Could Protecting Levees Be a Key in Halting Global Warming?

By Joan Melcher, Miller-McCune.com. Posted January 6, 2009.

Methane gas is released when a methane molecule uses carbon dioxide to produce energy. When sulfate is available in a wetland, however, the production of methane is inhibited.

Because sulfate is generally available in tidal wetlands, fluxes of methane are low in these areas. Marine wetlands, therefore, are getting attention as carbon sinks, while freshwater wetlands, such as those of the Sacramento-San Joaquin River Delta, continue to be viewed as marginal.

Chmura researches carbon sequestration in tidal marshes and has authored a paper on carbon accumulated in the Atlantic Ocean's Bay of Fundy marshes that recommends marine wetlands be included in global carbon budgets. She described the rate of accretion in the USGS project as "phenomenal," noting "it's four times the average for marine tidal wetlands."

However, "if they release methane, they need to have more like 70 times the carbon stored."

Scott Bridgham, an ecosystem ecologist at the University of Oregon, also doubts the overall benefit when it comes to global warming. He has seen some of Miller's data and thinks the rate of carbon sequestration will end up being "a wash" once methane emissions are factored in.

Bridgham is the lead author of the chapter on wetlands in The First State of the Carbon Cycle Report of the U.S. Climate Change Science Program and a leading researcher in methane release from wetlands. His work has shown freshwater wetlands are not likely to sequester enough carbon to offset methane release.

But It's Different Here

Miller counters that Bridgham's study does make a possible exception for certain peat soils because restoration keeps carbon in the soil while adding new carbon through plant growth and decomposition.

Another factor Miller identifies is high rates of sulfate in her study area compared with other freshwater wetlands, likely because the delta is hydrologically connected to the sea. This, Miller said, may give it some marine wetland advantages.

To date, measurements of methane release have varied widely at Twitchell Island, but the USGS plans to conduct a complete greenhouse gas inventory. "We'll have a lot of that up and running by the next growing season," she said.

Other concerns the researchers will be addressing are nitrous oxide emissions and production of methyl mercury.

Miller believes that the greenhouse gas balance from the project, when the huge amounts of carbon sink are tallied with methane emissions, will show carbon capture outweighing the release of methane gases.

Chmura and Bridgham will no doubt be interested in the results. There is one thing all the scientists will agree on: There are many ecosystem services associated with saving wetlands, among them ensuring a safe water supply, providing habitat to many species, coastal protection, flood regulation and recreation.

Carbon capture may be another, but the bets are still out.

Miller, for one, is quick to note that what may work at Twitchell Island won't work in many other wetlands. "We have specific conditions that occur in our wetlands that don't occur everywhere," she said. In her 15-acre study area, the 10-month growing season, peat soils and biogeochemical conditions favor high carbon production and low methane release.

What she is most pleased with is that she has been able to study this wetland for 10 years, a rare occurrence in scientific research: "It's been incredibly interesting to study it and access it as long as we have - to see it become a much more carbon-capturing environment."

It helps that she may be killing two birds with one study.

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