Water-logged land areas such as marshes, bogs and fens are the world's largest natural source of methane. Even the smallest of wetlands emit this powerful greenhouse gas. In a new study from The University of Texas at Austin, researchers have identified tens of millions of easily overlooked small wetlands across the globe and found that they have a substantial collective impact, accounting for 24% of the world's total non-forested wetland emissions of methane.
Using high-resolution satellite imagery and machine learning, researchers identified roughly 160 million small wetlands that have been difficult to detect and remain underrepresented in global methane assessments due to their relatively small size.
"Small wetlands are easy to overlook on a map, but they are not small in the methane budget," said the study's lead author Fa Li, an assistant professor at the Department of Earth and Planetary Sciences at UT's Jackson School of Geosciences.
This research was published in Nature Climate Change .
The small wetlands in this study range in size from as small as an Olympic swimming pool to about 250 acres — almost as large as Austin's Zilker Park. Although these areas may seem large to a human, they make up only a tiny fraction of a coarse-resolution satellite pixel, making them difficult to capture in traditional wetland maps used for global methane modeling. This has allowed small wetlands to fly under the radar in global assessments for decades. Larger wetlands are typically detected at this scale through coarse resolution satellite data, which uses passive microwave sensors — a method scientists have used for years. These sensors can penetrate dense tree canopies, providing consistent coverage of wetlands regardless of visibility. However, they can miss small wetlands.
In this study, researchers turned to a different data source to find the missing wetlands: years of high-resolution satellite images that can identify smaller wetlands ranging in size from 1,000 square meters (about a quarter of an acre) to one square kilometer. Researchers measured how these small wetlands shrunk or expanded from 2003 to 2022, then combined that data with field-based methane measurements and used machine learning to calculate their emissions. Researchers found that small wetland methane emissions have increased by 9.9% over this time frame.
And the newly identified wetlands are almost certainly an undercount, Li noted. There are other wetlands out there that are even smaller in size, or that are in a forested location, like swamps. Unlike the microwave sensors, the high-resolution satellite data used cannot pick up the presence of a wetland beneath dense tree canopies.
The reason why wetlands produce so much methane is due to microbes. The soils in wetlands, being saturated with water, block the transfer of oxygen from the air into the ground. Particular microbes that thrive in these oxygen-poor environments produce a significant amount of methane, a greenhouse gas that is 80 times more potent than carbon dioxide over a 20-year period after it is released. This microbially produced gas is what makes wetlands the largest natural source of methane worldwide.
About two-thirds of the world's methane emissions come from a variety of anthropogenic sources, such as fossil fuels, commercial livestock like cows, human waste management, and rice farming. While these sources are ostensibly more directly controllable, it is still important to know how natural sources contribute to total methane emissions, Li said.
"Natural sources respond strongly to climate dynamics, in turn influencing the climate system. For example, as the planet warms, these emissions may increase, further amplifying warming and partially offsetting mitigation efforts," Li said. "The concerning reality is that atmospheric methane concentrations have increased substantially in recent decades, yet there is still no clear consensus on the dominant causes of this long-term increase."
Li is also a co-author on a recently-published policy forum in Science , which makes the case that a global methane observation system is needed to track how emissions, from natural sources in particular, impact the climate.
Li is part of the team working on a global flux-tower network called FLUXNET-CH4 that provides frequent direct measurements of methane emissions across an array of ecosystems. Still, he notes that flux towers alone will not completely solve this knowledge gap. To get a holistic picture of methane dynamics across the globe, researchers will need to integrate satellite observations, aircraft measurements, atmospheric concentration towers, and direct flux measurements from sites around the world, he said.
