Global Atmospheric Methane Cycle
Besides carbon dioxide, methane is the most important anthropogenic greenhouse gas in the atmosphere. Global atmospheric CH4 concentration increased by more than a factor of two since the beginning of industrialisation. Although the atmospheric CH4 abundance is still very small compared to that of CO2 the additional radiative forcing of additional CH4 since 1750 is about 1/3 of that of the CO2 increase. Methane has both, natural and anthropogenic sources and is mainly produced by bacteria in anaerobic environments that have high organic matter content. Such bacterial sources are natural wetlands, rice agriculture, animal breeding (ruminants), landfills, termites as well as aquatic sediments. Other so called thermogenic sources are incomplete combustion of biomass and energy generation where methane is released by extraction, processing and distribution of coal, gas and oil. About 40% of total CH4 emissions come from natural, 60% from anthropogenic sources. The main sink of methane is chemical destruction in the troposphere through reaction with OH-radicals (about 90%). Other sinks which contribute to the removal of tropospheric CH4 are oxidation by soil bacteria and transport into the stratosphere. Here methane is destroyed by reactions with OH, Cl and O(1D) radicals. The stratospheric water content - which plays an important role in the ozone depletion process - originates to about 30% from stratospheric methane oxidation. As most of the sources are located on continents and in the northern hemisphere we observe an interhemispheric gradient in atmospheric methane mole fraction of about 100 ppb (see figure). Atmospheric methane reveals a seasonal cycle, which in the northern hemisphere is in anti-phase to that of the southern hemisphere. These seasonal cycles are caused by changing emissions of the sources as well as changing destruction rates of the sinks.
Methane itself occurs as different isotopologues: 12CH4, 13CH4, 12CH3D and 14CH4 (D = Deuterium = 2H). Other isotopologues are negligible. The atmospheric ratios 13CH4/12CH4 and 12CH3D/12CH4 do, however, not reflect the natural abundance of the constituents because all formation and destruction processes of methane are fractionating isotopes: different sources of methane have different isotopic signatures and the different sinks have different fractionation factors. Therefore the investigation of the long term trend of the isotopic composition of methane provides important additional information about the development of individual methane sources and sinks. In the IUP carbon cycle group we perform continuous measurements of methane mole fraction in ambient air (Heidelberg) since 1994. Measurements of methane mole fraction and isotope-ratios on samples from Neumayer station, Antarctica are performed since 1988, from Alert, Nunavut, Canada since 1990 and from Izaña, Tenerife, Spain from 1991 to 1998 (Levin et al., 2012). The most recent trends of CH4 in the Arctic and Antarctica are displayed in the Figure below.
While CH4 was steadily increasing in both hemispheres until about 2000 it stayed surprisingly stable for almost one decade but in the last decade we again observe global increase rates similar to those in the 1990s. The reasons for this latest development are not perfectly understood, which is also true for the inter-annual variations in the last decades (Schäfer et al., 2016).
Long-term measurements of methane mixing ratios in high Northern (Alert) and high Southern (Neumayer) latitudes.
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