On Thursday, Wal-Mart is going to announce its new effort to create a comprehensive "sustainability index" for every product it sells. The plan is to give every microwave, bean bag, book, and Transformer a score that reflects its full environmental impact, from manufacture through use and disposal, taking into account greenhouse gas emissions, use of materials and natural resources, and impacts on "people and communities."Wal-Mart has enormous power over companies who want to sell their goods in its stores. Its buying policies can all but force a supplier to change its products or packaging. This new sustainability index, in other words, is going to be a number that Proctor and Gamble and General Mills care about.And it looks like the index has been designed well. The details are being hammered out by a "consortium" including Wal-Mart, Wal-Mart competitors like Target, consumer goods companies, and academics. Importantly, the consortium is being led by those latter folks: professors of sustainability from University of Arkansas and Arizona State University.This is really big news. Wal-Mart has the clout to force businesses to dig up and release real data. And Wal-Mart has the reach to create a standard the average American pays attention to. Their index probably won't be perfect, but it will be our first real starting point in developing a comprehensive national measure of the actual cost of our stuff. That's pretty exciting.