Historical and Contemporary Global Methane Cycling

Abstract

Methane (CH4) has been studied as an atmospheric constituent for more than 200 years. However, the first modern measurements of atmospheric CH4 concentration were made by using the infrared absorption and estimated an atmospheric concentration of 2.0 ppmv in 1948. The development of gas chromatography (GC) and the flame ionization detector (FID) in the 1950s led to observations of vertical CH4 distributions in the troposphere and stratosphere , and to establishment of time-series sampling programs starting from 1979. Results from these sampling programs led to suggestions that similar to atmospheric CO2, the concentration of atmospheric CH4 has been increasing. The data indicated that the atmospheric CH4 concentration has increased almost exponentially from 722 ± 4 ppb at the onset of Industrial Revolution in 1750 to 1650 ppb by mid 1980s when global CH4 emissions monitoring started. The current (2015) global annual surface mean abundance is 1845 ± 2 ppb, and a relative abundance of 256% compared to 1750. The mean annual absolute increase during the last 10 years is estimated at 6 ppb CH4 yr−1. The role of CH4 as one of the greenhouse gas (GHG) causing global warming stimulated further research on sources and sinks of CH4, which is emitted from a variety of sources, both of natural and anthropogenic origin. Natural sources account for about 40% of the total, while anthropogenic emissions contributing 60% of the global emissions. The anthropogenic sources fall under the main categories of agriculture, energy, waste, and industry. CH4 is also of interest to microbiologists, but findings from microbiology have entered the larger context of the global CH4 budget only recently. CH4 is the most abundant hydrocarbon in the atmosphere, and its increase by a factor of 2.5 since the Industrial Era has raised concerns due to the potential effects on atmospheric chemistry and climate. It plays important roles in atmospheric chemistry and the radiative balance of the Earth. Stratospheric oxidation of CH4 provides a means of introducing water vapor above the tropopause. The CH4 also reacts with atomic chlorine in the stratosphere, forming HCl, a reservoir species for chlorine. Some 90% of the CH4 entering the atmosphere is oxidized through reactions initiated by the OH radical, mostly in the troposphere. These reactions are important in controlling the oxidation state of the atmosphere. The CH4 absorbs infrared (IR) radiation in the troposphere and is an important GHG. On per mole basis, CH4 is more effective a GHG than CO2. Current atmospheric burden is estimated at 4954 ± 10 Tg CH4, with the annual increase estimated at 14 ± 3 Tg CH4 yr−1 from both natural (36.3%) and anthropogenic (63.7%) sources.