It was with great interest that I discovered the recent article entitled "Metabolic Teamwork between Gut Microbes and Hosts" in the July 2009 issue of Microbe (p. 323). Microbiologists, like myself, who work with gastrointestinal tract anaerobes are always interested in seeing this area of science highlighted in publications such as Microbe. Thanks for the article.
However, there is one serious misstatement in the article that needs to be corrected. On page 326, it is stated that "The 1.5 billion cattle on the planet generate more than 100 million tons of methane per year, which is about 20% of emissions contributing to global warming." The 20% figure cited, which I assume was obtained from the FAO report (Steinfeld et al, 2006, ftp://ftp.fao.org/docrep/fao/010 /a0701e/a0701e00.pdf) is a misstatement as written. It in fact represents 20% of global methane emissions, not total global greenhouse gas (GHG) emissions. There is a difference. The actual contribution of "enteric" methane directly emitted by ruminants and other animals to total global GHG emissions is only 3-5%, a relatively small proportion. By comparison, carbon dioxide from all sources including internal combustion engines continues to make up the bulk of total GHG emissions (approximately 80%).
Data from the Environmental Protection Agency (EPA) supports my statements. For the U.S., total GHG emissions for 2006 were estimated at 7,073 million metric tons CO2 equivalents (Mt CO2 equiv.). Emissions of methane from all sources were estimated at 605 Mt CO2 equiv. (about 8.5%) and as a subset, enteric methane sources contributed one-fifth of the methane total or only 2% of total GHG emissions (http://www.eia.doe.gov/bookshelf/brochures/greenhouse/Chapter1.htm). Similar data for 2007 is also available from EPA (http://epa.gov/climatechange/emissions/downloads/2009GHGFastFacts.pdf). Similarly, total global GHG emissions in 2005 have been estimated at 44,153 Mt CO2 equiv. with total methane emissions estimated at 15% of this total. Livestock and manure sourced methane was estimated at 5% of total GHG emissions or about 2,200 Mt CO2 equiv. (http://www.wri.org/image/view/11147/_original).
Dr. Jouany and Martin (INRA France) who have conducted research in this area also present useful information about rumen methane production and its relative contribution to global warming (http://gaia.agraria.unitus.it/ceuroghg/cftalk9.pdf). They calculate a value of less than 2% for ruminant methane and point out that the carbon dioxide emitted by ruminants contributes as much or more towards global warming than the methane expelled by these animals!
I take exception to the 20% misstatement in the Microbe article because it has been repeated so often that it has taken on the characteristics of an urban myth. Accurate public perception requires that numbers be cited correctly and in the right context so as not to be misleading. If we expect policy makers and the public to address this problem, then it is important that they understand the relative dimensions of the issue. The recent debate concerning a methane tax on cattle is but one example of the public confusion that results from inaccurate and misstated numbers. While reductions in methane emissions can help with the problem of climate change, carbon dioxide has been and continues to be the primary anthropogenic climate forcing agent on which we need to remain focused.
Mark Rasmussen
Odenton, Md.
Dr. William Karasov responds: Thanks to Dr. Rasmussen for this useful correction. His values are in line with the model estimate by the researchers in Australia that shifting from methane-producing ruminant cattle and sheep to nonmethane- producing kangaroos for meat could be useful for decreasing by 3% the Australian contributions to CO2 equivalents (G. R. Wilson and M. J. Edwards, Conservation Lett. 1:119-128, 2008).
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