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.
“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].”