For people who want to reduce their carbon footprints, shopping can present a dilemma. Is it greener to visit a physical store, to have that store deliver an order or to place one with an online-only retailer? The answer depends on several factors. In a new attempt to compare the greenhouse gas emissions associated with each shopping method, researchers have quantified how often one option ends up being better for the environment than another. In most cases, delivery from a local store wins out—but this result can vary.

“We used a probabilistic approach—actually, that was one of the novelties of our work,” says study co-author Sadegh Shahmohammadi, an environmental scientist at Radboud University in the Netherlands. “Previous studies showed contradicting results [as to] whether online is better or offline is better. We wanted to improve the reliability of those models and see what is exactly going on.”

Whether they are purchased via Amazon or a local mom-and-pop business, consumer goods often go on a long journey to reach buyers’ hands. Most items must travel from the factory to the manufacturer’s warehouse to a distribution center or physical store—and each leg of the trip requires greenhouse gas–emitting transportation. “We have modeled [this process] from the moment the products leave the factory up until they arrive to the consumer’s home,” says Shahmohammadi, who is also now at the Netherlands Organization for Applied Scientific Research (TNO). In order to keep that model as streamlined as possible, the researchers focused on so-called fast-moving consumer goods, a category that includes food, cleaning supplies and personal-care products (but not clothing or electronics). They left out refrigerated food, because cooling methods contribute to greenhouse gas emissions as well.

The model incorporated various factors, including the number of products purchased, packaging, storage and greenhouse gas emissions for various types of transportation. In some previous studies, researchers had developed models that used an average value for each of these parameters. In the new paper, Shahmohammadi says, he and his colleagues entered a range instead of a single number. “We have probability distributions for all those input variables,” he explains. The scientists’ result, published this week in Environmental Science & Technology, showed a span of potential emissions for each shopping method.

“Their analysis is the range of those outputs, and that’s great,” says Jason Mathers, director of vehicles and freight strategy at the advocacy nonprofit Environmental Defense Fund, who was not involved in the research. “It gives you a fuller picture than just running one set of assumptions for each case.”

That fuller picture suggests the three shopping methods examined—visiting a physical store or receiving a delivery from a physical store or from an online retailer—produced roughly the same amount of emissions throughout most of the process. The big disparity came from the “last mile,” the final part of a purchased item’s journey to a home. “The last mile part is the main phase that causes the differences between the different shopping channels,” Shahmohammadi says. He and his colleagues calculated the last-mile carbon footprint in four countries: the U.K., the U.S., the Netherlands and China.

Credit: Amanda Montañez; Source: “Comparative Greenhouse Gas Footprinting of Online versus Traditional Shopping for Fast-Moving Consumer Goods: A Stochastic Approach,” by Sadegh Shahmohammadi et al., in Environmental Science & Technology. Published online February 26, 2020

Most of the time, having a physical store deliver products was greener than traditional shopping—which, in turn, was usually more environmentally friendly than online retail. This ranking shifted a great deal, depending on specific circumstances, however. In particular, the number of objects purchased on each shopping trip, the distance of that last-mile journey and the method of transportation made a large impact on the calculated volume of greenhouse gas emissions. For example, bundling more items together on a single purchase reduced shoppers’ carbon footprint because it required fewer trips to the store or package deliveries. And in countries such as the Netherlands and China, where many shoppers walk or bike to stores, visiting a brick-and-mortar location was much more environmentally friendly than in the U.S., where driving is more prevalent. “In the U.S., the last-mile footprint for brick and mortar is, like, 32 times greater than that of the Netherlands,” Shahmohammadi says.

To shop greener, he suggests, consumers might bundle purchases into fewer packages, take low-emission vehicles to stores and forgo same-day delivery. But the onus should not be entirely on individuals. “How can we have policies and corporate behavior structured in order to make sure that we’re coming out on the low side of the impact?” Mathers asks. For example, Shahmohammadi notes that companies switching to lower-emission conveyances would make a big difference. “We actually modeled that replacing delivery vans with electric cargo [vehicles] could reduce the last-mile transport footprint by 42 percent and the total footprint by 26 percent in the U.K.”

The question is not whether online or in-person shopping is more environmentally friendly. “It’s really complex when you try to tease out which is better,” Mathers says. “And ultimately, it’s somewhat irrelevant. We, as modern consumers, are buying through all these channels. And what we need to be focused on is: How do we make each one of them as low-polluting as it can be?”

Editor’s Note (2/27/20): This story was updated to include a graphic showing “Shopping’s Carbon Footprint.”