? Filling up your tank with ethanol - The corn-based fuel is popular with politicians looking for votes from Iowa farmers, but environmental groups have opposed the subsidies because of all the land, water and energy needed to produce it. While using ethanol instead of gas may reduce greenhouse emissions by about 10 percent, the benefit is swamped by the adverse environmental consequences, according to an exhaustive study of biofuels last year by Swiss researchers.

? Recycling everything - While it can make economic sense to recycle aluminum and paper, towns frequently lose money recycling glass and plastics because they're expensive to collect and aren't worth much. Go ahead and recycle plastic if it gives you pleasure -- you can feel virtuous about the energy savings. But there are easier and cheaper ways to reduce greenhouse emissions. And you may not be saving energy if you're making a trip to the recycling center to haul a few bottles.

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? Buying local food - If you want to support local farmers and love fresh food, fine, but don't assume you're helping the planet. Foods from farther away may be grown and shipped so much more efficiently (and cheaply) that they produce fewer greenhouse gases. "There are lots of good reasons to eat local," says David Victor, director of the Program on Energy and Sustainable Development at Stanford University. "But energy savings don't top the list, because local production often requires more trips than mechanized food production."

? Going organic - Buying organic food makes sense if you believe it's tastier, more nutritious or safer than conventionally grown food (despite conflicting opinions on these points). But there is one major environmental downside: Since organic farms often yield less per acre than factory farms, organic food requires more land, leaving less room for forests that absorb carbon dioxide and wilderness areas that promote biodiversity.