It's just one study, so I'm not taking it as gospel, but it's interesting. And there's good news: you CAN make a difference in pollution by shifting what you eat. We eat no beef or chicken and just a bit of fish (of course environmental issues around fishing are a whole other... um.. kettle of fish...) and I try to eat locally in the summer when it's feasible, but I pretty much feel zero guilt about having fruit from South America in the middle of winter. If you live in the Midwest, it's pretty close to impossible to eat entirely local unless you want to eat venison jerky and canned peaches all winter.
http://news.nationalgeographic.com/news/2008/04/080422-green-food.html
http://news.nationalgeographic.com/news/2008/04/080422-green-food.html