amount of land flooded for growing rice, and political and sociological changes “are the three factors that are difficult to quantify,” Khalil says. “All of them are reducing emissions” for the moment, he adds.
Changes in these three factors in China, in particular, could be responsible for a major methane slowdown, Khalil says. Anoxic conditions in flooded fields make rice paddies perfect for producing methane. The gas percolates up through soils or through plants themselves, which act like drinking-straw conduits to the atmosphere. Organic fertilizer (such as hay or manure) provides carbon inputs that can stimulate methane production.
Khalil’s team has confirmed previous research showing that flooded fields release six times more methane than intermittently flooded fields. Straw applied as fertilizer leads to twice as much methane from intermittently flooded fields than from bare, drained fields, according to initial data presented by Khalil at the American Geophysical Union annual meeting last December.
Anecdotal evidence shows that Chinese farmers now let their fields dry out and that they are no longer applying as much organic fertilizer, Khalil notes. As-yet-unpublished surveys, he says, show that farmers are also using modified rice plants that require less time to grow. Once farmers meet their government quotas, they may choose not to recultivate that acreage with rice that year, further cutting down on the time that land is flooded and producing methane.
But future changes in human land use remain challenging to predict, says Hugo Denier van der Gon, of TNO Built Environment and Geosciences (The Netherlands). One confounding factor will be the growing demand for rice, he comments. Current methane production estimates use agricultural data and best guesses to scale up to the global level. Satellites do not have the technology to document water well, whether flooded rice fields or wetlands. And although greenhouse and field trials have been important in the past, they are “nothing like what a real farmer does,” Denier van der Gon adds.
Nevertheless, says Ed Dlugokencky, a researcher at the National Oceanic and Atmospheric Administration, the detailed results from Khalil and colleagues’ rice mesocosm and field studies will be useful for models. But the conclusions in their review remain to be confirmed, he says.
The researchers present tentative evidence for what seems to be a 7.7-year cycle in the global fluxes of methane. That’s a surprising finding, Dlugokencky says, and “I don’t think it is really a cycle.” The anomalies in methane’s growth rate could be attributed to unrelated events.
