The world's largest retailer will help suppliers buy renewable energy via power purchase agreements. : LONDON — When Hurricane Katrina devastated New Orleans in 2005, it proved to be a tipping point for Walmart, the world's largest retailer. While the business was doing well at the time, it was focused on customers and employees over the broader environment, according to CEO Doug McMillon. "We were a large company but had not fully understood what that meant or what was required of us socially and environmentally," he said, addressing the Climate Week NYC conference in an online broadcast last month.