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

A new study in Science Improvements led by Tianle Yuan of UMBC used satellite data from 2003 to 2020 to determine the effect of fuel regulations on cargo air pollution. Data from the research team revealed significant changes in air sulfur pollution after regulations came into effect in 18 and

. Their large dataset may also help answer a larger problem: 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 cooling effect on the planet due to the way they interact with clouds . However, estimates of the magnitude of this effect vary by a factor – not very precise 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 , associate researcher at the Goddard Earth Sciences Technological innovation and Exploration (GESTAR) II Centre.

Sea of ​​data

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

Satellite instruments can detect these droplet size differences. 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, then some of it is polluted by the ship, which creates a contrast,” says Yuan.

Although the vessel 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 “has 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 ( year). Then they’ll share it with the world, opening the door for anyone to dig into the data and make new discoveries.

Disappearing act

Even before the implementation of regulations limiting pollution, Yuan and his colleagues found that ship tracks did not occur everywhere ships traveled. Only areas with some forms 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, traces of ships have almost disappeared in these regions, demonstrating the effectiveness of these regulations in reducing air 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” accumulated along the ECA boundaries as ships changed their routes to pass the fewest miles possible inside restricted areas.

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

The climate conundrum

It seems obvious that the reduction of air pollution by ships would produce an internet profit. However, since particles (such as air pollution from the sea) have a cooling effect when they interact with clouds, their significant reduction could contribute to a problematic increase in global temperatures, says Yuan.

This is another reason why it is crucial to determine to what extent particulate pollution cools the planet. While the cooling effect of these pollutants and other particles is essential, humans will need to balance the need to prevent vital warming with the need to reduce pollution where people and other species live. which creates difficult choices.

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

The Ocean air pollution is also an outsized factor in 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

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,” Yuan explains, “vehicle ship tracks can be used as mini- laboratories”.

By analyzing data from a relatively uncomplicated and well-controlled system – narrow ship tracks traversing very clean clouds – they can arrive at conclusions they can be sure of.”

Other research teams can also use the team’s dataset and algorithm to draw their own conclusions, thus 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 air pollution and temperature change.


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