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First global map of cargo ship pollution reveals impact of fuel regulations: ‘Ship tracks’ in clouds also help explain how particles interact with clouds and affect global temperatures

A new study in The progress of science led by UMBC’s Tianle Yuan used satellite data from 2003 – 2020 to determine the impact of fuel regulations on pollution from cargo ships. The research team’s data revealed significant changes in sulfur pollution after regulations took effect in 2015 and 2020. Their extensive data set may also help answer a larger question: How do pollutants and other particles interact with clouds to affect global temperatures overall?

Small particles in the atmosphere, called aerosols and include pollutants, can harm human health, but they also often have a cooling effect on the planet because of how they interact with clouds. However, estimates of the magnitude of that effect vary by a factor of 10—not very precise for something so important.

“How much cooling aerosols cause is a big unknown right now, and that’s where ship tracks come in,” said Yuan, an associate scientist at the Goddard Earth Sciences Technology and Research (GESTAR) II Center.

Sea of ​​data

When pollutant particles from ships enter clouds low in the atmosphere, they reduce the size of individual cloud droplets without changing the total volume of the cloud. That creates more droplet surface area, which reflects more energy entering Earth’s atmosphere back into space, cooling the planet.

Instruments on satellites can detect these differences in droplet size. And the air over the ocean is generally very clean, making the relatively narrow ship tracks snaking across the ocean easy to pick out. “Most of the original cloud is uncontaminated, and then some of it is contaminated by the ship, so it creates a contrast,” explains Yuan.

Although ship tracks can be relatively obvious in satellite data, you need to know where to look and have the time and resources to search. Before advances in computing power and machine learning, says Yuan, Ph.D. the students were able to focus their entire thesis on identifying a group of ship tracks in satellite data.

“What we did was automate this process,” says Yuan. His group “developed an algorithm to automatically find these ship tracks from the ocean of data.”

This huge advance allowed them to create a comprehensive global map of ship tracks over an extended period (18 years) for the first time. They will then share it with the world – opening the door for anyone to dig into the data and make further discoveries.

Disappearing action

Even before pollution control regulations were introduced, Yuan and his colleagues found that ship tracks did not occur everywhere ships traveled. Only areas with certain types of low cloud cover had ship tracks, which is useful for adjusting the role of clouds in climate models. They also found that after Europe, the US and Canada established Emission Control Areas (ECAs) along their coasts in 2015, ship tracks almost disappeared in those regions, demonstrating the effectiveness of such regulations in reducing pollution in port cities.

However, the shipping companies have not necessarily reduced their pollution production across the board. Instead, they made changes to adapt to the new rules. Ports in northern Mexico (not part of the ECA system) saw increased activity, and pollution “hot spots” built up along the borders of the ECAs as ships changed their routes to spend as few miles as possible within the restrictive zones.

But in 2020, an international agreement set a much more restrictive standard for transporting fuel across the world’s oceans, rather than just near coastlines. After that, the only ship tracks the team’s algorithm could detect were those in the cleanest clouds. In clouds with even mild background pollution, the supposed ship tracks blended in.

Climate problems

It seems clear that reducing pollution from ships would provide a net gain. But because particles (such as shipping pollution) have a cooling effect when they interact with clouds, reducing them could significantly contribute to a problematic increase in global temperatures, Yuan says.

That’s another reason why it’s important to boost the rate at which particulate pollution cools the planet. If the cooling effect of these pollutants and other particles is significant, humans will have to balance the need to prevent widespread warming with the need to reduce pollution where humans and other species live — creating difficult choices.

“Ship pollution alone can create a significant cooling effect,” says Yuan, “because the atmosphere over the ocean is so clean.” There is a physical limit to how small cloud droplets can get, so at a certain point adding more pollutants does not increase the cloud’s cooling effect. But over the ocean, since the background is largely unpolluted, even a small amount of pollution from ships has an effect.

Marine pollution is also a major driver of the cooling effect of aerosols, as low clouds, which are most favorable for creating ship tracks, are more common over water than over land. And, as Yuan reminds us, “the ocean covers two-thirds of the Earth’s surface.”

The bigger picture

In the future, Yuan and his colleagues will help address this conundrum by continuing their work to more precisely define the role clouds play in climate. “We can take advantage of the millions of ship track samples we have now to start getting at the overall aerosol-cloud interaction problem,” says Yuan, “because ship tracks can be used as mini-labs.”

By analyzing data from a relatively simple and well-controlled system – narrow ship tracks going through very clean clouds – they can come to conclusions that they can be sure of.”

Other research teams may also use the team’s dataset and algorithm to reach their own conclusions, amplifying the potential public impact of this work. That spirit of collaboration will help scientists and communities determine how best to address global challenges such as pollution and temperature change.

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