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When you think of plastic pollution, you might imagine ocean “garbage patches” swirling with tens of millions of plastic bottles and shopping bags. But unfolding alongside the “macroplastic” pollution crisis is another threat caused by much smaller particles: microplastics.

Microplastics — tiny plastic fragments that are less than 5 millimeters in diameter, a little less than one-third the size of a dime — have become ubiquitous in the environment. They form when larger plastic items like water bottles, grocery bags, and food wrappers are exposed to the elements, chipping into smaller and smaller pieces as they degrade. Smaller plastic fragments can get down into the nano territory, spanning just 0.000001 millimeter — a tiny fraction of the width of a human hair.

These plastic particles do many of the same bad things that larger plastic items do: mar the land and sea, leach toxic chemicals into the food chain. But scientists are increasingly worried about their potential impact on the global climate system. Not only do microplastics release potent greenhouse gases as they break down, but they also may be inhibiting one of the world’s most important carbon sinks, preventing planet-warming carbon molecules from being locked away in the seafloor.

Matt Simon, a science journalist for Wired, details the danger in his forthcoming book on microplastics, A Poison Like No Other. He told Grist that it’s still early days for some of this research but that the problem could be “hugely important going forward.”

To understand the potential magnitude, you first have to understand an ocean phenomenon called the “biological carbon pump.” This process — which involves a complex network of physical, chemical, and biological factors — sequesters up to 12 billion metric tons of carbon at the bottom of the ocean each year, potentially locking away one-third of humanity’s annual emissions. Without this vital system, scientists estimate that atmospheric CO2 concentrations, which recently hit a new record high of 421 parts per million, could be up to 250 parts per million higher.

“The biological carbon pump helps to keep the planet healthy,” said Clara Manno, a marine ecologist at the British Antarctic Survey. “It helps the mitigation of climate change.”

The pump works like this: First, carbon dioxide from the atmosphere dissolves into water at the surface of the ocean. Using photosynthesis, tiny marine algae called phytoplankton then absorb that carbon into their bodies before passing it onto small ocean critters — zooplankton — that eat them. In a final step, zooplankton excrete the carbon as part of “fecal pellets” that sink down the water column. Once these carbon-containing pellets reach the ocean floor, the carbon can be remineralized into rocks — preventing it from escaping back into the atmosphere.

So where do microplastics come in? Unfortunately, at every step of the process.

Perhaps most concerning to scientists is the way microplastics may be affecting that final stage, the sinking of zooplankton poop to the bottom of the seafloor. Once ingested, microplastics get incorporated into zooplankton poop and can cause fecal pellets to sink “way, way more slowly,” said Matthew Cole, a senior marine ecologist and ecotoxicologist at the Plymouth Marine Laboratory in the U.K. In a 2016 paper he published in Environmental Science & Technology, he documented a 2.25-fold reduction in the sinking rate for the fecal pellets of zooplankton that had been exposed to microplastics. Other research has shown that plastic-contaminated krill fecal pellets can sink about half as quickly as their purer counterparts.

Teaser image credit: Grist/Getting images (used with permission)