In searching for the answer he came across an article by Christopher Weber and H. Scott Matthews in the Environmental Science and Technology journal, which concluded:
"We find that although food is transported long distances in general (1640 km delivery and 6760 km life-cycle supply chain on average) the GHG emissions associated with food are dominated by the production phase, contributing 83% of the average U.S. household’s 8.1 t CO2e/yr footprint for food consumption. Transportation as a whole represents only 11% of life-cycle GHG emissions, and final delivery from producer to retail contributes only 4%. Different food groups exhibit a large range in GHG-intensity; on average, red meat is around 150% more GHG-intensive than chicken or fish. Thus, we suggest that dietary shift can be a more effective means of lowering an average household’s food-related climate footprint than 'buying local.' Shifting less than one day per week’s worth of calories from red meat and dairy products to chicken, fish, eggs, or a vegetable-based diet achieves more GHG reduction than buying all locally sourced food."
Here is Dubner's flawed conclusion:
"This is a pretty strong argument against the perceived environmental and economic benefits of locavore behavior—mostly because Weber and Matthews identify the fact that is nearly always overlooked in such arguments: specialization (which Michael Pollan mostly dislikes, and which has been around for a long, long time) is ruthlessly efficient. Which means less transportation, lower prices—and, in most cases, far more variety, which in my book means more deliciousness and more nutrition. The same store where I blew $12 on ice cream ingredients will happily sell me ice cream in many flavors, dietetic options, and price points."
Dubner is clearly a first-rate economist and original thinker, but he doesn't know how to make ice cream and he doesn't know delicious. First of all, in no way does variety equal deliciousness, and even more importantly, in no way do agribusiness practices resulting in the efficient economies of scale he mentions produce tastier, more nutritious food. In fact, it's just the opposite.
I'll continue to eat as much local food as is feasible and affordable, and eat more eggs, fish, chicken, and vegetables. In other words I'll eat more like Michael Pollan and less like Stephen Dubner.