Sunday, May 11, 2008

Shifting diets vs. eating local

I finally had the chance to read this terrific paper by Christopher Weber and Scott Matthews of Carnegie Mellon that compares greenhouse gas emissions from the production different types of food and the delivery of that food to your plate. As was reported by some news agencies and blogs, Weber and Matthews conclude that transportation represents only 11% - on average, it depends on the food – of the total life-cycle GHG emissions of U.S. food (there’s little reason to expect a dramatically different result in Canada).

The take home message is shifting your diet will do far more to reduce greenhouse gas emissions than buying local. From the paper:

The results of this analysis show that for the average American household, “buying local” could achieve, at maximum, around a 4-5% reduction in GHG emissions due to large sources of both CO2 and non-CO2 emissions in the production of food. Shifting less than 1 day per week’s (i.e., 1/7 of total calories) consumption of red meat and/or dairy to other protein sources or a vegetable-based diet could have the same climate impact as buying all household food from local providers.

The authors did some simple calculations to demonstrate this point:

To put these figures into perspective, driving a 25 mi/gal (9.4 L/100 km) automobile 12 000 miles/yr (19 000 km/yr) produces around 4.4 t CO2/ yr. Expressed in this manner, a totally “localized” diet reduces GHG emissions per household equivalent to 1000 miles/yr
(1600 km/yr) driven, while shifting just one day per week’s calories from red meat and dairy to chicken/fish/eggs or a vegetable-based diet reduces GHG emissions equivalent to 760 miles/yr (1230 km/yr) or 1160 miles/yr (1860 km/yr), respectively. Shifting totally away from red meat and dairy toward chicken/fish/eggs or a vegetable-based diet reduces GHG emissions equivalent to 5340 mi/yr (8590 km/yr) or 8100 mi/yr (13 000 km/yr), respectively.

It is important to note that macro-scale GHG “accounting” studies always come with a number of caveats. The calculations or model requires a number of simplifying assumptions and often some more complicated factors are often excluded. The two central limitations to this particular study appear to be the simple treatment of direct GHG emissions from animal and crop production (i.e. N2O from manure, fertilizer and the animals themselves) and the exclusion of land use impacts and the “land use cascade” (i.e. carbon released from directly or indirectly transforming land for crop or animal production). Improving those components of their mode should increase the share of GHG emissions from food production and the relative GHG emissions from red meat production. In other words, that provides even more support for the conclusion that eating less beef is one of the best ways to reduce personal greenhouse gas emissions.


Anonymous said...

Not that we should concentrate solely on carbon emissions, of course. There are other resources involved in the production of food which also need to be taken tinto account. Labour costs, land usage, that sort of thing.

This is why the price of goods is so much more important than the carbon footprint.

Unknown said...

A valid point, though maybe for a different post. This paper is not about the food crisis or prices. It addresses a particular technical question, raised by the common argument that "eating local" reduces GHG emissions.

Anonymous said...

Good Job! :)