Photo by Kris Krüg

(Updated below with Michael Pollan's response)

You want some petroleum with that Big Mac?

Journalist and food writer Michael Pollan broke down the hidden cost of America's best-known burger on Saturday to an eager audience at the Poptech conference. He traced the Big Mac's origins all the way back to the oil fields, used to make fertilizer that is crucial to the corn grown for cows in massive feeds lots.

“Our meat eating is one of the most important contributors we make to climate change," said Pollan, who is best known for his book "The Omnivore's Dilemma."

"A vegan in a Hummer has a lighter carbon footprint than a beef eater in a Prius.”

It's a great line and quite a mental image, one that wowed the audience and quickly spread on Twitter. Too bad it's not true.

Gidon Eshel and Pamela Martin of the University of Chicago published a 2005 paper in the journal Earth Interactions that looked at the relative carbon footprints of plant-based and red-meat diets.