Shifting less than one day per week’s worth of calories from red meat and dairy products to chicken, fish, eggs, or a vegetable-based diet achieves more greenhouse-gas reduction than buying all locally sourced food, according to a recent study by Christopher Weber and H. Scott Matthews, two Carnegie Mellon researchers. Every time a Prius or other hybrid owner drives to the grocery store, she may be cancelling out its emissions-reducing benefit, at least if she shops in the meat section. Emission from cows, as well as sheep and other ruminants, are 25 times more potent as a greenhouse gas than the carbon dioxide released by cars and humans.