美国国家公共电台 NPR Can Anyone, Even Walmart, Stem The Heat-Trapping Flood Of Nitrogen On Farms?（在线收听）

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A lot of companies have promised to do their part to reduce their releases of greenhouse gases that are trapping heat and giving the planet a fever. Among them are big food companies, including the biggest American grocer, Walmart. But companies are finding that reducing their climate impact is complicated because the emissions from food production come from unexpected places. NPR's Dan Charles reports.

DAN CHARLES, BYLINE: Decisions by a company like Walmart about what products it will sell can leave their mark on fields and forests and factories around the globe. The company is so huge and influential that 10 years ago, the advocacy group Environmental Defense Fund set up an office near Walmart's headquarters in Bentonville, Ark. Here's Suzy Friedman, a senior director at EDF.

SUZY FRIEDMAN: And we really saw that working with companies could be transformative at a scale that was pretty unmatched.

CHARLES: EDF wants to persuade companies that going green could be good for business. And Walmart often sounds like a convert to the cause. Laura Phillips, a senior vice president there, says Walmart is committed to fighting climate change.

LAURA PHILLIPS: It's really one of the most important issues we work on. Our customers are interested in climate and many of our stakeholders.

CHARLES: Last year, Walmart promised to reduce emissions of greenhouse gases by a billion tons of carbon between now and 2030. Now, that's way more than what's released by Walmart's own stores and trucks. Most of those reductions will have to come from Walmart's suppliers, all those other companies that make the products it sells. So Walmart's been calculating the climate price tags of those products, figuring out how much making each one contributes to global warming. When Phillips looked at that data, she was startled to see how high the price tag was on simple food items that didn't seem like they'd require burning a lot of fossil fuels - baked goods, for instance.

PHILLIPS: Why is that? You know, why are we seeing bread have high emissions?

CHARLES: Other food companies are asking the same question because a lot of them have made their own climate promises. They've set up an organization called Field to Market to measure and reduce the environmental impact of their operations. Allison Thomson is the group's research director.

ALLISON THOMSON: It has been a process of discovery mapping out the emissions and understanding there's a huge portion of the footprint that comes from the farm.

CHARLES: From the farm, not factories or fleets of trucks. And on the farm, the big greenhouse source is something most people don't think about very much. It's the fertilizer, mainly nitrogen, that farmers spread on their fields to feed their crops. Manufacturing this fertilizer releases a lot of carbon dioxide. And then when you spread nitrogen on a field, bacteria in the soil feed on it and release an even more powerful greenhouse gas called nitrous oxide. Add it all up and fertilizer is the biggest part of the global warming price tag of a loaf of bread or a box of Corn Flakes. According to one study, greenhouse emissions from fertilizer are the biggest single piece of the global warming price tag for almost half of the top-selling items on the shelves at Walmart. I asked Walmart executive Laura Phillips what her company could even do about that.

You see that. You see fertilizer's really significant. But do you have any control over that?

PHILLIPS: No. We don't make the product ourselves. We work with our suppliers. And I think we would want to work with our suppliers towards a path of continuous improvement.

CHARLES: But those suppliers don't control fertilizer use either. Bakers just buy the grain that the farmers grow. Meatpackers buy the cattle that ate that grain. They don't apply fertilizer to the fields. This is the big hitch in the whole strategy that environmental advocates like Suzy Friedman have been pursuing in working with big companies like Walmart.

FRIEDMAN: So that was one really big eye-opener. This is a whole lot more complicated than we even thought in the beginning.

CHARLES: You sort of had the idea in the beginning that Walmart can just do it.

FRIEDMAN: I think even Walmart had that idea in the beginning. And we really learned that you need to engage the whole supply chain.

CHARLES: They're trying to do that now, but it's not easy. It's a new frontier for environmentalists who are trying to fight climate change, not just dealing with the CEOs of big companies, but also fertilizer dealers and farmers in small towns where the food on store shelves gets its start. Dan Charles, NPR News.