But how much difference does it make? Here’s some perspective: To offset the greenhouse impact of one passenger’s round-trip flight between New York and London, you’d have to recycle roughly 40,000 plastic bottles, assuming you fly coach. If you sit in business- or first-class, where each passenger takes up more space, it could be more like 100,000.

Even those statistics might be misleading. New York and other cities instruct people to rinse the bottles before putting them in the recycling bin, but the E.P.A.’s life-cycle calculation doesn’t take that water into account. That single omission can make a big difference, according to Chris Goodall, the author of “How to Live a Low-Carbon Life.” Mr. Goodall calculates that if you wash plastic in water that was heated by coal-derived electricity, then the net effect of your recycling could be more carbon in the atmosphere.

To many public officials, recycling is a question of morality, not cost-benefit analysis. Mayor Bill de Blasio of New York declared that by 2030 the city would no longer send any garbage to landfills. “This is the way of the future if we’re going to save our earth,” he explained while announcing that New York would join San Francisco, Seattle and other cities in moving toward a “zero waste” policy, which would require an unprecedented level of recycling.