Shopping online surged during Covid. Now the environmental costs are becoming clearer.

The most recent research is starting to incorporate more of the complexities of retail. In January, MIT’s Real Estate Innovation Lab published a study that simulated hundreds of thousands of those kinds of scenarios and found online shopping to be more sustainable than traditional retail 75 percent of the time.

But consumers today aren’t choosing one or the other, underscoring just how tricky this assessment is. So the MIT researchers recommended how shoppers and policymakers could instead help reduce carbon footprints at various steps of the supply chain, because either way, people are buying more.

“This is so much more complicated than, ‘E-commerce is better than brick and mortar,’” said Andrea Chegut, director of the lab. “We’re not on a good trajectory, because everyone is using both strategies. So on the aggregate, there will be more emissions.”

However theoretical it might seem, the question of the environmental impact of shopping has real consequences. The entire supply chain of everything we consume — from the extraction and processing of natural resources into products that are shipped to us and then used and disposed of — accounts for half of global emissions, according to the United Nations. The U.N. also estimates that global material use could double in the coming decades.

Brands and retailers are at the nexus of those supply chains. And only recently have major companies started mapping the entire carbon footprint of their sprawling networks, identifying sources of emissions and setting goals to reduce them. For many, third-party suppliers and customers account for the majority of their climate pie.

There are millions of retailers in the U.S. Of those, nearly 40 top companies have either set science-based targets to slash their total carbon footprints in alignment with the Paris agreement, or pledged to do so, the National Retail Federation found. Those retailers include Amazon, H&M, Ikea and Walmart.

At first blush, it appears that there are three key areas where e-commerce and traditional retail diverge: the last mile (whether a product was delivered or a consumer made a trip to buy it), the buildings (storefronts or warehouses) and the packaging waste.

Most research suggests that ordering goods for delivery is more beneficial for the environment because it means people are making fewer individual shopping trips. The average U.S. consumer goes to the grocery store at least 300 times a year. If they drove there, it was likely in a gas-powered vehicle. Plus, there tends to be higher energy demands at storefronts compared to warehouses.

But that scale “could easily tip in the other direction,” according to a study of the U.S. market published last spring by the sustainable investment firm Generation. The firm’s researchers found that e-commerce is 17 percent more carbon efficient than traditional retail, but could change with a few tweaks to their assumptions, such as the number of items purchased in a single visit, the amount of packaging and the efficiency of last-mile delivery.

In January, the World Economic Forum also found that growing demand for delivery could spike emissions and traffic congestion by more than 30 percent in the world’s top 100 cities by the end of the decade. The report accounted for the emissions saved from fewer individual shopping trips but didn’t consider packaging, and recommended that companies switch to electric vehicles, consolidate hubs for packages and boost nighttime deliveries.