Easy Answer: Neither. Bring your own. But if you forget, you may want to choose the type of bag you're most likely to reuse.
San Francisco is considering expanding its plastic bag ban to include almost all retailers. The state of California is advancing a law that would ban plastic bags and make customers pay for paper sacks. China banned plastic bags last year. Which got me wondering--why all the hate for plastic? Are plastic sacks that much worse than paper?
Nope. Turns out both are pretty bad for the environment.
I got in touch with the National Resources Defense Council--to get their opinion.
If I forget my reusable shopping bags at home, is it better to choose plastic or paper? In answering that question, does it matter where in the country I live? If so, why?
"The production of both plastic and paper bags has environmental consequences; there are a number of studies out there devoted to the question of which one causes the most environmental damage, and no definitive answer (which is why reusable bags are still the best option - but I agree that it's important to have a strategy for when you just don't have one). For both plastic and paper bags, though, incorporating recycled content helps to reduce environmental impacts associated with manufacture. Many paper shopping bags have recycled content, but not very many plastic bags do. And most plastic bags are derived from fossil fuels, which are a non-renewable resource, while paper is made from renewable resources. On the other hand, plastic is lighter in weight than paper, meaning less fossil fuels are needed to transport a larger amount of plastic bags. Plastic also tends to be stronger and more waterproof than paper, which can mean greater efficiency in packaging (e.g., you might not need as many plastic bags as paper bags if you're packing heavy or moist goods). In San Francisco, like many other communities in the U.S., it's a lot easier for the typical consumer to recycle paper bags, since you can often do that in your home curbside paper recycling, but many grocery stores and other locations also collect plastic bags for recycling. However, plastic bags are trickier to recycle, and consumers don't always to remember to bring bags back to the store for recycling. So there are a number of issues which can affect which bag has the greater environmental impact, including how you use it, how you make it, and which environmental factors you're considering as most significant.
I think the best option if you have to take a paper or plastic bag is to take the one you personally are more likely to reuse. Reusing bags helps reduce environmental impacts overall, especially if by reusing a bag you're avoiding buying a new one. For example, many people reuse plastic shopping bags for trashcan liners, food storage, or pet waste disposal. Others may use paper bags for storage or for collecting recyclables, for example. If you're equally likely to reuse either a plastic or paper bag, then I'd pick the one that has more renewable/recycled content (typically paper) and/or is more recyclable in your community (which in San Francisco and many other communities is also typically paper)."
San Fran is considering banning most plastic bags from retail establishments. CA might also limit plastic bags? Why does policy seem to favor paper?
"Again, it's absolutely true that the production of both plastic and paper bags has negative environmental consequences, and that the negative consequences are worse when it's designed to be a single-use bag (because you need to produce so many of them). Often, communities consider bans on plastic bags not just because of the environmental consequences of production, but because they represent a litter and/or disposal problem in that community. In coastal communities, for example, plastic bags can find their way into the water, where marine wildlife can choke on them or become entangled in them. Plastic bags are easily airborne, and are not biodegradable, so they end up blowing around streets and neighborhoods and getting caught in trees and so forth, and they don't decompose. So the litter incentive drives many communities to legislate specifically on plastic bags.
It probably doesn't make sense for most communities to outright remove all single-use bag options without some kind of transition period, just because people are used to having free single-use bags offered to them, and it will take a while to transition to a society where we all remember to bring our reusable bags every time we visit a store. If some single-use bags are to continue to be available, though, we should figure out how to reduce their environmental footprint as much as possible. As I mentioned above, it's typically easier to reduce the environmental impact of paper bags, by incorporating postconsumer recycled content and by making sure there's a way to recycle them in a particular community - and paper bags don't typically represent as serious a litter problem as plastic bags do. For these reasons, more communities are choosing to keep paper bags for the remaining single-use bag option. Our approach is to support legislation which requires recycled content standards (and recyclability) for paper bags, whether that legislation places an equal fee on all single-use (both plastic and paper) bags, or whether it's legislation to ban plastic bags with limited distribution or sales of paper bags permitted. The proposed CA legislation does require postconsumer recycled content standards for paper bags, and also requires them to be sold at no less than five cents to consumers - reusable bags are also required to be available for sale. This way, consumers who forget their reusable bags have options to purchase a paper bag - but having a cost (or fee) hopefully provides some incentive to try to remember to bring reusable bags in the future."