Wetlands are a hidden source of rising methane emissions

Rising temperatures may influence a hidden battle happening in the muddy soils of wetlands. This quiet struggle pits microbes that produce methane against those that consume it.

With climate change intensifying, the natural balance that keeps methane levels in check could start to unravel. In a recent study, researchers at the Smithsonian Environmental Research Center have explored this phenomenon.

While wetlands are known for removing carbon dioxide (CO₂), they’re also the world’s largest natural source of methane – a gas that packs far more heat-trapping power.

Conflict beneath the wetlands

The National Oceanic and Atmospheric Administration estimates that 19% of global warming can be attributed to methane. This highlights the significance of understanding how wetlands behave under a changing climate.

“If there is a large amount of methane emissions from wetlands, and if we don’t know anything about that, then our carbon reduction target for mitigating climate change is going to be off track in the future,” said study lead author Jaehyun Lee.

Wetland soils host two kinds of microbes. One group produces methane, a greenhouse gas up to 45 times more potent than carbon dioxide.

The other group consumes it, converting methane into CO₂ using oxygen. This process is one of nature’s most important ways of slowing climate change.

Microbes in flooded wetlands

The study focused on a special kind of microbe that lives in oxygen-free, or anaerobic, environments. These microbes are common in flooded wetlands.

For a long time, scientists believed they couldn’t remove methane because of the lack of oxygen. Even after discovering that anaerobic microbes could use sulfate molecules to consume methane, their role was considered minor.

“They thought that the anaerobic methane [consumption] process is going to be too slow to remove a significant amount of methane,” Lee said.

But that assumption didn’t hold up. Most methane is produced in low-oxygen zones – exactly where these anaerobic microbes live.

In the studied wetland, these microbes removed about 12% of the methane. In saltier environments, with more sulfate, that number jumped to 70 percent. Then the team turned up the heat.

Rising carbon dioxide and temperatures

The researchers ran an experiment at the Smithsonian Environmental Research Center in Maryland called SMARTX – Salt Marsh Accretion Response to Temperature eXperiment.

Using infrared lamps and underground cables, they warmed certain parts of the wetland by 5.1°C. In some plots, they also increased carbon dioxide to mimic a more realistic future.

“You’re never going to get a warmer world without also having higher CO₂ in the atmosphere. What SMARTX is doing is trying to mimic that warmer world, with the aboveground and belowground heating,” said Genevieve Noyce, senior scientist at SERC.

“But because that’s not going to happen independent of CO₂, we also cross it with CO₂, so we have a real future that has both.”

Methane production in warmer wetlands

As temperatures rose, methane emissions spiked. But not because the methane-consuming microbes weakened. In fact, they worked harder and removed more methane.

The problem? The methane producers became even more active. The methane consumers simply couldn’t keep up.

The kind of plant growing in the soil made a big difference. In plots dominated by thick sedges, methane emissions rose nearly fourfold. In areas with smaller grasses, the increase was only about 1.5 times.

Higher carbon dioxide levels eased the increase, but didn’t erase it. In sedge-heavy plots, emissions doubled instead of quadrupling when both CO₂ and heat were added.

Plants with bigger roots

The researchers think this happens because CO₂ helps plants grow bigger roots. More roots lead to more oxygen entering the soil, which supports the methane-consuming microbes.

“Warming is going to have a really big effect on increasing methane emissions,” Noyce said. “But when you add elevated CO₂, it kind of brings it back down a little bit.”

Methane production across wetlands

This isn’t just a one-time event. In 2021, the same team found a similar pattern in oxygen-rich soils. Warmer conditions made methane-producing microbes outpace those that remove it. It’s a trend that could affect wetlands all over the world.

Still, the researchers emphasized that wetlands are worth protecting. They provide vital defense against storms and flooding. They also lock away carbon in soil and plants. One acre of coastal wetland can store more carbon than an acre of tropical rainforest.

“There is great value in protecting and restoring coastal wetlands to benefit climate, especially when we consider the many ecosystem services they provide to people,” said Pat Megonigal, senior author and associate director of research at SERC.

Preparing for the future

Looking ahead, policymakers need better data to prepare for the future. Knowing how much methane wetlands will emit in coming decades is key to hitting climate targets.

The scientists stress that climate change isn’t just about warmer air – it’s also about tiny microbial processes that could quietly shift the balance of greenhouse gases.

“We also have to consider, how is climate change going to affect these delicate microbial processes, such as methane oxidation and methane production?” Lee said.

The full study was published in the journal Science Advances.

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