Methane emissions offset some blue carbon burial benefits

by Mary Caperton Morton
Monday, September 10, 2018

A new study measuring emissions from tropical mangroves, like this one in Australia, suggests that the mangroves' methane emissions are higher than previously thought. Credit: Gentlefriends, CC BY-SA 4.0.

Wetlands are prolific sinks for atmospheric carbon. They pull carbon dioxide out of the air and sequester the carbon in plants, soils and sediments. But there’s a catch: Wetlands also emit methane, an even more potent, albeit far less abundant, greenhouse gas than carbon dioxide. Two new studies, one measuring methane emissions from a rehabilitated freshwater peatland in California and the other looking at emissions from tropical mangrove forests in Australia, are revealing that these so-called “blue carbon” sinks may emit much more methane than previously thought.

The Sacramento-San Joaquin Delta in Northern California’s Central Valley is an expansive estuary that covers 2,800 square kilometers at the confluence of the Sacramento and San Joaquin rivers. Organic matter built up over the last 5,000 years has produced layers of carbon-rich peat up to 15 meters thick. But human intervention in the form of water diversion, levee building and agricultural development over the last 150 years has drained the marsh and fundamentally changed the landscape, resulting in extreme subsidence of the delta’s land surface. “In many places in the delta, the land surface is up to 10 meters below the river, which is held back by a levee,” says Kyle Hemes, an ecologist at the University of California, Berkeley, and lead author of a new study published in Geophysical Research Letters.

In recent years, efforts have been underway to rehabilitate the Sacramento-San Joaquin Delta through a method known as wet restoration, in which the drained peatlands are flooded to produce wetlands, creating new habitat and slowing the release of sequestered soil carbon.

Wet restoration of drained peatlands to wetlands has been promoted as a way to help mitigate climate change. Previous studies have estimated that such wetlands could sequester thousands of tera­grams (1 teragram is equal to 1 trillion grams) of carbon each year on a global scale. But few studies have attempted to quantify methane emissions from such wetlands. To assess the methane flux of restored wetlands in the Sacramento-San Joaquin Delta, scientists installed a network of eight micrometeorological towers throughout the delta to take near-continuous measurements — 20 times a second — of the exchange of methane between the wetlands and the atmosphere.

The 14-year study found that the delta emits as much as 590 milligrams of methane per square meter per day, with annual sums ranging from 27 to 63 grams of methane per square meter per year. “These rates are incredibly high. It’s really a striking study,” says Benjamin Poulter, an environmental scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., who was not involved in the new study.

But while the numbers are higher than expected, they represent a fraction of global methane emissions, which have been increasing in the last 10 years, Poulter says. “At the local scale, the methane emissions are quite high, but when you scale that up and look at it in the context of all the other methane sources, it’s unlikely that the biogenic methane emissions from wetlands are contributing to this renewed growth in methane concentrations we’re seeing in the atmosphere,” he says. Rather, this increase is thought to be due to the biogenic contributions of agriculture — particularly large-scale rice and livestock production — as well as fossil fuel emissions.

Studies like this that provide detailed observations of methane emissions at local scales are really helpful to determine what affects the global methane budget, Poulter says.

The ongoing monitoring of the rehabilitation of the Sacramento-San Joaquin Delta represents one of the longest-running studies of its kind, Hemes says. Long-term studies are important as little is known about how the carbon cycle evolves in a rehabilitated ecosystem like the delta. “Within an ecosystem, there’s going to be some amount of interannual variability. For example, how do methane and carbon fluxes change in a drought year versus a wet year? Every year we get a new understanding of how these wetlands are changing and how they’re affecting the climate and atmosphere.”

In a separate study, published in Science Advances, an Australian team tallied methane emissions in three tropical mangrove creeks. “Our work shows that combined mangrove water and sediment methane emissions have the potential to partially offset blue carbon burial rates in mangrove sediments across the globe on average by 20 percent,” says lead author Judith Rosentreter, a biogeochemist at Southern Cross University in New South Wales, Australia. She and her colleagues also found that the offsets may be as high as 60 percent at midlatitudes, driven by both lower mangrove sediment burial rates and higher methane emissions.

The team combined its findings with previously published studies to estimate global methane emissions from mangrove habitats. They calculated that the 137,760 square kilometers of mangrove forests around the world emit about 0.27 teragrams of methane per year while sequestering about 31 teragrams of carbon a year. Since methane is a more potent greenhouse gas than carbon, relatively small methane emissions offset the effects of greater amounts of carbon dioxide sequestration. (For example, the 0.27 teragrams of methane released per year equates to about 6.4 teragrams of “carbon dioxide-equivalent” carbon when considered over a 20-year time frame, or 2.5 teragrams of “carbon dioxide-equivalent” carbon on a 100-year time frame.) While those are significant findings, Rosentreter says, more work is needed to refine these estimates and produce more accurate global carbon budgets. “We need to further explore methane emissions and carbon burial offsets by methane in other blue carbon habitats, such as sea grasses and salt marshes,” she says. “Most of the carbon dioxide and methane fluxes in coastal blue carbon ecosystems are probably underestimated.”

The findings of both studies suggest that, despite methane releases from wetlands, restoration of such ecosystems should not be scaled back to avoid methane emissions, Poulter says. “These wetlands systems provide tremendous biodiversity benefits, water storage and flood mitigation services.”

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