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Tracking Plastics in the Ocean By Satellite

Turns out microplastic concentrations fluctuate with the seasons

3 min read
Still from a video showing high levels of microplastic pollution being dispersed from the mouth of the Yangtze River in the South China Sea.
Still from a video showing high levels of microplastic pollution (red) being dispersed from the mouth of the Yangtze River (left) in the South China Sea.

It's well known that microplastics have infiltratedmuch ofthe world's oceans, where these persistent pieces of garbage threaten the livelihood of marine life. But truly quantifying the extent of this problem globally remains a challenge.

Now, a pair of researchers are proposing a novel approach to tracking these minute scraps of plastic in the world's oceans—remotely from space. They describe their new satellite-based approach in a study published this month in IEEE Transactions on Geoscience and Remote Sensing, which hints at seasonal fluxes of microplastic concentrations in the oceans.

The problem of plastic persisting in environments is only growing. The annual global production of plastic has been increasing steadily every year since the 1950s, reaching 359 million metric tons in 2018. “Microplastic pollution is a looming threat to humans and our environment, the extent of which is not well-defined through traditional sampling alone," explains Madeline Evans, a Research Assistant with the Department of Climate and Space Sciences and Engineering, University of Michigan. “In order to work towards better solutions, we need to know more about the problem itself."

Evans was an undergraduate student at the University of Michigan when she began working with a professor of climate and space science at the institution, Christopher Ruf. The two have been collaborating over the past couple of years to develop and validate their approach, which uses bistatic radar to track microplastics in the ocean, using NASA's Cyclone Global Navigation Satellite System (CYGNSS).

The underlying concept behind their approach hinges on how the presence of microplastics alters the surface of the ocean. “The presence of microplastics and surfactants in the ocean cause the ocean surface to be less responsive to roughening by the wind," says Ruf.

Therefore, Ruf and Evans explored whether CYGNSS measurements of the ocean's surface roughness that deviated from predicted values, given local winds speeds, could be used to confirm the presence of microplastics. Indeed, they validated their approach by comparing it to other models for predicting microplastics. But whereas existing models provide a static snapshot of the extent and scope of microplastic pollution, this new approach using CYNGSS can be used to understand microplastic concentrations in real-time.

Using their approach to analyze global patterns, the researchers found that microplastic concentrations in the North Indian Ocean tend to be highest in late winter/early spring and lowest in early summer. These fluxes coincide with monsoon season, prompting the authors to suggest that the outflow patterns of rivers into the ocean and the diluting power of the rain may be influential for microplastic concentration, although more research is needed to confirm this. As well, they detected a detected a strong seasonal pattern in the Great Pacific Garbage Patch, whereby concentrations of microplastics are highest in summer and lowest in winter.

“The seasonal shift in our measurements surprised me the most," says Evans. “Before working on this project, I conceptualized the Great Pacific Garbage Patch as a consistent, mostly static mass. I was surprised to witness these large accumulations of microplastics behaving in such a dynamic way."

Ruf emphasizes the novelty of being able to track microplastic concentrations in real time. “Time lapse measurements of microplastic concentration have never been possible before, so we are able to see for the first time some of its dynamic behavior, like the seasonal dependence and the outflow from major rivers into the ocean," he says.

However, he notes that not being able to validate the model with direct sampling of microplastics remains a major limitation. Raf and Evans are now serving on global task force dedicated to remote sensing of marine litter and debris, which aims to improve these measurements in the future.

“It is very exciting to be involved in something this new and important," says Raf. “[This work] will hopefully lead to better awareness of the problem of ocean microplastics and other marine debris and litter, and maybe someday to changes in industrial practices and lifestyle choices to help reduce the problem."

This article appears in the September 2021 print issue as "Satellites Track Ocean Microplastics."

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Europe Expands Virtual Borders To Thwart Migrants

Our investigation reveals that Europe is turning to remote sensing to detect seafaring migrants so African countries can pull them back

14 min read
A photo of a number of people sitting in a inflatable boat on the water with a patrol ship in the background.

Migrants in a dinghy accompanied by a Frontex vessel at the village of Skala Sikaminias, on the Greek island of Lesbos, after crossing the Aegean sea from Turkey, on 28 February 2020.


It was after midnight in the Maltese search-and-rescue zone of the Mediterranean when a rubber boat originating from Libya carrying dozens of migrants encountered a hulking cargo ship from Madeira and a European military aircraft. The ship’s captain stopped the engines, and the aircraft flashed its lights at the rubber boat. But neither the ship nor the aircraft came to the rescue. Instead, Maltese authorities told the ship’s captain to wait for vessels from Malta to pick up the migrants. By the time those boats arrived, three migrants had drowned trying to swim to the idle ship.

The private, Malta-based vessels picked up the survivors, steamed about 237 kilometers south, and handed over the migrants to authorities in Libya, which was and is in the midst of a civil war, rather than return to Malta, 160 km away. Five more migrants died on the southward journey. By delivering the migrants there, the masters of the Maltese vessels, and perhaps the European rescue authorities involved, may have violated the international law of the sea, which requires ship masters to return people they rescue to a safe port. Instead, migrants returned to Libya over the last decade have reported enslavement, physical abuse, extortion, and murders while they try to cross the Mediterranean.

If it were legal to deliver rescued migrants to Libya, it would be as cheap as sending rescue boats a few extra kilometers south instead of east. But over the last few years, Europe’s maritime military patrols have conducted fewer and fewer sea rescue operations, while adding crewed and uncrewed aerial patrols and investing in remote-sensing technology to create expanded virtual borders to stop migrants before they get near a physical border.

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