As Leslie Dach, a top Wal-Mart executive, put: "It is really a Walmart approach to solving a problem.... The size and scale of this company can be put to use to make a real difference in the world."

That's no doubt a good thing. Better, as one of my sources told me, to improve practices at 10,000 factories around the world than simply to make WMT's operations more efficient. "Sensational" was how Fred Krupp, the president of Environmental Defense Fund, described it, during a lovefest with Walmart CEO Mike Duke, which was webcast on Treehugger, of all places.

Duke praised EDF, saying: "Our NGO partners have pushed us and been patient with us."

Krupp returned the favor. EDF has planted two staffers in Bentonville, Arkansas, to work closely with WMT, and he said: "I don't think there's any better money we are spending anywhere."

Specifics about how the carbon reduction effort would work were few, understandably so since it is new. "This is uncharted waters," said Elizabeth Sturcken of EDF. (Here is her excellent analysis, with some detail on initiatives in packaging and around dairy products.) Right now, there's little data available to measure the carbon impacts of the products that Wal-Mart sells, particularly if you want to include how they are made, shipped, used and thrown away, as WMT does.

Walmart said it would start with the products that have the most "embedded carbon" and seek GHG reductions that are "economically viable." The company has already had success getting suppliers to use smaller packages, from concentrated detergents to lighter-weight DVD cases.

Walmart itself, though, wants to get bigger. Duke was straightforward about this. "We are a growth company," he said. "We want to add square footage. That's the reality of our business."

Critics are unsatisfied. Here's reaction from Stacy Mitchell, a senior researcher with the New Rules Project, a program of the Institute for Local Self-Reliance:

By focusing on suppliers, Wal-Mart continues to deflect attention from the enormous greenhouse gas implications of its own business model. Wal-Mart is rapidly expanding in China, Mexico, and other countries, where it is destroying neighborhood businesses and replacing them with an auto-oriented form of big-box shopping that is highly polluting. Under Wal-Mart, local and regional systems of economic production and distribution are giving way to global supply chains, which almost invariably means longer distances and greater fuel consumption.

She's got a point, but the story is more complicated, for a couple of reasons. (Warning: geeky analysis ahead.) First, look at the graphic on the far right, above. Walmart is reducing its GHG emissions per unit of sales, meaning that it's more efficient. So, if its competitors are not doing as well in terms of efficiency, and if it takes market share away, then it's possible that WMT can sell more stuff and the planet will be better off. For example, if Walmart sells lots of Fair Trade coffee, and the locally-owned convenience store around the corner sells less conventional coffee, that's a good thing. Local isn't necessarily better.

Second, and paradoxically, Walmart is actually becoming more local. For example, Walmart has made a concerted effort to buy more from local farmers. Corby Kummer has a terrific article about this in the current Atlantic, in which he asks: Will Walmart and not Whole Foods save the Small Farm and Make America Healthy? The company, he reports, "wants to revive local economies and communities that lost out when agriculture became centralized in large states." Best quote in the story is from Michelle Harvey of EDF who says: "It's getting harder and harder to hate Walmart."

True enough. Nevertheless, in my ideal world, Walmart would set a cap on its own emissions, sell used goods as well as new, nudge people to buy vegetables instead of meat, and share profits with its workers. In today's world, Walmart will try to grow (profits) and shrink (pollution) at the same time. That's about all we can ask of a big company until we, collectively, can find a way to decouple economic growth from environmental harm. That's a job too big even for Walmart.

GreenBiz.com Senior Writer Marc Gunther is a longtime journalist and speaker whose focus is business and sustainability. Marc maintains a blog at MarcGunther.com. You can follow him on Twitter @marcGunther.