Ships moving in shallow waters cause huge hidden methane leaks

Ships have long connected the world’s economies. They bring goods and people across seas and rivers. But their impact on the environment goes beyond the fuel they burn.

A new study from Chalmers University of Technology has uncovered a surprising threat linked to ship movement itself.

This research reveals how ships stir up methane emissions, a potent greenhouse gas, simply by traveling through shallow waters.

The findings are both alarming and unexpected. They show that methane emissions in busy shipping lanes are twenty times higher than in undisturbed waters nearby. This means that global methane estimates may have been far too low for many years.

Methane has always been a cause for concern among climate scientists. It traps heat far more effectively than carbon dioxide. Despite its powerful effects, methane emissions from ship movements have been mostly ignored.

Methane escapes as ships pass

Amanda Nylund, a researcher at Chalmers University of Technology and the Swedish Meteorological and Hydrological Institute (SMHI), explains how these emissions occur.

“Our measurements show that ship passages trigger clear pulses of high methane fluxes from the water to the atmosphere,” she said.

“This is caused by pressure changes and mixing of the water mass. Even if the pulses are short, the total amount during a day is significant.”

This new finding changes how we think about emissions from ships. Previously, the main concern involved ships that run on liquefied natural gas, or LNG.

In this case, however, the methane has no connection to the fuel used by ships. Instead, it comes from the seafloor itself, disturbed by ship traffic. This means that all ships, regardless of their fuel, can cause these emissions.

Shallow seas emit more gas

The study focused on shallow marine environments rich in organic matter. These areas have oxygen-free sediments, ideal for producing methane.

Methane in such regions builds up naturally within the seafloor. In many cases, it slowly escapes in small bubbles or leaks through cracks into the water above.

But when a ship passes through, the story changes. The passing vessel causes sudden pressure changes at the seafloor. This triggers methane to burst out from the sediments in large amounts.

The effect grows stronger when combined with the mixing action from the ship’s wake. This mixing allows methane to travel more quickly from the water to the surface, where it escapes into the air.

As ships pass repeatedly throughout the day, the methane pulses accumulate into significant emissions.

Methane link found by chance

The research team stumbled upon this phenomenon by accident. They were conducting unrelated measurements in Neva Bay, part of the Baltic Sea, when they noticed strange methane spikes.

This accidental finding became the starting point for a new line of inquiry. Their work has now been published in Nature Communications Earth & Environment.

Johan Mellqvist is a professor of Optical Remote Sensing at Chalmers. His group made the unexpected discovery that forms the basis for the new results.

“The discovery of the hitherto unknown impact of ships is important for improving global estimates of methane emissions, not least considering that nine of the world’s ten largest ports are located in waters with similar conditions as Neva Bay,” he said.

This finding holds global significance because many of the world’s busiest ports are in shallow waters similar to Neva Bay.

Smaller ships, bigger leaks

The study analyzed emissions from different ship types. Large cruise ships and container ships produced the highest and most frequent methane releases.

Surprisingly, smaller ropax vessels, which transport both passengers and cargo, also caused major emissions. These vessels, though smaller than cruise or container ships, had high methane release rates.

Interestingly, larger bulk carriers showed lower methane emissions than expected. This challenges the assumption that bigger ships always cause greater emissions.

Researchers believe that ship design may influence these results.

“A possible explanation for the high emissions of ropax vessels is that they have double propellers,” says Rickard Bensow, Professor of Hydrodynamics, Chalmers, and responsible for the study’s modeling of ship traffic.

Big ports hide methane risks

The findings suggest a need for urgent changes in how methane emissions of ships are monitored and reported. The authors stress that more attention must be given to coastal areas where natural and human activities interact.

The researchers now plan to expand their investigation to include large ports in river deltas – these locations likely face similar methane risks.

“The next step is to estimate how large these effects can be globally,” said Ida-Maja Hassellöv, a professor of Maritime Environmental Science. She will lead the follow-up project that will start this autumn.

“Major ports in China, Singapore, and South Korea, as well as European ports such as Rotterdam, Antwerp, and river systems in Germany, have similar conditions to the Neva Bay,” she said. “It is very likely that we underestimate methane emissions there as well.”

Shipping rules could tighten

This research could reshape how the world calculates methane emissions. Current models may leave out critical methane sources hidden beneath the waves.

If similar emissions are found in other shallow ports worldwide, it may prompt major changes in global climate strategies. It could also affect shipping regulations, urging stronger controls on ship routes and port operations in sensitive areas.

This surprising discovery proves that hidden climate risks can emerge from unexpected places. Ships do not just carry cargo and passengers – they may also be pushing greenhouse gases into the air, one wave at a time.

The study is published in the journal Nature Communications Earth & Environment.

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