As concern over plastic waste grows, researchers are raising red flags about another problem: plastic’s rapidly growing carbon footprint. Left unchecked, greenhouse gas emissions associated with plastics will be nearly four times greater by mid-century, when they are projected to account for nearly one-sixth of global emissions.

Not all plastics have the same carbon footprint, though. What they are made from, the source of the energy that powers their production, and how they are disposed of at the end of their life cycle all make a difference.

In a study published Monday in the scientific journal Nature Climate Change, researchers calculated the life cycle emissions of different types of plastics, made from fossil fuels and from plants, and looked for ways to lower their total greenhouse gas emissions.

They found that there is no silver bullet. Every combination of plastics production and end-of-life disposal generates greenhouse gas emissions. But by combining four different approaches, they found they could lower emissions up to 93 percent compared to business as usual by 2050 if each measure was taken to the extreme.

The researchers found that using 100 percent renewable energy in plastics production with fossil fuel-based feedstock could reduce greenhouse gas emissions by half compared to business as usual in 2050, though it would still be roughly twice the emissions generated from plastics in 2015.

Adding both aggressive recycling that keeps plastics out of landfills and incinerators and efforts that reduce the growth in demand for plastics by half to that scenario could further lower greenhouse gas emissions from plastics to roughly the amount produced in 2015.

Using plant-based feedstocks instead of fossil fuels could lower it even more, the researchers found.

The most effective combination the researchers found was to use a plant-based feedstock (sugarcane in this case), with 100 percent renewable energy for production, recycling of all plastics rather than incinerating or dumping them in landfills, and reducing the annual growth in demand for plastics by half.

That combination could theoretically reach a 93 percent reduction compared to business as usual in 2050, or about a 74 percent reduction from 2015 levels, the researchers found.

The Unprecedented Scale and Pace Required

The authors readily acknowledge that this would require implementing these strategies at “an unprecedented scale and pace” in an industry that is projected to have sustained 4 percent annual growth through 2050.

Less than 1 percent of the plastic produced globally was made from biological feedstocks and only 18 percent of plastic waste was recycled as of 2015.

Shifting all plastic from petroleum to bio-based feedstocks such as corn or sugarcane would also require as much as 5 percent of all arable land, the researchers said. Such a shift under current agriculture practices would put added pressure on food security and freshwater resources, though other plant-based feedstocks, such as crop residue like leaves and stalks, may yet emerge.

“The strategies that we tested are anywhere between unrealistic to ridiculous, to be honest,” said Sangwon Suh a professor of environmental science and management at the University of California, Santa Barbara, and a co-author of the study. “What we realized as a result of the study was the magnitude of the challenge that we are facing really requires an unprecedented level of effort to mitigate greenhouse gas emissions.”

To limit global warming to 2 degrees Celsius or less, a target in the Paris climate agreement, scientists say greenhouse gas emissions must be reduced to near zero by mid century.

“That is a tremendous challenge, and there is no room for us to increase our greenhouse gas emissions,” Suh said.

Plastics Also Release Methane as They Degrade

The study looked at plastics’ life cycle greenhouse gas emissions—emissions associated with every aspect of plastics, from extracting oil and natural gas for fossil fuel-based plastics, to manufacturing, to end of life processes, such as dumping plastic waste in landfills, recycling it or incinerating it.

Though not accounted for in the current study, roughly 3 percent of plastic waste produced each year ends up in oceans at the end of its life.  A study published last fall concluded that as ocean plastic slowly degrades, it emits methane and other pollutants into the atmosphere. Methane, a short-lived climate pollutant, is also emitted during oil and gas extraction and is many times more potent than carbon dioxide.

The amount of methane currently released from such decomposing ocean plastic in the world’s oceans is tiny, likely less than 1 percent of total methane emitted into the atmosphere each year from natural and manmade sources according to Sarah-Jeanne Royer, a marine scientist at the University of Hawaii and lead author of the ocean plastics study. Unless waste management practices improve, however, the amount of plastic entering oceans could be 10 times greater in 2025 than they were in 2015 according to a 2015 study in the journal Science.

Cost an Issue for Plant-Based Plastics

Efforts so far to replace polyethylene and polypropylene, two of the most common types of plastic used in everything from plastic bags and bottles to pipes and containers, have fallen short, said Geoffrey W. Coates, a chemistry and chemical biology professor at Cornell University.

“There is not a bio-based polymer that gets anywhere close to replicating those materials,” he said.

Coates added, however, that for more niche applications, other solutions are being developed. Novomer, a company Coates co-founded, uses carbon dioxide as a feedstock to make polyurethane, a polymer used in paints, varnishes, and foams. 

Cost also poses a challenge as plant-based plastics try to gain a foothold in an industry where petroleum-based plastics benefit from economies of scale and a half-century head start over newer bio-based alternatives. Developing bio-based plastics that can compete with petroleum-based plastics will come at a cost, said Marc Hillmyer director of the Center for Sustainable Polymers at the University of Minnesota and co-founder of a company that is exploring new bio-based plastic materials.

“The economics of bio-based resources are not at parity with fossil fuel-based sources,” Hillmyer said. “The real question is are we willing to pay for it?”