(Mitloehner’s talk was based on a peer-reviewed paper he co-authored last fall. The press release issued by UC Davis noted that the report “was supported by a $26,000 research grant from the Beef Checkoff Program, which funds research and other activities, including promotion and consumer education, through fees on beef producers in the U.S. Since 2002, Mitloehner has received $5 million in research funding, with 5 percent of the total from agricultural commodities groups, such as beef producers.” The more recent release from the American Chemical Society did not include this detail, and neither did many news articles, with the exception of a blog post in the Guardian. The post made it clear that the author did not think Mithloehner was “in the pocket of Big Beef,” and that his criticisms of the FAO merited further investigation, but rightly criticized other outlets for overlooking an important detail of the story.)

So what about that 18 percent figure, then? Mitloehner is indeed skeptical about its accuracy, but he does not claim to be certain that it is wrong. He suspects that the FAO did not do a lifecycle analysis of the transportation industry because the data needed to do so is simply not available (which the FAO later confirmed). Without that information, Mitloehner argues, a comprehensive analysis is difficult.

“If you want a global assessment, then you really need those detailed emissions numbers,” he said. “We might have them for the United States, Germany, and Britain, and so on, but I question whether we have them for Paraguay, Ethiopia, and Afghanistan. And if you don’t know what the pieces of the pie are, then how do you know what 100 percent is? And if you don’t know what 100 percent is, then how can you say livestock is 18 percent?”

Gerber, the livestock policy officer at the FAO and co-author of “Livestock’s Long Shadow,” conceded that the comparison between livestock and transportation emissions was deeply flawed, but defended his organization’s estimate of the former’s relative contribution to total global emissions. “We still stand entirely behind the 18 percent figure,” he said.

The FAO will post a “technical note” (and issue a press release) on its Web site this week, responding to Mitloehner’s criticism and explaining what was and wasn’t included in its assessment. Gerber argues that the Intergovernmental Panel on Climate Change’s figure for the total global emissions, which the FAO used for its calculations, is accurate, and that because the FAO used the same methods as the IPCC to do its lifecycle analysis of the livestock industry, comparing its subset to the total is fair. The lack of a lifecycle analysis for the transportation industry doesn’t change that, he added, arguing that such analyses have a greater effect on how the emissions pie is carved than on the size of the pie.

“In the IPCC’s analysis, you have everything—all transport, all agriculture, deforestation, manufacturing. You name it; it’s there,” Gerber said. “But if you were to add up lifecycle emissions for every industry, you would come up with more than 100 percent because of overlap [in other words, part of the livestock industry involves transportation, and part of the transportation industry involves livestock, so certain activities get counted more than once]. In depends on how they’re partitioned. So, lifecycle analyses are mostly a problem only when trying to compare different sectors.”

For instance, Gerber said the IPCC attributes only about 5 percent of global emissions to livestock (out of roughly 14 percent from agriculture overall) because it did not perform a lifecycle analysis like the FAO did.

Nonetheless, a recent report from the National Research Council (which advises the government on matters of science), titled “Verifying Greenhouse Gas Emissions: Methods to Support International Climate Agreements,” lent some credence to Mithloehner’s concerns about uncertainty in the IPCC’s estimate of total global emissions:

In many developed countries, uncertainties are reported to be less than 5 percent for national CO2 emissions from fossil-fuel use, which is the dominant source. With the exception of a few minor sources in the industrial sector, uncertainties are much higher for other greenhouse gases and sources and vary greatly by country. Uncertainties for the net CO2 emissions from agriculture, forestry, and other land uses and for emissions of CH4, N2O, PFCs, HFCs, CFCs, and SF6 from all sectors can be less than 25 percent in some countries and greater than 100 percent in others.

Stephen Pacala, an ecologist at Princeton University who chaired the committee that published the report on verifying emissions, couldn’t be reached to comment on the merits of Mitloehner’s and Gerber’s arguments.

[Update, 3/30: Reached today, Pacala said that we know, unequivocally, how much greenhouse gas is in, and is being added to, the atmosphere each year.

Curtis Brainard writes on science and environment reporting. Follow him on Twitter @cbrainard.