Humans produced nearly 300 million tons of plastic in 2012, but where does it end up? A new study has found plastic debris in a surprising location: trapped in Arctic sea ice. As the ice melts, it could release a flood of floating plastic onto the world.

Scientists already knew that microplastics—polymer beads, fibers, or fragments less than 5 millimeters long—can wind up in the ocean, near coastlines, or in swirling eddies such as the Great Pacific Garbage Patch. But Rachel Obbard, a materials scientist at Dartmouth College, was shocked to find that currents had carried the stuff to the Arctic.

In a study published online this month in Earth’s Future, Obbard and her colleagues argue that, as Arctic ice freezes, it traps floating microplastics—resulting in abundances of hundreds of particles per cubic meter. That’s three orders of magnitude larger than some counts of plastic particles in the Great Pacific Garbage Patch. “It was such a surprise to me to find them in such a remote region,” she says. “These particles have come a long way.”

The potential ecological hazards of microplastics are still unknown. But the ice trap could help solve a mystery: Industrial plastic production has increased markedly in the last half-century, reaching 288 million tonnes in 2012, according to Plastics Europe, an industry association. But ecologists have not been able to account for the final disposition of much of it. The paper shows that sea ice could be an important sink—albeit one that is melting, says Kara Lavender Law, an oceanographer at the Sea Education Association in Woods Hole, Massachusetts, who was not part of the study. “There could be freely floating plastics, in short order.” The authors estimate that, under current melting trends, more than 1 trillion pieces of plastic could be released in the next decade.

Obbard and her colleagues based their counts on four ice cores gathered during Arctic expeditions in 2005 and 2010. The researchers melted parts of the cores, filtered the water, and put the sediments under a microscope, selecting particles that stood out because of their shape or bright color. The particles’ chemistry was then determined by an infrared spectrometer. Most prevalent among the particles was rayon (54%), technically not a synthetic polymer because it is derived from natural cellulose. The researchers also found polyester (21%), nylon (16%), polypropylene (3%), and 2% each of polystyrene, acrylic, and polyethylene. Co-author Richard Thompson, a marine biologist at the University of Plymouth in the United Kingdom, says it’s difficult to pinpoint the source of these materials. Rayon, for instance, can be found in clothing, cigarette filters, and diapers.

Abundances are likely to grow as scientists learn to sift more finely. Law points out that microplastic estimates for the Great Pacific Garbage Patch are based on phytoplankton nets that catch only particles bigger than 333 microns. Obbard, who used a much smaller 0.22 micron filter, says she still probably missed many particles herself; searching by eye, she easily could have missed brownish or clear plastic particles that were masquerading as sand grains.

What is the consequence of all this plastic floating around? At this point, it is hard to say. Plastic is chemically inert. But the plastic can absorb organic pollutants in high concentrations, says Mark Browne, an ecologist at the University of California, Santa Barbara. Browne has performed laboratory experiments with marine organisms showing not only how the microplastics can be retained in tissues, but also how pollutants might be released upon ingestion. “We’re starting to worry a bit more,” he says.