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METHANE & MICROBES With ruminants getting bad press at the moment over their methane-filled belches, Sally Morgan looks at the science and the critical role of methane-eating microbes Are ruminants really to blame for the rapid increase in methane levels over the last decade? 22 Organic Farming Winter 2020 There is no denying methane is a significant contributor to climate change. Its global warming potential is said to be as much as 28 times that of carbon dioxide, but it has a much shorter life of around 12 years. The concern about methane is the sudden surge in levels since 2008. It came out of nowhere and scientists are still unsure of the cause. Current levels are just under 1,900 parts per billion, contributing an estimated 20 per cent of total atmospheric warming to date. Commentators have been quick to put the blame on ruminants, a blame game that started back in 2006 when the UN Food and Agriculture Organisation published a study entitled Livestock’s Long Shadow. The study claimed that livestock production was responsible for 18 per cent of all greenhouse gas emissions (a figure which has since been revised down considerably). Since then, many organisations have called for a reduction in meat consumption and a switch away from extensive systems to intensive ones, in which animals are raised quickly on grain and soya-based diets that generate
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less methane. However, it’s becoming increasingly apparent that the methane story is far from clear cut. Methane sources There are various sources of methane, both natural and anthropogenic, with the latter being responsible for about 60 per cent of global methane emissions. Methane is released by microbes called methanogens during microbial decomposition of organic matter, especially that which takes place under water in anaerobic conditions as found in wetlands and paddy fields. It’s also released by microbes living in the gut, in particular, the rumen of ruminant mammals and the gut of insects, such as termites. Methane is released during the decomposition of natural gas, the socalled geological methane, which is stored in underground reservoirs and reaches the surface via seepage, geothermal and hydrothermal activity etc, along with climate-active ethane and propane. Other sources include fossil fuel extraction, fracking, mining, landfill, waste management and the manufacture of nitrogen fertiliser (see Organic Farming 131, Summer 2019). Interestingly, atmospheric methane levels did not rise between 1999 and 2006 despite a 70 per cent increase in livestock numbers and looking further back to times when vast herds of ruminants were grazing grasslands across the world, methane levels ranged between 350 and 750 ppb. So what’s caused the dramatic rise over the last 12 years? There's much finger 23 Organic Farming Winter 2020

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less methane. However, it’s becoming increasingly apparent that the methane story is far from clear cut. Methane sources

There are various sources of methane, both natural and anthropogenic, with the latter being responsible for about 60 per cent of global methane emissions. Methane is released by microbes called methanogens during microbial decomposition of organic matter, especially that which takes place under water in anaerobic conditions as found in wetlands and paddy fields. It’s also released by microbes living in the gut, in particular, the rumen of ruminant mammals and the gut of insects, such as termites. Methane is released during the decomposition of natural gas, the socalled geological methane, which is stored in underground reservoirs and reaches the surface via seepage, geothermal and hydrothermal activity etc, along with climate-active ethane and propane. Other sources include fossil fuel extraction, fracking, mining, landfill, waste management and the manufacture of nitrogen fertiliser (see Organic Farming 131, Summer 2019). Interestingly, atmospheric methane levels did not rise between 1999 and 2006 despite a 70 per cent increase in livestock numbers and looking further back to times when vast herds of ruminants were grazing grasslands across the world, methane levels ranged between 350 and 750 ppb. So what’s caused the dramatic rise over the last 12 years? There's much finger

23 Organic Farming Winter 2020

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