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Methane is a gas formed of carbon and hydrogen (CH4). In its fossil form, it is extracted as natural gas and sold through the gas network. It is also produced through the fermentation of organic material in wetlands, landfills, anaerobic digesters, rice paddies and the guts and manures of ruminants. Some 85% of methane emissions in Ireland come from agriculture.
How is methane’s contribution to climate change measured?
The Intergovernmental Panel on Climate Change (IPCC), which brings together scientists appointed by all governments in the world, has set global warming potential (GWP) values for all greenhouse gases. These figures are all relative to carbon dioxide (CO2), the main gas associated with man-made climate change. They result from complex calculations assessing how much heat each gas retains in the atmosphere as radiation passes through it. The global warming potential over 100 years (GWP100) is most closely watched because scientists and governments say the next century is the timescale we need to watch to bring climate change under control.
Methane’s GWP100 used to be 21, and the IPCC increased it to 28 in 2013. This number is used to convert methane emissions into CO2 equivalent (CO2e). Current EU legislation setting climate change targets for Ireland gives methane a GWP of 25. This means 1t of methane is deemed to cause as much warming as 25t of CO2. Under these rules, 1kg of Irish beef had a carbon footprint of 11.58kg CO2e in 2016, while 1kg of fat and protein-corrected milk emitted 1.14kg CO2e, according to Bord Bia.
Scientists have made new discoveries since the latest GWP values were set – this is where it gets technical.
How long does methane affect the climate?
Methane decays in the atmosphere in about 12 years, compared with centuries for CO2. University of Oxford-led academics have shown that GWP does not reflect methane’s rapidly decreasing greenhouse effect during this period. In a paper published in June, they suggested a new measuring unit, GWP*, which would better reflect the actual effect of cumulative emissions. GWP* uses a formula, rather than a fixed number, to calculate the actual warming effect of each gas over time. They found that unlike CO2, methane increases global warming only if its emissions increase. If they remain stable, no further climate change occurs. If they reduce, this has a cooling effect on the planet. Using GWP* would allow “fairer and more effective design of domestic policies and measures”, the paper concludes.
How does methane trap heat radiation?
Another recent discovery is that methane retains more heat than previously thought. While IPCC calculations account for the trapping of infrared radiation coming up from the Earth, “previous studies had not taken into account that methane absorbs sun rays as they pass through the atmosphere”, Keith Shine, professor of meteorology and climate science at the University of Reading, told the Irish Farmers Journal. His team found that including this effect would boost methane’s GWP100 by 14%.
What does this all mean for farmers?
If the latest scientific findings are confirmed by further research and used in future legislation, the rules of the game could change. Acceptance of GWP* would mean that climate concerns become confined to any additional methane emissions above an existing threshold – the benchmark in current EU legislation is 2005 levels. However, any emissions above this will likely be linked to higher global warming. This could mean that livestock numbers in a reference year such as 2005 become a new quota, unless technologies such as breeding, feed additives or vaccines successfully reduce emissions from ruminants.
The Department of Agriculture’s environment chief inspector Bill Callanan told the Oireachtas climate committee last week that the Department was considering the “evolving science” on methane’s decay in the atmosphere.
“If that’s the case, provided that methane emissions are not going up, it doesn’t have an additional global warming potential,” he confirmed, warning: “That is not at a point when it’s translating into any targets, let’s be very clear on that.” EU legislation in place for 2020 targets and agreed for the next decade remains based on GWP100.
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