Wetlands are custodians of a greenhouse gas heavyweight—methane. Pound for pound, methane is 45 times more powerful than carbon dioxide at trapping heat in the atmosphere. Understanding which types of wetlands emit methane, where they are, and whether these emissions are natural or caused by people is critical information for wetland managers seeking to justify and support restoration and conservation projects.

To address such questions, it helps to have access to a network of study sites that reflect the diversity of wetlands around the country. That’s why scientists from the Smithsonian Institution collaborated with the National Estuarine Research Reserve System (NERRS) as part of the Blue Methane Project. This ambitious effort combined satellite imagery with on-the-ground monitoring data to lay the groundwork for an inventory of the methane stored in all of the nation’s wetlands.

“We can’t take ground samples everywhere, so we need mapping techniques,” says James Holmquist, project lead and research scientist with the Smithonian’s Environmental Research Center. 

The Blue Methane Project was a first step toward building the map needed to map coastal methane-producing properties across the entire U.S. Given that most of the work to understand wetland emissions has been at the site scale, the study focused on scaling up to provide useful information.

With support from the NASA Carbon Monitoring System, the Smithsonian scientists designed and distributed sampling kits to 16 Reserves in different regions. Reserve scientists set to work, sampling the quantity and movement of methane in their local wetlands over two field seasons. 

“It’s been a great partnership,” observes Holmquist. “The NERRS is one of the only national coastal networks that represents such a diversity of wetlands, and Reserves are staffed by people who have intimate, expert knowledge of their sites. That made it possible for us to troubleshoot our protocol so it could work in so many different places.”

The Smithsonian team calibrated the local data sets provided by Reserves and tested their capacity to hold up at the satellite scale—or as Holmquist explains, “the resolution of a baseball diamond.” Ultimately, the goal was to test whether the NERRS data could support predictions of how much methane is stored in wetland soils in areas beyond Reserve boundaries. 

“When you scale up from a small site to make generalizations about a larger one, you can run into problems,” says Holmquist. “A lot can vary, even within a parcel the size of a baseball diamond. Through this study, we are able to find trends driving the presence of methane in wetland soils at Reserves that hold up at the larger scale.” 

Most significantly, the scientists found that one factor stood out above other potential drivers of methane emissions—the saltier the water, the less methane was present in the wetland soil. The project’s findings are already informing models that will support a national inventory of methane in wetlands and wetland restoration criteria for places beyond the Reserve System.

“Coastal carbon quantification and storage is a priority for NOAA,” says Chris Kinkade, physical scientist with NOAA’s Office for Coastal Management. “By helping to bring wetlands into the U.S. National Greenhouse Gas Inventory, Reserves support that priority, while advancing coastal restoration, conservation, and science.