The tiny size of microparticle plastic also makes it ever more difficult to detect—possibly lower historical estimates—and tougher to detect. The team is skeptical that there are any feasible collection strategies for what has already leaked into the ocean.
"How can we collect plastic from the ocean floor when the average depth of the ocean is 14,000 feet?" asked researcher Roland Geyer of the University of California, Santa Barbara.
The countries releasing the most trash into the oceans have long coastlines and large populations, as one might expect. But middle-income countries that have lately seen fast economic growth—including China, Indonesia and Brazil—account for 16 of the top 20 producers. The top five countries—China, Indonesia, Philippines, Vietnam, and Sri Lanka—account for roughly half of the total among top 20 countries.
"This isn't about pointing fingers at certain countries," lead researcher Jenna Jambeck of the University of Georgia told CNBC in a phone interview. "I am sure if you looked back a half century or so, the biggest waste generators would be other countries. Waste generation is coupled with economic growth—as you grow you produce more waste."
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Many of the top producers also do not have adequate waste management systems in place, the study said.
The United States barely makes the list, coming in 20th place. Taken as a whole, the European Union would rank a couple of places ahead, in 18th place.
The scientists were quick to note that all their data are only estimates, and they pointed out that, for example, their numbers do not include plastic trash that comes from maritime vessels.
But the team was confident enough in its findings to suggest that if the top 20 countries improved plastic disposal by 50 percent, the total mass of newly plastic waste would decrease by almost half by 2025.
"Humans have a long track record of causing large environmental problems, but we are starting to build a track record of solving them," Geyer said.