The most significant environmental risk from banning plastic bags is the increase in energy use. Plastic bags are the most energy-efficient form of grocery bag. The U.K. Environment Agency compared energy use for plastic, paper and re-usable bags. It found the “global warming potential” of plastic grocery bags is one-fourth that of paper bags and 1/173rd that of a reusable cotton bag. In other words, consumers would have to use a reusable cotton bag 173 times before they broke even from an energy standpoint. Thus, even if consumers switched to reusable bags, it is not clear there would be a reduced environmental impact.
Ironically, many of the cities leading the charge against plastic bags are signatories to the U.S. Conference of Mayors Climate Protection Agreement. Yet, few of these cities even attempt to assess the climate impact of switching from the least energy-intensive grocery bag to bags that use far more energy to produce.
The U.K. Environment Agency study is echoed by other research as well, and the reason is simple – grocery stores began using plastic bags in part because they are cheaper to produce, in part because they use less energy to manufacture.