NASA: IMO’s global sulphur cap slashed air pollution from shipping

The International Maritime Organization’s (IMO) global sulphur cap has reduced artificial “ship track” clouds to record-low levels, a new study conducted by the National Aeronautics and Space Administration (NASA) shows.

Ship tracks, the polluted marine clouds that trail ocean-crossing vessels, are a signature of modern trade. 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 IMO, requiring an 86% reduction in fuel sulphur content – likely reduced ship track formation. 

Recently, the IMO also pointed out that there has been an estimated 77% drop in overall sulphur oxide emissions from ships since the entry into effect of the IMO 2020 regulations. The 2020 Sulphur Cap has limited the use of sulphur in fuel oil used on board ships operating outside of designated emission control areas to 0.50 percent m/m down from 3.50 percent m/m limit.

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NASA scientists emphasized that by capping fuel sulfur content at 0.5% (down from 3.5%), IMO’s global regulation in 2020 changed the chemical and physical composition of ship exhaust. Less sulphur emissions mean there are fewer aerosol particles released to form detectable ship tracks, according to NASA.

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

Moreover, ship-based tracking data indicated that the COVID-19 pandemic played a role in decreasing global shipping traffic by 1.4% for a few months. They noted that the pandemic-related disruptions played a secondary role. But this change alone could not explain the large decrease in observed ship tracks, which remained at record-low levels through several months of 2021.

Credit: NASA Earth Observatory

“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.

Furthermore, Yuan and colleagues found that fluctuations in economic activity leave distinct traces in the satellite record. In particular, Trans-Pacific ship track patterns between Asia and the Americas reflect dips and spikes in trade.

As outlined in the study, 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 reflected a slowdown in Chinese imports and exports of raw materials and commodities.

The researchers applied 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.

“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. That is a key uncertainty we face in terms of what drives climate right now,” Yuan concluded.