24th February 2011.
Polar ice core records show that around 5000 years ago (late Holocene) the concentration of methane in the atmosphere started to rise significantly. This increase is considered anomalous, in that it coincides with a decrease in Northern Hemisphere insolation. The normal pattern observed in ice core records going back over 400,000 years is that increases in atmospheric methane concentrations correspond to increases in orbital precession-related insolation.
One controversial theory, proposed by Ruddiman et al. (2003), suggests that early human activities including forest clearance and rice agriculture account for the extra methane. However, a recent study published in Nature by Singarayer et al. (2011) challenges Ruddiman’s ‘Early Anthropocene Hypothesis’ and proposes instead that the late Holocene anomaly can be explained by natural changes in methane emissions with no anthropogenic input required.
Singarayer and co-authors used the coupled ocean-atmosphere Hadley Centre climate model (HadCM3) to predict 65 climate ‘snapshots’ over the last 130 kyr glacial cycle. They then used these snapshots as inputs to a coupled vegetation and wetlands methane emission model, to predict changes in methane emissions and reconstruct atmospheric methane concentrations over the period. In a series of novel experiments using the model the authors showed that an increased source from South America, arising from an insolation signal out of phase with the normally dominant Northern Hemisphere methane signal, can account for the late Holocene methane ‘anomaly’.
However, Bill Ruddiman says he is not convinced by the new study. “I respect the work of Singarayer and her colleagues but in this case I think they are wrong”. He notes recent work by UCL archaeologist Dorian Fuller, who has mapped the spread of rice agriculture and domestic livestock throughout Asia in the late Holocene, coincident with the increase in atmospheric methane. Fuller believes that, on the basis of the archaeological evidence, Ruddiman’s hypothesis is “strongly supported” and notes more generally that “archaeology has lots of important front-line data to contribute…” and that “the Early Anthropocene should be a wake up call for archaeologists to stick their heads above their trenches and think beyond sites and local regions and on broader interregional and long term patterns”. Stepping back from the specifics of the debate, he appeals for greater interdisciplinary collaboration between archaeologists and those researching the dynamics of land and climate systems and their human inputs.
Commenting in Nature on the Singarayer paper, MethaneNet member Eric Wolff lauds the novel use of a well-resolved GCM for this type of experiment. He tells MethaneNet that the authors “… do remarkably well in getting the pattern of change over the whole period..” whilst also noting that “the balance between the different sources is quite subtle so it’s hard to know how robust the result is” and that, regarding the proposal of a human influence to explain the observations, the paper “doesn’t prove there wasn’t one, but removes the compelling need for one”.
Wolff also notes the wider significance of this modelling work, that it demonstrates the overall sensitivity of the methane system and emphasises the exceptional size of imbalance of the last 200 years.
Singarayer, J.S., Valdes, P.J., Friedlingstein, P, Nelson, S., and Beerling, D.J. (2011) Late Holocene methane rise caused by orbitally controlled increase in tropical sources. Nature, 470, 82-86.
Ruddiman, W. F. (2003). The anthropogenic greenhouse era began thousands of years ago. Climatic Change, 61(3), 261-293.
Wolff, E. W. (2011) Methane and monsoons. Nature, 470, 49-50.