The only thing you might need to look out for are the claims for local, small producers.

http://freakonomics.com/2011/11/14/the-inefficiency-of-local...

http://blogs.ei.columbia.edu/2012/09/04/how-green-is-local-f...

+ There are economies of scale.

+ There are also people against genetic engineering even if it would reduce pesticides or other pressure on the environment tremendously.

+ Food safety might be more efficiently regulated in a centralized manner.

+ Old-fashioned greenhouses might have large yields and might be properly scaled to reduce costs per produced crop as well as support recycling systems for CO2 and other "waste" products.

In other words, the situation around food seems more complicated than just "buy local".

I drive a car to work, because the summers are already too hot and humid for me to bike with a respiratory condition.

I work in the office 99% of the time because my bosses have a 19th century mentality

I fly at least twice a year to escape my miserable existence somewhere far away

I never take transit because I'm stuck in an endless suburban hell and the nearest bus stop/grocery store is over a mile away

I only shop online because traditional retail has a horrible selection and partly sells the same Chinese shit that Amazon does, just at a higher markup

I eat beef because food is one of the few pleasures I have in life

All of my food comes from supermarkets because it's cheaper.

No idea how this compares to the scale of taking a bus or a solo car, but there is an increased cost. Growing food (often with fertilizers, harvest machinery, etc) in order to bring it to the market for you to consume and convert into motion doesn't seem like it's particularly energy efficient.

I bike a lot, it's better for me physically and mentally, but I do wonder how the carbon impact is affected. (Of course, this ignores the fact that petroleum is a limited supply.)

fermatslibrary.com gets the credit for introducing me to this paper.