A new study in scientific advances led by UMBC’s Tianle Yuan, used satellite data from 2003 to 2020 to determine the impact of fuel regulations on pollution from cargo ships. The research team’s data showed significant changes in sulfur pollution after the regulations went into effect in 2015 and 2020. Their extensive dataset may also help answer a larger question: How do pollutants and other particles interact with clouds to affect global temperatures overall?
Tiny particles in the atmosphere known as aerosols, which include pollution, can harm human health but often also have a cooling effect on the planet due to their interaction with clouds. However, estimates of the magnitude of this effect range by a factor of 10 – not very accurate for something this important.
“How much cooling the aerosols cause is a big unknown right now, and that’s where ship tracks come in,” says Yuan, an associate research scientist at the Goddard Earth Sciences Technology and Research (GESTAR) II Center.
sea of data
When pollutant particles from ships enter clouds deep in the atmosphere, they reduce the size of individual cloud droplets without changing the overall volume of the cloud. This creates a larger droplet surface area that reflects more of the 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 it easy to spot the relatively narrow ship tracks that meander across the ocean. “Most of the original cloud is unpolluted, and part of it is polluted by the ship, so there is a contrast,” Yuan explains.
While ship tracks can be fairly obvious in satellite data, you need to know where to look and have the time and resources to look. Before advances in computing power and machine learning, Yuan, Ph.D. Students could focus their entire thesis on identifying a cluster of ship tracks in satellite data.
“We have automated this process,” says Yuan. His group “developed an algorithm to automatically find these ship tracks from the sea of data”.
This tremendous advance allowed them to create, for the first time, a comprehensive, global map of ship tracks over an extended period (18 years). Next, they will share it with the world – opening the door for anyone to dig into the data and make more discoveries.
act of disappearance
Even before regulations to limit pollution were introduced, Yuan and his colleagues found that ship tracks didn’t appear 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 in those regions had all but disappeared, demonstrating the effectiveness of such regulations in reducing pollution in port cities.
However, the shipping companies have not necessarily reduced their pollutant emissions 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” formed along the borders of ECAs as ships changed routes to spend as few miles as possible within the restricted zones.
In 2020, however, an international agreement set a much more restrictive standard for transporting fuel across 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 of clouds. In clouds with even light background pollution, the suspected ship tracks interfered directly.
It seems obvious that there would be a net benefit in reducing pollution from ships. However, since particles (like ship pollution) have a cooling effect when interacting with clouds, a significant reduction could contribute to a problematic rise in global temperatures, Yuan says.
That’s another reason why it’s important to cement the degree to which particulate matter pollution is cooling the planet. When the cooling effect of these pollutants and other particles is significant, people must balance the need to prevent excessive warming with the need to reduce pollution where people and other species live – leading to difficult choices.
“Ship pollution alone can produce a significant cooling effect,” Yuan says, “because the atmosphere over the ocean is so clean.” There is a physical limit to how small cloud droplets can get, so beyond a certain point adding more pollution will not increase the cooling effect of the clouds. But over the ocean, because the background is largely unpolluted, even a small amount of pollution from ships makes an impact.
Marine pollution is also an outsized driver of the cooling effect of aerosols because low clouds, which are most conducive to ship-track formation, are more common over water than on land. And, as Yuan reminds us, “the ocean covers two-thirds of the earth’s surface.”
The Bigger Picture
Going forward, Yuan and his colleagues are helping to tackle this mystery by continuing their work to more precisely define the role clouds play in climate. “We can use the millions of ship track samples we now have to tackle the general problem of aerosol-cloud interaction,” 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 running through very clean clouds – they can come to conclusions they can trust.”
Other research teams can also use the team’s dataset and algorithm to reach their own conclusions, increasing the potential public impact of this work. This collaborative spirit will help scientists and communities determine how best to address global challenges such as pollution and temperature changes.