‘Ship Tracks’ Study Reveals IMO 2020’s Impact on Reducing Pollution from Ships

The International Maritime Organization’s (IMO) low sulphur fuel regulation was a marked shift in the shipping industry’s efforts to reduce air pollution from ships.

Known as “IMO 2020”, the regulation put a new global limit on the sulphur content of bunker fuel used by ships, in turn reducing sulphur oxide (SOx) emissions which are harmful to human health, by some 85%.

Now a new NASA study helps to visualize the benefits by showing studying artificial “ship track” clouds, serving as pollution fingerprints.

The narrow clouds, which were first observed as “anomalous cloud lines” in early weather satellite images acquired in the 1960s, form when water vapor condenses around microscopic pollution particles that ships emit as exhaust.

Drawing on nearly two decades of satellite imagery, researchers found that the number of ship tracks fell significantly after the new low sulphur fuel rules came into effect in 2020. COVID-19-related trade disruptions may have also played a smaller role.

Less sulfur emissions means there were fewer aerosol particles released to form detectable ship tracks.

Scientists used advanced computing to create the first historical measurements of ship tracks. Using artificial intelligence, they were able to automatically identify ship tracks across 17 years of daytime images (2003-2020), as 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 Tianle Yuan, an atmospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland, Baltimore County.

According to the Yuan, similar but more targeted sulphur 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.

While analyzing 2020 data, however, the researchers found that ship-track density fell that year in every major shipping lane.

While ship-based tracking data indicated that COVID-19 played a role by decreasing global shipping traffic for a few months in the spring of 2020, the change alone could not explain the large decrease in observed ship tracks throughout the year. Through several months of 2021, ship track clouds remained at record-low levels even though global shipping had picked back up.

The researchers therefore concluded that the new global fuel regulation played the dominant role in reducing ship tracks in 2020.

The researchers also found that fluctuations in economic activity left distinct traces in the satellite record. In particular, Trans-Pacific ship track patterns between Asia and the Americas reflect dips and spikes in trade. Looking back further, the study outlined that a general upward trend in shipping activity between 2003 and 2013—reflected in ship-track clouds—dropped for about a year in the aftermath of the 2008 global financial crisis. An even sharper decrease between 2014 and 2016, likely reflecting a slowdown in Chinese imports and exports of raw materials and commodities.

 “Ship tracks are great natural laboratories for studying the interaction between aerosols and low clouds, and how that impacts the amount of radiation Earth receives and reflects back to space,” Yuan said. “That is a key uncertainty we face in terms of what drives climate right now.”

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