USGS - science for a changing world

Understanding Our Planet Through Chemistry

Recent Methane Emissions from Gulf Coast Marshes

The Earth's atmosphere is still changing. Natural environmental processes (geological, biological, and geochemical) produce carbon dioxide (CO2) and methane (CH4). These gases, along with water vapor, are responsible for trapping heat at the Earth's surface.

Because biological processes are responsible for the production of methane in environments where organic matter ferments, wetlands (swamps, bogs, etc.) were previously the principal source of methane. Now, however, the combination of rice cultivation and cattle raising have taken over as the principal contributor. Studies of methane sources help us to understand their relative contributions and the factors that control the methane production and release to the atmosphere.

The studies show that when coastal wetlands are flooded by sea-level rise, salt marshes are inundated, up-slope brackish marshes become saltier, and some fresh marshes near the coast become brackish. Consequently, total methane emissions decrease because salt marshes do not produce as much methane as fresh marshes.

Photo of field set-up for sampling methane emissions.Fifteen miles inland from the Gulf of Mexico in a brackish marsh in Terrebonne Parish, Louisiana, methane emissions are collected in inverted buckets and measured with a portable gas analyzer. Using these measurements, scientists can determine one effect of global sea-level rise. [61k]

USGS studies of methane in Gulf Coast Louisiana indicate that brackish marshes emit between one-fourth and one-half the methane of the fresh marshes they replace during sea-level rise. The results of these local measurements in Louisiana can be used to project the world-wide effects of sea-level rise on methane emissions. By the year 2050, projected world-wide, sea-level rise will replace 50 percent of coastal fresh-water marshes with brackish water marshes. This will reduce the world's methane emissions by 2 percent.

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