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Plastics degrade into greenhouse gases

Researchers at the University of Hawaii at Manoa show that plastics release methane and ethylene when exposed to sunlight

By: Katie Boon

10 December 2018


Plastic has unseen gas-releasing consequences, University of Hawaii at Manoa researchers have found.


Plastic pellets and other plastics gathered from ocean pollution were monitored by researchers from UH Manoa’s Center for Microbial Oceanography: Research and Education (C-MORE). They found a correlation between plastics exposed to sunlight and two greenhouse gases: methane and ethylene.


According to the Center for Science Education, methane is responsible for about one-fifth of the warming effects contributed by greenhouse gases in the Earth’s atmosphere. Although methane makes up a relatively small concentration of these gases, it is a powerful climate change contributor.


The Tree Fruit Research & Extension Center at Washington State University refers to ethylene as a ripening hormone. Commercially, it is used to ripen produce, such as bananas.


C-MORE director and senior researcher David Karl said that the purpose of the study was to document the gas-emitting properties of plastic and show that all plastics, not just those degrading in the ocean, are responsible for these emissions.


Everyday commodities are constantly releasing gases that are invisible to the human eye. Karl mentioned car bumpers as one unassuming contributor.


“They don’t have to change in appearance to be emitting these gases,” said Karl.


Many experiments went into finding these emission results.

Seven common use plastic types were tested to evaluate which types produce which gases and how much of them. Saltwater, size and solar radiation were used to change conditions exposed to each plastic as measurements were taken.


All of the tested plastics released both methane and ethylene; however, one type trumped them all: low-density polyethylene plastic (LDPE).


Everyday uses of LDPE include single-use plastics, packaging and bottles, such as cartons that hold milk. According to the 3D printing company Fabbaloo, LDPE makes up about 17.5 percent of produced plastics.


Plastics are made when different polymers are combined; often times, they are mixed with additives. The researchers used virgin LDPE pellets in their experimentation (or those made without additives).


For 212 days, recordings were taken bi-monthly on LDPE pellets exposed to solar radiation. A similar experiment over 152 days was done using plastic pollution found in the ocean.


Though the pellets seem smooth, pits and rough surfaces emerge during degradation. Larger surface areas allow for greater gas emissions.


Even when removed from light, the plastics continued to release gases. However, the recently published study states that the rates of emissions are likely an insignificant contributor to worldwide methane production.

On Aug. 7, the PLASTICS Industry Association responded to the report by stating that all unmanaged materials that are littered can have detrimental effects on the environment.


“Plastics do offer many sustainability advantages which significantly reduce emissions of greenhouses gases,” said PLASTICS Industry Association communications director Jacob Barron. “For instance, plastics contribute to lightweighting automobiles and improved fuel economy. Construction products increase energy efficiency. Plastic packaging helps prevent food waste. Bottles use fewer GHGs and less energy than other material, and products like flexible coffee pouches produce much less waste and take less energy to make than cans or canisters.”


Principal author and post-doctoral researcher Sarah Jeanne Royer wrote on her personal website:  “Greenhouse gases directly influence climate change, affecting sea level, global temperatures, ecosystem health on land and in the ocean, and storms, which increase flooding, drought, and erosion.”

According to the Science History Institute, synthetic plastics were invented in 1907 and grew in popularity as they were seen as one way to “protect the natural world from the destructive forces of human need.” Through the world wars and the Great Depression, plastics became a suitable substitute for depleted resources.

In the 1960s, plastic pollution in the ocean became obvious. Awareness of the issue grew in the following decades because of the threat that plastics cause to human life.


Such plastic pollution includes the accumulation of garbage in different parts of the ocean.


The National Oceanic and Atmospheric Administration explained that these masses, better known as “garbage patches,” are brought together by ocean currents known as gyres. There are five large underlying currents that circulate water within and between different oceans. These currents carry plastic pollution to accumulation points, making up the five major garbage patches in the world.


The UH experiment shows the consequences they have in the world. Compared to plastic pellets in the experiment, global plastic pollution has higher surface area and more factors contributing to degradation.

Royer completed her research through C-MORE but is also involved with the UH Manoa’s International Pacific Research Center and Sustainable Coastlines Hawaii, which is the largest beach clean-up organization in the Hawaiian islands.


“Considering the amounts of plastic washing ashore on our coastlines and the amount of plastic exposed to ambient conditions,” Royer said, “the findings provide further evidence that to protect against climate change, we need to stop plastic production.”


Brady Thomas, a Hawaii resident, has become dedicated to reducing both plastic production and pollution. He owns Keko Beverage Bottles company, which produces reusable bottles. He also sits on the board of the Surfrider Foundation, whose mission is to “protect and preserve the world's oceans, waves and beaches.”


Thomas said that one reason he became concerned with marine health is because he has seen a visible change in ocean pollution over the years.


“I love being at the beach and in the ocean,” said Thomas. “There’s a responsibility we have to take care of what we have. It’s a responsibility we, collectively as people, need to step up and be accountable to.”

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