Lettuce a bigger environmental threat than beef?

On Monday a press release began making the rounds claiming that "Vegetarian and 'healthy' diets are more harmful to the environment" and the story has generated provocative headlines like, "Lettuce three times worse than bacon for the environment."  The results are based on this paper published in Environment Systems and Decisions by Michelle Tom , Paul Fischbeck, and Chris Hendrickson.

Here's a portion of the abstract:

This article measures the changes in energy use, blue water footprint, and greenhouse gas (GHG) emissions associated with shifting from current US food consumption patterns to three dietary scenarios, which are based, in part, on the 2010 USDA Dietary Guidelines . . . . This study finds that shifting from the current US diet to dietary Scenario 1 [same mix of foods but fewer calories] decreases energy use, blue water footprint, and GHG emissions by around 9%, while shifting to dietary Scenario 2 [holding calories consumed constant but shifting away from meat to fruit and veg] increases energy use by 43%, blue water footprint by 16%, and GHG emissions by 11 %. Shifting to dietary Scenario 3, which accounts for both reduced Caloric intake and a shift to the USDA recommended food mix, increases energy use by 38 %, blue water footprint by 10 %, and GHG emissions by 6 %. These perhaps counterintuitive results are primarily due to USDA recommendations for greater Caloric intake of fruits, vegetables, dairy, and fish/seafood, which have relatively high resource use and emissions per Calorie.

So, what's going on here?  Didn't the authors of the latest (proposed) Nutritional Guidelines suggest less meat eating due to "sustainability" concerns?  Haven't we repeatedly read things like this quote from a story in Time in  2008? 

It’s true that giving up that average 176 lb. of meat a year is one of the greenest lifestyle changes you can make as an individual.

A few comments about this latest study and how it relates to the "received wisdom."

First, meat eating often looks bad in aggregate because the industry is so big.  In the US, we eat a lot of meat.  As a group, animal products probably represent the largest share of food expenditures of any food category.  As a result, we need to put the outcomes on some kind of units where foods can be compared on an even playing field.  There are a lot of options: lbs of food produced, dollars spent, acres used, or as the current study uses, calories.  As I've discussed before:

. . . meat is relatively (relative to many fruits and vegetables) inexpensive on a per calorie or per gram of protein basis, although meat looks more expensive when placed on a per pound basis. If you want really inexpensive calories eat vegetable oil or crackers or sugar; if you want real expensive calories, eat zucchini or lettuce or tomatoes.

For what it's worth, this discussion is related to the debate on whether eating healthy is more or less expensive.  As it turns out: the answer depends how you measure it ($/lb or $/calorie) .

Second, it is important to think on the margin, or think in terms of changes.  Meat is bad.  Compared to what?  As I discussed in the Food Police,  many fruits and vegetables are big users of water and pesticides.  So, if we eat less meat, what will we eat more of instead, and what are the impacts of the items we switch to?  This recent study produces results that look a bit different because it looks not just at aggregates but at marginal changes.  

Third, as this study makes clear, "the environment" is not a single thing.  It is multidimensional.  Here's a graph from the paper.

Some foods are better water users, others worse in terms of GHS emission, others produce more calories/acre and spare more land, and so on.  Very rarely are there "have your cake and eat it too" moments, and there are typically tough tradeoffs between health, environment, and taste (and even tradeoffs within each of those categories).  One problem with studies like this is that they don't count the consumer welfare cost from eating a different mix of foods or different number of calories from what they normally consume, so we don't have any sense for the tradeoff between taste and cost on the one hand and health and the environment on the other.  

The last comment is that it's tough for me to evaluate the "quality" of this paper, and one should probably be a bit careful of suffering from confirmation bias.  A lot of the assumptions driving the result actually come from other papers and analyses.  Moreover, the work was published in a relatively new journal (started in 2012) and I know nothing about it. [update: a follower on Twitter informed me that the journal has been around since the 1980's but recently changed its name]  However, it does appear that the authors' results are similar to those in another paper that the Nutritional Guidelines Committee actually discussed in their report.  The committee writes: 


a report from Heller and Keoleian suggests that an isocaloric shift from the average U.S. diet (at current U.S. per capita intake of 2,534 kcals/day from Loss-Adjusted Food Availability (LAFA) data) to a pattern that adheres to the 2010 Dietary Guidelines for Americans would result in a 12 percent increase in diet-related GHG emissions. This result was modified, however, by their finding that if Americans consumed the recommended pattern within the recommended calorie intake level of 2,000 kcal/day, there would be a 1 percent decrease in GHG emissions.

But, of course, people don't just follow the guidelines to a tee.  That's one reason why these sorts of guidelines and recommendations should consider consumers' behavioral responses to the policies in question.