"At Wal-Mart, sustainability is core to our mission", explains Kathleen McLaughlin, Chief Sustainability Officer. "The biggest challenge in the U.S. is making it economic." The scale of the company has meant that investing in clean energy, such as solar power, has come at a huge expense. Back in 2005, Wal-Mart's CEO Lee Scott pledged to reduce the company's Carbon emissions, and to switch to 100% renewable energy. 12 years on, the company is powered from just 17% renewable sources. Fred Krupp of the Environmental Defence Fund believes they can meet their goals. "The scale of Wal-Mart is hard to wrap your head around...In the United States, they sell about a third of all the food that we buy at retail stores. But what they have shown so far is a serious commitment."