"When I met Nathanael, I thought, 'This is a writer who could do this really well,' " recalls Rosenberg.

Johnson came to this assignment with the kind of skepticism about green orthodoxy that comes from growing up immersed in it.

"I was born into the cult of organic," he wrote in one of his first Grist columns. His parents were "Berkeley hippies" who moved back to the land in Nevada City, Calif. Growing up there "gave me a front-row seat from which to observe how faith in nature could spin into fuzzy thinking and paranoia," Johnson wrote. "I loved science because it could separate plausible and crazy."

Johnson dove into his GMO assignment with that same devotion to weighing the evidence. If you're confused by the long-running GMO fight — and even more if your mind is already made up — it's worth reading.

Be warned, though: You'll need some stamina. There are several notable things about this online experiment in explanatory journalism, and the first is its length. Johnson covers questions both obvious (Are GMO foods tested for safety?) and obscure (Are independent scientists banned from carrying out research on GMOs?).

More unusual, though, is its approach. In an online world of snark and smack-downs, Johnson presents his work as a humble quest for truth. For the most part, he convinces you that it's an honest search, and that he's not sure, himself, where he'll end up.

Where he ends up, in fact, is the final surprising thing about this series. Instead of preaching to the deep-green choir, Johnson questions its faith. He challenges many of the anti-GMO views that Grist's readers are used to seeing.

Which doesn't put him squarely in the pro-GMO camp, either. Unlike British polemicist Mark Lynas, who switched sides in this debate, Johnson is all about nuance. Are GMOs bad for the environment? Well, yes and no. They've allowed farmers to cut their use of insecticides while increasing the use of chemical weed-killers. More effective weed control is good for crop yields but bad for the Monarch butterfly. And so on.

Some of Grist's readers haven't appreciated the nuance, and reacted to Johnson's series with the kind of anger that's reserved for those who betray a righteous cause.

"I've got news for Johnson," wrote Claire Robinson, from GM Watch. "Misleading the readers of an until-now respected publication like Grist doesn't make him exciting, creative or cool. It just makes him an unreliable source." Instead of being, as Grist's slogan promises, a "beacon in the smog," Robinson accused the site of "operating more like a smog machine."

"I was unprepared for the ugliness" and the personal attacks, Johnson admits. "I'd never felt anything like that before" in 10 years of journalism.

Many proponents of GMOs, meanwhile, praised Johnson's efforts. Jon Entine, who writes for the Forbes site, described it as a partial repudiation of Grist's previous "embarrassing coverage" of the issue.

Those GMO supporters, however, weren't quite so happy with Johnson's final wrap-up post this week, which carried the headline: "What I learned from six months of GMO research: None of it matters."

"The most astonishing thing about the vicious public brawl over GMOs is that the stakes are so low," Johnson wrote. Essentially, if you care about the environment or human welfare, it just doesn't matter very much whether farmers are planting GMOs.

An essential personal disclosure here: In that final essay, Johnson quotes approvingly from my own book about GMOs, published way, way back in 2001. So perhaps I'm paying extra attention to Johnson's work because we agree.

Yet 13 years ago, when I argued that GMOs didn't matter all that much for the environment or human welfare — certainly not compared to old-fashioned things like tilling grasslands or draining wetlands — people mostly looked puzzled. I wonder whether Johnson's work will have more resonance — whether, for instance, apocalyptic rhetoric about GMOs will give way to a calmer understanding of what's at stake.

Johnson isn't sure.

"When I look at the comments, I get the feeling that nothing will ever change," he says.

On the other hand, there are occasional personal exchanges that leave him more optimistic. He cited an online conversation with another writer who's been a frequent GMO critic: "I felt like we were building a slender bridge across the chasm."

Copyright 2014 NPR.

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A Green-Movement Website Shakes Up The Debate Over GMOs