Half of Global Methane Emissions Are From Aquatic Sources

Atmospheric methane has tripled since pre-industrial times. It traps heat far more effectively than carbon dioxide and accounts for 25% of atmospheric warming to date. And much of that methane is coming from aquatic ecosystems, with human activities contributing to the emissions levels, a new paper published in Nature Geoscience has found.

The global contribution and importance of aquatic ecosystems as methane emitters has been underestimated, says Judith Rosentreter, postdoctoral associate at the Yale School of the Environment (YSE) who led the study with a team of 14 researchers worldwide.

The study authors reviewed methane fluxes from 15 major natural, human-made, and human-impacted aquatic ecosystems and wetlands, including inland, coastal, and oceanic systems. They found that when methane emissions are combined from these aquatic ecosystems, they are potentially a larger source of methane than direct anthropogenic methane sources, such as agriculture or fossil fuel combustion. Aquatic ecosystems and wetlands contribute at least as much as half of the total methane emissions budget.

"An accurate accounting of the sources of methane from aquatic ecosystems, and if they are impacted by human activities, is important to understanding atmospheric methane concentrations,'' says Peter Raymond, professor of ecosystem ecology who co-authored the study.

One issue that stood out is how humans have impacted methane emissions from aquatic sources.

"Anything human-driven or human-impacted had much higher fluxes than more natural sites,'' says Rosentreter, a Yale Institute for Biospheric Studies Hutchinson Fellow.

Globally, rice cultivation releases more methane per year than all coastal wetlands, the continental shelf and open ocean combined. Fertilizer runoff causes nutrient-rich lakes and reservoirs to release methane. Coastal aquaculture farms have methane fluxes per area that are 7-430 times higher than from non-converted coastal habitats, such as mangrove forests, salt marshes or seagrasses.

But the study notes that there are opportunities to reduce human-impacted emissions with the right management techniques.

"The intense methane emissions from aquatic ecosystems offers opportunities for intervention providing potential quick wins in reducing greenhouse emissions, provided the much large role per molecule emitted of methane compared to carbon dioxide,'' says study co-author Carlos M. Duarte, professor at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia.

Fluctuating between flooded and non-flooded conditions in aquaculture farms and rice paddies; restoring tidal flow in degraded coastal wetlands; and reducing nutrient and organic matter in freshwater lakes, reservoirs, and rivers can all help reduce emissions, the study notes.

"Reducing methane emissions from aquatic systems will be an important part of stabilizing the Earth's temperature,'' says co-author Bradley Eyre Director, Centre for Coastal Biogeochemistry at Southern Cross University in Australia.

Bringing awareness to the amount of methane emissions coming from aquaculture and other water systems can help inform new monitoring and measurements that identify where and how methane emissions are being produced and change over time.

"With this awareness is also the possibility of helping to keep our waters cleaner,'' Rosentreter says.

Reference: Rosentreter JA, Borges AV, Deemer BR, et al. Half of global methane emissions come from highly variable aquatic ecosystem sources. Nat. Geosci. 2021;14(4):225-230. doi: 10.1038/s41561-021-00715-2

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