A global standard limiting sulfur in ship fuel reduced artificial “ship track” clouds to record-low levels in 2020, according to a study conducted by NASA.
Ship tracks, the polluted marine clouds that trail ocean-crossing vessels, are a signature of modern trade. Like ghostly fingerprints, they trace shipping lanes around the globe, from the North Pacific to the Mediterranean Sea. But in 2020, satellite observations showed fewer of those pollution fingerprints.
Drawing on nearly two decades of satellite imagery, researchers found that the number of ship tracks fell significantly after a new fuel regulation went into effect. A global standard implemented in 2020 by the International Maritime Organization (IMO) – requiring an 86 percent reduction in fuel sulfur content – likely reduced ship track formation, the study says.
COVID-19-related trade disruptions also played a small role in the reduction. Ship-based tracking data indicated that during the height of the pandemic, global shipping traffic decreased by 1.4 percent for a few months.
Scientists used advanced computing techniques to create the first global climatology (a history of measurements) of ship tracks. They used artificial intelligence to automatically identify ship tracks across 17 years of daytime images (2003-2020) captured by NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite.
“Without this kind of complete and large-scale sampling of ship tracks, we cannot begin to completely understand this problem,” said lead author of the study Tianle Yuan, an atmospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland, Baltimore County.
Ship tracks are formed by water vapor coalescing around small particles of pollution (aerosols) in ship exhaust. The highly concentrated droplets scatter more light and therefore appear brighter than non-polluted marine clouds, which are seeded by larger particles such as sea salt.
By capping fuel sulfur content at 0.5 percent, IMO’s global regulation in 2020 changed the chemical and physical composition of ship exhaust. Less sulfur emissions mean there are fewer of the aerosol particles released to form detectable ship tracks.
According to Yuan and colleagues, similar but regionally defined sulfur regulations – such as an IMO Emission Control Area in effect since 2015 off the west coast of the U.S. and Canada – had not had the desired effect because operators altered their routes and charted longer courses to avoid designated zones.
The researchers concluded that the new global fuel regulation played the dominant role in reducing ship tracks in 2020.
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