The study, published Thursday in the journal Science, estimates that 4-12 million tonnes of plastic are dumped every year by coastal countries.
"It is an enormous, staggering amount of material that we believe might be entering the ocean every year," Roland Geyer, one of the authors of the report, said in an interview with CBC News.
Scientists have known since the 1970s that plastic debris carried by currents is creating floating islands of garbage in the world's oceans, but until now they haven't studied the source.
The study estimates an average of about eight million metric tonnes of plastic — everything from water bottles to garbage bags to food packaging —is improperly disposed of.
"If you spread it out on the ground, eight million metric tonnes would be enough plastic waste to cover 34 times the area of Manhattan ankle-deep in uncompacted plastic waste," Geyer said.
China tops list
The research was conducted by scientists at the University of California Santa Barbara's National Center for Ecological Analysis and Synthesis. They looked at 192 coastal countries in 2010 and how they disposed of their plastic.
They found that 20 countries were responsible for 83 per cent of the litter, or so called "mismanaged plastic." In order, the top contributors were:- China.
- Sri Lanka.
- South Africa.
- North Korea.
- United States.
Geyer, an associate professor at UCSB's Bren School of Environmental Science and Management, says these are countries where economies have grown faster than their ability to handle the garbage produced by a growing middle class.
"As economies develop, typically what we see is [the amount of plastic in] their solid-waste generation …increases … but at the same time they're still lacking the infrastructure to make sure this waste doesn't move around and end up in the environment."
Dumping plastic illegal at sea
It's against the law for ocean vessels to dump plastic at sea.
Researchers think the plastic is often blown out of overflowing garbage dumps into estuaries or rivers where its carried into the oceans by tides. And it'soften simply dumped on beaches and along the coasts.
However, developing countries aren't the only ones to blame. The United States is No. 20 on the list of plastic polluters even though it has garbage collection and recycling.
It still has a "large mass of mismanaged plastic waste because of large coastal populations and … high per-capita waste generation," the report concludes.
Reducing the amount of plastic garbage can directly affect the amount that gets into the ocean, the researchers found, but it's a massive job.
"To achieve a 75 per cent reduction in the mass of mismanaged plastic waste, waste management would have to be improved by 85 per cent in the 35 top-ranked countries," the study found.
Geyer says that means better landfill systems, increased recycling, reduced plastic packaging and replacing plastic with other materials.
In the meantime, scientists need to figure out where the plastic is going. While some is floating on oceans, large amounts have disappeared — either sinking or disintegrating into tiny shards known as microplastics.
"An important question is 'where is it all,' and then of course, 'what does it do to the natural environment and in the long run what does it do to us?,'" Geyer said.