Since 1999 atmospheric methane concentrations have leveled off while the world population of ruminants has increased at an accelerated rate. Prior to 1999, world ruminant populations were increasing at the rate of 9.15 million head/year but since 1999 this rate has increased to 16.96 million head/year. Prior to 1999 there was a strong relationship between change in atmospheric methane concentrations and the world ruminant populations. However, since 1999 this strong relation has disappeared. This change in relationship between the atmosphere and ruminant numbers suggests that the role of ruminants in greenhouse gases may be less significant than originally thought, with other sources and sinks playing a larger role in global methane accounting.
On November 17, 2003 National Oceanic and Atmospheric Administration reported that the concentration of the potent greenhouse gas methane in the atmosphere was leveling off and it appears to have remained at this 1999 level (Figure 1). The Intergovernmental Panel on Climate Change in 2007 acknowledged that methane concentrations have plateaued, with emissions being equivalent to removals. These changes in methane atmospheric dynamics have raised questions about the relative importance of ruminant livestock in global methane accounting and the value of pursuing means of further suppressing methane production from ruminants. At this time there is no relationship between increasing ruminant numbers and changes in atmospheric methane concentrations changes, a break from previously assumed role of ruminants in greenhouse gases (Figure 1).
Over the last three centuries, atmospheric methane concentrations have increased by approximately 2.5 times pre-industrialization levels, reaching concentrations not seen in at least 650,000 years. In atmospheric samples collected between 1979 and 1999 methane concentrations increased by 10.8 ppb/year. This elevation in methane concentrations has been attributed to expansion in agricultural and industrial activities, including livestock farming, rice cultivation, mining of fossil fuels, reticulation of natural gas, and large scale burning of forest and grassland biomass.
Increasing methane concentrations have been identified as the second largest contributor to global warming, with one ton of methane being equivalent to 21 tons of carbon dioxide in the atmosphere with respect to changes in global temperatures. Consequently, when the shorter half life of methane is accounted for, methane has eight times the effect of carbon dioxide in the atmosphere. Thus, methane was included in the Kyoto Protocol with 1990 chosen as the base year for future decisions concerning the impact of mitigation efforts. Professor Aslam Khalil, at the Portland State University, in an analysis of more than 20 years of atmospheric sampling, concluded that “global emissions and the lifetime of methane in the atmosphere have been constant, so the buildup of methane in the atmosphere has been slowing for as long” Currently the level of emissions is equivalent to the removals and atmospheric concentrations are leveling off at the historically higher concentrations. This report also states that “major sources such as rice agriculture and cattle have little or no potential for large increases in the future and have already shown reduction in emissions from some regions. Seeing that the total source has remained constant for at least the last two decades, it is questionable whether human activities can cause methane concentrations to increase greatly in the future.” Since 1999, there has been a non significant atmospheric increase of 0.3 ppb methane/year. This contrasts with the 10.8 ppb/year for the previous time period of 1979 to 1999.
World ruminant numbers are still increasing after a temporary pause in the late 1990’s. This temporary pause is most likely a result of a turn down in the wool industry and subsequent reductions in wool sheep flocks around the world (Figure 2). For the time period 1979 to 1999 the large ruminant equivalent population increased at the rate of 9.15 million head/year but since 1999 the growth rate has increased to 16.96 million head/year.
In the global balance of methane, ruminants accounted for 15.7% of global and 25.7% of anthropogenic methane production. Between 1979 and 1999 period there was a strong relationship between large ruminant numbers and increasing atmospheric methane concentrations, with methane levels rising by 1.15 ppb for every one million increase in large ruminant equivalents.
The Kyoto agreement and its subsequent outcomes placed significant pressure on the ruminant industries to reduce the amount of methane produced. Considerable time and research effort has been directed towards measuring and decreasing ruminant methane. To date, these research efforts to reduce ruminant methane production have not produced an industry-wide means of suppressing methane production for a given diet; although our knowledge of methane production by ruminants has increased considerably. For example, researchers have concluded that increasing the quality of the diet decreases the amount of methane produced by rumen micro-organisms.
Despite this lack of success in reducing ruminant methane production, since 1999, the link between atmospheric methane and ruminant population growth seems to have broken down. This has occurred despite accelerated increases in ruminant numbers without an equivalent increase on global methane concentrations within the current time frame. One explanation for this trend may be the improvements in animal husbandry and feeding that have occurred since the Second World War in developing countries. The IAEA has supported a number of projects to evaluate local feeds in Member States and formulate higher quality rations. These projects have resulted in improved productivity of livestock, perhaps they may also be contributing to reducing the impact of ruminants on the environment. This suggestion is not meant to dispute the issue of global warming. However, it does suggest a need reassess the contribution of livestock production to the entire process.