What are the greenhouse gas benefits of vegetarianism?

Question

I'm looking for a range, or better yet, likelihood distribution, compared to:

1. The average US meat consumer.
2. The average world meat consumer.

How significantly are those averages affected by:

1. Distance the meat travels to plate.
2. Food source of the meat. For instance, are these numbers greatly impacted if the livestock is fed with organic feed, or range grass vs. meal
3. The types of meat consumed. I know beef much more GHG intensive than chicken. But how much and where does pork fit in?
4. Any other factors you can identify.

Answer 1

Vegetarian Diet vs Avg US Meat Consumer

Brighter Planet did a report on the American Carbon footprint.

It gives the following averages for US diet types:

• omnivore - ~6500 lbs CO2 per person per year
• vegetarian ~5100 lbs CO2 per person per year
• vegan ~4400 lbs CO2 per person per year

That's over a 20% improvement for vegetarianism over a meat diet and over a 30% improvement for vegans (this assumes equal caloric intake). Note I eyeballed those numbers off graphs.

Also, fairly important, I saw no mention of emissions associated with deforestation from clearing land for pasture. This could be significant and imply a more significant GHG reduction from a vegetarian diet.

Vegetarian vs Avg Global Meat Consumer

A report undertaken by two World Bank environmental advisers, estimated that the global livestock industry (lifecycle supply chain) accounts for 51% of global CO2 emissions. This seems really high. The UN FAO put the figure at 18%, but report cited above includes respiration, land use change, undercounted methane, increased meat consumption between the dates of the reports (2002 and 2009), and several other smaller factors.

I couldn't find an emissions per calorie figure for the globe, however, the ratios probably aren't too different from above. What's important at the global scale is meat consumption now stands at 322 g/day/capita in the US. The global figure is 115 g/day/capita. That means the world outside of the US is relatively vegetarian. However, as incomes climb, consumers eat more meat (though at a diminishing rate).

All in all, the world is shifting towards a more meat dense diet and thereby raising food emissions significantly (stats above from UNEP GHG Emissions Associated with Meat Production.

Vegetarianism vs Locavorism

A paper from Environmental Science and Technology, which earned ACS honors for top paper in environmental policy, claims that 83% of food emissions are in production - only 11% of lifecycle emissions are caused by transportation. They conclude:

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.

Shifting away from red meat, especially to a vegetarian diet, is a stronger lever towards reducing GHG emissions.

Importance of Livestock Feed This could actually be pretty significant for beef. Haven't found a scientific figure, a prof from Dalhousie University claimed an interview with Science News, the significance could be on the order of 50%. That would make it competitive with some fruits and vegetables.

Relative emission intensities of meat products The same report from Brighter Planet gives the following GHG intensities per calorie (g CO2/calorie) by food group:

• red meat : 11
• poultry: ~5.5
• fish ~ 7.5
• dairy ~ 6.5
• vegetables and fruits 5-6

Conclusions

• Vegetarianism has non-negligible GHG benefits (less so with poultry)
• The difference is most stark with red meat
• Differences in meat type and agricultural practices may be as significant as the difference between the average meat diet and vegetarianism.
• Composition of diet is more important than location of food sources.

Answer 2

This is a partial answer, but it's too long to be a comment.

It is not clear to me that there are greenhouse benefits.

1. Transport: Most of the weight in transported food is water. A truckload of carrots is going to cost close to the same as a truckload of hamburger. A vegetarian diet tends to have a lot more fiber in it. I would expect that a such a diet would result in increased transport.

2. Range vs feedlot. Feedlot beef obviously has all the vegy costs plus all the meat costs. Range fed meat is often done on land that is not suitable for farming, either because of terrain (too steep, rocky) or climate (too dry, too wet, too cold)

3. As a lead in to the GHG component, look at the conversion ratios -- how many pounds of feed does it take to produce a pound of meat? Pigs are between chickens and cows. Rabbits and guinea pigs are better than chickens. Cows in particular produce methane in large quantities, but I suspect that all animals that can digest cellulose will do the same. Methane has a residence time of about a century in the atmosphere, so while it is a greenhouse gas, it is not here forever.

World Vs. US

It may be more appropriate to make this comparison on the basis of First world vs rest of world

Optimizing factors:

1. Range fed meat is better than vegetarianism for GHG
2. Local obviously trumps long distance. I would hate to be on a vegetarian diet on what can be grown locally. Never have an avocado or a peach again? 5 months a year without green stuff? Only greenhouse tomatoes (Is that a win? Depends on greenhouse design, but in our climate (central Alberta) I suspect that shipping from California is a Greener choice than a greenhouse in winter.)

The more I look at this, the less I see that vegetarianism is the main factor. Regionality, appropriate land use, availability of transport all are at least as significant.

So perhaps you want to start over and rework the question as

"How can our diet be optimized to reduce to production of greenhouse gasses?"