From Quora: I was surprised when I was attacked for saying that beef production contributes to global warming worldwide. The attacks came in the form of misinformation, primarily from a white paper by Frank Mitloehner, a professor at UC Davis. Mitloehner’s opinions are published in the form of a white paper, not an article in a peer-reviewed journal. But a group of scientists from Johns Hopkinshas provided a detailed response that fills in some of the details that Mitloehner omitted from his paper.
In looking over their response, I find their points well-taken. Mitloehner concentrates on improvements the meat industry has made in their production methods over the past 50–70 years. But these improvements do not address major problems that will always exist in an industry that slaughters 15 billion animals each year (total of all animals, including chickens).
The US meat industry fails to account for the fact that the US imports large quantities of beef from overseas. Thus, some of the efficiencies claimed by the industry merely result from the export of problems—such as emissions from deforestation and feed-crop production—to other countries. The US currently imports about 2 billion pounds of beef annually.
Mitloehner’s figure of 4.2% greenhouse gas emissions resulting from livestock refers to US figures and doesn't represent the true scale of the problem. GHG is a world-wide problem where it is impossible to separate the US contribution from the rest of the world. World-wide, animal agriculture accounts for 14.5% of GHG, while world-wide production of GHG from transportation is slightly less.
The reason I described that cutting beef consumption is “low-hanging fruit” is because cutting consumption does not require an onerous life-style change. Clearly, since according to Dr. Mitloehner’s paper transportation accounts for 27% of US GHG production, there are many larger cuts that must be made in the transportation sector. These necessary cuts, however, will require lifestyle changes that Americans will find it difficult to make.