Cargo ships carrying truckloads of clothing, cheap furniture, toys, and other goods destined for Walmart stores in the U.S. emitted an estimated 3.7 million tons of carbon dioxide in 2019—more than a coal-fired power plant running for a year, according to a new report. The report, from the nonprofits Pacific Environment and Stand.earth, calculates the carbon footprint and air pollution from fossil-fueled ships for the top importers of products sold in the U.S.

The world’s shipping fleet has quadrupled since the 1980s as manufacturing moved overseas. Outsourcing “is predicated on a reliance on fossil-fueled maritime shipping,” says Madeline Rose, climate campaign director at Pacific Environment. “It’s intrinsic to the business model of manufacturing overseas. And we felt like there has never been a legitimate public accounting of the public health and environmental externalities of that decision.”

Walmart tops the list in the report, followed by the furniture retailer Ashley, Target, Dole, Home Depot, Chiquita, Ikea, Amazon, Samsung, Nike, LG, Redbull, Family Dollar, Williams-Sonoma, and Lowes. Collectively, the report found, the companies were responsible for as much climate pollution from shipping imports to the U.S. as the energy used by 1.5 million homes. Cargo ships are also a major source of air pollution, including sulfur oxide, which is linked to asthma and an increased risk of cancer in the low-income communities living near ports. The report found that imports for the 15 companies on the list generated as much sulfur oxide as 2 billion cars and trucks, or more than 7 times more vehicles than are currently on the road in the U.S.

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