Microplastics are one of the biggest environmental issues you may have never heard of. In March, researchers estimated that the Great Pacific Garbage Patch, a massive vortex of trash between Hawaii and California, contains over 87 thousand tons of plastic, about seven tons of which are microplastic fragments, smaller than 5 millimeters across. Because they’re so tiny, note researchers in Nature Communications on Tuesday, these bits of plastic can easily spread, sometimes hitching a ride on icebergs to contaminate new bodies of water.

In the paper, a team of researchers at the Alfred Wegener Institute for Polar and Marine Research in Germany explains how polar sea ice can trap microplastics in the winter and release them again when the weather warms and the ice melts. This unique vector for microplastics makes it easy for them to spread across the Arctic Ocean, swept along in the sea ice. Researchers suspect that, as oceans continue to warm as a result of climate change, sea ice will release more and more of these frozen, hitchhiking microplastics.

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Researchers used ice core samples to track the extent of microplastics distribution in the Arctic.

To conduct this study, researchers took three expeditions to collect core samples of sea ice — once during summer, once in the spring, and once in the fall — from three different sites in the Arctic. They examined these core samples and found very high concentrations of microplastics in the sea ice, suggesting that Arctic sea ice is important as a sink or storage site for microplastics, which sounds like it could be a good thing. But what happens when the ice melts?

“Even during winter large fractions of the sea ice are exported southward and eventually doomed to melt,” write the authors. “Hence, Arctic sea ice can be considered a temporary sink, a source and an important transport vector of [microplastics], at least to the Fram Strait and North Atlantic.” The conventional view of microplastics is that they’re only transported out of populated areas into the Arctic, but this research suggests that it actually works both ways.

This paper represents the first detailed analysis of how microplastics move through Arctic sea ice, so more research will be necessary to determine how ecologically significant this process is. To put it simply, though, these findings suggest that Arctic sea ice is a microplastics time bomb, just waiting for warming seas to melt the ice and release all this trash.

For now, this paper shows us that there really is no part of the Earth that’s unaffected by human activity, even the most remote, unpopulated stretches of the Arctic. Time will tell just how bad the problem is.