Carbon is the foundation of the Delta’s peat soils and a key component of two major greenhouse gases: carbon dioxide, which persists in the atmosphere indefinitely, and methane, a short-lived but highly potent greenhouse gas. Since the 1850s, draining the Delta’s soils has released over a billion tons of carbon dioxide into the atmosphere—equivalent to about a quarter of the United States’ annual emissions today. However, the process could potentially be reversed, making Delta wetland restoration a powerful ally in the quest to reduce greenhouse gas emissions. The critical role of Delta wetlands in addressing these challenges was a key focus of a panel discussion at the September 2025 meeting of the Delta Stewardship Council.
Oxidation of the Delta’s peat soils has not only contributed to climate change but also increased pressure on Delta levees, raising flood risks and threatening California’s water supply, while reducing farmland.
“However, re-wetting our Delta soils can reverse that, and we know that now, because of long-term high-quality data,” said Dr. Lisamarie Windham-Myers, Delta Lead Scientist. “Those climate benefits may seem like they’re far out in the future, but they’re actually coming faster than we expected.”
The Delta as a carbon sink
Delta wetlands are among the world’s most significant carbon sinks, with tremendous long-term potential to absorb carbon and mitigate climate change, Dr. Kyle Delwiche, a research scientist at UC Berkeley, explained.
“As the plants grow in the wetland, they take up carbon dioxide, turning it into plant material. The plants die, get buried in the sediment, taking the carbon with them, and as long as they stay there and are covered by water, they won’t return to the atmosphere. So you create this really powerful, long-term net carbon sink,” said Dr. Delwiche.
The climate benefits of wetlands are multifaceted. Flooding the land prevents CO2 emissions, while plants sequester carbon. However, methane—a potent greenhouse gas—is naturally produced in wetlands, temporarily offsetting some of the carbon sequestration benefits. Unlike carbon dioxide, which remains in the atmosphere indefinitely unless removed, methane degrades over time due to atmospheric chemistry.
“At first, after the wetland is constructed, it is actually net warming of the climate,” said Dr. Delwiche. “This is because methane is such a powerful greenhouse gas on relatively short time horizons that it overpowers the effect of that carbon sink from a climate perspective.”
The benefits are not static; they change over time due to atmospheric chemistry dynamics. The graphic shows the climate benefits of a project, with the line representing the breakeven point; if it is above the line, the project is net cooling of the atmosphere. If the project is below, it’s net warming. Initially, restored wetlands may contribute to net warming because of methane’s short-term potency. As methane breaks down in the atmosphere, the wetland transitions to a net cooling effect, provided it remains flooded.
Extended monitoring in the Delta reveals methane and carbon trends
Long-term data is demonstrating the carbon benefits of Delta wetland restoration and delivering insights that shorter-term monitoring would have missed. In the article, Methane emissions in a restored Delta wetland highlight the value of long-term data, Delwiche et al. (2025) studied greenhouse gas exchanges on the restored Mayberry wetland, a 300-acre area of Sherman Island in the Sacramento-San Joaquin Delta that was reflooded in 2010. As part of the restoration, an eddy covariance tower was installed to continuously measure and analyze the exchange of energy, water vapor, carbon dioxide, and methane between the wetland and the atmosphere. This system provides ongoing data to calculate carbon fluxes in and out of the wetland.
The slide shows data from the Mayberry wetland project on Sherman Island. The top plot shows the carbon dioxide sink, and the bottom shows methane emissions.
The data shows the seasonal patterns; more happens in the summer, when it’s warm. She noted the decreasing emissions trend. “Over the past decade, methane has been steadily going down, except for last year, but it’s been steadily going down, which is good news for the climate and really shows how important it is to keep monitoring these wetlands, because they change so much after you restore them.”
She pointed out that if they had only five years of data, they would have calculated the breakeven point at 150 years to reach the breakeven point and start cooling the climate. With the new long-term data, that calculation has come down to about 40 years.
“So this is good news for the climate, and it really shows us the value of these long-term data sets and the continued monitoring.”
Xavier Mascareñas / California Department of Water Resources
Delta wetlands a key solution to meeting climate goals
California has set ambitious climate goals, aiming to cut emissions to 40% below 1990 levels by 2030 and achieve carbon neutrality by 2045. The Department of Water Resources (DWR) has an even more aggressive target, striving for carbon neutrality by 2035—ten years ahead of the state’s timeline.
“DWR considers Delta wetland restoration as one of its key nature-based climate solutions to meet both DWR and statewide climate goals,” said Dr. Tyler Anthony, Greenhouse gas emissions reduction specialist & lead specialist on carbon sequestration strategies at DWR. “So DWR is leading this restoration to reverse subsidence, sequester carbon, enhance habitat, and essentially protect water quality with targeted restoration across the Delta.”
Since 1997, DWR has been working with other agencies, academic institutions, and NGOs on subsidence and carbon research in the Delta. These efforts have produced long-term datasets, including eddy covariance data, which is the gold standard for measuring greenhouse gas exchange with the atmosphere. These datasets were instrumental in developing a protocol, created by the Delta Conservancy and adopted by the American Carbon Registry (ACR) in 2017, to generate carbon credits at scale.
“Today, we are generating credits from Sherman and Twitchell subsidence reversal wetlands. Our first round of verification, through 2018, generated about 52,000 carbon credits, which is equivalent to about 52,000 tons of carbon dioxide either avoided or sequestered. We’re in the middle of verification projects for another 60,000 that we hope to get by the end of the year.”
So, what is a carbon credit? In simple terms, a carbon credit is a verified reduction in greenhouse gas emissions. Delta wetland restoration can generate these credits through the American Carbon Registry, a global registry for the voluntary market. The credits represent both the net sequestration of carbon and the prevention of emissions from alternative land uses, such as drained peat soils, which release significant amounts of carbon. Organizations can purchase these credits on the voluntary market to offset their emissions and meet their net-zero or sustainability goals.
So far, an acre generates about six credits per year. The proceeds of any sales of Delta carbon credits could be used to fund further Delta restoration projects and offset long-term management costs; however, DWR has yet to sell any.
The value of these credits is currently uncertain. DWR is the only entity to have generated wetland restoration credits in the Delta, and because they haven’t been sold, their true value is unknown. “These are voluntary market credits, and the price of a voluntary market credit is really up to the seller and the buyer. So we need to have both a price that we’re willing to sell them for and a buyer willing to pay that price to sell these credits.”
To scale carbon credit projects across the Delta and benefit the wider community, Dr. Anthony said it is essential to leverage high-quality datasets, models, and remote sensing to reduce costs and upscale estimates, as measuring everywhere is impractical. By targeting areas with the highest emissions and sequestration potential, restoration efforts can maximize benefits and generate revenue.
Classifying wetland restoration as “carbon farming” could reduce implementation barriers and broaden participation, he said. Additionally, fostering public-private partnerships can address capital investment gaps on public lands and attract external funding, accelerating the implementation of these projects as viable climate solutions and revenue streams.
“We are trying to explore selling these credits for another reason,” said Dr. Anthony. “CARB has suggested that they would like to see some supply and demand for these credits on the voluntary market before you can adapt this methodology into the regulatory market. And the regulatory market automatically demands a higher value for these credits and continued demand for these into the future because of the recent extension of the program.”
FEATURED PAPER: Dynamic methane emissions in a restored wetland: Decadal insights into uncertain climate outcomes and critical science needs
Authored by Kyle Delwiche, Jaclyn Hatala Matthes, Ariane Arias-Ortiz, Sara H. Knox, Patty Oikawa, Cove Sturtevant, Joseph Verfaillie, Daphne Szutu, Trevor F. Keenan, Dennis Baldocchi
Wetland restoration is increasingly viewed as a strategy for long-term carbon sequestration. However, methane (CH₄) emissions from restored wetlands can significantly offset their climate benefits. In this study, we analyzed one of the longest quasi-continuous methane eddy covariance datasets, spanning over a decade from the Mayberry wetland in the Sacramento-San Joaquin Delta, CA. Methane emissions at Mayberry initially spiked post restoration but have since declined, and this interannual trend positively affects the natural climate change solution potential of the restoration project. Using random forest analysis we find that the decadal trend in decreasing emissions aligns with a decadal trend in vegetation infill. A recent uptick in methane emissions is aligned with a decrease in porewater conductivity, indicating that porewater chemistry may also play a dominant role in driving methane fluxes. The isotopic signal of methane accumulated in sediments were remarkably stable over the past decade, indicating minimal changes in carbon lability of the re-flooded peat. We find that on a diel scale, latent heat is by far the dominant predictor for methane emissions, highlighting the role of diurnal patterns in plant transpiration. On seasonal timescales changes in water table depth and surface water conductivity help explain methane emissions. Our results emphasize the unique value of eddy-covariance and ancillary measurements initiated at the start of restoration in elucidating long-term methane dynamics in restored wetlands. They also highlight the critical need for expanded environmental data, such as porewater chemistry and vegetation changes, to comprehensively capture the factors driving methane flux.
