Australia: The Land Where Time Began

A biography of the Australian continent 

Methane- Shifting Atmospheric Sources

Concentrations of atmospheric methane, a potent greenhouse gas, have been steadily rising since the Industrial Revolution. Between 1999 and 2006 there was an interruption in this steady rise, a period during which the rise plateaued. As the concentration of atmospheric methane is controlled by both the magnitude of methane being emitted and the availability of the reactants that break down the methane, the cause of this plateau is the subject of debate. For the period of this plateau a full account of the methane budget requires an understanding of volumes of methane that was being emitted to the atmosphere by various sources, as well as the availability of the chemicals in the atmosphere that decompose the atmospheric methane.

According to Newton the carbon isotope composition of the methane goes some way to identifying the sources; largest sources, the burning of fossil fuel, burning biomass and agriculture and emissions from wetlands, all produce methane that has a different isotopic composition. The measurements of the isotopic composition of methane found in ice cores and air samples that have been archived, as well as the composition that has been recorded by monitoring stations were utilised (Schaefer et al., 2016) in order to isolate the factors that contributed to the methane plateau (Science, 352, 80-84; 2106).

Atmospheric concentrations of methane and isotopic composition rose in tandem before 1999, indicating increasing emissions of methane with a relatively higher ratio of ¹³C to ¹²C. This isotopic ratio is consistent with a fossil fuel-like source. During the plateau pereiod this trend broke down and after 2006, methane concentrations started rising again, though the isotopic composition fell. It is indicated by this that the emitted methane had less ¹³C, which is consistent with methane being emitted from a biogenic source. It has been confirmed by box-modelling that the trend that has been observed is best explained by fossil fuel-related emissions being reduced beginning between 1992 and 1993, with possible contributions being from changes in atmospheric reactants, namely hydroxyls, whereas the methane concentrations rise after 2006 is best explained by an increase in biogenic emissions.

It is not possible to readily discriminate between different sources of biogenic emissions, such as ruminant livestock, cultivation of rice, tropical wetlands and Arctic permafrost. Satellite measurements of methane do, however, rule out a substantial increase in methane emissions from the high latitudes; over this time tropical climate changes would be expected to increase wetland emissions, though not in the areas in which the highest levels of emissions have been observed. Contrasting with this, livestock inventory increases and the expansion of cultivation of rice have been reported to have occurred over this interval, and it has been recorded by methane inventories that emissions of methane from agriculture were increasing over the period 2000-2006. Therefore it probably isn’t surprising that such a large influence on the concentrations of methane in atmosphere has been attributed agriculture.

However, it is surprising that fossil fuels appear to have contributed less, especially as there has been recent exploitation of both coal and unconventional gas reserves. It is suggested by these findings that carbon budgeting, and efforts at mitigation, must consider closely the expansion of agriculture, especially if the global trend towards more meat consumption continues.

Sources & Further reading

1. Newton, A. (2016). "Atmospheric methane: Shifting sources." Nature Geosci 9(5): 346-346.