Monday, December 28, 2015

Is Lettuce Actually Worse for the Environment than Bacon?

A newly published paper was recently circulated in the media with headlines claiming that ‘lettuce produces more greenhouse gas emissions than bacon’ or that ‘a vegetarian diet produces more greenhouse gas emissions’ (or some variety of this) [1].  As usual, those headlines were dishonest

The paper in question (behind a pay wall) [2] reviewed the energy use, blue water footprint (use of surface water rather than rainwater) and greenhouse gas emissions of various food groups (see below), and used this to estimate the environmental impact of 3 different diet scenarios:

“The three dietary scenarios we examine include (1) reducing Caloric intake levels to achieve ‘‘normal’’ weight without shifting food mix, (2) switching current food mix to USDA recommended food patterns, without reducing Caloric intake, and (3) reducing Caloric intake levels and shifting current food mix to USDA recommended food patterns, which support healthy weight”

In other words:

1.      Lower calorie intake, same foods
2.      Same calorie intake, USDA guidelines
3.      Lower calorie intake, USDA guidelines

It found that lowering calorie intakes (scenario 1) would reduce the environmental impact (obviously), but both scenario 2 and 3 would increase energy use, the blue water footprint and GHG emissions

Energy use
Blue water footprint
GHG emissions
Scenario 1
Scenario 2
Scenario 3

This surprising result is due to the assessment of added sugars and fats as having a lower environmental impact than the fish, dairy, fruits and vegetables that are recommended in the USDA guidelines

One of the important aspects of this paper is that they compare foods on environmental impact per calorie, rather than environmental impact by weight.  Much of the discussion on environmental impact of foods has compared them by weight (100g of X vs. 100g of Y).  The problem with this is that the energy content of food, rather than its weight or volume, is the far more important driver of satiety, and therefore consumption.  This is a major reason (rightly or wrongly) as to how fruits and vegetables score badly, while added sugars and fats score so well.  While environmental impact should be judged per calorie rather than by weight, this outcome shows that the ‘per calorie’ measure isn’t flawless either.  We need energy, but we also need adequate protein and micronutrients.  Insufficient micronutrient intakes are quite prevalent in the US [3] and added sugar and fats provide little to no micronutrients

Finally, it’s surprising that fruits and vegetables scored so badly.  If people grew these at home or purchased them the small local farmers I imagine they would have scored much better, and perhaps would be considered among the most sustainable food groups.  In respect it’s important to remember that this study looked at three different scenarios in the context of the environmental impact of foods in the US food supply.  Whereas in an ideal world the environmental impact of these would all be a lot lower.  1) The energy required for food production and transport would come from wind/solar/etc.  2) Water intensive crops would only be grown in areas with consistently high rainfall, or at least produced without unsustainable irrigation.  3) There wouldn’t be GHG emissions from fuel or fertilisers.  (CO2 emissions produced by plants and livestock are part of the natural carbon cycle and shouldn’t count as GHG emissions.  Methane from enteric fermentation is different, but may only contribute towards 1.2% of global greenhouse gases in CO2 equivalents and 2.5% of US greenhouse gases in CO2 equivalents [4]) 

* Another issue regarding diet and environmental impact is food waste, which has increased in the US in recent decades to about 40% (or ~1400kcal per person per day) [5]

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