Satellite image of clouds and ship tracks in the Pacific Ocean south of Alaska
Scientists are taking a closer look at emissions associated with ship tracks, like these crossing the Pacific Ocean south of Alaska. Credit: NASA

A total of 99,800 commercial ships sail the seas, carrying around 90% of the world’s trade in goods. Their operation is vital—the transport of COVID-19 vaccines, for instance, wouldn’t have been possible without them.

The fuel oil that runs these immense vessels can contain up to 3,500 times more sulfur than the diesel that is used for land vehicles. When the fuel is burned, large amounts of sulfur oxides and other aerosols are released into the atmosphere. These aerosols increase the concentration of droplets in marine low clouds, which makes them appear brighter. These bright clouds trail ships and can be seen in satellite images.

In 2018, Tianle Yuan, a University of Maryland, Baltimore County, atmospheric scientist at NASA’s Goddard Space Flight Center, and his team began developing a machine learning algorithm that automatically identifies these “ship tracks.” After years of refining the algorithm with satellite imagery, they created the first global map of ship tracks with data spanning the past 18 years.

“Before our study, the largest samples contained about 5,000 ship tracks, and that’s already a huge effort,” Yuan said. “But now we have hundreds of thousands, maybe millions.”

The team’s first finding after analyzing the monitoring map is good news for both people and ocean health.

Regulations Are Working

Sulfur oxides are harmful to people’s respiratory health and damaging to ocean ecosystems. When the particles reach the atmosphere, they quickly become surrounded by water and promote cloud creation. Those clouds produce acid rain, which affects crops and forests near the coasts, and contribute to ocean acidification, which likely reduces marine ecosystem services.

“Most of the ship tracks just disappeared. Regulations are working.”

As a response to shipping-related air pollution, the United Nations’ International Maritime Organization (IMO) started implementing limits on the sulfur content in fuels. In 2015, a sulfur content limit of 0.1% was established for ships sailing within Emission Control Areas (ECAs). Outside ECAs, the sulfur content limit was reduced from 3.5% to 0.5% in 2020. Together the latest regulations account for an annual reduction of 8.5 million metric tons of sulfur oxides—77% of total emissions, according to IMO agency data.

The results obtained by Yuan and his team, published in Science Advances, put those numbers into perspective. “Within the ECAs, most of the ship tracks just disappeared,” Yuan said. “Regulations are working.”

Although the COVID-19 pandemic affected maritime trade, the number of ship tracks in ECAs continued to decline even as trade began to recover between late 2020 and early 2021, Yuan said.

However, when analyzing data from the U.S. West Coast, the scientists noted that since the implementation of the ECA limits in 2015, several ships changed their routes to the south, outside the ECA. While shipping traffic from Los Angeles and San Diego ports decreased, hot spots were found in Baja California, just behind the ECA.

Although there is no hard evidence, Capt. Ricardo Valdés of Mexico’s Secretariat of the Navy “would also not doubt” that ships may be evading ECAs to avoid buying cleaner fuels, which are also more expensive. Valdés has been sailing for 35 years and is very familiar with shipping policies and practices.

“The tools and legislation exist, but political will is needed.”

In fact, in 2018, Mexico’s Ministry of Environment and Natural Resources (SEMARNAT) sought to create its own ECA because the lack of regulation on emissions was causing vessels to use cheap, polluting fuel when arriving in Mexican national waters. “We want to control the emissions from the ships that transit our waters [75% of which are foreign], so that these emissions are as few as possible,” said Rodolfo Lacy of SEMARNAT.

Ultimately, the IMO declined Mexico’s initiative because the country is not a signatory to Annex VI of the International Convention for the Prevention of Pollution from Ships, the mechanism through which emissions limits are implemented.

Although the IMO already has an emissions inventory based on automatic identification system data that lets it identify ship emissions, fuel consumption, and energy use, Yuan’s team’s algorithm “is useful for validating some of the bottom modeling that’s used as the basis for IMO decisionmaking,” said environmental scientist Bryan Comer, who leads the International Council on Clean Transportation’s marine program.

The algorithm could also be “an excellent tool” for monitoring evading ships and their emissions, Valdés said, but it will be useful only if Mexico works to adhere to IMO agreements. “[Mexico] can’t require a foreign ship to comply with some standards if its own ships don’t…The tools and legislation exist, but political will is needed.”

The Road to Decarbonization

In addition to emitting sulfur aerosols, bright ship tracks reflect the Sun’s rays, a process that generates a cooling effect in the atmosphere. However, measuring this effect is extremely difficult because environmental conditions in each region affect clouds in different ways.

The Intergovernmental Panel on Climate Change (IPCC) has stated that aerosol-cloud interactions constitute the biggest uncertainty in climate projections. Recent studies have shown that although global air pollution has decreased by up to 30% from 2000 levels, cleaner air may have increased warming by 15%–50%. Still, the IPCC report points out that “strong, rapid and sustained reductions in methane [and other greenhouse gas] emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality.”

“Humanity is in the process of doing a large-scale experiment with the planet.”

The IMO is aware of this. Comer explained that decarbonizing the maritime sector would produce benefits to both health and climate. The main issue, he said, is that decarbonization will reduce the atmospheric concentration of greenhouse gases. “We should not take any steps backward,” he said.

For Columbia University climate scientist James Hansen, Yuan’s work is an important element that will help scientists better understand the complex role of aerosols in climate change. The constant change of clouds makes it extremely difficult to study how they interact with aerosols. But ship tracks are perfect laboratories to study these interactions. There is very little change in and out of the plume of exhaust emitted by ships as they sail under the clouds, so scientists can attribute changes in the clouds more directly (though never entirely) to the aerosols.

“The radiation balance of the planet seems to be affected as we expected, but the record needs to be longer to quantify the impact,” Hansen said. “Humanity is in the process of doing a large-scale experiment with the planet that potentially can provide important information about future climate.”

—Humberto Basilio (@HumbertoBasilio), Science Writer

Citation: Basilio, H. (2022), Tracking climate through ship exhaust, Eos, 103, Published on 27 September 2022.
Text © 2022. The authors. CC BY-NC-ND 3.0
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