Q: Is there a link between our plastic use and climate change?

Asked by Raul Nava, '05, Monterey, Calif.

Carbon Calculations

According to the EPA, approximately one ounce of carbon dioxide is emitted for each ounce of polyethylene (PET) produced. PET is the type of plastic most commonly used for beverage bottles. Other sources pin the production ratio of carbon emissions to plastic production closer to 5:1. Worldwide, we consume approximately 100 million tons of plastic each year. From the EPA's more conservative estimate to the more liberal one, that's anywhere from 100 million tons of carbon dioxide emitted to 500 million tons. With the more conservative estimate, plastics are on par with the annual emissions of 19 million vehicles, a number of drivers equal to the entire population of New York state. You can do your own car emission comparisons with the EPA's energy and emissions conversion calculator for annual passenger vehicles emissions. The liberal estimate puts us closer to the emissions equivalent of 92 million vehicles, or the number of drivers equal to the populations of every state west of the Rockies and Texas. Put another way, our passion for plastics resulted in emissions ranging from 10 to 45 percent of the annual emissions from the approximately 200 million licensed drivers in the United States. Using the conservative estimate of 30 percent carbon savings for recycled plastic (though some peg the savings at 70 to 80 percent, below), recycling could save between 30 and 170 million tons of carbon each year, or the approximate equivalent of removing between six and 30 million vehicles from U.S. roads.

Production Components

Plastics are made most commonly from petroleum and natural gas. The hydrocarbon starting points are refined into ethane and propane (among other petrochemical products). Both of these gases are "steam cracked" into ethylene and propylene, a process in which the saturated hydrocarbon gases are combined with steam at temperatures of 900 degrees Celsius or more and break down into their lighter, unsaturated monomers—their chemical building blocks. A little more processing and you get a wide range of monomers for use in production. Add chemical catalysts to these petroleum-based feedstocks, and voilà!, a polymer is born, ready for melting, processing and shipment as basic plastic pellets. From there it's up to the plastic manufacturers to turn these pellets into the plastic bottles, films, pipes, you-name-it via extrusion, injection molding and a variety of other methods. The bottom line? Making virgin plastic is fuel intensive and carbon heavy, so recycling is a key component to reducing plastic's environmental impact.

Role of Recycling

Some recycling naysayers claim that recycling plastic is more trouble than it's worth from the perspective of sheer resource efficiency. Yet study after study reveals that hands down, recycling wins over virgin production on all environmental measurements, especially when it comes to carbon emissions. Estimates vary with the type of recycling process used, but researchers agree that recycling and re-manufacturing plastic saves at least 30 percent of the carbon emissions that original processing and manufacturing produces. That ould mean an annual savings of 30 to 150 million tons of CO2, given our previous calculations of carbon emissions from plastics production.

For example, a group of  Italian scientists performed a life-cycle analysis of different recycling methods, as compared to landfilling or combustion. Comparing greenhouse gas emissions from post-consumer plastic production (recycling) with the equivalent amount of virgin plastic production, the researchers quantified emissions savings of 70 to 80 percent in the recycling scenario (see Figure 1). 

Figure 1Fig. 1. Comparison of Carbon Emissions of Plastic Manufacture or Recycling. A group of Italian scientists performed a life-cycle analysis of different recycling methods, as compared to landfilling or combustion for disposal. Because recycling plastic produces comparably fewer greenhouse gas emissions than would otherwise be produced with the equivalent amount of virgin plastic production, these "offsets" are factored into the carbon calculations. In contrast, landfilling and combustion represent the amount of carbon emitted from the production and transport of once-used virgin plastic. (Figure from Perugini et al 2005.)

Yet we are nowhere near this level of resource efficiency, as evidenced by EPA data from the United States alone (see Figure 2). Although our plastics consumption has grown by a factor of 30 since the 1960s, our recycling—or recovery—has grown by a factor of just two.

Figure 2
Fig. 2. Plastics Generation & Recovery in the U.S. 1960-2007 (Figure from Municipal Solid Waste in the U.S.: Facts & Figures. EPA 2007.)

Why are these recovery rates so low? Part of the blame lies quite glaringly on consumers, because we often fail simply to separate recyclable plastics from general waste. Yet the recycling infrastructure is also at fault, whether for its inability to recycle some hard-to-recover plastics or to make the processes that do exist cost-competitive at various scales.

All recycling is not created equal, and there are major engineering advances underway to improve plastics recovery. Although mechanical recycling is most common, it is not without problems, and new alternatives may begin to replace this traditional method.

Mechanical Recycling

Most municipal recycling programs in the United States began in the 1980s, when state-level bottle-deposit requirements were passed. Policymakers saw the mounting piles of trash emanating from our quickly escalating use of plastic, and they ushered in a new era of municipal waste management to match.

Mechanical recycling, a method which involves sorting, washing and melting the separate types of plastic, was the first method used for recovering plastics. The recovered material is remelted and reprocessed into new products, which saves the initial oil extraction, processing and polymerization steps involved in virgin plastic production (see graph, above). Yet this method is hampered by its inability to produce high-grade, valuable plastics. That's because thermoplastics, the type of soft, food-packaging plastic with which we're most familiar, degrade during the remelting process. As a result, most recycled plastic has limited, and less valuable, uses, such as playground equipment and building-insulation fiber.

Advances in Recycling

There is an increasing demand for more efficient plastics recycling processes. And results may not be far off. For example, chemical engineers at North Carolina State University are working on a depolymerization technique. Instead of the conventional two to four hours that it would take to mechanically sort, wash and melt thermoplastics, this technique uses chemicals to break plastics into their constituent monomers in a matter of minutes. Not only is the process less time consuming, but it may also be able to produce a higher-grade end product than mechanical recycling. That means that recycled plastic, already an environmental win, could also become more cost-competitive than it is today. And that's the bottom line that will ultimately determine whether or not municipal recycling programs will accept all of our synthetic gizmos and gadgets. If there's a viable market for post-consumer plastics, recycling rates will soar.

Samantha Staley plans to receive her master's degree in earth systems in 2009.