The scale of the global food waste problem is almost as hard to grasp is it is to address.

But the payoff from reducing it could have a significant impact, not only on food supplies but on greenhouse gas emissions, as the U.N. Food and Agriculture Organization sought to make clear in a report released this fall.

It takes a staggering 3.5 billion acres of land globally to raise all the food that goes uneaten in a given year, according to the report. That’s close to 30 percent of the world’s total agricultural land and more than three times the total crop and rangeland in the United States.

A lot of water is lost as well with that global food waste — enough each year to keep the Volga River in Russia flowing.

Food waste ranks as the third-largest source of greenhouse gas emissions in both the United States and China.

That counts everything from emissions from the fertilizer used to grow crops such as corn to the methane rising from the landfills where the household garbage ends up, the FAO says.

More than half of global food waste, about 54 percent, occurs either on the farm or in storage.

This is a major problem in much of the developing world, where it’s not uncommon for weevils to get a farmer’s corn before his family does. The rest of the spoilage happens further down the food chain, from processors to household kitchens and restaurants.

“All of us — farmers and fishers, food processors and supermarkets, local and national governments, individual consumers — must make changes at every link of the human food chain to prevent food wastage from happening in the first place, and re-use or recycle it when we can’t,” FAO Director-General José Graziano da Silva said in a statement. “We simply cannot allow one-third of all the food we produce to go to waste or be lost because of inappropriate practices, when 870 million people go hungry every day.”

Not all waste is of equal concern.

Meat accounts for only 4 percent of the total food that spoils, according to FAO. But raising livestock has such a significant carbon footprint that uneaten meat accounts for 20 percent of the total economic cost of the global food waste.