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.
