  Ship-tracks are produced by ship-emitted aerosols interacting with marine low
clouds. Here we apply deep learning models on satellite data to produce the
first multi-year global climatology map of ship-tracks. We show that
ship-tracks are at the nexus of cloud physics, maritime shipping, and fuel
regulation. Our map captures major shipping lanes while missing others,
reflecting the influences of background cloud and aerosol properties.
Ship-track frequency is more than 10 times higher than expected from a previous
survey. Interannual fluctuations in ship-track frequency reflect variations in
cross-ocean trade, shipping activity, and fuel regulations. Fuel regulation can
alter both detected ship-track density and pattern of shipping routes due to
cost economics. The new fuel regulation, together with the COVID-19 pandemic,
reduced ship-track frequency in 2020 to its lowest level in recent decades
across the globe and may have ushered in a new era of low ship-track frequency.
We estimate the aerosol indirect forcing induced by the fuel regulation to be
between 0.02 and 0.22 Wm-2.
