By James Dinneen
Restoring dried-out wetlands could avoid emissions equivalent to more than 100 billion tonnes of carbon dioxide by the end of the century, about a tenth of all expected human-caused emissions over the same period.
The amount of greenhouse gases emitted by wetlands depends on the amount of water in them. When wetland soil is covered in water, it emits large amounts of methane. When dry, wetlands emit less methane, but the dead plants and other organisms within them decompose and release carbon dioxide and potent nitrous oxide.
Zhenzhong Zeng at the Southern University of Science and Technology in China and colleagues calculated the precise water level at which wetlands produce the fewest net emissions.
The researchers considered 3704 records of water levels and emissions from wetlands around the world, including peat bogs in the UK and Indonesia, swamps in China and floodplains in the south-eastern US.
They found that the amount of carbon stored in wetlands offset nearly all the methane emitted when the water level was a few centimetres beneath the surface. In tropical wetlands emissions were lowest with slightly higher water levels.
The researchers estimated that restoring the more than 4 million square kilometers of global degraded wetland to an ideal water level would avoid between 100 and 400 gigatonnes of carbon-equivalent emissions by the end of the century, a greater reduction than all the forest restoration projects that countries have committed to around the world would achieve.
Preventing degradation of intact wetlands would avoid an additional 150 to 650 billion tonnes of emissions by the end of the century. Regions with the greatest potential for emissions reductions include Siberia, Canada, the Congo, Brazil and Indonesia.
"Even if we don't fully restore a wetland, but make some progress in raising the water level we can have some real impact," says Joseph Holden at the University of Leeds in the UK. Peatlands alone store more carbon than all the world's forests, he says.
Agriculture and urban development are responsible for draining most wetlands. Prolonged drought can also dry wetlands, says Zeng.
Restoring wetlands competes with other ways these lands are used, but effective restoration is happening, says Chris Evans at the UK Centre for Ecology and Hydrology. A project in Europe is researching ways to restore wetlands without displacing agriculture by growing food on peatlands. China has a new law to protect more than half of its wetlands by 2025. In the US, beavers are helping to dam water and rewet wetlands, too.
Journal reference: Nature Geoscience, DOI: 10.1038/s41561-022-00989-0