Millions of Americans will head to the post office this month, unwanted holiday gifts in hand. But while it may be convenient to return that ugly sweater from your mom, that ease comes at a huge cost to the environment.

The United Parcel Service (UPS) predicts that it will process a record 1.9 million returns on 2 January, which it has dubbed National Returns Day. More than half (55%) of Americans said they planned on returning unwanted holiday gifts within a month of receiving them, according to a survey published by the National Retail Federation.

The spike in returns is largely the result of more Americans doing their holiday shopping online: e-commerce sales were up 19% over last year, according to Mastercard. Amazon, the country’s largest online retailer, saw its stock rise last week after the company reported a “record-breaking” holiday season.

However, what many shoppers may not know is that shipping returned goods contributes to greenhouse-gas emissions – and much of what they send back ends up in landfills. Each year Americans return about 3.5 billion products, and five billion pounds of returned goods ended up in US landfills, according to Optoro, a technology company that helps retailers process returns.

“Our data shows that 88% of consumers think that returns go right back on the shelf and are resold to the next consumer,” says Ann Starodaj, head of sustainability at Optoro. “But in reality, the majority of returned items cannot be resold as new.”

The company says the landfill waste from returns alone contributes 15 million metric tons of carbon dioxide to the atmosphere.

Clothing sales have a particularly high return rate, between 40 and 50%. That’s because, as one market research firm puts it, “the bedroom is the new fitting room”.

“The easier you make it for the customer to return something, the more likely it is that the customer thinks there’s no cost to the return,” says Miguel Jaller, co-director of the Sustainable Freight Research Center at University of California, Davis. In fact, he says, “There are untold externalities in terms of packaging and resources, just to move the things we don’t like to another home or discarding facility or to another country.”

Companies opt not to return goods to the market for several reasons. In some cases, returned goods have been damaged or are no longer in season – for example, a winter coat returned in July. Often the value of the product is so low that it’s simply cheaper to throw it away than to handle the logistics of resale. Sellers sometimes have contracts with suppliers that prohibit them from selling an item more than once.

Jaller has found that consumers are more than twice as likely to return items bought online than in a store. And returns made in stores typically have less of an environmental impact than those made to e-retailers. “With e-retailers, there’s no store to drive to. A person drives to UPS, the product is then shipped to a warehouse. That return requires several different movements, whereas with a traditional retailer that product will likely stay at the store,” he says. Jaller estimates that between 20 and 25% of e-retailers’ environmental externalities are due to returns.

There are steps governments, retailers, and consumers can take to reduce waste.

Cities and states can impose tolls or congestion pricing on delivery trucks, while local law enforcement can restrict where and for how long trucks are allowed to idle while making deliveries. Earlier this year, the French government banned the destruction of unused consumer goods.

Retailers can minimize packaging, offer clearer descriptions of products, and use energy-efficient warehouses, among other things. They can also increase the number of dropoff sites, like Amazon lockers, that can streamline the returns process. Some companies, like Ikea, are using virtual reality to help customers envision their products in their homes; many clothing brands use TrueFit technology to help consumers more accurately choose their size.

More sellers could also charge for returns, but experts say that is unlikely in the short term as companies compete for market share. “According to our research, 97% of consumers say that a positive returns experience will encourage them to shop at that retailer again,” wrote Starodaj. “Amazon understands that owning and excelling at the returns moment is critical to their customer satisfaction.”

Jaller says that means much of the responsibility for reducing waste will fall to the consumer. He suggests fundamentally rethinking what we purchase in the first place, adding: “If you’re not sure you need it, don’t buy it.”