Fossilised farts (and other agroGHGs)! Part 3: The critiques of fart records: it’s ALL NATURAL.

Now both articles (Fuller et al., 2011; Ruddiman et al., 2011) and their side of the debate have critiques. From these graphs (from Ruddiman et al., 2011) they become evident:

In the first (A) graph you can see that the relationship between CH₄ concentrations and population is not constant. Initially CH₄ per capita increased proportionally, then methane rose steadily whilst population was rising exponentially. This decoupling is down to (what Ruddiman et al. 2011 note from Ellis and Wang in 1997) different land production efficiencies and priorities. With increasing intensification techniques, like rearing cattle, more land and plants are needed as well as primitive ruminants who haven’t been selectively bred to maximise meat or milk production yet. These inefficiencies which increase CH₄ release (IPCC, 2006) where only dealt with during the latter half of the Holocene, this is just one argument supporting anthropogenic methane emissions prior to the Anthropocene; this decouples methane and population, whilst explain the change in rates. Also land use per capita dropped, as seen in the second graph, that is not to say that the early human pastoralists had large herds of cows farting across the once green, bread-basket of the Sahara, it just highlights primitive techniques of farming. Quantifying the contributions of CH₄ into rice agriculture and livestock rearing category is hard as more research needs to be undertaken (Fuller et al., 2011; Singarayer et al., 2011).

Picking up on the point of the inter-polar gradient (IPG), Chappellaz et al. (1997) investigated the difference between the polar records of methane concentrations. Studying the Arctic GRIP ice core and the Antarctic BYRD and D47 ice cores, they attributed the changes in the IPG to initially (5.7 and 2.5 – 5 ka) lower atmospheric CH₄ levels due to the on-going drying of the tropical regions combined with massive peat land growth in the northern boreal regions after 5 ka. With a recent period (ca. 1 ka) increases due to increased wetness and significant anthropogenic emissions. Harder et al. (2007) investigates this further, coupling a GCM with information on the sinks of methane; volatile organic compounds (VOC) and the sea (changes in sea surface temperature, SST). Another vital sink, the largest in fact (and one I hope to investigate further is the hydroxyl radical (­^●OH). Stressing the importance of changes in the various other sources and sinks, Harder et al.’s research show that the IPG changes are the result of dynamics within the ‘methane cycle’, between the balance between the sources/sinks. However, they draw attention to the necessity to improve understanding about how methane may react with other GHGs especially considering the fact that the hydroxyl radical is the sink for many other GHGs. Any anthropogenic influence on the changing methane concentrations either at 5 ka or in the IPG has been sidelined.

This point is underlined by Singarayer et al. (2011) as concluding remarks describe the lack of model evidence successfully calibrating predicted and observed data sets, with an anthropogenic input providing a correct outcome. It goes even further saying, and I quote:

“The late Holocene increase in methane can be primarily ascribed to increasing emissions from the Southern Hemisphere tropics. In the late Holocene, unlike the last interglacial, these increases are not counteracted by equivalent decreases in Northern Hemisphere emissions. We suggest therefore that direct anthropogenic influences are not necessary to explain the late Holocene methane record.”

Rather than the idea of cows farting (as it is quite hard to believe!); Singarayer et al. (2011) looks into possible overlooked variables. Exploring such variables like: glacial extent, and how it may effect subtle changes in the source regions; seasonality of the SH tropical wetland, and the resulting emissions; but most importantly, the link to the Eemian period where the orbital configuration is comparable to the present (and where models attempting to show the anthropogenic link fall short). They reaffirm their point that SH emissions were not counteracted with NH CH₄ emission decreases.

Even Burns (2011) discusses the possibility of an ‘all-natural’ 5 ka methane rise due to tropical produce methane causing the deviation from the expected. Burns looks at speleothem records to infer monsoonal strengths. It shows that the monsoons did migrate southwards, so making the highly productive tropics and areas south of the equator increasingly waterlogged and, ergo, greater CH₄ productive. It does seem that it is a one or the other theory approach… Neo can only take either the red or blue pill. There is no such thing as a purple one. But here, I would suggest that even though evidence is in favour of an all-natural approach. In my opinion one cannot exclusively write out the other, and the debate will go on for ages; but archaeological evidence shows the techniques expansion. Whether you like it or not, ruminants fart, producing methane, as well as humans might I add!

I would like to think that thousands of years ago my ancestors around the Mediterranean were herding farting sheep, farting cows and farting chickens, contributing to increasing methane concentrations in the atmosphere. It was a simpler time; it was a less fartier time!

Reference list for the 3 parts of Fossilised Farts (and other agroGHGs)!

Brook, E. J., Sowers, T. and Orchardo, J., 1996, Rapid variations in atmospheris methane concentration during the past 110,000 years, Science, 273, 1087-1091 pp.

Burns, S. J., 2011, speleothem records of changes in tropical hydrology over the Holocene and possible implications for atmospheric methane, The Holocene (special issue), 1-7 pp.

Chappellaz, J., Blunier, T., Kints, S., Dallenbach, A., Barnota, J., Schwander. J., Raynaud, D. and Stauffer, B., 1997, Changes in the atmospheric CH­₄ gradient between Greenland and Antarctica during the Holocene, Journal of Geophysical Research, 102, D13, 15,987-15,997 pp

Ellis, E. C. and Wang, S. M., 1997, Sustainable traditional agriculture in the Tai Lake region of China, Agriculture Ecosystems and Environment, 61, 177-193 pp.

Fuller, D. Q., Manning, K., Castillo, C., Kingwell-Banham, E., Weisskopf, A., Qin, L., Sato, Y. and Hijmans, 2011, The contribution of rice agriculture and livestock pastoralism to prehistoric methane levels: An archaeological assessment, The Holocene, 21, 743-759 pp.

Harder, S. L., Shindell, D. T., Schmidt, G. A. and Brook, E. J., 2007, A global climate model study of CH₄ emissions during the Holocene and glacial-interglacial transitions constrained by ice core data, Global biogeochemical cycles, 21, GB1011, 1-13 pp.

IPCC, 2006, CH4 Emissions from enteric fermentation,in Guidelinesfor National Greenhouse Gas Inventories Volume 4 Agriculture, Forestry andOther Land Use, writtenby Gibbs, M. J., Conneely, D., Johnson, D., Lasse, K. R. and Ulyatt M.J., , 297-320 pp. 

Ruddiman, W. F., Kutzbach, J. E. and Vavrus, A. J., 2011, Can natural or anthropogenic explanations of late-Holocene CO₂ and CH­₄ increases be falsified? The Holocene, 21, 865-879 pp.

Schlit, A., Baumgartner, M., Schwander, J., Buiron, D., Capron, E., Chappellaz, J., Loulergue, L., Schupbach, S., Spahni, R., Fischer, H. and Stocker, T., 2010, Atmospheric nitrous oxide during the last 140,000 years, Earth and Planetary Science Letters, 300, 33-43 pp.

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 pp.

Sowers, T., 2010, Atmospheric methane isotope records covering the Holocene period, Quaternary science Reviews, 29, 213-221 pp.

Wolff, E. W., 2011, Methane and Monsoons, Nature, 470, 49-50 pp.