Protect a Levee, Protect the World

A method of buttressing California's aging levees shows promise for capturing carbon dioxide.
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A method of buttressing California's aging levees shows promise for capturing carbon dioxide.

It's obvious that carbon is stored in wetlands. But could it be stored at a rate that would merit their inclusion in carbon cap-and-trade programs?

That question has been asked since researchers looking at the safety of levees uncovered a promising way to capture atmospheric carbon. The preliminary answer is a definite ... maybe.

Well before Katrina, scientists studying central California's Sacramento-San Joaquin River Delta speculated that restoring wetlands on abandoned farmland would mitigate the hydraulic force on miles of delta levees, which in some places hold back 20 feet of water. Then, Katrina's devastation of New Orleans drew national attention to concerns about the delta's aging levees and the potential for another catastrophic failure.

Exacerbating the problem was the likelihood of certain disasters (such as California's looming "Big One") allowing saltwater intrusion from San Francisco Bay, a threat to millions of acres of farmland in the state's Central Valley as well as freshwater supplies for some 25 million Californians.

As U.S. Geological Survey scientists studied the subsidence of land drained for agricultural uses in the delta, they began to notice surprisingly high rates of carbon captured — or accreted — in their study plots.

Could restoring these freshwater wetlands not only help save the levees, protect farmland and save freshwater supplies but also address global climate change? That was something USGS scientist Robin Miller said people inside and outside her 10-year-old project on Twitchell Island have started asking.

Let's Find Out

The USGS recently launched a three-year, $12.3 million project that will attempt to answer the question. Scientists from the USGS California Water Science Center and the University of California, Davis have joined Miller at Twitchell Island. If they find that "carbon capture" farming in the delta is a viable idea, farmers could be paid for restoring wetlands while helping save the planet.

Specifically, Miller's research has shown that as tule (a species of sedge grass also known as bulrush) and cattails grow in these soggy study plots, the land surface rises an average of 4 to 6 centimeters a year — from decomposing plants that form a peat soil — with some areas rising about 0.63 meter, or two feet, over 10 years. Typical rates documented in scientific literature are a centimeter or less a year, she said.

That decomposed matter contains lots of stored carbon. The project has shown an average of 1,000 grams of carbon per square meter per year has been captured over the past five years. That dwarfs the rate of carbon sequestration achieved in reforested agricultural land, which is typically less than 100 grams of carbon per square meter per year, according to a study by Gail Chmura, associate professor of geography at McGill University and director of the Global Environmental and Climate Change Center of Quebec.

The idea of looking at wetlands as carbon sinks is relatively new, and little scientific quantification has been made of wetlands' potential to offset global warming, Chmura said.

Scientists working with Miller put some numbers to what carbon-capture farming could do for California's efforts to offset greenhouse gas emissions. They claim restoring an area the size of subsided lands in the delta could see greenhouse gas reduction equal to turning all the SUVs in California into small hybrids or turning off all residential air conditioners in the state. They're looking to find a place for carbon farming in California's carbon-trading market, which state law requires be online by 2011.

But There's a Catch

Wait a minute, say other scientists, who note methane is typically released from freshwater wetlands, and methane is a much more potent greenhouse gas than carbon dioxide. To put it in perspective, one molecule of methane is equal to 70 molecules of carbon dioxide in the atmosphere, Chmura said.

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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