Blog: Of Mammoth Significance?

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By Gail Riekie. 9th May 2011.

Methane researchers will all be accustomed to the old jokes about flatulent cows, but fewer will be familiar with the debate about the possible role of methane emissions from other large bodied herbivores in an earlier era.

Prior to the arrival of humans, around 12,500 years ago, a vast population of now extinct megafauna (mammoths, camelids, giant ground sloths, to name but a few) roamed the Americas.  The megafauna’s dramatic demise is considered to be the earliest catastrophic event attributable to our own species.

A Nature Geoscience article published last year by Smith et al. (1) suggested that the drop in atmospheric methane concentrations observed in the ice core record at the onset of the Younger Dryas, ~12,800 years ago, occurred at an unusually rapid rate, and could be explained, at least in part, by the loss of methane emissions from megafauna.  Using estimates of the amount of methane emitted by the 114 species of herbivores known to have become extinct by the end of the Pleistocene epoch, Smith and co-authors calculated that the loss of megafauna could account for between 12.5 and 100% of the observed methane decrease. They also suggested that the onset of the ‘Anthropocene’ should be recalibrated to 13,400 years before present, when humans first started migrating on a large scale into the Americas.

More recently in Nature Geoscience, doubt has been cast on some of these claims (2). Brook and Severinghaus assert they are inconsistent with constraints imposed by the ice core record, noting that the drop in concentration of atmospheric methane discussed by Smith and co-authors and more usually attributed to decreased wetland emissions, is not in fact uniquely rapid in rate, and that furthermore it is likely that the megafauna extinctions occurred too early to be the cause of the observed methane decrease.

The debate about changes in atmospheric methane concentration (causes, timings and rates) in the atmosphere over the past 125,000 years is doubtless not over yet.  The ‘Early Anthropocene’ hypothesis, originally proposed by William Ruddiman (3) in the context of the onset of human-related activities such forest clearance (8000 years ago) and rice agriculture (5000 years ago) is also still a live topic, as discussed elsewhere on this website in the news item ‘Early Anthropocene doubt’. Watch this space.

References

(1) Smith, F.A., Elliot, S.M. and Lyons, S.K. (2010) Methane emissions from extinct megafauna. Nature Geoscience, 3, 374-375.

(2) Brook , E.J. and Severinghaus, J.P. (2011) Methane and megafauna. Nature Geoscience, 4, 271-272.

(3) Ruddiman, W. (2003) The anthropogenic greenhouse era began thousands of years ago. Climate Change. 61, 261-293.

Early Anthropocene Doubt?

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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.

References:

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.