Wal-Mart Stores, Inc., is putting the onus on its suppliers as part of a goal to cut 20 million metric tons of greenhouse gas emissions from the retailer’s global supply chain over the next five years.

The world’s biggest retailer is partnering with the Environmental Defense Fund to work with Wal-Mart suppliers on identifying ways to reduce pollution and energy costs, according to a joint statement today.

Wal-Mart’s global supply chain carbon footprint is “many times” larger than its operational footprint and “represents a more impactful opportunity to reduce emissions,” the Bentonville, Ark.-based company said in the statement.

“We know that we have an opportunity to do more and the capacity to do more,” Wal-Mart Chief Executive Officer Mike Duke said in the statement.

The 20 million metric ton reduction target would be the equivalent of taking 3.8 million cars off the road for a year, Wal-Mart said.

Wal-Mart has already been buying more fresh produce from local growers in recent years, lowering transportation costs. Meat processors are among hundreds of Wal-Mart suppliers, and what steps they may take as part of the new effort isn’t yet clear.

Such direct-from-farm purchasing “not only reduces energy use and emissions, but also delivers a superior product to the consumer,” Wal-Mart’s Senior Vice President of Sustainability Matt Kistler said during a media conference call today. “It’s fresher and we experience less waste in our stores.”

Emission-reducing efforts will focus on product categories “with the highest embedded carbon,” today’s statement said, without listing any specific categories. Wal-Mart wants suppliers to curb emissions from raw materials sourcing, manufacturing, transportation, customer use or “end-of-life” disposal.

The effort will “transform a vast supply chain here at home, and around the world,” Fred Krupp, president of the New York-based Environmental Defense Fund, said in the statement.