The study on 192 coastal countries found between 4.8 and 12.7 million metric tonnes of plastic entered the ocean in 2010 from people living within 50 kilometres of the coastline.
That year, a total of 275 million metric tonnes of plastic waste was generated in those countries, researchers said. Researchers explain the amount of plastic moving from land to ocean each year using 8 million metric tonnes as the midpoint.
"Eight million metric tonnes is the equivalent to finding five grocery bags full of plastic on every foot of coastline in the 192 countries we examined," said Jenna Jambeck, from the University of Georgia and the study's lead author.
To determine the amount of plastic going into the ocean, Jambeck "started it off beautifully with a very grand model of all sources of marine debris," said study co-author Roland Geyer, from the University of California, Santa Barbara.
Researchers began by looking at all debris entering the ocean from land, sea and other pathways. After gathering rough estimates, "it fairly quickly emerged that the mismanaged waste and solid waste dispersed was the biggest contributor of all of them," he said.
With the mass increase in plastic production, the idea that waste can be contained in a few-acre landfill or dealt with later is no longer viable.

With between 4.8 and 12.7 million metric tonnes going in, researchers are only finding between 6,350 and 245,000 metric tonnes floating on the ocean's surface.
Jambeck forecasts that the cumulative impact to the oceans will equal 155 million metric tonnes by 2025. The planet is not predicted to reach global "peak waste" before 2100, according to World Bank calculations.
The study was published in the journal Science.


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