Mapping methane reduction potential of tidal wetland restoration in the United States

Effective management of ecosystem carbon sinks is crucial for climate change mitigation. Natural climate solutions (NCS), such as the intentional management of carbon sinks in coastal ecosystems (blue carbon), offer advantages including rapid implementation, relatively low cost, and co-benefits for ecosystem health. While uncertainties exist, NCS are widely recognized as essential for climate change mitigation alongside technological decarbonization efforts. Coastal blue carbon, encompassing salt marshes, mangroves, and seagrass ecosystems, demonstrates potential for climate mitigation due to high rates of soil carbon storage driven by feedbacks between soil formation and sea-level rise. A key advantage of saline tidal wetlands is their minimal methane (CH₄) emissions, owing to sulfate reduction dominating methanogenesis in sulfate-rich soils. Restoring tidal seawater exchange to wetlands currently impounded by structures like dikes, which leads to artificial freshening and increased CH₄ emissions, represents a significant opportunity for greenhouse gas management. This restoration strategy offers substantial mitigation benefits via avoided emissions that are not reversible, unlike stock accumulations in other systems. The US, having included coastal wetlands in its National Greenhouse Gas Inventory, is actively investigating wetland restoration for climate mitigation. Previous estimates of US blue carbon NCS potential were based on limited data and lacked detailed maps of potential restoration sites. This study addresses these limitations by using GIS to combine multiple public datasets to map impounded wetlands and other potentially restorable habitats, estimating new emission factors, and quantifying the net-cooling potential of different restoration types at a national scale.

Several studies highlight the importance of coastal blue carbon in climate change mitigation. Chmura et al. (2003) demonstrated the significant carbon sequestration capacity of tidal saline wetland soils. McLeod et al. (2011) provided a blueprint for understanding the role of vegetated coastal habitats in CO₂ sequestration. Subsequent research (Ouyang & Lee, 2013; Howard et al., 2017) further emphasized the carbon storage potential of these ecosystems. Research on the resilience of tidal wetlands to sea-level rise (Morris et al., 2002; Kirwan & Megonigal, 2013; Gonneea et al., 2019; Rogers et al., 2019; Wang et al., 2019; Herbert et al., 2021) underlines the importance of their long-term carbon storage capacity. The influence of salinity on methane emissions from tidal marshes has been studied by Bartlett et al. (1987) and Poffenbarger et al. (2011), demonstrating the potential for methane reduction through salinity restoration. Kroeger et al. (2017) specifically highlighted the climate change mitigation potential of restoring tidal flow to impounded wetlands to reduce methane emissions. Previous assessments of US blue carbon potential (Fargione et al., 2018) utilized limited data and upscaling techniques, lacking the spatial detail and comprehensive analysis presented in this current study. This study builds upon existing literature by providing a refined and spatially explicit assessment of methane reduction potential from tidal wetland restoration across the contiguous United States.

This study utilized a Geographic Information System (GIS) approach to map and quantify the potential for methane reduction through tidal wetland restoration in the contiguous United States. The methodology involved several key steps: 1. **Data Compilation:** Multiple publicly available datasets were integrated, including coastal land cover classification from the Coastal Change Analysis Program (C-CAP), tidal elevation data, impoundment status from the National Wetlands Inventory (NWI), and protected areas data from the USGS Protected Areas Database. 2. **Impoundment Mapping and Accuracy Assessment:** The NWI data on impoundments underwent an independent accuracy assessment using a stratified random sampling design focused on protected areas. This assessment involved on-the-ground verification by experts to determine the accuracy of the NWI's impoundment classification. This allowed the researchers to estimate the true extent of impounded wetlands, which proved to be significantly underestimated in existing maps. 3. **Reference Wetland Salinity Mapping:** A reference wetland salinity map was created using C-CAP data for non-impounded wetlands. The salinity of the nearest non-impounded wetland was then assigned as the potential restored salinity for impounded areas and low-elevation uplands. This provided a spatially explicit estimate of potential salinity changes after restoration. 4. **Emissions Factor Estimation:** Updated methane (CH₄) emission factors, incorporating salinity levels, were used, drawn from literature and refined using a survey of coastal managers. The survey helped account for the complexities of restoration outcomes (i.e. not all restorations will result in increased salinity). A zero-inflated approach was used to account for scenarios where restoration might not result in a net cooling effect. 5. **Scenario Development:** A detailed scenario analysis was conducted using GIS and the manager survey to consider different restoration types and their likelihood of success. A two-class salinity scale (estuarine and palustrine) was primarily used in the analysis, while a finer six-step classification was used for the expert survey. Monte Carlo simulations were employed to propagate uncertainty in area estimates, emissions factors, and other parameters. 6. **Net-Cooling Probability Estimation:** The likelihood of each restoration type resulting in a net-cooling effect (i.e., greater greenhouse gas emission reduction than any associated increases) over a 100-year timeframe was determined using additional Monte Carlo simulations. This ensured that only net-beneficial restoration scenarios were included in the final estimations. 7. **Candidate Site Mapping:** A map of potential restoration sites was generated, focusing on parcels within protected areas that had a high probability of achieving at least 1 metric tonne CO₂e y⁻¹ reduction. The spatial proximity of these parcels to reference wetlands was also analyzed. 8. **Uncertainty Analysis:** Monte Carlo simulations were used throughout the analysis to propagate uncertainty in area estimates, emissions factors, and other parameters, producing robust estimates of total emissions reduction and confidence intervals.

This study yielded several key findings: 1. **Underestimation of Impounded Wetlands:** The study found that existing maps significantly underestimated the area of impounded coastal wetlands in the contiguous US by approximately 50%. This highlights a critical data gap in previous assessments of blue carbon potential. 2. **High Potential for Methane Reduction:** The study estimated the potential for methane reduction from tidal wetland restoration to be 0.91 Tg CO₂e y⁻¹ (95% confidence interval: 0.42–1.60 Tg CO₂e y⁻¹). This is a more conservative estimate than previous assessments, which often relied on less spatially explicit data and potentially overestimated the area of suitable restoration sites. 3. **Greatest Benefit from Palustrine to Estuarine Conversions:** Restoring impounded palustrine (freshwater) wetlands to estuarine (brackish or saline) conditions showed the highest potential for methane reduction per unit area. While these conversions were less prevalent than other types, their high emission reduction potential makes them high priorities for future restoration. 4. **Geographic Distribution of Potential:** The study identified Louisiana, California, and South Carolina as states with the highest concentrations of potential for methane reduction. This spatial information allows for targeted restoration efforts in regions where the potential impact is greatest. 5. **Land Management Implications:** A substantial portion (54%) of the identified restoration potential lies within existing protected areas. The study also highlighted the key roles of the US Fish and Wildlife Service and various state agencies in managing areas with high potential for emissions reduction. This emphasizes the importance of collaboration between government agencies and conservation organizations in implementing restoration projects. 6. **Comparison with Prior Assessments:** This study's estimate of 0.91 Tg CO₂e y⁻¹ is significantly lower than previous estimates (12.0 Tg CO₂e y⁻¹). This difference stems from a more accurate assessment of impounded wetland area and more refined emission factors, highlighting the need for rigorous data and methods in blue carbon assessments. The study discusses several potential biases in both prior and current methods, including the accuracy of the C-CAP and NWI data and the possibility of under-representing incidental impoundments. 7. **Potential for Future Revisions:** The researchers acknowledge the possibility of further revisions to national assessments based on ongoing data collection and refined understanding of emissions factors, particularly those related to CH₄ emissions from impounded wetlands and accounting for carbon burial within impoundments. The inclusion of additional data or improvements to the accuracy of existing data, such as the C-CAP and NWI datasets, will improve the precision of future estimates.

This revised assessment of coastal methane emissions reduction potential in the contiguous US highlights three key takeaways: the more conservative estimate compared to previous studies, the potential for future revisions given data limitations and new information, and the widespread availability of potential restoration opportunities, many of which are already planned. The lower estimate (0.91 Tg CO₂e y⁻¹) compared to earlier studies (12.0 Tg CO₂e y⁻¹) is primarily attributed to a more accurate assessment of artificially freshened wetland area and slightly different emission factors. The study meticulously explores potential sources of bias in both this and prior studies, including possible underestimation of impoundment area due to limitations in available data and methodological approaches. The expert survey may have also underestimated the number of potentially salinizing restorations. Further investigation is needed to refine the accuracy of both past and present estimates. The study addresses the limitations of previous research through an improved data collection process and improved analysis techniques. The findings highlight the importance of detailed, spatially explicit assessments that account for uncertainty in determining the true potential of blue carbon interventions. The spatially explicit approach revealed that while high potential emission reductions are geographically dispersed, certain areas, organizations, and land management entities have a higher concentration of sites with the highest potential to benefit from such efforts.

This study provides a refined, spatially explicit, and more conservative assessment of the potential climate benefits of restoring tidal flow to impounded coastal wetlands in the contiguous US. The estimated methane reduction potential of 0.91 Tg CO₂e y⁻¹ represents a significant, though not massive, climate benefit, comparable to a portion of the total annual carbon burial in US coastal wetlands. The highest potential reductions per unit area are found in relatively rare conversions of palustrine to estuarine conditions. Future studies should focus on improving the accuracy of impounded wetland area estimates and refining methane emission factors. The provided map of 1796 parcels in protected areas with high restoration potential serves as a valuable resource for prioritizing and guiding future blue carbon projects.

While this study significantly advances the understanding of methane reduction potential through tidal wetland restoration, several limitations should be acknowledged. The accuracy assessment of impoundment maps was limited to protected areas, potentially leading to an underestimation of the national extent of impoundments. The reliance on existing datasets, such as C-CAP and NWI, introduced uncertainties that may affect the overall accuracy of the estimations. The study focused primarily on methane emissions, while other greenhouse gases (e.g., nitrous oxide) were not considered. Finally, the analysis did not account for the economic and political feasibility of restoration projects, focusing solely on the potential climate benefits. Future research should strive to address these limitations.