Direct human changes to natural aquatic ecosystems can increase methane emissions, new research has shown.
Atmospheric methane has tripled since pre-industrial times. It traps heat much more efficiently than carbon dioxide and makes up 25 percent of atmospheric warming to date. And most of that methane comes from aquatic ecosystems, and human activities contribute to the level of emissions, a new paper published in the journal Nature Geoscience showed.
The global contribution and importance of aquatic ecosystems as methane emitters is underestimated, says Judith Rosentreter, a postdoctoral fellow at the Yale School of Environment (YSE) who led the study with a team of 14 researchers around the world.
The study authors reviewed methane flows from 15 major natural, aquatic ecosystems and man-made wetlands, including terrestrial, coastal and oceanic systems. They found that when methane emissions combine from these aquatic ecosystems, they are potentially a greater source of methane than direct anthropogenic sources of methane, such as agriculture or the burning of fossil fuels.
Aquatic ecosystems and wetlands account for at least half of the total methane emission budget.
“Accurate accounting of methane sources from aquatic ecosystems and if they are affected by human activities is important for understanding atmospheric methane concentrations,” says Peter Raymond, professor of ecosystem ecology, who co-authored the study.
The question of how people affected methane emissions from water sources was highlighted.
“Everything that is human motion or human impact has had much larger fluxes than more natural sites,” says Rosentreter, an associate at the Yale Institute of Biosphere Studies, Hutchinson.
Globally, rice farming releases more methane per year than all coastal wetlands, the continental shelf and the open ocean combined. Leaking manure causes lakes and reservoirs rich in nutrients to release methane.
Coastal aquaculture farms have methane flows per area that are 7-430 times higher than unregulated coastal habitats, such as mangrove forests, salt marshes, or seagrass. But the study notes that there are opportunities to reduce man-made emissions using the right management techniques.
“Intensive methane emissions from aquatic ecosystems offer opportunities for interventions by providing potential quick wins in reducing greenhouse gas emissions, provided a large role per molecule of methane emissions is relative to carbon dioxide,” says study co-author Carlos M. Duarte, a professor at the University of science and technology King Abdullah (KAUST) in Saudi Arabia.
Fluctuations between flooded and non-flooded conditions on aquaculture farms and rice paddies; restoration of tidal flow in degraded coastal wetlands; and reducing nutrients and organic matter in freshwater lakes, reservoirs and rivers can help reduce emissions, the study notes.
“Reducing methane emissions from aquatic systems will be an important part of stabilizing Earth’s temperature,” says co-author Bradley Eyre, director of the Center for Coastal Biogeochemistry at the University of Southern Cross in Australia.
Raising awareness about the number of methane emissions from aquaculture and other aquatic systems can help inform new monitoring and measurements that identify where and how methane emissions are produced and change over time. “With this awareness is also the ability to keep our water cleaner,” says Rosentreter.