I was asked in email if I'd calculated the carbon footprint of milk, along with the footprint of a cheeseburger, and I realized that I had most of the figures I'd need already in hand.
[Caveat: In order to fully calculate the carbon footprint of milk, I'd need to have numbers on energy consumed in growing cattle feed for dairy cows, processing the milk (pasteurization, bottling, etc.) and transporting the results; however, the main carbon impact will be from the methane produced by the cattle, so that's where I'll focus.]
This calculation should be reasonably straightforward -- a first pass estimate would be
(Amount of methane produced by a milk cow in a year) / (Amount of milk produced by a milk cow in a year)
(242 lbs of methane)* / (19,951 lbs of milk)** = 0.012 lbs of methane per lb of milk
1 gallon of milk = ~8.5 lbs, so
1 gallon of milk = 0.102 lbs of methane
1 unit methane = 23 units of CO2 in terms of greenhouse impact, so
1 gallon of milk = 2.35 lbs of CO2 equivalent
Add in a pound of CO2 equivalent for processing and transportation (as a very crude figure), and that ends up to be about 3.35 pounds of CO2 equivalent per gallon of milk. Call it three-and-a-half pounds of CO2 equivalent per gallon of milk.
Or, in metric:
1 liter of milk = 282 grams of CO2 equivalent in methane. With transport & processing, call it about 400 grams of CO2 equivalent per liter of milk.
As with the cheeseburger figures, these are very fuzzy numbers, and the actual amount will vary on the basis of how much carbon is produced farming and transporting cattle feed, collecting and pasteurizing the milk, and trucking the gallons to your local grocery store -- along with how much milk a given cow produces (which can vary considerably).
Repeat after me: everything we do has some level of a carbon footprint. The achievable goal is to make smart choices about what kind of footprint we'll make, and what we'll choose to avoid. The first step in making a smart choice is understanding one's options. So drink milk. Eat a cheeseburger. Heck, drive a big truck. But don't do it without accepting the consequences.
* Methane amount: http://www.epa.gov/rlep/faq.html (in kilos, 1 kilo = 2.2 lbs)
** Milk per cow: http://usda.mannlib.cornell.edu/usda/current/MilkProdDi/MilkProdDi-04-27-2007.txt