There is now a widespread scientific consensus on the profound influence of human activity on the global climatic system, particularly through increased emissions of greenhouse gases like CO2 and CH4 since the pre-industrial era. Although CH4 is roughly 200 times less abundant in the atmosphere than CO2, it is a more potent greenhouse gas than CO2. The comparative impact of CH4 on climate change is more than 86 times greater than CO2 over a 20-year period (IPCC, 2013). However, due to its relatively short lifetime of ~9 years, it is now recognized that one of the most efﬁcient methods to mitigate warming due to greenhouse gases on decadal time frames is to cut CH4 emissions (Shindell et al., 2012).
Atmospheric CH4 concentrations have more than doubled since the pre-industrial period, reaching a new high of 1833±1 ppb in 2014, as shown by the latest analysis of observations from the WMO Global Atmosphere Watch (GAW) Programme (WMO News Bulletin, 2015). This odourless, transparent gas is released into the atmosphere by a wide variety of both natural and anthropogenic processes, see FIgure 1. About 40% of the emitted CH4 is of natural origin (wetlands, freshwaters, wildfires, termites, geological sources..). The remaining 60% comes from anthropogenic sources, such as waste management, fossil fuel exploitation, ruminants and rice agriculture. Removal of CH4 is primarily due to oxidation by OH, resulting in an atmospheric lifetime of CH4 of ~9 years.
Figure 1 : Anthropogenic and natural sources of CH4. Figures taken from the Global carbon project. Figure from http://www.globalcarbonproject.org; based on Kirschke et al., 2013.
Although its main sources and sinks have been identified, there remain many uncertainties regarding the balance between those sources and sinks as we can see from Figure 1 (Kirschke et al., 2013). In addition, CH4 is a challenging atmospheric component to study as its non-monotonous changes in the last decades and its interannual variability remain not fully understood (Nibet et al., 2014).
IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Cia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp.
Kirschke et al. (2013), Three decades of global methane sources and sinks, Nature Geoscience, doi:10.1038/ngeo1955.
Nisbet, E. G., Dlugokencky, E. J. and Bousquet, P.: Methane on the rise-again., Science, 343(6170), 493–5, doi:10.1126/science.1247828, 2014.
WMO News Bulletin (2015): http://www.wmo.int/pages/prog/arep/gaw/ghg/GHGbulletin.html