16th August 2011.
Methane has featured prominently on the pages of the journal Nature in recent weeks.
Two different explanations for unexpected changes in methane concentration in the atmosphere in the last twenty years were proposed in the same volume (Aydin et al., and Kai et al., 2011). A commentary on these two papers (Heimann, 2011) and an earlier review paper ‘Non-CO2 greenhouse gases and climate change’ (Montzka et al., 2011) provided useful context to the debate.
Aydin and colleagues took the novel approach of measuring the atmospheric variability of ethane in the firn (perennial snowpack) in Greenland, and used this to reconstruct estimates of methane from fossil fuel sources. They concluded that the amount of methane emitted from fossil fuels during the twentieth century is “strikingly different from bottom-up estimates”. Their inferred pre-1980 methane emissions from fossil fuel sources are double those from standard databases based on fossil fuel production, and a subsequent sharp post-1980 decline of 30% explains, according to their model, the observed slow down in the rate of increase in atmospheric methane.
By contrast, Kai and colleagues suggest that the late twentieth century pattern is best explained by reduced methane emissions from microbial source, in particular from rice paddy fields. They constrained atmospheric methane models with carbon isotope data, using the fact that microbially sourced methane is relatively depleted in 13C whereas that from fossil fuels is relatively enriched. They also used 2H/1H isotope ratios in the atmospheric methane to control for possible changes in the atmospheric sink. According to their models, the observed changes in methane concentration are incompatible with a fossil fuel source.
The review paper by Montzka (a co-author on the Aydin paper) and colleagues gives some insight into why explaining changes in global atmospheric methane concentrations is such a vexed issue. Methane has a relatively short lifetime of methane the in the atmosphere (~9 yr). There is a delicate balance between methane sources and sinks. The major sources are highly sensitive to both climate change and human activity, and both positive and negative feedback mechanisms operate. The magnitude of the hydroxyl radical sink is influenced by complex atmospheric processes. All these factors combined result in high short-term variability in the amounts of methane in the atmosphere, which will need to be better understood if future attempts to reduce the amount of methane in the atmosphere are to be effective.
Aydin, M., Verhulst, K.R., Saltzman, E.S., Battles, M.O., Montzka, S.A., Blake, D.R., Tang, Q. and Prather, M.J. (2011). Recent decreases in fossil-fuel emissions of ethane and methane derived from firn air. Nature, 476, 198-201.
Kai, F.M., Tyler, S.C., Randerson, J.T. and Blake, D.R. (2011). Reduced methane growth rate explained by decreased Northern Hemisphere microbial sources. Nature, 476, 194-197.
Heimann, M. (2011). Enigma of the recent methane budget. Nature, 476, 157-158.
Montzka, S.A., Dlugokencky, E.J. and Butler, J.H. (2011). Non-CO2 greenhouse gases and climate change. Nature, 476, 43-49.