Methane is extremely harmful to the environment: it is the second most abundant greenhouse gas in the air after carbon dioxide, and its heat-trapping capacity is about 28 times that of carbon dioxide. Methane is a major by-product of coal production and livestock fermentation, and it also comes from someUnexpected sources such as cattlehiccups, and - as Swedish researchers found - boats traveling through shallow water.
A study led by Chalmers University of Technology, stemming from an unexpected observation more than a decade ago, shows that ship traffic can trigger distinct pulses of high methane fluxes. "This is caused by changes in pressure and mixing of the water," explains researcher Amanda Nylund. "Even if the pulse is short, the total for a day is quite significant." In fact, methane emissions in these areas are 20 times higher than in nearby undisturbed areas.
The findings show that we are underestimating the amount of methane emissions produced by the global shipping industry and its impact on the atmosphere, meaning we are ill-equipped to address this environmental challenge.
As the ship passes over methane-rich sediments, it causes pressure changes at different depths and turbulent mixing of water, triggering the release of methane
It also highlights a previously unknown aspect of the shipping industry’s greenhouse gas emissions. This is particularly important, especially in the current climate, where reports indicate improvements in the carbon intensity of the freight fleet, but a concomitant increase in shipping emissions, and forecasts of future emissions are alarmingly high.
"Discovering a hitherto unknown impact of ships is important for improving estimates of global methane emissions, especially given that nine of the world's ten largest ports are located in waters with similar conditions to the Neva Bay," said Chalmers University professor John Melquist, whose team stumbled upon the emissions and launched the investigation.
So where do these emissions come from? How do ships emit them into the atmosphere? Methane is produced by organisms and occurs in shallow ocean areas where sediments are rich in organic matter and lack oxygen. The methane then bubbles up into the seawater above the sediment. As ships move through these waters, propellers vertically mix methane-rich water from the seafloor, causing changes in seafloor pressure. These factors work together to draw dissolved methane gas into the water—first into seawater and then into the atmosphere.
A team of researchers from Chalmers University of Technology first discovered this phenomenon back in 2011, when they were studying air emissions from burning ship fuel. They noticed that the methane emissions were surprisingly large and appeared slightly delayed compared to what they observed as ship exhaust; this delay suggested that the emissions were not coming from the ship's combustion system.
It was only after further study of turbulent ship wakes that Nylund began to delve deeper into the sources of these methane fluxes, and worked with colleagues to build data models to uncover the truth. The team's findings were published in the journal Nature Communications: Earth and Environment in May.
Notably, container ships and cruise ships produce greater methane emissions than similarly sized bulk carriers (which transport unpackaged bulk goods such as grain, coal and ore). This may have something to do with the ship's hull design and propeller system.
Interestingly, cargo ships loaded with containers like this release more methane than bulk carriers (carrying individual items such as grain or ore).
The team noted that their findings underscore the importance of measuring and monitoring these emissions more closely in shipping lanes around the world, while acknowledging that doing so is difficult for several logistical reasons. If we don't know how much damage we've done, it will be very difficult to repair it before it's too late.
Source: Chalmers University of Technology