In a letter to the editor of Environment Systems and Decisions – the original publisher Carnegie Mellon University's study that prompted the greens vs grease debate – the Johns Hopkins team takes issue with the assessment of diets studied and, particularly, the press release announcing the findings.
Released as 'Vegetarian and "healthy" diets could be more harmful to the environment: Carnegie Mellon Study Finds Eating Lettuce Is More Than Three Times Worse in Greenhouse Gas Emissions Than Eating Bacon', the press announcement states that 'following the USDA recommendations to consume more fruits, vegetables, dairy and seafood is more harmful to the environment because those foods have relatively high resource uses and greenhouse gas (GHG) emissions per calorie'.
"Eating lettuce is over three times worse in greenhouse gas emissions than eating bacon," said Paul Fischbeck, Professor of social and decisions sciences and engineering and public policy, in the press release.
The Johns Hopkins letter has this to say in response: "In the press release, the authors illustrate an extreme case of a misleading per-calorie comparison by describing lettuce as 'three times worse in greenhouse gas emissions' than bacon. Given that bacon and lettuce have drastically different caloric densities (4.68 and 0.15 kcal/g, respectively) (USDA 2015), a person would need to eat 22 cups of lettuce to match the calories in two slices of bacon.
"This comparison makes for an attractive headline, but relies on unrealistic assumptions about dietary substitutions."
Well, yeah. In the Carnegie Mellon press release Professor Fischbeck goes on to say: "Lots of common vegetables require more resources per calorie than you would think. Eggplant, celery and cucumbers look particularly bad when compared to pork or chicken." Which clearly few people do – as pointed out by various journalists and readers at the time.