Grow wetlands, fight global warming?

In the Sacramento-San Joaquin Delta, just north of the San Francisco Bay, canals and cultivation long ago replaced the region’s original ecosystems. Once a huge freshwater marsh at the confluence of the Sacramento and San Joaquin rivers, the Delta is now a maze of slow-moving water, agricultural fields, and constructed islands protected by hundreds of miles of levees.

On one of these levee-protected islands, wetlands are reemerging. On a tract of land set amongst winding dirt roads, a group of scientists affiliated with the US Geological Survey (USGS) have been growing wetlands just as neighboring farmers do alfalfa and olives. These scientists say creating wetlands has the potential to mitigate climate change, bolster the safety of the state’s water supply, and create new economic potential for the region.

Since the mid-1990s, a fifteen-acre tract of Twitchell Island in the western Delta region has hosted their research. The Twitchell Island Wetlands Pilot Project was originally conceived to investigate the possibility of reversing land subsidence, the steady loss of land elevation due to compaction and soil erosion that threatens the stability of the Delta’s levees. This has created serious flooding potential if the levees fail, positioning the Delta as the next New Orleans.

The pilot project was designed to study how wetland growth might combat subsidence trends. As wetland plants die and decompose, their root mats and other decayed materials form layers over which new plants grow, increasing land surface over time. Elevation gains of more than ten inches were recorded in sections of the Twitchell Island wetlands from 1997 to 2005.

But scientists behind the pilot project soon realized that wetlands also offer another environmental gain: carbon sequestration. Wetland vegetation like cattails and tules use photosynthesis to capture carbon dioxide from the atmosphere, storing it in their plant tissue. Wetlands can sequester as many as 25 metric tons of atmospheric carbon dioxide per acre. Forests and marshlands only sequester six and nine tons per acre, respectively.

Three of these scientists—research chemists Roger Fujii and Brian Bergamaschi of the USGS and Dr. Stuart Siegel of the private consulting firm Wetlands and Water Resources, Inc.—are now proposing an investigation of the potential of “carbon-capture farming” in the Delta, which would entail shifting sections of farmland from low-value crops such as corn and alfalfa to carbon-sequestering wetland vegetation. So far, the concept is mostly untested—state budget cuts halted a large-scale experiment planned for 2009—but the scientists believe it has enormous potential. Carbon-capture farming, Bergamaschi says, is a “solution that restores land surface in the Delta, protects levees, and provides an income to farmers, sustaining our farming communities in the Delta. It’s better than a win-win situation.”

Fujii came up with the concept of carbon-capture farming in the early ‘90s after realizing that wetlands sequester carbon dioxide at impressive rates. It would work much like ordinary farming, except that profits would be generated by selling sequestered carbon credits rather than food crops. In order to grow wetlands, farmers would install water control structures and flood their fields. As wetlands mature, they would sequester carbon at faster rates, “meaning they would capture carbon dioxide out of the air and fix it into biomass, which is then pushed into their root system,” Bergmaschi explains. A farmer would make money by selling the credits, similar to emission-reduction credits, on the market to industries or companies that find reducing greenhouse gas emissions too expensive or impractical.

Managed wetlands would be very similar in plant distribution and habitat function to their natural counterparts, except that carbon-capture farms will be constantly flooded. “In natural wetland systems, there tends to be a succession of vegetation once the soil grows to knee-high waters,” Bergamaschi says. “That wouldn’t occur in [carbon-capture farms] because we are going to keep flooding them.” Keeping land flooded, Bergamaschi believes, has the potential to increase land elevation at faster rates.

Growing wetlands for profit is a novel concept, and its newness poses complications. “There are no other wetland projects that we are aware of that are in any sort of carbon market,” Bergamaschi says, and there’s also no system yet for verifying how much carbon a farm might sequester. As Bergamaschi points out, “There are no registries out there, no markets that are willing to accept wetlands, there’s no way to price it because there’s no one to gauge what the risk is with wetlands.”

If the market forces work, however, Bergamaschi and his colleagues believe that farming wetlands would not only benefit farmers and absorb greenhouse gases, but would also help shore up the Delta levee system, protecting California’s water supply. When the Delta marshlands were first diked and drained in the late 1880s, the flow of rich mineral sediment onto the Delta plains during the rainy season stopped. Sediment still flows through canals, however, which means that land elevations of the diked plains have dropped while elevations of adjacent river channels have risen. Soil loss from farming exacerbates these trends. As soil is exposed to the atmosphere for cultivation, oxygen promotes soil microbial activity. Those microbes then oxidize the organic matter in peat soils, causing soils to erode away. Today, nearly half of the Delta’s 740,000 acres are at least fifteen feet below sea level.

This has led to instability in the levee system. To compensate for the rising elevations of river channels, levees have also been raised. This expands their bases onto adjacent marsh, sand, and peat beds, materials that are inherently weak and unstable. The elevation differences between river and drainage ditches result in water seepage through and under levees. Compaction of the Delta’s peat soils, which form the foundations for many levees, has led to cracking and slumping. Because much of the region sits below sea level, were these levees to fail, saltwater from the bay would most likely be drawn into the Delta, turning the statewide water hub into an unusable saline lake.

According to a 2007 report by the Public Policy Institute of California, approximately 1,100 miles of Delta levees are fragile. “The cumulative likelihood of islands flooding over time… [is] very high and increasing. When you factor in sea level rise that increases the potential risks,” warns Dr. Ellen Hanak, the institute’s senior research fellow. Between 1967 and 1992, fifteen major islands were flooded due to levee failure.

Levee failure threatens not only the state’s agriculture but its drinking water. According to Hanak, Delta water supplies “about a third of the Bay Area’s urban use, about a third of the San Joaquin Valley’s agricultural use, and about a third of urban Southern California’s water use.” Altogether, over 23 million Californians rely on the Delta levee system for water.

To Siegel, Bergamaschi, and Fujii, carbon-capture farming is a long-term solution to this problem, too. They believe its widespread adoption would rebuild the Delta’s unique peat soils and lessen pressure on the levees. “There would be tremendous benefits from doing this,” Siegel said. “We’d have better levee protection, local climate moderation effects, and huge reduction in waste water input into Delta waterways.”

But not everyone is as convinced that carbon-capture farming is the solution to the Delta’s subsidence woes. Hanak calls for more effective management of the existing water infrastructure and the construction of a peripheral canal. And Bruce Blodgett, executive director of the Delta Farmers Association, says that wetlands “would be a pretty disastrous turn” for agriculture. “I can see where people would say ‘This is a real feel-good project,’” he says. “It’s actually not if you’re looking from a food production capacity standpoint, the ability of our country to feed itself, our state to feed itself.”

According to Blodgett, turning over agricultural land to wetland production would be a serious blow to local and state economies. Irrigated agriculture generates $27 billion annually for California. Delta land, used to grow everything from wheat to blueberries, is a significant contributor to that revenue. Food production also supports
associated industries like manufacturing, trucking, and packaging. The only commodity produced by farming wetlands would be carbon credits, which don’t require transportation, processing, or packaging, and would leave those industries high and dry.

Worse, Blodgett adds, growing wetland plants like cattails would lead to higher demand for water in an already drought-stricken state. According to USGS research, managed wetlands use about ten percent more water than common Delta crops like corn. “We’re not in a situation where we can start adding a lot more water-intensive crops,” Blodgett says. If wetland production starts up, he says, “you’re going to have a lot higher water usage, and you’re going to have no economic productivity for related industries. In a county like this where we have record unemployment… it’s an absolute disaster.”

As Bergamaschi sees it, the future well-being of the Delta depends on selecting the best use for Delta lands. “We’re proposing to apply wetland farming in an area that’s under threat” of levee failure, he says. “We’re not proposing that wetland farming replace agriculture in the Central Valley.” There must be a balance, he says, between food production
and using active agricultural land to store carbon.

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