Tidal wetlands are extremely important for the environment, not only because of the role they play in preserving biodiversity, preventing erosion and supporting fisheries, but also because they help to remove carbon dioxide from the atmosphere and slow the decomposition of organic matter in soils that are wet and oxygen-poor.

However, these conditions also promote the release of methane, a greenhouse gas that is much more potent than carbon dioxide and has a greater potential to trap heat in the atmosphere. The effect of methane emissions counteracts the effect of carbon sequestration, so accurately determining and forecasting methane gas emissions in tidal wetlands is essential to assessing the climatic effects of restoring or degrading these natural environments.

The study, led by Ariane Arias-Ortiz, a UAB Department of Physics researcher and a member of the Marine and Environmental Biogeosciences research group at ICTA-UAB, analyzed methane flux data from 109 tidal wetlands across the U.S., focusing on factors such as climate, vegetation and sediment chemistry. It is the first time that such a large dataset of these emissions, along with a wide range of environmental and biogeochemical parameters, has been made available to the entire scientific community in a unified manner.

The study identified important spatial and temporal predictors of methane emissions that have interactive effects among environmental variables documented for the first time. Salinity was observed to be a dominant factor: saltier marshes emitted low levels of methane, while fresher marshes showed variable emissions. In freshwater marshes, warmer marshes emitted more methane, while marshes located above the floodplain (less flooded) emitted less.

The study also found that seasonal variations in methane emissions in the same ecosystem are strongly influenced by temperature—the higher the temperature, the higher the emission—and by plant carbon fixation and photosynthesis. Unlike inland wetlands, tidal marshes exhibit significant diurnal variations in methane emissions, which are influenced by plant activity, which can enhance root secretion during active photosynthesis, stimulate methane-producing microbes, or facilitate their transport through cavities in plant tissue. Furthermore, in zones with significant tidal activity, the highest emission levels occur as a periodic release of stored gas after each ebb. Using the data from the study, we can improve models to predict and facilitate the simulation of methane gas in tidal wetlands under a changing climate.

“Methane emissions have a huge impact, and their variability in tidal wetlands poses challenges when it comes to determining the proportion of greenhouse gases produced by these ecosystems. Predicting methane emissions is important for achieving environmental goals and improving climate models,” Ariane explains. “With this study, we can offer data and methods to improve estimates of methane emissions from tidal wetlands and can improve national and global greenhouse gas inventories.” In the last decade, there has been growing interest in restoring coastal wetlands to mitigate climate change. Tidal marshes can sequester more carbon dioxide per unit of soil area than other ecosystems, such as terrestrial forests. Ariane emphasizes that this means that “the implications of the study are important for improving the precision of methane emission predictions from tidal wetlands and for carefully assessing how restoring these ecosystems can help mitigate climate change.”

The study offers practical guidance for estimating whether methane emissions from a particular marsh are, or could be, significant enough in the future to be included in greenhouse gas inventories for mitigation projects. The study provides “more detailed estimates of methane fluxes in ecosystems than the global values ​​provided by the IPCC,” he explains. Understanding the mechanisms that drive the emissions we observed “is critical to accurately estimating methane emissions under future climate scenarios, especially as tidal marshes are exposed to increasing human pressures and climate change impacts such as sea level rise and global warming,” he concludes.