Thirty million metric tons of plastic pollution have already accumulated in the world’s oceans, research suggests, with the number projected to rise in coming years. But tracking litter as it floats on the sea surface has historically proved difficult for scientists because the trash covers a relatively small fraction of Earth’s surface, making it almost impossible to accurately monitor from space.

In a bid to find another way to track ocean trash, an international group of researchers analyzed 300,000 images of the Mediterranean Sea collected by the Copernicus Sentinel-2 mission, a European Space Agency-sponsored satellite mission that takes multispectral images of Earth’s surface from orbit. They were on the hunt for litter windrows — long, ephemeral clusters of floating trash that, researchers suspected, might be a good indicator of litter density in a given ocean area.

Between July 2015 and September 2021, the satellites captured images of 14,374 litter windrows covering about 36 square miles of sea surface. The longest were up to 14.3 miles.

The researchers used the imagery to calculate the density of the litter in windrow areas, identifying hot spots near Algeria, Libya, southwest Italy and the northern Adriatic Sea. Areas with denser populations had higher nearby litter densities, they found.

The analysis suggests “much of the litter remains near its land-based source,” the researchers write, though ocean currents and winds affect the litter, and floods and rainstorms in particular pushed the litter farther out to sea. These “pulses” of litter caused densities to peak in the spring and autumn and decline during winter.

Though the satellites weren’t designed to track trash, they helped identify the location and magnitude of waste while “opening new prospects” for litter tracking and considering litter windrows’ potential roles as habitats for marine life, the researchers write.

The researchers propose that future satellite missions include litter-specific tracking devices. More accurate plastic tracking capabilities could also improve ship navigation, search-and-rescue operations or lost cargo searches, they write.