Helen Chappell / McClatchy-Tribune News Service

RALEIGH, N.C. — New plastics designed to break down naturally have been hailed as environmentally friendly alternatives to traditional plastics. Instead of taking decades or even centuries to decompose, they vanish in a few years.

But new research at North Carolina State University suggests they may not be so green after all.

The study, led by NCSU doctoral student James Levis, found that biodegradable plastics can release large amounts of methane gas when they break down in landfills. Methane is one of the most problematic greenhouse gases, able to trap much more heat than carbon dioxide, making it a major contributor to global warming.

“Everybody assumes that biodegradable is desirable. This study calls that into question,” says NCSU’s Dr. Morton Barlaz, an author on the study.

Biodegradable plastics are commonly used to make yard-waste bags and disposable drink cups. Unlike plastic bottles that are designed to be recycled, these materials were created with composting in mind. They’re also able to break down quickly if they end up as litter.

Though they might have been intended to go into the compost, many are destined to end up in a landfill. “You can say a cup is compostable,” Levis says. “But here in Raleigh and most places, there’s no separate bin, and it’s going to end up going in the garbage.”

In a well-managed compost system, Levis says, biodegradable plastics don’t release much methane at all. As long as oxygen is present, they will give off mostly carbon dioxide and water.

But that’s not the case in landfills, where garbage is starved of oxygen.