First global map of cargo ship pollution reveals fuel regulatory effects: 'ship footprint' in clouds also helps explain how particles interact with clouds and affect global temperatures

A new study in Science Advances led by Tianle Yuan of UMBC used satellite data from 2003 – 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 the regulations took effect in 2015 and 2020. Their extensive data set could also contribute to answering a larger question: How do pollutants and other particles interact with clouds to affect overall global temperatures?

Small particles in the atmosphere, called aerosols and include pollution, can be harmful to human health, but they also often have a cooling effect on the planet because of the way they interact with clouds. However, an estimate of the extent of that effect by a factor of 10 — isn’t very precise for something so important.

“How much cooling is caused by aerosols is still massively unknown at this time, and that’s where the ship trails come in,” said Yuan, associate research 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 surface area for the droplets, which reflects more of the energy entering Earth’s atmosphere back into space and cooling the planet.

Instruments on the satellite can detect this difference in droplet size. And the air over the oceans is generally very clean, making the relatively narrow paths of ships crossing the ocean easy to detect. “Most of the original clouds are not polluted, and some are polluted by ships, thus creating a contrast,” Yuan explained.

While the ship’s trail is relatively clear in satellite data, you need to know where to look and have the time and resources to search. Prior to advances in computing power and machine learning, said Yuan, Ph.D. students can focus their entire thesis on identifying a group of ship paths in satellite data.

“What we are doing is automating this process,” Yuan said. His group “developed algorithms to automatically find traces of these ships from a sea of ​​data.”

This major advance allowed them to produce a comprehensive global ship path map over an extended period (18 years) for the first time. Next, they will share it with the world — opening the door for anyone to dig into data and make further discoveries.

Action disappear

Even before pollution-limiting regulations were put in place, Yuan and his colleagues found that ship paths did not occur wherever ships traveled. Only areas with certain types of low cloud cover have ship trajectories, which is useful for adjusting the role of clouds in climate models. They also found that after Europe, the US and Canada implemented Emission Control Areas (ECAs) along their coastlines in 2015, ship paths almost disappeared in the region, demonstrating the efficacy of the regulations for reducing pollution in port cities.

However, shipping companies do not necessarily reduce their overall pollution output. Instead, they make changes to adapt to the new rules. Ports in northern Mexico (not part of the ECA system) are experiencing increased activity, and pollution “hotspots” form along the ECA border as ships change their route to spend as few miles as possible inside the restriction zone.

However, in 2020, an international treaty sets far stricter standards for the delivery of fuel across global oceans, than just near coastlines. After that, the only ship paths that the team’s algorithm can detect are those in the cleanest clouds. In the clouds with even light background pollution, the traces of the alleged ships coalesce.

climate puzzle

It seems clear that reducing pollution from ships will result in a net profit. However, because particles (such as shipping pollution) have a cooling effect when interacting with clouds, reducing them significantly could contribute to a problematic increase in global temperatures, Yuan said.

That’s another reason why it’s important to emphasize the extent to which particulate pollution cools the planet. If the cooling effects of these pollutants and other particulate matter are significant, humans will need to balance the need to prevent widespread warming with the need to reduce pollution in the places where humans and other species live — creating difficult choices.

“Ship pollution alone can create a substantial cooling effect,” Yuan said, “because the atmosphere over the ocean is very clean.” There’s a physical limit to how small a cloud drop can get, so at some point, adding more pollution doesn’t increase the cooling effect of the cloud. But over the ocean, because the backdrop is largely unpolluted, even a small amount of pollution from ships matters.

Ocean pollution is also a major driver of the cooling effect of aerosols, as low clouds, which are most conducive to creating ship passages, occur more frequently on water than on land. And, as Yuan reminded, “the oceans cover two-thirds of the earth’s surface.”

Bigger image

Going forward, Yuan and his colleagues are helping to solve this conundrum by continuing their work to define more precisely the role clouds play in climate. “We can take advantage of the millions of ship path samples that we have now to start tackling the aerosol-cloud interaction problem as a whole,” Yuan said, “because the ship path can be used as a mini laboratory.”

By analyzing data from a relatively simple and well-controlled system — a narrow ship path crossing a very clean cloud — they were able to come to a conclusion they could trust.”

Other research teams can also use the team’s datasets and algorithms to arrive at their own conclusions, amplifying the potential public impact of this work. This spirit of collaboration will help scientists and society determine how best to deal with global challenges such as pollution and temperature change.

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