An untold amount of plastic pollution finds its way into the ocean every year. No one knows for sure what becomes of all that garbage. Much of it most likely erodes into microplastic, tiny flecks smaller than five millimeters in diameter, which can take up pollutants and are often ingested by marine animals, including fish and crustaceans.

Unexpectedly, trillions of those particles end up in Arctic sea ice, according to a paper published in May in the scientific journal Earth's Future. The study found that sea ice contains up to 240 microplastic particles per cubic meter—as much as 2,000 times the density of the particles that are estimated to float in the Great Pacific Garbage Patch. “We know that microplastic is found in oceans worldwide, but it is surprising that it's found in such an abundance in Arctic sea ice,” says Rachel Obbard, a materials scientist and engineer at Dartmouth College and lead researcher of the study. When ice forms at the surface of the ocean, it traps anything that happens to be floating there. The freezing process, she says, seems to be concentrating the trapped particles, which otherwise would eventually sink to the seafloor.

Obbard did not set out to examine sea ice for plastic. Instead she and a student were looking for algae in four ice cores collected from remote locations in the Arctic Ocean. When she melted and filtered the samples, however, she found blue, red, green and black bits. “These brightly colored things,” she says, “just jumped right out at me.”

Extrapolating from the samples, Obbard and her colleagues estimate that up to seven trillion pieces of microplastic in total could be released as Arctic sea ice melts because of climate change. Some researchers say summer in the Arctic may be ice-free around 2100. Others project it could happen within the next decade.