Global Warming Could Cause Wetlands to Emit More Methane, Study Finds

As temperatures rise, wetlands, which naturally act as both a sink and source of methane, may tip to emit more methane, according to new research from Smithsonian scientists.

In a study published in the journal Science Advances, scientists examined anaerobic microbial activities in brackish coastal wetlands. Anaerobic microbes are found in areas without free oxygen, such as flooded wetlands, and they can draw oxygen from sulfate molecules to consume methane.

Based on the research, the microbes could remove up to 12% of methane around the wetlands in normal conditions, even more than the microbes in zones with more free oxygen available. In salty areas with more sulfate, the anaerobic microbes were able to remove up to 70% of methane, Smithsonian reported.

To test how the microbes would react to global warming scenarios, the scientists established an experiment in the wetlands at the Smithsonian Environmental Research Center (SERC) in Maryland.

Using infrared lamps and underground cables, the scientists were able to increase temperatures in parts of the wetland area by 5.1 degrees Celsius in an experiment they dubbed “Salt Marsh Accretion Response to Temperature eXperiment,” or SMARTX. 

They also increased the carbon dioxide levels in areas of the wetland for a more accurate portrayal of global warming.

“You’re never going to get a warmer world without also having higher CO2 in the atmosphere… What SMARTX is doing is trying to mimic that warmer world, with the aboveground and belowground heating,” Genevieve Noyce, co-author of the study and senior scientist at SERC, said in a statement. “But because that’s not going to happen independent of CO2, we also cross it with CO2, so we have a real future that has both.”

After scientists increased the temperature of the wetland, the anaerobic microbes could not keep up with removing the increase in methane produced by other soil microbes, which were more productive under the higher temperature scenario. As such, methane emissions ultimately increased by up to four times as much compared to normal.

In areas with higher carbon dioxide levels, the methane emissions rose by about twice as much as normal. This could be because the higher carbon dioxide leads to larger root system growth in plants, and those roots can bring more oxygen to the soil, the scientists explained.

Wetlands are important ecosystems that sequester carbon and can both store and emit methane. However, with rising temperatures, the balance of storing and emitting methane could tip, causing wetlands to become an even larger source of methane emissions. This could then become a feedback loop, as methane is the second-biggest contributor to global warming behind carbon dioxide.

“If there is a large amount of methane emissions from wetlands, and if we don’t know anything about that, then our carbon reduction target for mitigating climate change is going to be off track in the future,” Jaehyun Lee, lead author of the study who was a postdoctoral fellow at SERC during the time of research, said in a statement.

According to a previous study, wetlands have been emitting more methane since 2000, with particularly high levels of methane emissions in 2020 and 2021. At the same time, these vulnerable and important ecosystems are disappearing, with over 1,500 square miles of wetlands lost over the past two decades, another study found.

With the results of the Smithsonian study, scientists noted that conservationists and policymakers will need to understand how warming will impact methane sequestering and emissions in wetland areas to better conserve these ecosystems.

“There is great value in protecting and restoring coastal wetlands to benefit climate, especially when we consider the many ecosystem services they provide to people,” Pat Megonigal, senior author of the study and associate director of research at SERC, said in a statement. “We also have to consider, how is climate change going to affect these delicate microbial processes, such as methane oxidation and methane production?”