Single-use plastic bags are the worst environmental choice at the supermarket. Wrong: they’re the best choice. These high-density polyethylene bags are a marvel of economic, engineering, and environmental efficiency …  the fact that they’re not biodegradable is a plus, not a minus, because they don’t release methane or any other greenhouse gas, as decomposing paper and cotton bags do. The bags’ tiny quantity of carbon, extracted from natural gas, goes back underground, where it can be safely sequestered from the atmosphere (and the ocean) in a modern landfill with a sturdy lining.