Total cost after the price drop: the equivalent of $4.63.

About one-third of the food produced and packaged for human consumption is lost or wasted, according to the Food and Agriculture Organization of the United Nations. That equals 1.3 billion tons a year, worth nearly $680 billion. The figures represent more than just a disastrous misallocation of need and want, given that 10 percent of people in the world are chronically undernourished. All that excess food, scientists say, contributes to climate change.

From 8 to 10 percent of greenhouse gas emissions are related to food lost during harvest and production or wasted by consumers, a recent report by the Intergovernmental Panel on Climate Change found. Landfills of rotting food emit methane, a gas that is roughly 25 times more harmful than carbon dioxide. And to harvest and transport all that wasted food requires billions of acres of arable land, trillions of gallons of water and vast amounts of fossil fuels.

For consumers, cutting back on food waste is one of the few personal habits that can help the planet. But for some reason, a lot of people who fret about their carbon footprint aren’t sweating the vegetables and rump steak they toss into the garbage.

“There’s been a lot of focus on energy,” said Paul Behrens, a professor in energy and environmental change at the University of Leiden in the Netherlands. “But climate change is as much a land issue and a food issue as anything else.”

Reducing waste is a challenge because selling as much food as possible is a tried, tested and ingrained part of all-you-can-eat cultures. Persuading merchants to promote and profit from “food rescue,” as it is known, is not so obvious.

“Consumers are paying for the food, and who wants to reduce that?” said Toine Timmermans, director of the United Against Food Waste Foundation, a nonprofit in the Netherlands composed of companies and research institutes. “Who profits from reducing food waste?”

A growing number of supermarkets, restaurants and start-ups — many based in Europe — are trying to answer that question. The United States is another matter.

“Food waste might be a uniquely American challenge because many people in this country equate quantity with a bargain,” said Meredith Niles an assistant professor in food systems and policy at the University of Vermont. “Look at the number of restaurants that advertise their supersized portions.”

Nine of the 10 United States supermarket chains that were assessed by the nonprofit Center for Biological Diversity last year were given a C grade or lower on food-waste issues. Only Walmart did better, largely for its efforts to standardize date labels and to educate staffers and customers.

Some of the most promising food waste efforts are apps that connect food sellers to food buyers. Think Tinder, except one party in this hookup is a person and the other is an aging loaf of bread.

Among the most popular is Too Good to Go, a company based in Copenhagen, with 13 million users and contracts with 25,000 restaurants and bakeries in 11 countries. Consumers pay about one-third of the sticker price for items, most of which goes to the retailer, with a small percentage paid to the app.