16th October 2013.
A review of global methane sources and sinks has recently been published in Nature Geoscience by an international group of scientists. They attempted to explain decadal changes in atmospheric methane concentrations using a combination of bottom-up and top-down estimates. Methane concentrations increased by 12 ppb per year in the 1980s, but this rate dropped to 6 ppb per year in the 1990s, before stabilising between 1999-2006. Since then, concentrations have once again started to rise.
Global methane sources are grouped into three categories: biogenic (methanogens), thermogenic (released from natural seeps and fossil fuel usage) and pyrogenic (released from soil and biomass during wildfires, as well as from fossil fuels and biofuels). The main methane sink (90%) is oxidation by hydroxyl radicals in the troposphere. Other small sinks include soil methanotrophy, reactions with stratospheric chlorine and oxygen radicals, and reactions with tropospheric chlorine radicals from sea salt.
The results of the review showed that the main natural emitter of methane during the 2000s was natural wetlands (a bottom-up approach estimated 217 Tg per year), with lakes and rivers (40 Tg per year), and geological sources (54 Tg per year) also being important contributors. Considering anthropogenic sources, agriculture and waste (200 Tg per year) released more methane than fossil fuels (96 Tg per year). Tropospheric hydroxyl reactions, the primary sink, were estimated to consume 528 Tg per year, whilst soil methanotrophy was calculated as 28 Tg per year.
It seems that the yearly variations in methane emissions have been driven by wetlands, but the drivers of decadal variations are more enigmatic. One suggestion for the stabilisation of methane emissions during the early 2000s is that changes occurred due to decreasing and stabilising methane emissions from the fossil fuel industry, coupled to the same trend in emissions associated with rice growing. A second possibility is a combination of stabilising fossil fuel emissions and decreasing microbial emissions. The explanation for the renewed increase in methane concentrations after 2006 is that wetland methane emissions were stimulated by high temperatures in northern latitudes and increased rainfall over the tropics. Additionally, fossil fuel emissions increased due to shale gas extraction in the US, and coal exploitation in China and India.
The paper concludes by discussing uncertainties in their calculations. They point to the need to improve wetland mapping, as well as a general shortage of datasets for wetland methane fluxes that are needed to validate models. They also note that decadal trends for both natural and anthropogenic methane emissions are extremely uncertain, making it difficult to correctly attribute the source for atmospheric changes.
Reference: Kirschke, S., et al. 2013. Three decades of global methane sources and sinks. Nature Geoscience, 6, 813-823.
Image: AIRS 2011 annual mean upper troposphere(359Hpa) methane mixing ratio.