Scientists writing in the journal PLOS One said Wednesday that plastics emit the greenhouse gases methane and ethylene when they are exposed to sunlight and degrade.

The researchers carried out tests on such common plastic products as water bottles, shopping bags and food containers.

"Plastic represents a source of climate-relevant trace gases that is expected to increase as more plastic is produced and accumulated in the environment," said senior researcher David Karl of the University of Hawaii.

Methane gas, both man-made and naturally occurring, is said to be a major cause of climate change.

The manufacturing and use of plastics have come under scrutiny in recent years after environmentalists discovered a massive island of garbage floating in the Pacific Ocean — threatening sea life and the food supply.

Plastic drinking straws are the latest target of activists, who say they are used once and discarded because most cannot be recycled.

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A graphic video of doctors pulling a plastic straw out of the bloody nose of a large sea turtle has prompted some U.S. cities and companies to phase out the straws. (VOA)