The fuel regulation reduced air pollution caused by shipping
Ship tracks, the polluted sea clouds that follow in the wake of ships crossing oceans, are a hallmark of modern commerce. Like eerie fingerprints, they trace shipping lanes around the world from the North Pacific to the Mediterranean. But in 2020, satellite observations showed fewer of these pollution fingerprints.
Based on almost two decades of satellite images, the researchers found that the number of ship tracks decreased significantly after the entry into force of the new fuel regulation. A global standard introduced by the International Maritime Organization (IMO) in 2020, requiring an 86 percent reduction in fuel sulfur content, likely reduced ship derailment. Trade disruptions related to the COVID-19 virus also played a small role in the decline.
Scientists used advanced computing techniques to create the first global climate (measurement history) from ships’ rolls. They used artificial intelligence to automatically identify the tracks of the ships in daily photos taken over a period of 17 years (2003-2020). Medium Resolution Imaging Spectroradiometer (MODIS) at NASA Aqua satellite. The map at the top of this page shows the density of ship tracks identified between these years, calculated as a percentage of the total amount of marine low cloud.
“Without such a complete and large-scale sampling of the ship’s wake, we cannot begin to fully understand this problem,” said lead author Tianle Yuan, an atmospheric scientist at NASA’s Goddard Space Flight Center.
Ship tracks were first observed as “anomalous cloud lines” in early-weather satellite images acquired in the 1960s. They are formed when water vapor merges around small pollution particles (aerosols) from ships’ exhaust gases. Highly concentrated droplets scatter more light and therefore appear brighter than unpolluted sea clouds seeded with larger particles such as sea salt. Crossed ship tracks along the Pacific coast of North America can be seen in the natural color image above, acquired on December 7, 2021. Visible Infrared Imaging Radiometer Suite (VIIRS) is Finland’s nuclear power plant satellite.
By limiting the sulfur content of fuel to 0.5 percent (from 3.5 percent), the IMO’s global regulation in 2020 changed the chemical and physical composition of ship exhaust gases. Lower sulfur emissions mean that fewer aerosol particles are released to form detectable ship wakes.
According to Yuan and colleagues, similar but regionally defined sulfur regulations – such as the IMO Emissions Control Area since 2015 along the west coast of the United States and Canada – had not had the desired effect, as operators changed their routes and charted longer courses to avoid the designated zones.
Analyzing the data from 2020, the researchers found that ship track density decreased in the same year in all major shipping lanes. (See map above.) Ship-based tracking data showed that the COVID-19 pandemic played a role in reducing global shipping traffic by 1.4 percent for a few months. But this change alone could not explain the large decline in observed ship tracks, which remained at record low levels for several months in 2021 (the latest data analysed). The researchers concluded that the new global fuel regulation played a dominant role in reducing ship derailments in 2020.
During their lengthy analysis, Yuan and colleagues also found that fluctuations in economic activity leave clear traces in the satellite record. In particular, the trans-Pacific shipping lanes between Asia and the Americas reflect the ebbs and flows of trade. As the study noted, the general upward trend in shipping activity between 2003 and 2013—reflected in ship track clouds—reversed for about a year after the 2008 global financial crisis. An even sharper decline between 2014 and 2016 was likely due to a slowdown in China’s imports and exports of raw materials and commodities.
In addition to the importance of world trade, ship tracks can serve as case studies for climate change. “Ship tracks are great natural laboratories for studying the interaction between aerosols and low clouds and how that affects the amount of radiation that the Earth receives and reflects back to space,” Yuan said. “That’s the key uncertainty we face about what’s driving the climate right now.”
NASA Earth Observatory images based on data from Joshua Stevens Yuan, T., et al. (2022) and VIIRS data from NASA EOSDIS LANCE, GIBS/World Viewand Finland’s national Arctic Circle Partnership. Story by Sally Younger/NASA’s Earth Science News Team.