“Ship tracks” in clouds also help explain how particles interact with clouds and affect global temperatures


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

Tiny particles in the atmosphere, called aerosols and pollutants, can harm human health, but they also often have a chilling effect on the planet due to the way they interact with clouds. However, estimates of the magnitude of this effect vary by a factor of 10 – not very accurate for something this large.

“The amount of cooling caused by aerosols is a big unknown right now, and that’s where ship tracks come in,” says Yuan, a research associate at Goddard Earth Sciences Technology and Research (GESTAR) II. Center.

sea ​​of ​​data

When polluting particles from ships enter clouds low in the atmosphere, they decrease the size of individual cloud droplets without changing the total volume of the cloud. This creates more surface area for droplets, which reflects more energy entering Earth’s atmosphere back into space and cools the planet.

Satellite instruments can detect these differences in droplet size. And the air over the ocean is usually very clean, making relatively narrow ship tracks winding through the ocean easy to spot. “Most of the original cloud is unpolluted, and then some of it is polluted by the ship, which creates a contrast,” Yuan says.

Although ship trajectories may be relatively obvious in satellite data, you need to know where to look and have the time and resources to research. Before advances in computing power and machine learning, says Yuan, Ph.D., students could focus their entire thesis on identifying a cluster of ship trajectories in satellite data.

“What we’ve done is automate this process,” says Yuan. His group “developed an algorithm to automatically find these ship tracks from the sea of ​​data.”

This enormous advance allowed them to generate for the first time a global and complete map of the routes of ships over a long period (18 years). Then they’ll share it with the world, opening the door for anyone to dig into the data and make new discoveries.

Disappearance certificate

Even before pollution-limiting regulations were put in place, Yuan and his colleagues discovered 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 United States and Canada instituted Emission Control Areas (ECAs) along their coasts in 2015, ship tracks almost disappeared in these areas, demonstrating the effectiveness of these regulations in reducing pollution in port cities.

However, shipping companies have not necessarily reduced their pollution output at all levels. Instead, they made changes to accommodate the new rules. Ports in northern Mexico (not part of the ECA system) saw increased activity and pollution “hotspots” built up along ECA boundaries as ships changed their routes to pass the fewest miles possible within restricted areas.

In 2020, however, an international agreement set a much more restrictive standard for transporting fuel across all of the world’s oceans, rather than just near shore. After that, the only ship tracks the team’s algorithm could detect were those in the cleanest clouds. In clouds with even light background pollution, the ship’s presumed trajectories blended in perfectly.

The climate conundrum

It seems obvious that reducing pollution from ships would produce a net benefit. However, since particles (such as marine pollution) have a cooling effect when they interact with clouds, their significant reduction could contribute to a problematic increase in global temperatures, Yuan says.

This is another reason why it is important to determine how much particulate pollution cools the planet. If the cooling effect of these pollutants and other particles is significant, humans will need to balance the need to prevent significant warming with the need to reduce pollution where people and other species live. which creates difficult choices.

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

Ocean pollution is also an outsized contributor to the aerosol cooling effect, as low clouds, which are most conducive to creating ship paths, are more common over water than on earth. And, as Yuan reminds us, “the ocean covers two-thirds of the Earth’s surface.”

The bigger picture

Moving forward, Yuan and his colleagues are helping solve this conundrum by continuing their work to further define the role clouds play in climate. “We can take advantage of the millions of ship track samples we now have to begin to understand the global aerosol-cloud interaction problem,” says Yuan, “because ship tracks can be used as mini-labs.” .

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

Other research teams can also use the team’s dataset and algorithm to reach their own conclusions, amplifying the potential public impact of this work. This 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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