At the grocery store, when they ask “Paper or plastic?” (and you have left your eco-friendly organic cotton tote bag at home), which is the lesser of two evils as far as total pounds of pollutants per bag (including solid waste, hazardous waste, and air and water pollution), and as far as ecological damage from raw material extraction and processing?
I am beginning to believe that true national environmental consciousness will be heralded by the cessation of this constant concern over tiny little bags. Stop your worrying. Take the energy you spent writing to me and use it to organize your durable bag collection so that you remember to take it to the store. If you forget your bags? Flip a coin. The quantifiable difference between paper and plastic bags is minimal. One trouble with gauging the comparative environmental impact of different products is the lack of a universally accepted evaluative framework. (Another is the participation of industry analysts in environmental impact studies.) Scientists are moving closer to a system called Life Cycle Analysis, but it has gaps and flaws, so folks often develop models for their specific research needs. The result? I might answer your question one way, only to have someone disprove me (or claim to disprove me) tomorrow.
As of this writing, though, it is generally understood that the plastic bag production process generates less water pollution, less air pollution, and less solid waste. Making something out of plastic just seems to be easier than making something out of wood, which goes a long way toward explaining why we have so much plastic packaging today: It’s cheap. In addition, and you can bet the plastic industry is cackling over this, the bags take up less space in landfills. As to which manufacturing process uses less energy, they may be equal. It depends in part on whether you calculate two plastic bags for each paper bag, because store clerks frequently double-bag when using plastic. See? Comparative Environmentalism 101 is tougher than you think.
Of course, paper bags trump plastic in the renewable resource realm, being made from trees rather than oil or gas, and being more commonly accepted by community recycling programs. Also, paper bags are biodegradable and far less likely to disturb natural ecosystems in the manner of plastic bags, which have a nasty tendency to be mistaken for jellyfish and wind up doing bad things to the innards of turtles — to cite just one example.
So the short answer to your which-is-the-lesser-of-two-evils question is that there is no clear lesser, and neither is exactly evil. Picture this scenario: You’re driving down the street and your eye is caught by a plastic bag snared in a tree. “Gross!” you think, “Some jerk has been littering again! Those plastic bags are a wasteful, American consumerist eyesore! Turtles are going to die!” Guess what? That flapping plastic bag is a distraction from the much more important, yet all-too-often overlooked fact that you are driving. Driving a particulate-spewing, gas-devouring, pavement-loving machine. And that is the role of the paper/plastic debate for far too many casual environmentalists: an eye-catching distraction from far more threatening concerns.