Unexpectedly strong methane emissions have been detected at several locations in Lake Siljan, Sweden, according to a new study from Chalmers University of Technology. The findings, based on a novel measurement technique developed by the researchers, reveal persistent and concentrated methane leaks never before observed in a lake environment. Scientists will now investigate whether these emissions are unique to Siljan—or part of a broader phenomenon that could occur in lakes worldwide.

Lake Siljan is the largest of several lakes in the Siljan Ring area, a 370-million-year-old impact crater in central Sweden formed by a meteorite strike. Natural methane seepage from lakes in this region has long been known, and local residents have reported persistent winter ice holes—”ice wakes”—reappearing in the same spots year after year. The Chalmers study focused on precisely these sites, as methane leaks could be the underlying cause of the recurring openings in the ice.

The research, led by Professors Johan Mellqvist and Vladimir Conde at Chalmers, measured methane emissions from lakes in the Siljan Ring during two field campaigns in 2023 and 2024. The results surprised the scientists in several ways.

Extensive emissions came as a surprise

Researchers identified several “hotspots”—clearly localized sites of intense methane leakage, forming visible gas plumes in the air. When the researchers returned over six months later to repeat the measurements, the strong emissions remained, estimated at roughly 3.5 tons of methane per year, corresponding to 85 tons of CO₂-equivalents.

“The natural methane emissions from the Siljan Ring lakes appear to be far more extensive than previously known. At certain sites, we recorded emission rates up to 300 times higher than what is typically seen in lakes,” says Mellqvist, Professor of Optical Remote Sensing at Chalmers.

He points out that methane is a potent greenhouse gas with a global warming potential significantly higher than carbon dioxide.

“That the methane emissions at Siljan are so substantial provides important insights into Sweden’s natural greenhouse gas budget and emission sources. While the individual sources we identified may be minor in the national context, we don’t yet know how many more exist or how they fluctuate over time. If future studies reveal even larger-scale emissions, it could justify efforts to reduce them,” says Johan.

Highly concentrated emissions

Methane emissions from lakes usually occur as “bubble emissions”: sporadic, scattered leaks that pop up here and there on the water’s surface. These result from organic matter decaying in the lakebed, forming methane that bubbles up. But the researchers were surprised to find the emissions in Siljan were far more concentrated than typical bubble emissions.

“It’s unusual—and quite strange—that the emissions were so extremely localized. To our knowledge, this kind of tightly concentrated methane leak has never been measured in a lake before. And we’ve only examined a small part of the Siljan Ring lakes so far. It’s possible we’ve only seen the tip of the iceberg,” says Mellqvist.

To conduct the measurements, the researchers partly used a new method developed at Chalmers. It involves releasing a tracer gas to understand the dispersion of methane in the air and quantify how much is leaking out.

“The method allows us to measure concentrated emissions much more effectively than traditional approaches, such as floating chambers, which are designed for homogeneous surface emissions across wide areas. One of our key questions now is whether similar localized emissions exist in other lakes, but have gone undetected because our previous methods weren’t suited to finding them,” he says.

Origin of the emissions still unknown

A crucial question is the origin of the methane. One possibility is that it’s so-called “deep gas” from underground—caused by the meteorite impact at Siljan, which may have carried organic material deep into the Earth where it continues to generate leaking methane. Another possibility is that the emissions come from methane pockets—trapped gas in sediments below the lakebed that gradually leaks out.

“If it turns out to be deep gas, then this may be unique to the Siljan Ring and the impact crater. But if it comes from more shallow sediment pockets, then this type of emission might be present in many more places,” Mellqvist explains.

The researchers now plan to map a larger part of Siljan and nearby lakes, including greater depths than previously studied.

“We need to understand how many hotspots the area contains and identify the origin of the emissions. We also want to explore possible strategies to limit them. Our findings could have implications far beyond the Siljan region and help us reassess how we view natural gas seepage in geological formations worldwide,” he concludes.