It can be hard to fathom, but Earth's oceans are teeming with plastic trash. From tiny specks to bottles, bags and fishing nets, this sea of debris is now common near shore and in remote, open waters, posing a variety of threats to wildlife. Scientists have been tracking the problem since the first garbage patch was found in 1997, but trying to quantify it is a tall order in 321 million cubic miles of ocean.

Yet a new study does just that, offering the most comprehensive picture of ocean plastic ever produced. Based on data from 24 trash-gathering voyages over six years, an international team of researchers used an oceanographic model to estimate how much plastic the planet's oceans really contain. Their answer is at least 5.25 trillion pieces, a motley trash blend that weighs about 269,000 tons in all.

That's an average of more than 15,000 pieces of plastic per cubic mile of ocean. The actual trash isn't so evenly spaced, but it is surprisingly cosmopolitan, enduring epic adventures after reaching the sea by river, beach or boat. Rather than being trapped in ocean gyres, garbage patches are more like garbage blenders, the new study suggests, churning plastic into smaller bits until it escapes or gets eaten.

"Our findings show that that the garbage patches in the middle of the five subtropical gyres are not final resting places for floating plastic trash," says lead author Marcus Eriksen, research director for the 5 Gyres Institute. "Unfortunately, the endgame for microplastic is dangerous interaction with entire ocean ecosystems. We should begin to see the garbage patches as shredders, not stagnant repositories."


A sample from the South Atlantic gyre includes crabs and an array of microplastics. (Photo: Jody Lemmon/5 Gyres)

Previous research has shown that microplastics pervade the oceans, showing up not just in surface garbage patches but also in sea ice, coastal sediments, seafloor mud, zooplankton, lugworms and the circulatory systems of mussels, among other places. And while many earlier estimates of plastic pollution relied on either visual counting or trawling for debris, the new study used both methods, helping it count large items like buoys and nets as well as the microplastics caught more easily by trawling.

The researchers divided the plastic into four size classes: two for microplastics (one equivalent to a grain of sand and one to a grain of rice), one for mesoplastics (up to the size of a water bottle) and one for macroplastics (anything larger). They had expected to find mostly sand-sized particles, but were surprised to learn the smallest fragments are outnumbered by the next largest size, and that more tiny pieces exist outside the garbage patches. That suggests macroplastics are crumbling faster than microplastics, and hints at how the latter can seemingly vanish once they get small enough.

"What's new here is that looking at all sizes gives us a better picture of what's out there," Eriksen tells MNN. "It lets us look at the life cycle of ocean plastics — it starts with coastal generation, then migration to the gyres, shredding in the gyres, and consumption by marine organisms. Or microplastics might sink down and get caught in deeper currents. So the life cycle of plastic is a new way to look at the gyres."

ocean plastic map

The global density of ocean plastics, in pieces per square kilometer, from four size classes. (Image: Laurent Lebreton/5 Gyres)

Despite the vast travels of plastic debris, some garbage patches still have trademark trash. The North Pacific is the "fishing gear gyre," for example, while the North Atlantic is the "bottlecap gyre." The three Southern Hemisphere gyres are linked by the Southern Ocean, however, making them less distinct.

Any ocean plastic can endanger wildlife, including large items like fishing gear that entangles dolphins or plastic bags that clog sea turtles' stomachs. But microplastics are especially insidious, absorbing a cocktail of ocean pollutants and then passing them on to hungry seabirds, fish and other marine life. This can be a "frightfully efficient mechanism for corrupting our food chain," Eriksen says.

The wide dispersal of microplastics likely rules out any large-scale cleanup efforts, he adds, but there is a silver lining to these findings. Although it's not entirely clear what happens to microplastics when they disappear, oceans do have ways of cleaning themselves — but only if we let them.

"If we can focus on not adding more plastic, the oceans kind of take care of it over time," Eriksen says. "It may be a long time, but the oceans will deal with this trash. The sea surface isn't the final resting place for plastic. It starts to shred, and marine organisms take it in. The entire ocean is filtering through marine life, from microorganisms to whales taking huge gulps of water. And some of it is sinking. It could be that when it gets that small, it responds more to water temperature than its own material buoyancy."

Countless marine animals will die from eating plastic, of course, and since some experts believe garbage patches will continue growing for centuries, this clearly isn't an ideal solution. Eriksen isn't saying the oceans can bear all our waste, though; he's just suggesting time and resources would be better spent preventing new plastic from reaching the sea than trying to remove what's already there. And that's a job for everyone on Earth, including both the makers and the users of plastic products.

"For the average person, most plastic has no value after it leaves their hands," he says. "So one challenge for the consumer is to see if you can be plastic-free. But what really has to happen is an across-the-board design overhaul. There has to be a careful consideration of how plastic is used in all products. Not just recyclability but recovery. If you can't recover it, recycling becomes meaningless. And if you can't recycle it, go back to paper, metal or glass. Plastic becomes hazardous waste once it's out there, and it has to be looked at in that light when we design products in the first place."

For more information about the new study, check out this video overview of its findings:

Related plastic pollution stories on MNN:

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.