A note from Shea: I'm out-of-pocket this week and I've asked a few of my green blogger friends to write some guest posts. Enjoy this piece from MNN's Bryan Nelson.

Several months ago food author and all around eco-guru Michael Pollan was caught making the claim that "a vegan in a Hummer has a lighter carbon footprint than a beef-eater in a Prius.”

The tantalizing comparison immediately made meme-waves across the environmental world. If he was right, it could finally vindicate vegans and vegetarians everywhere who insisted that anyone serious about being an environmentalist had to do more than just make responsible transportation choices. They had to give up their meat, too.

The only problem? Pollan was wrong.

It turns out that a study published out of the University of Chicago back in 2006 found that the difference between a heavy meat-eating diet and a vegan diet was, give or take, about 2 tons of carbon dioxide equivalent per person per year. Meanwhile, the difference between driving a Prius and a Hummer-sized SUV can be as much as 4.76 tons per year given average American driving habits.

In other words, Hummer drivers are still much bigger jackasses than meat-eaters, contrary to Pollan's claim.

Shortly after being corrected, Pollan (being the honorable journalist that he is) publicly retracted his quote. Environmentalist meat-eaters everywhere all slept a little better at night.

Unfortunately, the mix-up may have allowed the bigger point to be lost. The University of Chicago study didn't exonerate meat-eaters, except maybe in comparison to deplorable Hummer drivers. For instance, what if we tinker with the comparison slightly? Instead of using Hummer-drivers and Prius-drivers as a basis for comparing the environmental impacts of vegans and meat-eaters, let's compare a Prius-driving vegan against a meat-eating bicyclist.

In other words, a carbon-conscious transportation lifestyle might make up for the carbon-costly choice to eat meat when compared to the unholiest of Hummer-drivers. But how does the meat-eater match up against the more modest Prius driver?

The answer, in short: not so well. Using the University of Chicago study's calculations with the current EPA mileage statistics, a 2010 Prius getting 50 miles per gallon has a footprint of roughly around 1.4 tons annually. That means the Prius-driving vegan beats the meat-eating bicyclist by about half a ton in annual carbon impact.

Of course, the results can be tweaked. For instance, if you're the meat eater, it depends on the type of meat you eat, and how frequently you eat it. And regardless of your food preferences, the comparison depends on how the food was produced and whether or not it was delivered from a local source, etc.

Though even in a best-case scenario, the comparison should stay relatively close. And it gets worse for the meat-eater when generalized on a global level. According to U.N. data, meat production accounts for about 18 percent of worldwide greenhouse gas emissions compared to only 13 percent of emissions being accounted for by transportation. That includes all transportation: planes, trains, cars, and yes, Hummers too.

Now, I know a fair share of environmentalist-boasting bicyclists who still take pride in their carnivorous ways. While there's hardly anyone who could claim to be a perfect environmentalist (for the purposes of full disclosure, I'm a compact car-driving pescatarian), a moment of reckoning is probably in order for those of us who still make only minimal dietary choices.

And besides, it's a false dichotomy anyway. The vegan bicyclist bests us all.

Bryan Nelson is a science and environmental journalist, freelance writer and regular contributor to MNN. You can keep up with those contributions at his MNN profile, or you can follow him on Twitter.

Shea Gunther is a podcaster, writer, and entrepreneur living in Portland, Maine. He hosts the popular podcast "Marijuana Today Daily" and was a founder of Renewable Choice Energy, the country's leading provider of wind credits and Green Options. He plays a lot of ultimate frisbee and loves bad jokes.

The Prius-driving vegan vs. the meat-eating bicyclist: Who's the better environmentalist?
Environmentalists come in all kinds of lovable varieties. But what really matters is: which of us are best?