Forecasting microplastics

Eyes in the sky are providing University of Michigan scientists with a new perspective on an old pollutant. Using an array of NASA satellites, the researchers have developed a new method for tracking the tiny pieces of plastic piling up in certain regions of the ocean.

For decades, microplastics—flecks of broken-down plastic smaller than 5 millimeters—have piled up in garbage patches in the world’s oceans and infiltrated coastal waterways. But the vastness of the ocean and the smallness of the chunks of plastic have made it hard for scientists to understand the scope of the problem.

Michigan researchers Madeline Evans and Christopher Ruf leveraged data from NASA’s Cyclone Global Navigation Satellite System to hunt for the tiny flotsam floating at sea and published the data in IEEE Transactions of Geoscience and Remote Sensing. NASA uses the eight satellites to track wind speeds by measuring the roughness of open ocean waters. Rough seas portend high wind, helping the agency monitor storms.

But Evans and Ruf say the satellites can also detect plastics by identifying their effects. The scientists postulated that high volumes of plastics on the sea surface might deaden the waves. They looked for still waters that should be choppy due to high winds.

“We’d been taking these radar measurements of surface roughness and using them to measure wind speed, and we knew that the presence of stuff in the water alters its responsiveness to the environment,” Ruf said. “So I got the idea of doing the whole thing backward, using changes in responsiveness to predict the presence of stuff in the water.”

To confirm their work, Evans and Ruf consulted scientists who trawl the oceans measuring plankton. These plankton trawlers, whose equipment scoops up the tiny lifeforms as well as microplastics, have been one of the few reliable data sources for scientists investigating plastics in the sea. But data from trawlers provides an incomplete picture.

With the ability to see microplastics from space, the scientists found patterns of microplastic movement and concentrations that vary throughout the year. Plastics concentrate into the Great Pacific Garbage Patch, a convergence zone in the North Pacific, during June and July. During summer months in the Southern Hemisphere, microplastics are found in greater concentration in southern convergence zones.

Prior to two U.S. environmental laws passed in in 1972 and 1988, garbage and sewage routinely ended up in the ocean. And according to researchers, a lot of the plastic dumped prior to the 1990s is still out there floating through the seas. In 2019, scientists discovered a plastic bag dating back to 1965 ensnared on a monitoring device designed to keep track of plankton off the coast of Ireland.

Plastics don’t just swirl around the open ocean. A group of Woods Hole researchers recently found heavy concentrations of plastics dating back to the 1950s in salt marshes near Massachusetts’ Waquoit Bay. “This contamination is very pervasive; it’s everywhere,” lead author Javier Lloret told the Mashpee Enterprise. “It doesn’t really matter if you have people [living in the watershed] or not; you’re still going to find microplastics.”

While monitoring data from NASA satellites, Evans and Ruf found fresh sources of microplastics. For some time, scientists have speculated that China’s Yangtze River spews plastics that eventually wind up in the Great Pacific Garbage Patch. When reviewing their findings, the Michigan researchers spotted several spikes in microplastic concentrations at the mouth of the Yangtze.

“It’s one thing to suspect a source of microplastic pollution, but quite another to see it happening,” Ruf said. “The microplastics data that has been available in the past has been so sparse, just brief snapshots that aren’t repeatable.”