
On September 26, 2022, several explosions caused leaks on the Nord Stream pipelines in the Baltic Sea. Large amounts of methane escaped. However, exactly how much was unclear until now. Now, using model calculations, researchers have quantified that around 465,000 tons of methane likely escaped from the pipelines into the atmosphere – the largest reported amount of methane released by a single human-caused event to date. An accompanying study also shows how methane dissolved in the water spread in the Baltic Sea.
After the Baltic Sea pipelines Nord Stream 1 and 2 were destroyed by explosive charges, the natural gas contained in the pipelines bubbled to the surface for about a week. Previous estimates of how much methane was released were based either on calculations based on the volume of the destroyed pipelines or on measurements from the air, from ships or with the help of satellites. However, the results showed a wide range. Depending on the method used, the estimates provided values between 75,000 and 509,000 tons of methane. It was also unclear what proportion of the released gas initially dissolved in the water and what potential effects this had.
Largest amount of methane ever recorded from a single event
A team led by Stephen Harris from the International Methane Emissions Observatory of the United Nations Environment Program (UNEP IMEO) in Paris has now created a model for the emission rates after pipeline ruptures and used it to calculate how much methane likely escaped from the Nord Stream pipelines. “According to our calculations, 465,000 tons of methane were emitted into the atmosphere,” the team calculates. This value is consistent with emission estimates based on airborne and satellite measurements.
“To our knowledge, this is the largest reported amount of methane ever released in a single, transient, human-caused event,” Harris and his team explain. The previous largest comparable event, the Aliso Canyon gas storage leak in California in 2015, released about 100,000 tons of methane, less than a quarter of the emissions from the Nord Stream leaks.
Small share of total emissions
However, the Nord Stream explosion only accounted for a small proportion of the total emissions of the greenhouse gas methane: “The methane released corresponds to only 0.1 percent of anthropogenic methane emissions for 2022,” say Harris and his colleagues. The global oil and gas industry alone releases the same amount of methane within two days as a standard, and agriculture releases it in little more than a day. “The impact of the leaks on the global atmospheric methane budget brings into focus the numerous other anthropogenic sources of methane that need to be reduced worldwide,” said the research team.
But even apart from the impact on the greenhouse effect, the large quantities of methane escaping from the pipelines could have had an impact on the environment. “In order to understand the response of the ecosystem, estimates of methane dispersion and concentration are important,” explains a second research team led by Martin Mohrmann from the Voice of the Ocean Foundation in Sweden. “Much of the methane quickly escaped into the atmosphere, while an unknown amount was dissolved in the water.”

Methane dissolved in water
To estimate the amount of gas distributed in the Baltic Sea, Mohrmann and his team used high-resolution long-term observations of methane concentrations in the water around the leaks and in other locations in the Baltic Sea. Over three months, they measured how the methane spread using autonomous underwater robots and ships. “Our measurements show that the methane has spread across large parts of the southern Baltic Sea, from the coast of Denmark in the west to the Polish Bay of Gdańsk in the east,” reports Mohrmann. Shortly after the leak, the methane levels in the water were sometimes a thousand times higher than normal.
Only gradually was the methane diluted in the seawater, consumed by bacteria or released into the atmosphere. Nevertheless, months later, Mohrmann and his team still found significantly elevated methane levels in some parts of the Baltic Sea. “Overall, we estimate that over 14 percent of the entire Baltic Sea was exposed to methane pollution that was at least five times higher than normal,” says Mohrmann. The affected areas also included 23 marine protected areas. “We now know the areas where methane emissions may have had an impact. This will make it easier to determine whether a future problem in the Baltic Sea ecosystems is related to the Nord Stream leak or not, for example,” says co-author Bastien Queste from the University of Gothenburg.
Sources: Stephen Harris (United Nations Environment Program’s International Methane Emissions Observatory, Paris, France) et al., Nature, doi: 10.1038/s41586-024-08396-8; Martin Mohrmann (Voice of the Ocean Foundation, Västra Frölunda, Sweden) et al., Nature Communications, doi: 10.1038/s41467-024-53779-0