Scientists Dial Up Heat and Carbon Dioxide in Futuristic Experiment

Rising temperatures could tip the scale in an underground battle that has raged for millennia. In the soils of Earth’s wetlands, microbes are fighting to both produce and consume the powerful greenhouse gas methane. But if the Earth gets too hot, a key way wetlands clamp down on methane could be at risk, according to a Smithsonian study published April 23.

Methane is responsible for roughly 19% of global warming, according to the National Oceanographic and Atmospheric Administration. And while wetlands are champions at removing carbon dioxide (CO₂)—the more abundant greenhouse gas—they are also the world’s largest natural source of methane. As nations set targets to bring down methane emitted from human activity, it is crucial to understand how much methane wetlands emit naturally—and how much more they could emit in the future. 

“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 lead author Jaehyun Lee. Lee, who now works at the Korea Institute of Science and Technology, did the study while a postdoctoral fellow at the Smithsonian Environmental Research Center.

Microbial Tug-of-War

In wetland soils, two types of microbes are locked in competition. Some microbes produce methane, a greenhouse gas up to 45 times stronger than CO₂. But other microbes consume that methane, using oxygen to turn it into less-harmful CO₂. That simple transformation is one of nature’s most powerful ways to keep greenhouse gas emissions in check.

The new study, published in the journal Science Advances, focused on a special class of microbes known as anaerobic. Anaerobic microbes live in places without free oxygen—zones that are quite common in flooded wetlands. For a long time, they have been the underdogs in the methane wars. With no free oxygen to draw from, these microbes were believed to be unable to consume methane. When scientists finally discovered they could (by pulling oxygen from nearby sulfate molecules), they still thought it was a minor background effect compared to the microbes in oxygen-rich parts of the wetland. 

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

But as Lee pointed out, most methane production happens in these oxygen-starved environments. This essentially means anaerobic microbes are on the front lines. And they are pulling their weight. In the Smithsonian wetland Lee worked in, anaerobic microbes can remove up to 12% of the methane—far less than their oxygen-loving counterparts, but more than scientists previously suspected. And in saltier, sulfate-rich places, anaerobic microbes can remove up to 70% of the methane produced in oxygen-deprived soils.

However, things changed when scientists dialed up the heat.

A Fast-Forward Climate Experiment

In the new study, the team simulated a hotter future using an experiment on a wetland at the Smithsonian Environmental Research Center (SERC) in Maryland. The experiment goes by the name “SMARTX” (short for “Salt Marsh Accretion Response to Temperature eXperiment”). Scientists raised the temperature by 5.1 degrees Celsius in certain parts of the wetland by energizing rows of infrared lamps and underground cables. In some plots, the team also raised CO₂ to create 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, a coauthor and 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 emissions spiked under hotter temperatures alone. This was not because the helpful microbes became weaker. Warmer soils triggered them to remove even more methane than before. However, their competitors—the microbes that produce methane—became more active as well. And in a warmer world, the methane-removing microbes were unable to keep up. 

How much methane emissions went up depended on the plants. In areas dominated by thick sedges, methane emissions rose nearly four times higher. But where smaller grasses prevailed, methane emissions increased only 1.5 times.

Ironically, higher CO₂ lessened the impact—but not enough to cancel it out. Methane emissions in the sedge plots rose to just double normal levels, rather than nearly quadrupling, when scientists tested higher temperatures and higher CO₂ together. The researchers suspect this is because CO₂ triggers plants to grow bigger roots. Roots inject more oxygen into the soil, creating even more oxygen-rich sulfate compounds for the microbes to use.

“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.”

This pattern holds for microbes across the entire wetland. In 2021, the team discovered that microbes in oxygen-rich soils behave the same way as the oxygen-starved microbes in this study. When the environment heats up, microbes that remove methane fall further behind their methane-producing cousins.

Conserving wetlands is still a vital part of protecting the world from climate change, the authors said. They are lifesaving buffers from hurricanes and extreme weather. And despite the methane issue, wetlands excel at locking away planet-warming carbon in other forms. An 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, the senior author and associate director of research at SERC.

But to plan for the future, policymakers need to know how much methane wetlands will emit in the decades to come. At the end of the day, Lee said, climate change is not only about hotter temperatures. It is also about the invisible activities that could tip 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?” he said.

Yonsei University in Korea was also involved in the research. A copy of the study is available on the journal’s website. For photos or to speak with one of the authors, contact Kristen Goodhue at GoodhueK@si.edu.