Almost imperceptibly, plastics seem to emit tiny amounts of methane in the environment, according to a new study.
Researchers at the University of Hawaii found that most common plastics produce methane, a greenhouse gas, when exposed to sunlight. Among plastics tested, polyethylene — the largest volume plastic that finds use in applications ranging from packaging to toys — emitted the most methane.
"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 David Karl, senior author on the study and professor in UofH's Manoa School of Ocean and Earth Science.
"This source is not yet budgeted for when assessing global methane and ethylene cycles, and may be significant."
Researchers found that sunlight triggers the degradation of plastic, which results in emissions of methane. During a 212-day experiment, they tested the emission rate for low density PE in pellet form, in powder form and as debris collected from the ocean. They found that the rate of emission increased over the period of the study, and even occurred when LDPE was placed in a dark environment after being exposed to sunlight.
The results were unexpected but could be explained by researchers.
"We attribute the increased emission of greenhouse gases with time from the virgin pellets to photodegradation of the plastic, as well as the formation of a surface layer marked with fractures, micro-cracks and pits," said lead study author Sarah-Jeanne Royer, a post-doctoral researcher with the Center for Microbial Oceanography: Research and Education at the time of the study. Surface degradation of plastic likely increases the methane emission rate.
The study results were published in the Aug. 1 edition of PLOS One, a peer-reviewed journal. The science team tested a range of plastics, including polycarbonate, acrylic, polypropylene, PET, polystyrene, high density PE and LDPE. All emitted methane to some degree under study conditions.
The amounts of methane emitted were minuscule. At the end of the 212-day period, LDPE's emission rate was 92.8 millionths of a gram per kilogram of LDPE per day. Also emitted at comparable rates were ethylene, ethane and propylene.
Karl stated in an email that Sara Ferron, a postdoctoral researcher in his laboratory, made the initial discovery. The project was assigned to Royer when she joined Karl's research team. Samuel Wilson, the other study author, is a world-leading authority on methane measurements who recently led a United Nations Scientific Committee on Ocean Research, which worked on an international intercomparison of analytical methods.
Plastic-emitted methane appears to be minuscule compared to other sources of methane like natural gas and biomatter decomposition, but it is not insignificant, Royer said. Plastic powder emitted 488 times more methane than plastic pellets. Much of the plastic discarded to the environment breaks down into small particles. Huge amounts of plastic litter are already present in the environment, and more is predicted.
"Everybody uses plastics, from cellphones to packaging," Royer said in a phone interview. "One solution is to switch from single-use to reuse applications.".
Royer said she approached major plastics companies but "they didn't want to communicate with us."
The study results are interesting but maybe not alarming, the study researchers indicate.
"Based on the rates measured in this study and the amount of plastic produced worldwide [methane] production by plastics is likely to be an insignificant component of the global [methane] budget," the authors state.