Wal-Mart faces enormous challenges on its path to sustainability. One example: The company can operate its stores and its fleet more efficiently, but as it opens new locations and adds more trucks--in other words, as Wal-Mart keeps growing--it will tend to pollute more.
Last year, an internal Wal-Mart report says, the retailer's greenhouse gas emissions actually rose by 8.6 percent. Whether this is good or bad for the planet depends on whether Wal-Mart took business away from less efficient competitors. That's all but impossible to know, but clearly there's a tension between growing the company and reducing its environmental impact.
Another example: The company has a long-term goal of generating "zero waste," but to date doesn't have an accurate measurement of how much garbage it dumps. So, again, there's no way to know how much of a difference Wal-Mart is making.
Source: Fortune, http://money.cnn.com