Drowned Beringia: How Rising Seas May Have Released Methane at the End of the Ice Age

Carbon dioxide and methane are important parts of Earth's climate. Over the past 20,000 years, the amount of methane in the atmosphere has changed dramatically — with abrupt increases around 14,700 and 11,600 years ago — and why is not entirely clear.

The mouth of the Noatak River, where it meets Kotzebue Sound and the Chukchi Sea. USGS / Mike Records

The usual explanation, and a new one

One change since the Last Glacial Maximum is deglaciation. As glaciers and ice sheets retreated, lakes and peatlands formed, and both release methane.

Researchers from the National Park Service, the U.S. Geological Survey and several universities looked at a different cause: rising seas. As the ice melted, sea level rose and slowly flooded huge areas of the low-lying Beringian shelf. Land that had been tundra-steppe on permafrost became coastal wetland, then sea — and coastal wetlands are a new source of methane.

What they did

• Modelled sea-level rise and the area of Beringian coastal wetlands over the past 20,000 years, in 1,000-year steps.
• Used present-day measurements of methane from coastal wetlands to estimate what those past wetlands gave off as they spread.

What they found

• The wetlands reached their greatest extent around 14,000 years ago, when they may have released about 3.5 teragrams of methane a year (the estimate's range runs 4.0 higher to 1.9 lower).
• That moves the start of northern methane emissions earlier, toward the Bølling-Allerød warm period — before the peak from new thermokarst lakes and wetlands in the far north.
• Emissions from flooded Beringia match polar ice-core records of northern-hemisphere methane during the last deglaciation better — suggesting a link between rising seas, spreading coastal wetlands and more methane.

Coastal wetland formation in Beringia was, by this estimate, a major source of atmospheric methane. That matters now: future sea-level rise may also affect how much methane is in Earth's atmosphere.

The study

Fuchs, M., M. Jones, E. Gowan, S. Frolking, K. W. Walter Anthony, G. Grosse, B. Jones, L. Brosius, J. A. O'Donnell, and C. Treat. 2024. Potential methane flux from Beringian coastal wetlands during the last deglaciation. Quaternary Science Reviews 344: 108976.

Sources

Based on "Methane Changes in the Wetlands of Beringia," National Park Service, a work of the United States government in the public domain, and the study's abstract as quoted there, rewritten in hubnx's own words. The source's caption spells "Kotzebue Sounds," corrected here.