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.
This reduced sinking rate is a result of microplastics' buoyancy — especially those made of low-density polymers like polyethylene, the stuff used in grocery bags and likely the most common polymer in the surface ocean. Slower sinking rates mean fecal pellets may spend up to two or even three days more than usual drifting through the water column, presenting more opportunities to be intercepted.
"They're more likely to break apart, they're more likely to be eaten by other animals," Cole said, making it less likely that the carbon will reach the seafloor and become permanently sequestered.
There are other worries too, about the way microplastics can affect phytoplankton and zooplankton health — potentially compounding the stresses already posed by rising carbon dioxide concentrations, which are making the oceans warmer and more acidic, and are contributing to the expansion of oxygen-depleted "dead zones." High concentrations of microplastics in water are toxic to phytoplankton, and lab experiments have shown they can cause up to a 45 percent reduction in some species' growth. Cole's experiments on copepods, a common kind of zooplankton, have shown that ingested microplastics take up space in copepods' guts, causing them to eat less real food, produce smaller eggs that are significantly less likely to hatch, and live shorter lives.
Researchers are still trying to come to grips with what all these laboratory observations could mean on a global scale. But the worry is that a planet-wide population of smaller, shorter-lived ocean algae and zooplankton may not be able to take up as much carbon as their ancestors — exacerbating the problems associated with buoyant fecal pellets.
"There is something there that we should worry about," Manno said, stressing the need for more research. To that end, she's working on a multiyear field study, with research expeditions planned for the microplastics-laden Mediterranean Sea and the moderately cleaner Southern Ocean. Manno said she's hoping to collect real-world plastic and fecal pellet samples and get a better look at how microplastics interact with zooplankton in the open ocean.
The goal, Manno explained, is to quantify the decline in carbon sequestration related to microplastics and translate that into a dollar cost to society. "The ocean provides us this ecosystem service," she said. "If something stresses these processes … this is a kind of social benefit that we cannot use anymore."
If her hypothesis is correct — that microplastics are inhibiting the biological carbon pump — it will add even more weight to a growing recognition of plastic and microplastics as a major climate disrupter. Scientists already know that plastic production and incineration cause massive greenhouse gas emissions, and in A Poison Like No Other, Simon notes emerging research on the way microplastics release exponentially increasing amounts of planet-warming methane and ethylene as they break down.
"They continue emitting forever," said Sarah-Jeanne Royer, the oceanographer and postdoctoral researcher at the Scripps Institution of Oceanography who is conducting that research.
To mitigate their damage to ecosystems and the climate, Royer called for policymakers to double down on removing microplastics from the ocean. But that's a tall order. Despite some early-stage experiments involving plastic-eating mussels and bacteria, Simon said there are currently no viable, scalable ways to remove all the microplastics that have already accumulated in the environment.
"We have so much microplastic and nanoplastic in so many places on the planet — in the air and the land and the sea — that there's just no way to pull it all out," Simon said. "I wish there was a nice, happy solution like a magnet that you could drag through the environment and attract all the microplastics, but unfortunately that just doesn't exist."
Instead, he called for people to take steps to limit the release of microplastics into the environment — like by installing a filter on their washing machine — as well as government-mandated caps on plastic production. "We have to stop producing so much goddamn plastic," he said. "It is out of control at this point."
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