A study led by Chalmers Institute of Technology found that methane emissions during transport are caused by traffic travelling through shallow waters.
As a result, they observed 20 times higher methane emissions on the shipping lane compared to nearby areas that were not intrusted by traffic.
Methane is a greenhouse gas that is 27 times more powerful than carbon dioxide, but these emissions are often overlooked because of today’s measurement methods.
“Our measurements show that the ship’s passages cause clear pulses of high methane flux from water to the atmosphere,” explained SMHI, a researcher at the Chalmers University of Technology and the Swedish Meteorological Waterway Research Institute.
“The change in pressure and mixing of water masses causes this. Even if the pulse is short, the total amount during the day is important.”
Shipping Emissions Caused by Fuel Selection and High Activity
Methane is a powerful greenhouse gas, and concerns about methane emissions from several vessels running on liquefied natural gas (LNG) are a hot topic.
This study found that methane emissions during transport are primarily separated from the vessel’s fuel selection. This means that all ships can emit greenhouse gases, and the contribution of shipping to these emissions has previously been underestimated.
Researchers note that methane is naturally present in sediments, but ship activity leads to large-scale releases into the atmosphere.
This study focuses on shallow marine regions where sediments are free of oxygen and are rich in organic matter. In such an environment, methane is formed, and at high production levels, gas can leak or foam into the water above.
As the ship passes, the pressure on the seabed changes, causing methane bubbles to escape from the deposits more easily.
Combined with the mixing that occurs in the wake of the ship, methane can quickly rise to the surface and escape into the atmosphere.
Important vessel impacts to improve global methane estimates
The phenomenon of extensive methane emissions in shallow waters was first discovered by chance in connection with other measurements in the Gulf of Neva, part of the Baltic Sea.
“The discovery of previously unknown ship impacts is important to improve global estimates of methane emissions, especially given that nine of the world’s 10 largest ports are in waters with similar conditions as the Gulf of Neva.”
Two types of the largest ships, cruises and container ships, caused the most frequent and largest measured methane release. However, the slightly smaller Ropax vessels (combining cargo and passenger ferries) also explain the important methane release.
Larger (compared to Ropax and container) container type bulk carriers accounted for reduced emissions. This means it is more complicated than the size of the vessel that controls methane emissions during transportation.
“A possible explanation for the high emissions of Ropax vessels is that they have double propellers,” commented Rickard Bensow, professor of fluid mechanics, Chalmers and ship traffic modelling.
Rethinking where methane measurements are performed
Currently, the authors of the current study emphasize the need to rethink how and where methane measurements are performed, especially in coastal waters where natural and human factors interact. They are particularly focused on further investigation of the large ports on the Delta River.
Ida Maja Hasself, professor of maritime and environmental science at Chalmers, leading the follow-up project, concluded: “The next step is to estimate how much these effects will be globally.
“It’s very likely that we’re underestimating methane emissions there too.”
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