Go green, get lean! Scientists take a new tack - Diet & Nutrition - MSNBC.com How? They advise that people get out of their cars and walk or bike half an hour a day instead of driving. And also they suggest eating less red meat. That's how Americans can simultaneously save the planet and their health, say doctors and climate scientists. The payoffs are huge, although unlikely to happen. One numbers-crunching scientist calculates that if all Americans between 10 and 74 walked just half an hour a day instead of driving, they would cut the annual U.S. emissions of carbon dioxide, the chief greenhouse gas, by 64 million tons.