With its recent flurry of green initiatives, Wal-Mart has won the embrace of several prominent environmental groups. “If they do even half what they say they want to do, it will make a huge difference for the planet,” said Ashok Gupta of the Natural Resources Defense Council. Environmental Defense, meanwhile, has deemed Wal-Mart’s actions momentous enough to warrant opening an office near the retailer’s headquarters in Bentonville, Ark. “If [we] can nudge Wal-Mart in the right direction on the environment, we can have a huge impact,” said the organization’s executive vice president, David Yarnold.

Wal-Mart’s eco-commitments are not without substance. The two most significant are a pledge to make its stores 20 percent more energy efficient by 2013, which will cut annual electricity use by 3.5 million megawatt-hours, and a plan to double the fuel economy of its trucks by 2015, which will save 60 million gallons of diesel fuel a year.

Acting with unusual transparency, Wal-Mart has even published a benchmark calculation of its carbon footprint [Excel]. The company estimates that its U.S. operations were responsible for 15.3 million metric tons of CO2 emissions in 2005. About three-quarters of this pollution came from the electricity generated to power its stores.

This cannot be dismissed as greenwashing. It’s actually far more dangerous than that. Wal-Mart’s initiatives have just enough meat to have distracted much of the environmental movement, along with most journalists and many ordinary people, from the fundamental fact that, as a system of distributing goods to people, big-box retailing is as intrinsically unsustainable as clear-cut logging is as a method of harvesting trees.

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Here’s the key issue. Wal-Mart’s carbon estimate omits a massive source of CO2 that is inherent to its operations and amounts to more than all of its other greenhouse-gas emissions combined: the CO2 produced by customers driving to its stores.

The dramatic growth of big-box retailers, including Wal-Mart, Target, and Home Depot, over the last 15 years has been mirrored by an equally dramatic rise in how many miles we travel running errands. Between 1990 and 2001 (the most recent year for which the U.S. Department of Transportation has data), the number of miles that the average American household drove each year for shopping grew by more than 40 percent.

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It’s not that we are going to the store more often, but rather that each trip is an average of about two miles longer. The general trend toward suburbanization is only partly to blame: shopping-related driving grew three times as fast as driving for all other purposes. The culprit is big-box retail. These companies have displaced tens of thousands of neighborhood and downtown businesses and consolidated the necessities of life into massive stores that aggregate car-borne shoppers from large areas. During the 1990s, for example, about 5,000 independent hardware stores, dispersed across almost as many neighborhoods, were replaced by just 1,500 Home Depot and Lowe’s superstores, most erected on the outer fringes of our cities. The same trend is under way in virtually every retail sector. According to the market research firm Retail Forward, every time Wal-Mart converts one of its stores into a Supercenter with groceries, it leads to the closure of two existing grocery stores, leaving many residents with farther to drive for milk and bread.

Altogether, by 2001, Americans logged over 330 billion miles going to and from the store, generating more than 140 million metric tons of CO2. If we conservatively estimate that shopping-related driving over the last five years grew at only half the rate of the 1990s, that means Americans are now driving more than 365 billion miles each year and producing 154 million metric tons of CO2 in the process.

Since Wal-Mart accounts for 10 percent of U.S. retail sales, the company’s share of these emissions is at least 15.4 million metric tons — and likely higher, because Wal-Mart has led the way in auto-oriented store formats and locations. This amounts to more than all of its other domestic CO2 output combined.

Land-use consultant Kennedy Smith notes that another way to estimate these emissions is to start with the 100 million shoppers Wal-Mart says its stores attract each week, generously assume two shoppers per car, and then multiply by the average length of a shopping trip. This produces an almost identical result: over 15 million metric tons of CO2.

Shopping-related driving has been growing so fast that even a phenomenal improvement in the fuel economy of cars would soon be eclipsed by more miles on the road. Nor is CO2 the only environmental impact of all of this driving. Tens of thousands of acres of habitat have been paved for big-box parking lots, which, during rainstorms, deliver large doses of oil and other petrochemicals deposited by cars to nearby lakes and streams.

By embracing Wal-Mart, groups like NRDC and Environmental Defense are not only absolving the company of the consequences of its business model, but implying that this method of retailing goods can, with adjustments, be made sustainable.

Worst of all, they are helping Wal-Mart expand. In the Northeast and West Coast, where Supercenters are relatively few and environmental sentiment runs strong, a greener image is just what Wal-Mart needs to overcome widespread public opposition to new stores.

In January alone, Wal-Mart opened 70 U.S. stores. At current growth rates, by 2015 Wal-Mart will have enlarged its domestic footprint by 20,000 acres, turning CO2-absorbing fields and forests into stores and parking lots. Big-box stores make incredibly inefficient use of land. While 200,000 square feet of retail spread over several two-story downtown buildings with shared parking takes up about four acres, a single-story Superstore of this size, with its standard 1,000 parking spaces, consumes nearly 20 acres.

Wal-Mart’s new stores will use more electricity than its energy-efficiency measures will save. By making its existing outlets 20 percent more efficient, Wal-Mart says it will cut CO2 emissions by 2.5 million metric tons by 2013. But new stores built this year alone will consume enough electricity to add about 1 million metric tons of CO2 to the atmosphere.

It is not as though we need these stores. Between 1990 and 2005, the amount of store space per capita in this country doubled, while consumer spending grew at less than half that rate. The predictable result is that the U.S. is now home to thousands of dead malls and vacant-strip shopping centers. City planners are not the only ones alarmed. “The most over-retailed country in the world hardly needs more shopping outlets of any kind,” advised PricewaterhouseCoopers in a report to real-estate investors.

Yet Wal-Mart continues to build — consuming land, inducing more driving, and, perhaps most perilous of all, destroying what remains of small-scale, locally owned businesses. Tucked close to their customers in neighborhoods and downtowns, and sized to fit sidewalks rather than regional highway systems, it is these stores that are the true building blocks of a sustainable way of distributing goods. It is they, not Wal-Mart, that deserve the admiration and support of the environmental movement.