From an op-ed piece by Tyler Cowen in The New York Times.
More than 90 percent of American wine production occurs on the West Coast, but because the majority of consumers live east of the Mississippi, a large part of carbon-dioxide emissions associated with wine comes from simply trucking it from the vineyard to tables on the East Coast. A standard wine bottle holds 750 milliliters of wine and generates about 5.2 pounds of carbon-dioxide emissions when it travels from a vineyard in California to a store in New York. A 3-liter box generates about half the emissions per 750 milliliters. Switching to wine in a box for the 97 percent of wines that are made to be consumed within a year would reduce greenhouse gas emissions by about two million tons, or the equivalent of retiring 400,000 cars.
(via Daring Fireball).