A new paper suggests we may be overlooking a major source of global methane emissions
In a 2011 study of Neva Bay – a shallow, busy shipping lane leading from the Gulf of Finland to the port of Saint Petersburg – researchers unintentionally recorded a surprising phenomenon. Whenever a boat passed through the water, sensors detected a significant release of methane, with emissions measuring up to 20 times higher than in undisturbed parts of the bay. At the time, however, the researchers were focused on different emissions from ship exhausts. The collected data remained unstudied for more than a decade, the cause of these large methane emissions unresolved.
Amanda Nylund didn’t set out to study methane emissions from shipping when she began her PhD in marine biogeochemistry in 2017. Her goal was to investigate the impacts of ship-induced turbulence in stratified waters – places where water naturally separates into distinct, stable layers, usually due to differences in temperature or salinity. At the time, it seemed surprising how little research had already been done, especially given the scale of the shipping industry, which currently transports more than 80 per cent of all traded goods around the world. ‘We soon realised why there was a gap in the research,’ says Nylund. ‘It was classified.’
Nylund explains that most research on ship wakes has been done by the military. While this research has focused on detecting ships rather than measuring their environmental impact, it still provided Nylund and her colleagues with a valuable starting point for their own work. ‘Knowing that acoustic instruments can be used to detect gas bubbles, for example.’ One of Nylund’s first papers used this method to investigate how ship wakes might affect water stratification (she found that the physical disturbance could have a significant impact on nutrient dynamics and even lead to greater plankton mortality). ‘We also know from satellites that you can see ship wakes as temperature anomalies that are sustained for a really long time. It’s particularly pronounced in a shipping lane where you constantly have ships passing through.’
It was at a conference, while presenting her work, that a crucial connection was made. In the audience was Johan Mellqvist, a professor at Chalmers University, who remembered the strange methane measurements his team had taken from Neva Bay years before. Could Nylund’s research help explain the readings? ‘I was a PhD student, so I had a lot of available time,’ Nylund recalls with a laugh, describing how she took on the challenge of revisiting the decade-old data.
Methane is naturally found in sediments. In shallow marine environments, where sediments are oxygen-free and rich in organic matter, it forms at high levels and can leak or bubble up into the water above. Nylund’s research reveals that as a ship moves through the shallow water, the pressure change at the seafloor allows methane bubbles to escape more easily. The turbulent mixing in the ship’s wake then propels the methane to the surface, where it escapes into the atmosphere, causing the periodic fluxes first recorded in Neva Bay.
When it comes to measuring emissions of gases such as methane, we often separate human impacts and natural processes. ‘The more I learn about the world,’ says Nylund, ‘the more I realise that we influence these natural processes as well, and we don’t know enough about either to be able to guess what’s going to happen.’ Similarly, we don’t yet know enough about how ships trigger methane emissions – partly, Nylund explains, because taking accurate measurements is challenging – to estimate the magnitude of its impact on global methane emissions, but Mellqvist calls it an important discovery, ‘not least considering that nine of the world’s ten largest ports are located in waters with similar conditions as Neva Bay.’
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The shipping industry is already known to be a major and growing source of global carbon emissions, accounting for roughly three per cent of the worldwide total – a figure comparable to the emissions of a major economy such as Germany or Japan. Unlike carbon emissions, however, these newly discovered methane emissions can’t be mitigated by a different fuel source. ‘As long as we have ships moving, this impact will remain,’ says Nylund. ‘I think we have to consider whether we actually need to ship everything that currently gets shipped.’
