More data

Nothing gets by you guys! Patrick--who remains curiously astute after knocking back a few--rightly gets to the heart of the matter, which is the comparative greenhouse gas intensity of different kinds of foods. Luckily, the study (pdf) underlying the Ezra Klein post has a chart that captures precisely this information:
(You may need to click the image to read it without squinting.)

Now, I'm not any kind of trained statistics person but I'm pretty sure that by "normalization by X" they mean the greenhouse gas (GHG) output expressed as a rate of "CO2-equivalent emitted per X". So for example, the second dataset means that red meat results in about twice the amount of GHG per dollar as chicken/fish/eggs--i.e., $5 worth of red meat has the same footprint as $10 worth of chicken/fish/eggs.

I imagine the measure we ought to be most interested in is the GHG per kCal figure (third dataset), since we are trying to figure out how to lessen the GHG impact of our diets regardless of cost and, er, food weight. Viewed this way, cows are definitely the biggest culprit: red meat looks to be about 2.5 times more GHG-intensive per kCal than chicken/fish/eggs, and dairy is slightly worse than chicken/fish/eggs as well. So the biggest gains, it seems, can be had by replacing most of our red meat with white meat or vegetables, and replacing dairy with...not dairy.

(Note: some greenhouse gases affect the climate more than others--hence the generic unit of measurement "CO2e", or CO2-equivalent.)

Anyway, in case all that wasn't very clear, here's the abstract to give you the main idea:

Despite significant recent public concern and media attention
to the environmental impacts of food, few studies in the
United States have systematically compared the life-cycle
greenhouse gas (GHG) emissions associated with food production
against long-distance distribution, aka “food-miles.” We find
that although food is transported long distances in general (1640
km delivery and 6760 km life-cycle supply chain on average)
the GHG emissions associated with food are dominated by the
production phase, contributing 83% of the average U.S.
household’s 8.1 t CO2e/yr footprint for food consumption.
Transportation as a whole represents only 11% of life-cycle
GHG emissions, and final delivery from producer to retail
contributes only 4%. Different food groups exhibit a large range
in GHG-intensity; on average, red meat is around 150% more GHG-
intensive than chicken or fish. Thus, we suggest that dietary
shift can be a more effective means of lowering an average
household’s food-related climate footprint than “buying local.”
Shifting less than one day per week’s worth of calories
from red meat and dairy products to chicken, fish, eggs, or a
vegetable-based diet achieves more GHG reduction than buying
all locally sourced food.

Where does he get those wonderful charts?

You can always count on Ezra Klein for a good chart. Here is one that breaks down the greenhouse gas impact of different kinds of food as it moves from the production phase to the transit phase:

As you can see, the first three colors--the ones that cover the greenhouse gas impact of the transportation of the food from the farm to the supermarket--are just a small sliver compared to the amount of greenhouse gases emitted during food production. In fact, according to this report, a mere 11% of greenhouse gases emitted because of food consumption are attributable to the transportation of the food. And so this whole "locavore" movement, while tasty, does not have much of a ecological impact.

If you really want to cut back on the ecological impact of your diet, you'd do well to keep cows out of the picture, as they account for about half of all greenhouse gases emitted from food production:


So all you vegetarians out there, this is your free pass to take longer, hotter showers.

Ok that got weird.

EDIT: Eric sensibly asks how much of the total greenhouse gas emissions are caused by food. The report says: "For perspective, food accounts for 13% of every U.S. household's 60 t share of total U.S. emissions; this includes industrial and other emissions outside the home." So food transportation accounts for 11% of 13%, or 1.4% of every U.S. household's emissions.