We dump 8 million tons of plastic into the ocean each year. Where does it all go?

Plastic has become ubiquitous in our lives. Plastic bottles, plastic lids, finger-slicing clamshell packaging, plastic Donald Trump bobblehead dolls... So what happens when we throw all this plastic out?

In recent years, scientists have been puzzling out this very question. The vast, vast majority of plastic garbage just ends up in landfills, where it sits, benignly enough, taking thousands of years to degrade. A smaller fraction gets recycled; in the United States, the rate is about 9 percent.

There's also a large swath of plastic trash that eventually finds its way into the oceans, either from people chucking litter into waterways or from storm-water runoff carrying plastic debris to the coasts. That's not quite as benign: ecologists and biologists have long worried that all this plastic could have negative effects on marine life.

Now we can quantify the ocean problem. A recent study in Science calculated that between 5 and 13 million metric tons of plastic waste, worldwide, made it into the ocean in 2010. What's more, the authors estimate this amount could more than quadruple by 2025 without better waste management.

Plastic debris in the Mediterranean Sea. (Alan Bachellier/Flickr)

There's also a weird twist here: We don't actually know where in the ocean most of this plastic ends up.

A separate 2014 study published in the Proceedings of the National Academy of Sciences identified giant swirling garbage patches in each of the world's oceans, containing up to 35,000 tons of plastic. But those patches accounted for less than 1 percent of the plastic thought to be in the oceans — and no one knows, exactly, where the other 99 percent went. One possibility is that marine creatures are eating tiny bits of plastic, which then somehow worms its way into the food chain. But it's still a mystery.

China accounts for one-quarter of plastic ocean waste

(Jambeck et al 2015)

The Science study, published in February 2015 and led by Jenna R. Jambeck of the University of Georgia, was the first since the 1970s to quantify how much of our plastic waste ends up in the ocean each year.

The authors looked at global plastic production rates, as well as data on waste management and disposal in 193 different coastal countries. Putting this together, they deduced that the world threw out 275 million metric tons of plastic waste in 2010, with much of that from plastic packaging.

Next, the authors estimated that between 4.7 and 12.7 million metric tons of that trash made its way to sea — with a best estimate of 8 million tons. That's enough to cover the world's entire coastline.

China was the biggest contributor by far, accounting for roughly one-quarter of the marine debris produced each year. (Note that these figures only include plastic waste on land that ends up in the sea. It doesn't include things like plastic waste from fishing vessels, which makes up an unknown fraction of the total.)

What's more, the researchers calculated, the amount of plastic waste could quadruple (or worse) by 2025 unless better waste-management techniques are adopted, such as recycling or a reduction in packaging materials used.

Every ocean now has a massive plastic garbage patch

Concentrations of plastic debris in surface waters of the global ocean. Colored circles indicate mass concentrations (legend on top right). (Cozar et al, 2014.)

Okay. So once it's in the ocean, where does all the plastic go?

Many people have heard of the Great Pacific garbage patch — a massive patch of trash that's accumulated in a swirling subtropical gyre in northern Pacific Ocean. Ocean currents carry trash from far and wide into this vortex.

It turns out that there are at least five of these floating garbage patches around the world. That's according to a 2014 study in The Proceedings of the National Academy of Sciences, led by Andres Cózar of the Universidad de Cadiz and based on the results of a 2010 circumnavigation cruise.

These floating garbage patches aren't always visible from the air — or even from a passing boat — since most of the plastic is bobbing just beneath the surface, and most of the particles are smaller than 1 centimeter in diameter. Over time, the plastic bits get broken down into ever smaller pieces as they get battered by waves and degraded by the sun.

Even so, these gyres contain a lot of garbage, collectively holding some 7,000 to 35,000 tons of plastic in all. The patch in the North Pacific was by far the biggest, containing about one-third of all the floating plastic found. Much of the plastic debris from eastern China collects here.

Now the twist. What was most surprising to researchers was that these plastic garbage patches were so small, relatively speaking. After all, there should be millions of tons of plastic in the oceans. Yet these subtropical gyres only contained 35,000 tons, at most. In particular, the garbage patches seemed to contain far fewer plastic bits smaller than 1 millimeter than the researchers expected. So where did the rest go?

Bo Eide/Flickr

In the PNAS paper, the authors offer a couple of hypotheses for why they didn't find nearly as much floating plastic in the garbage patches as they expected. The most troubling possibility is that fish and other organisms are eating all the plastic:

One possibility is that plankton and fish are eating the plastic

1) Maybe much of the ocean plastic is just washing back ashore. The problem with this hypothesis is that most of the "missing" plastic is less than 1 millimeter in diameter. It's unclear why only the smaller bits would have washed up ashore.

2) Perhaps the plastic somehow breaks down into really, really tiny, undetectable pieces. This is possible, although the authors note that "there is no reason to assume that the rate of solar-induced fragmentation increased since the 1980s."

3) Maybe small organisms are growing on some of the plastic bits, causing them to get heavier and sink deeper into the ocean. This is also a possibility, although other studies have found that when these plastic pieces sink, the organisms on them typically die and the plastic should bob back up to the surface.

4) Plankton and fish are eating the plastic. This one's the most disquieting hypothesis. The tiny plastic bits that seem to have vanished are small enough to be eaten by zooplankton, who are known to munch on plastic. The authors also argue that mesopelagic fish beneath the surface may be eating a lot of plastic too — and, perhaps, pooping it out down to the ocean bottom. (This needs further testing though.)

Now, if fish are eating all that plastic and it's entering the food chain, it's still unclear how dangerous that is. Obviously some marine organisms, like seabirds, can get digestive problems (and can die) if they eat large pieces of plastic. But what about very tiny pieces? There's some evidence that toxic chemicals can cling to plastic in the ocean and accumulate, but there's still scant research on how much harm this might actually do as it passes through the food chain.

5) Plastic is accumulating in the ice caps. Meanwhile, a separate 2014 study published in the journal Earth's Future suggested that a great deal of microplastic is accumulating in the polar ice caps. As sea ice forms and expands, the argument goes, it essentially "scavenges" the plastic from the seawater. This, too, might be part of the story.

6) Someone's done their maths all wrong. Alternatively, it's always possible that scientists' best estimates of how much plastic is actually entering the oceans are incorrect. That might help explain the discrepancy.

In any case, something doesn't add up: the current numbers suggest that the vast majority of plastic trash in the ocean is vanishing, and no one seems to know where it's gone.

Further reading/listening

--Jenna Jambeck has a fascinating podcast about her research available here.

-- Dianna Parker of NOAA's Marine Debris Program talks about possible ways to clean up the Great Pacific Garbage Patch.

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