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NASA AI shows slashing sulfur in shipping fuel cut air pollution at sea

COVID-19 pandemic made a bit of a dent, too

A reduction of sulfur in shipping fuel reduced air pollution levels at sea to the lowest levels this century in 2020, according to an AI model built by NASA. The COVID-19 pandemic also helped here, too, natch.

A rule set by the International Marine Organization (IMO) significantly limiting sulfur content in fuel oil for ships travelling outside emission-control areas came into effect two years ago. The restriction was predicted to cut sulfur oxide emissions by 77 per cent – equivalent to 8.5 million metric tonnes. The noxious gas increases risk of acid rain and can cause respiratory, cardiovascular, and lung disease. 

Now, a study by NASA reveals that the IMO 2020 policy – and the decreased levels of shipping traffic during the early stages of the COVID-19 pandemic – brought shipping air pollution to the lowest levels since tracking started almost 20 years ago. Researchers crafted an AI model to identify shipping tracks in 17 years of images taken by the Moderate Resolution Imaging Spectroradiometer instrument (MODIS) aboard NASA's Earth-observation Aqua satellite from 2003 to 2020.

"Without this kind of complete and large-scale sampling of ship tracks, we cannot begin to understand this problem," Tianle Yuan, an atmospheric scientist at NASA's Goddard Space Flight Center and the University of Maryland, said Tuesday. The study's results were recently published in Science

The images revealed what the model detected as "anomalous cloud lines" at first. These clouds are formed when pollutant aerosols spewed from ships mix with water vapor. The concentrated droplets scatter more light and look brighter than other types of clouds at sea, which contain larger particles of salt.

The model helped researchers realize that ships started generating less air pollution in 2020, when the level of sulfur in fuel was capped.

"Ship-track density experiences strong reductions in every detected major shipping lane compared to climatology and reaches record lows in the nearly 20-year data record," the paper states.

"Except the trans-Pacific and trans–North Atlantic shipping lanes, other shipping lanes are not discernible any longer. Annual mean ship-track density decreases by 50 percent or more in five major shipping lanes compared to the climatological mean. The decline is even steeper if compared to 2019."

Global shipping traffic fell by 1.4 percent for a few months during the COVID-19 pandemic and remained low in 2021, though those fewer vessels did not account for the large drop in detected shipping tracks. The researchers believe the IMO 2020 fuel regulation had a larger impact in reducing air pollution.

They also managed to trace popular shipping routes across Asia and America at different times – such as the drop in global trade following the 2008 financial crisis. There were two other dips in shipping traffic in Asia between 2014 and 2016 when China was importing and exporting less resources. 

"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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