Decarbonisation Technology May 2022 Issue

Global methane emissions

Natural Agriculture Waste Fossil fuel Biomass and biofuel burning

18%

Global anthropogenic methane emissions

8% 9%

39%

Rice cultivation Livestock fermentation & manure Landll & waste Fossil fuel extraction and use Biomass & biofuel burning

31%

33%

Total emissions: 596Tg (range 572–614)

11%

19%

Total anthropogenic emissions: 354Tg (range 340–381)

27%

Figure 2 Global methane emissions, 2017 Source: (Saunois, et al., 2020): data used 2017 “top-down” (1Tg=1Mt)

by a separate US-China cooperation agreement to develop additional measures to enhance methane emission control before COP27. China intends to develop a comprehensive and ambitious National Action Plan on methane, aiming to significantly affect methane emissions control and reductions in the 2020s (US Department of State, 2021b). Atmospheric methane concentrations Atmospheric concentrations of methane (CH 4 ) averaged about 500 ppb (see Figure 1 , left) for hundreds of thousands of years but, with the onset of industrialisation, increased to over 1800 ppb by 2019 (see Figure 1 , right, EPA, 2021a). Although these concentrations are at least an order of magnitude lower than carbon dioxide (CO 2 ), the global warming potential of methane over 100 years (GWP 100 ) is 28-36 that of CO 2 . Natural sources of methane Methane is formed biogenically as a result of anaerobic decomposition of organic matter in wetlands and forests. Over time, substantial deposits of natural gas, primarily methane but with lesser amounts of ethane (C 2 H 6 ), propane (C 3 H 8 ), butane

(C 4 H 10 ), and pentane (C 5 H 12 ), have formed from thermogenic decomposition in underground rock formations under the surface of the Earth, often in association with other hydrocarbon reservoirs including coal and oil. Methane hydrates occur in oceanic sediments as methane clathrate (CH 4 )4(H 2 0) 23 . Methane can be released naturally from these deposits (Skarke et al ., 2014). Methane is also formed pyrogenically by the combustion of biomass, for example, from forest fires caused by lightning strikes. Natural methane sinks, such as the reaction with hydroxyl (OH - ) radicals within the troposphere, oxidation by methanotrophic bacteria and other chemical reactions with oxygen and free chlorine in the atmosphere, maintained an equilibrium with natural methane emissions (IPIECA, 2021a). By 2017, natural sources only accounted for 39% of global methane emissions (see Figure 2 ). Anthropogenic emissions of methane Anthropogenic emissions reached 62% of global methane emissions, or 1.6 times the natural emissions by 2017, while methane sinks removed an estimated 96% of total (natural and anthropogenic) emissions (Global Carbon Atlas, 2020). Anthropogenic emissions are considered