“If you use as the measure the atmospheric totals, or increase in atmospheric totals, then we know exactly what they are to within a couple percent for all these gases,” he said.

Some uncertainty comes into play when talking about the amount of certain gases that comes from anthropogenic versus natural sources, however. Take nitrous oxide. Scientists know exactly how much is in the atmosphere and that the amount is increasing steadily. “But we cannot attribute that global increase accurately to different sources, nor can we disaggregate that N2O geographically,” Pacala said. The situation is similar for methane and the so-called F-gases (HFCs, PFCs, etc.). Carbon dioxide, on the other hand, is very well understood and scientists have a firm handle and how much is being added to the atmosphere from various sources.

Because carbon dioxide accounts for slightly over three-quarters of the IPCC’s estimate of total global greenhouse-gas emissions (based on a 100-year global warming potential), that estimate is fairly reliable. Nonetheless, Pacala said, it might be better to base the relative contribution of industries like livestock on total carbon-dioxide emissions alone.

“If you really want to do people a service, you just take as a denominator something that is indisputably known, like fossil CO2 emissions,” he said. “And you say, okay, we’re going to evaluate everything relative to that. Then the uncertainty is in the thing you’re talking about – not in something else.”]

Given the availability of such information and perspective, it is unfortunate that reporters did not do more to analyze the dispute over the 18 percent figure. On the other hand, the conflict narrative isn’t ideal either, and there might be a richer story to be found in places where Mitloehner and Gerber see eye-to-eye.

For all his skepticism, Mitloehner concedes that, globally, the 18 percent may or may not be that far off. But, applied regionally, it is certainly a misleading number, he argues, and that is his main beef (pun intended) with the figure.

“In Paraguay, the contribution of livestock may be as high as 50 percent because they are clear-cutting a lot of forest, and that basically takes a unit of [greenhouse-gas] sequestration away and puts cattle, which is an emissions source, there instead. In the U.S. the contribution from livestock is only around 3 percent of the total,” Mitloehner said. “So, I don’t think that the 18 percent number is all that meaningful because it doesn’t apply regionally. It does not apply to Ethiopia, it does not apply to Paraguay, and it does not apply to the United States. Yet it’s being used in these countries [to influence policy and consumer choices].”

On this point, Gerber agrees. “[The 18 percent] is a global figure that covers a wide variety of farming systems and development objectives,” he said. “Different countries have different first priorities in terms of feeding their people, or preserving environment, or protecting public health. So, using that 18 percent figure to try to draw conclusions for the U.S., or for the U.K., or for the Sudan is of course wrong.

“This is why there is a major effort here now, which is requiring much more work, to disaggregate this 18 percent into various commodities – eggs, milk, beef, poultry, and so on – in different farming systems and different regions to understand where are the emissions and where are the big pockets of reduction potential. We’re also coupling that with economic work to see what the most cost-effective options are to reduce environmental impact.”

Mitloehner agrees that the FAO is taking admirable steps to improve its reports. He called the press’s single-minded focus on conflict between him and the organization unfortunate, and stressed that despite its faulty comparison between livestock and transportation emissions, the FAO is doing valuable research.

“We are predicting a massive increase in the demand for animal protein, so the question is: how do we satisfy that demand without having a major environmental impact? That’s why what the FAO is doing is so laudable,” Mitloehner said. “Livestock’s Long Shadow’ was a good first attempt and they are working on follow-up studies right now. I’ve had the privilege to look at some of them and they are much better because they are refined by region. So, the follow-up reports will talk about how the United States livestock industry compares to those in, say, sub-Saharan Africa, or India, or Brazil, and why some areas have a higher environmental impact per unit of meat or milk produced than others.”

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