NASA report: sulphur cap key factor in record-low 'ship tracks'

Scientists used advanced computing techniques to create the first global climatology measuring the history of ship tracks. They used artificial intelligence to automatically identify ship tracks across 17 years of daytime images spanning from 2003-2020 captured by NASA’s moderate resolution imaging spectroradiometer 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 NASA atmospheric scientist and lead author for the study, Dr Tianle Yuan.

"Ship tracks were first observed as ’anomalous cloud lines’ in early weather satellite images acquired in the 1960s. They are formed by water vapour 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," NASA said in a statement on the study.

By capping fuel sulphur content at 0.5% (down from 3.5%), IMO’s 2020 global sulphur regulation changed the chemical and physical composition of ship exhaust, meaning ships are emitting fewer sulphur particles around which water vapour can coalesce. In total, NASA calculated the reduction in fuel sulphur content to be 86%.

According to Dr Yuan and his colleagues, the data also reveals that global regulations – as opposed to similar but regionally defined sulphur regulations such as an IMO Emission Control Area (ECA) in effect since 2015 off the west coast of the US and Canada – were what brought ship track levels down.

Localised ECAs "had not had the desired effect because operators altered their routes and charted longer courses to avoid designated zones," NASA said.

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