Study: Ship traffic can release methane trapped underwater

ALBAY – It turns out that ships traveling through shallow, oxygen-poor waters can disturb methane trapped below and release it into the atmosphere. 

A 2025 study published in Nature Communications Earth & Environment found that ship traffic can cause methane emissions to spike up to 20 times higher than in undisturbed areas, even during short passages.

This discovery is relevant to the Philippines, where all five existing gas-fired power plants have started using imported liquefied natural gas (LNG) since 2023 following the decline of the Malampaya gas field. 

According to the think tank and grassroots mobilizer Center for Energy, Ecology, and Development (CEED),  LNG vessel traffic in the Verde Island Passage could surge from just one to two ships per week (equivalent to 85 shipments annually) to seven to eight weekly (equivalent to 387 per year). Each vessel, with its 267,000-cubic-meter capacity, is considered the largest.

This increase would occur if all planned, under-construction and existing gas plants operate at full capacity, the report said.

While the CEED report doesn’t assess methane-related sediment disturbance, the projected rise in vessel traffic should raise concern about release of this potent greenhouse gas, especially near gas-fired power plants located in coastal zones resembling Neva Bay in the Baltic Sea in Russia, where ship turbulence has been shown to unleash significant methane plumes.

What ship traffic does to methane-rich sediments

Researchers from Chalmers University of Technology in Sweden found that ship traffic in these areas can trigger the release of short but intense bursts of methane into the atmosphere.

These emissions are caused by pressure changes and water mixing as vessels pass through, disturbing the sediment and releasing the gas.

The researchers initially set out to study air pollution from ship exhaust but were surprised to detect elevated methane levels unrelated to fuel type, leading them to uncover the sediment disturbance phenomenon.

Their findings underscore the urgent need for real-time monitoring tools that can track methane emissions in dynamic coastal environments, especially as global shipping intensifies.

The study shows that these emissions are unrelated to the type of fuel ships use which means all vessels, not just those running on LNG, contribute to the problem.

“Even short pulses of methane triggered by ship movement can add up to significant daily emissions,” said Amanda Nylund, one of the study’s lead researchers.

The study challenges current climate accounting, which often overlooks methane released from disturbed sediments. Meanwhile, methane emissions from ship traffic are rarely measured or included in national inventories.

Why methane builds up in coastal sediments

Methane builds up in oxygen-poor sediments as organic matter sinks and decomposes. Anaerobic microbes break it down, and in the final stage, produce methane. With little oxygen or reactive chemicals to consume it, the gas stays trapped until disturbed.

But methane’s role isn’t purely destructive. In such conditions, it fuels microbial communities that regulate carbon and nutrient cycles through processes like anaerobic oxidation.

Disrupting these sediments not only releases methane into the atmosphere but also destabilizes these biological systems, risking long-term damage to the seafloor’s ecological balance.

Rising vessel traffic can therefore increase the risk of hypoxia: low oxygen levels that harm marine life and promote the buildup of decaying organic matter, compounding the environmental risks.

Why this matters for the Philippines

The researchers call for better monitoring, especially in major ports and river deltas with conditions similar to Neva Bay, where the phenomenon was first observed.

Johan Mellqvist, professor of Optical Remote Sensing at Chalmers, emphasized the global relevance: “Nine of the world’s ten largest ports are located in waters like Neva Bay.”

Cruise and container ships triggered the largest and most frequent methane releases, while smaller ropax vessels (ferries carrying both passengers and freight) also caused significant emissions.

Interestingly, larger bulk carriers produced less methane, suggesting that ship design, not just size, plays a role.

“A possible explanation for the high emissions from ropax vessels is that they have double propellers,” said Rickard Bensow, Professor of Hydrodynamics at Chalmers, who led the modeling of ship traffic.

While larger bulk carriers may cause less disruption, vessels in shallow, sediment-rich waters still threaten biodiversity hotspots like the Verde Island Passage which is already strained by fossil fuel expansion.

Batangas, home to five legacy gas plants, now hosts a new LNG-fired facility developed by San Miguel Global Power. 

This strengthens its position as the Philippines’ LNG hub and drives increased ship traffic through the fragile VIP, as all now almost depend on imported fuel. (JDS)

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