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DO CATTLE REALLY INCREASE METHANE IN ATMOSPHERE?

by: David South
Auburn University

Interested in greenhouse gasses produced by cows? If so, you might be interested in this news about methane. Water vapor, methane, carbon dioxide and nitrous oxide are the four primary greenhouse gasses. From 1990 to 2004, the total U.S. emissions of carbon dioxide increased by 19.9 percent while methane emissions decreased by 10 percent. During animal digestion, methane is produced through enteric fermentation, in which microbes residing in animal digestive systems break down food and produce methane. Ruminants, which include cattle, buffalo, sheep, and goats, have a large fore-stomach where methane-producing fermentation occurs. In 2004, methane produced by ruminates contribute only 1.6 percent of the major 'greenhouse gas emissions in the U.S. From 1990 to 2004, methane production by enteric fermentation declined by 4.5 percent.

Until recently, some claimed the level of atmospheric methane was related to the number of ruminants in the world (mostly cows, sheep and goats). It seemed as though for each additional billion large ruminants (where 8 sheep = 1 large ruminant), atmospheric methane increased by 1 ppb (parts per billion). Between 1979 and 1999, methane concentrations increased by 10.8 ppb per year, and large ruminant numbers increased by about 8.7 billion per year. However, this apparent correlation is unrealistic since, globally, ruminants only account for about 26 percent of the methane emissions that result from human activities. Methane is also produced by landfills (the largest US human-caused source of methane) and the production of natural gas (the second largest US source). In addition to animal produced methane (the third-largest US source), methane is produced by coal rnining, manure management, treatment of wastewater, rice cultivation, mining of fossil fuels and burning of forests and grasslands.

However, around 1999 the world apparently changed, and the increase in methane in the atmosphere has slowed. On November 17, 2003, the National Oceanic and Atmospheric Administration reported that the concentration of methane in the atmosphere was leveling off and it appears to have remained at about the same level as in 1999 (there has been a nonsignificant increase of 0.3 ppb/year). Now it seems the strong relationship between number of cows and methane that once existed has vanished. Since 1999, it seems as though for each additional billion ruminants, methane now increases by only 0.005 ppb.

So what caused the change? In short, we do not know. Perhaps the oceans are cooling slightly and releasing less methane. If our global warming models were good enough, we would know exactly why the increased slowed. However, even the best computer modelers did not predict this leveling off. They likely still assume a continued increase in atmospheric methane for the next 22 years (which some journalists are quick to report). For example, a recent report, by an organization that won the 2007 Nobel Peace prize, said "If methane emissions grow in direct proportion to increases in livestock numbers, then global livestock-related methane production is expected to increase by 60 percent up to 2030 (FAO 2003)," The slow increase in methane now raises questions about the accuracy of this prediction by Nobel laureates.

From 1979 to 1999 there appears to be a correlation between global atmospheric methane and number of cattle equivalents (includes sheep and goats). However, this relationship did not continue from 1999 to 2006. Data were adapted (with permission) from the Animal Production & Health Section Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Department of Nuclear Sciences and Applications International Atomic Energy Agency. For more information on this subject, go to www-naweb.iaea.org/nafa/ aph/stories/2008-atmospheric- methane.html

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