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A carbon sweet spot

For a minute, I thought I had stepped into that scene from Lawrence of Arabia where T.E. Lawrence, approaching the Suez Canal, sees a ship sailing across the sand. Only I saw it in a farm field with cattle.

I had parked on a levee at the north end of Twitchell Island, in the middle of the great Sacramento-San Joaquin river Delta, east of San Francisco. In front of me was prime farmland – a busy mosaic of pasture land and row crops as far as the eye could see – and just beyond a slight rise in the distance I saw a big cargo tanker plowing its way slowly across the farm field. Of course, it was plowing the middle of the San Joaquin River instead.

Like the ship in Lawrence, there was a great deal of symbolism in that image, including what it said about human industriousness. I didn’t drive all the way out to Twitchell Island, however, to park on a levee and muse on Progress – I went there to see a fascinating carbon experiment.

Once upon a time, the Delta was a vast freshwater marsh thick with tule reeds, cattails, and abundant wildlife. At least six thousand years old, the marsh caught sediment that washed down annually from the Sierra Nevadas, building up soil that eventually extended sixty feet deep in places. And what soil it was! When the delta began to be settled in the 1860s following John Marshall’s famous gold strike on the South Fork of the American River, the farmers couldn’t believe their luck. Since the soil had been often submerged, a consequence of flat terrain, frequent flooding, and tidal action, it had essentially become peat, rich in carbon and other organic minerals. Crops grew quickly and vigorously in this rich soil, and its owners grew equally rich. Soon, a new gold rush was on – to claim land in the Delta, drain it, and grow row crops by the bushel-load.

Here’s a photo of the Delta today:


Fast-forward to today, and the Delta is in big trouble. Innumerable dikes, ditches, and levees have broken up the marsh into 57 separate islands, 98% of which are now below sea level. Pumps work continuously to keep the roots of the crops dry enough to grow. Salt intrusion from the Bay is creeping inland, threatening not only the crops but the drinking water supply for two-thirds of all Californians and much of its agriculture in the Central Valley. Not many people know that California is a vast plumbing project, cris-crossed by a complex network of canals, ditches, water pipes, and pumping stations – and most of the water in this plumbing system has its origin in the southern part of the Sacramento-San Joaquin Delta. That means keeping saltwater away from these metal ‘straws’ is of paramount importance.

However, the islands are sinking, sea level is rising, and the 1100 miles of levees that protect it all are feeling the stress, literally. It’s called ‘subsidence’ and it places tremendous hydrostatic pressure on the levees, requiring that they be constantly raised and endlessly maintained – creating perpetual anxiety. What if a California-like earthquake struck the region? What if the levees were breached by a massive flood? What if salt water poured through, ruining crops and drinking supplies?

It’s the sort of scenario that keeps water managers up at night.

In an attempt to alleviate these worries, in 1997 a group of scientists with the U.S. Geological Service in Sacramento came up with a novel idea: employ nature, not technology, to reverse the subsidence. Here was their bright idea in a nutshell: when the early farmers drained and ditched the Delta, they exposed the peat soil to the atmosphere, causing the organic material – previously under water – to oxidize rapidly. The carbon in the soil literally blew away (adding to the CO2 load in the atmosphere, by the way), causing the land to compact and subside over time. That’s how the islands ended up below sea level – as much as 25 feet in some places! The scientists wondered: could this process be reversed? In other words, could the land be built back up if the marsh ecology, including periodic flooding, could be resurrected?

To find out, the scientists implemented an experiment on two 7-acre, side-by-side plots of farmland adjacent to a ditch that bisected Twitchell Island. They flooded the western plot to a depth of 25cm, and the eastern plot to 55cm. Tules were planted in a small portion of both plots. By the end of the first growing season, cattails had colonized both plots (the seeds arriving on the wind), which provided a screen for other plants, including duckweed and mosquito fern. Then things really took off. After just a few short years, the western plot had developed a dense canopy of marsh plants, as did the eastern plot, though it maintained some open water.

Then they took measurements of the soil. Peat is formed in an anaerobic (oxygen-less) environment – i.e., underwater – which means not all the plant material is “eaten up” by soil microbes. Some stays in place. A lot, in fact. The scientists were amazed to discover that after seven years the soil in both plots had risen 10 inches, the result of 15 tons of plant material growing and dying per acre per year, according to a report that I read. This answered their question: subsidence could be reversed by returning natural marsh processes to the land, quickly too. Here’s how one scientist put it: “Our results show that restored non-tidal, impounded wetlands with managed hydrology can produce large short-term rates of land-surface elevation gain.”

But the good news was just beginning. The researchers next tested the amount of CO2 that had been sequestered in this new soil as a result of their experiment. They suspected that 10 inches of dense, carbon-rich peat soil likely soaked up a lot of atmospheric CO2 – and they were right. In fact, as much as 25 metric tons per acre per year were sequestered in the study plots, according to their analysis. In comparison, a typical passenger vehicle emits 5 metric tons of CO2 per year. So, the fourteen acres in the study plots sequestered the equivalent emissions of 70 passenger vehicles per year! And that doesn’t even count the CO2 emissions eliminated by not farming the land. And that doesn’t count all the other ecosystem services generated by a functioning marsh, including water purification and wildlife habitat.

Of course, there are a lot of “what ifs” raised by this project, including a big one: why would a farmer trade profit-making farmland for a “valueless” marsh? The quick answer is: pay him or her to sequester CO2 instead. The researchers called what they did a “carbon-capture farm” and hoped that the project would demonstrate that it is highly feasible to use managed wetlands to sequester carbon and reduce subsidence at the same time. The key word is managed, which raised another big “what if.” As a USGS briefing paper said in a typically understated way: “Large scale efforts to manage the environment have a decidedly mixed record of success.”

The results of this project are also likely limited to the Sacramento-San Joaquin Delta, which is a phenomenally productive patch of the planet, requiring that inferences to other locations be made carefully. Nevertheless, it is a very good example of a “sweet spot” – a place where one can get a huge carbon return for a small investment. For example, the Twitchell Island project (1) reversed subsidence; (2) reduced the risk of levee failure; (3) demonstrated carbon sequestration; (4) provided wildlife habitat, especially for birds on the Pacific flyway; and (5) pointed the way for significant economic returns to the private landowner by growing carbon. If we can figure out a mechanism by which a landowner could be paid for doing this things, rather than paid to continue depleting the land of its carbon, then we could be looking at a whole different type of economy.

Right now, however, that sort of economy is a mirage, though perhaps no more so than a ship sailing across a farm field.

Here’s a dramatic photo of a Twitchell study plot:


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