Back in April I gave a talk at a workshop organized by the Institute of Medicine of the National Academies and the CDC entitled Exploring the Health and Environmental Costs of Food (you can read a summary of what went on at the workshop here).  

The premise of the workshop was that there are externalities in food production and our job was to quantify them and think about correctives.  There were many very smart people in the room but only a handful of economists.  If you read the summary report (see parts in chapters 6 and 7), you can get a bit of a sense of the disagreement that went on between economists on the one hand and nutritionists/epidemiologists/ecologists on the other (the report calls it "lively") about what constitutes an externality and whether "experts" can properly devise taxes to fix the problem.   

This video by Mike Munger should have been required viewing for everyone at the workshop.