Thanks for the post. It is a little disappointing, but makes sense.
As Dana points out, there are other reasons to eat locally. I get all of my meat directly from a farm 5 miles down the road. Part of the reason is that it supports a local economy. The farmer shares my interests and I am helping create demand for a good product. These all factor into my decisions.
The list from 7th Generation does provide some good guidelines. I would like to see more detailed studies of other parts of the food system, particularly added ingredients. It's relatively easy to compare orange juice in terms of environmental impact. When you start adding things to it is where it gets tricky. You can get orange juice with added vitamins, for instance. It should be considered that those extra vitamins are usually made at another facility and shipped to the juice manufacturer. Chemicals (vitamins) have a shelf life. Inevitably there will be waste. Some of these chemicals are very hazardous in high concentrations. Some are flammable. The equipment used to make these chemicals all need to be cleaned throughly quite frequently. This involves a lot of powerful chemical cleaners (more processing and transporting of these) and tremendous amounts of hot water. Some of these chemicals are bought and stored in bulk. In my company that involves a lot of moving pallets with gas-powered forklifts and movement between production and warehousing (sometimes off-site) facilities. And on and on it goes.
This does not even begin to consider those mysterious "natural flavors" found in nearly all foods, even organics. These flavors are comprised of a lot of different chemicals. Using what I have already shared, you can imagine the difficulty in measuring the environmental impact of dozens, sometimes hundreds, of chemicals all being produced and shipped to a facility to make the flavors, which then ship that (usually hazardous) product to the food manufacturer. It's quite a process!
Basically, the less ingredients the better. Another importnat factor in food buying, but one that is difficult to measure in terms of overall impact. I find it best to err on the side of caution!
To follow Michael Pollon's advice, eat food, mostly plants, and not too much!