But it doesn't say exactly how much "minimizing" it actually takes to qualify as "sustainable."

"Those metrics have to be developed nationally," says Alex Bjork, manager of agriculture supply chains at the World Wildlife Fund, a member of the Sustainable Beef Roundtable. Beef production raises different concerns in different countries.

In Brazil, "we'd like to see deforestation eliminated," says Bjork. In Australia, environmentalists want to stop sediment washing from grazing areas into the ocean, damaging the Great Barrier Reef. In the U.S., the goal may be to "keep ranchers ranching" in a way that preserves healthy grasslands.

On top of those complications, there are trade-offs between different goals. Raising cattle on grass is generally worse for the climate; cattle grow more slowly and emit more greenhouse gases per pound of beef produced. On the other hand, grazing cattle can help preserve diverse grassland ecosystems.

For all those reasons, actually getting to a consensus definition of sustainable beef may take a very long time. "Twenty years is kind of the time frame that we're looking at," Bjork says.

So why did McDonald's promise to start buying "verified sustainable" beef in 2016? As it happens, that's the year the Global Roundtable on Sustainable Beef plans to begin some pilot projects that will raise cattle in the major beef-producing countries. Researchers will measure the environmental and social effects of producing beef in those operations, and look for ways to do even better.

So technically, the beef won't be "verified sustainable" just yet.

And while some environmental groups have argued that reducing the demand and supply of meat should be the priority, Bjork is hoping that this effort to reform the beef industry can imitate the Forest Stewardship Council, an international initiative devoted to reducing the environmental damage caused by logging. That group, which is now 20 years old, also brings together private companies and their critics, and according to Bjork, it has led to a global consensus on how forests should be managed.

Copyright 2014 NPR.

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Food Giants Want 'Sustainable' Beef. But What Does That Mean?