A study led by Chalmers University of Technology discovered that methane emissions in shipping are caused by traffic moving through shallow waters.
Consequently, they observed 20 times higher methane emissions in shipping lanes compared to nearby areas that were undisturbed by traffic.
Despite methane being a greenhouse gas 27 times more potent than carbon dioxide, these emissions are often overlooked due to today’s measurement methods.
“Our measurements show that ship passages trigger clear pulses of high methane fluxes from the water to the atmosphere,” explained Amanda Nylund, researcher at Chalmers University of Technology and the Swedish Meteorological and Hydrological Institute, SMHI.
“Pressure changes and mixing of the water mass cause this. Even if the pulses are short, the total amount during a day is significant.”
Shipping emissions caused by fuel choices and high activity
Methane is a potent greenhouse gas, and concerns about methane emissions from the few ships that run on liquefied natural gas (LNG) are a very hot topic.
The study found that methane emissions in shipping are largely decoupled from the ships’ choice of fuel. This means that all ships can emit greenhouse gases, and shipping’s contribution to these emissions has previously been underestimated.
The researchers note that although methane is naturally present in the sediments, the activity of the ships leads to an extensive release into the atmosphere.
The study focuses on shallow marine areas where the sediments are oxygen-free and rich in organic matter. In such environments, methane is formed, and at high production levels, the gas can leak or bubble up into the water above.
When a ship passes, the pressure at the seafloor changes and methane bubbles make their way out of the sediments more easily.
In combination with the mixing that takes place in the wake of ships, the methane can quickly rise to the surface and escape into the atmosphere.
Impact of ships crucial for improving global methane estimates
The phenomenon of extensive methane emissions in shallow waters was first discovered by chance, in connection with other measurements in Neva Bay, part of the Baltic Sea.
“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,” said Johan Mellqvist, Professor of Optical Remote Sensing, Chalmers, whose group made the unexpected discovery that forms the basis for the new results.
Two of the largest ship types, cruise and container ships, triggered the most frequent and largest measured methane releases; however, slightly smaller ropax vessels (which combine freight and passenger ferries) also account for significant methane releases.
The larger (compared to ropax and container) vessel type, bulk carriers, accounted for lower emissions. This means that it is more complicated than the size of the vessels controlling methane emissions in shipping.
“A possible explanation for the high emissions of ropax vessels is that they have double propellers,” commented Rickard Bensow, Professor of Hydrodynamics, Chalmers, and responsible for the study’s modelling of ship traffic.
Rethinking where methane measurements are carried out
The authors of the current study now emphasise the need to rethink how and where methane measurements are carried out, especially in coastal waters where natural and human factors interact. They place particular focus on further investigating large ports in river deltas.
Ida-Maja Hassellöv, Professor of Maritime Environmental Science, Chalmers, who will lead the follow-up project, concluded: “The next step is to estimate how large these effects can be globally. Major ports in China, Singapore, and South Korea, as well as European ports such as Rotterdam and Antwerp, and river systems in Germany, have similar conditions to those of the Neva Bay.
“It is very likely that we underestimate methane emissions there as well.”
