Large rafts of floating trash can be used to track areas of ocean-litter concentration with standard satellite-based infrared imaging, according to a new study published by a team at Universidad de Cadiz and Barcelona's Institute of Marine Sciences.
Long lines of drifting plastic - litter windrows - are formed where currents converge on the ocean's surface, and can be readily spotted by infrared earth observation sensors on satellites like the Sentinel-2. This is good news for marine scientists, because it means that ocean pollution can be studied using existing satellite capabilities until a purpose-built sensor system can be commissioned.
The study looked for correlations between the formation of large-scale trash rafts and the outflow of litter from land-based sources (where the vast bulk of ocean plastic begins its journey). Heavy rain is the primary driver, sweeping street litter into rivers and out to sea. Satellite-based monitoring provides an "unprecedented source-to-sink view" of this phenomenon.
The team used six years of infrared imaging of the Mediterranean from the Sentinel-2 satellite, totaling 300,000 photos. This time series showed clear pulses of trash-pile formation following the rainy season, particularly during three periods of extreme rain in 2016, 2018 and 2021. After seasonal peaks in the spring and autumn, the windrows would clear out every winter - likely because of winds driving the plastic back onto beaches.
They spotted more than 14,000 litter windrows in the imaging over the six-year time series. A small number were huge, 10 meters wide by up to 20 kilometers long. The highest concentration of litter windrows was in the northern Adriatic - by far the most polluted area, thanks to abundant inland rainfall driving terrestrial plastic litter into surface runoff. (The opposite is true off Egypt or Libya, where there is very little ocean plastic litter found offshore, the authors suggested.)
