In 2006, the Food and Agriculture Organisation (FAO) of the UN published a report called Livestock's Long Shadow which concluded that emissions from livestock production make up 18% of global man-made greenhouse emissions, which is more significant that the emissions associated with fossil fuel use in transport. Despite the authors of that report subsequently admitting there was a major flaw in the methodology used, the 'number' has persisted, and continues to appear - even in papers recently published in peer-reviewed scientific journals.
Livestock's Long Shadow is a much-quoted FAO report, the conclusions of which are frequently referenced in papers and articles that are critical of livestock production for a range of reasons. Radical animal welfare organisation such as PETA jumped on the information in the report with glee, as did various organisations promoting vegetarianism. Environmental groups also joined the bandwagon, with some such as the Worldwatch Institute even trying to claim that the FAO report was a vast under-estimation, and that the livestock sector was the source of 50% of global emissions, not just the 18% claimed by the FAO.
However, a number of researchers including Professor Frank Mitloehner have subsequently highlighted a major flaw in the methodology used by the FAO and others, in that they do not compare like-with-like in arriving at total emissions attributed to livestock. The estimates generally assume all livestock production first involves emissions associated with clearing and burning areas of forest to grow the feed, and then includes all possible emissions associated with livestock production including both direct livestock emissions (between 3 and 9% 0f global emissions according to the IPCC) and any indirect emissions assumed to be be associated with any possible transport, processing, refrigeration and related emissions. They then compare that number with only the direct (tailpipe) emissions associated with other activities such as transport, ignoring all the indirect and related emissions. The livestock estimates are also gross emission estimates, and ignore the fact that the carbon emitted by animals was in fact recently sequestered from the atmosphere through plant growth, and if this amount of recently sequestered carbon was deducted from gross livestock emissions, the net emission figure would be considerably less.
One of the authors of the UN report has since admitted that the initial analysis was flawed, (and here), and it was reported that the FAO would re-do the analysis, and issue a corrected report by the end of 2011. While the revised report is yet to appear, the relevant FAO webpage now states that livestock account for 9% of global anthropogenic emissions, not the 18% originally claimed (or the grossly exaggerated 50 + % used by some groups).
However, that has not stopped reference being made to the original data (and the exaggerated 50% figure) with news that a new paper soon to be published in a 'peer-reviewed' scientific journal makes the statement that "Reducing consumption of red meat could have significant environmental benefits, the paper says, citing estimates that as much as 51 per cent of the world's greenhouse gas emissions come from livestock farming." If such a statement has passed peer-review, there must be some real questions about the quality of the reviewers involved.