The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 174

13 CH 4 was about 2‰ above expected values between the years

such that the δ ad 0 and

13 CH 4 values were about 2‰ below expected values in the sub-
sequent 700 years, up to

1000. Conversely, δ

1700. The MCA also shows up as expected in the methane
pattern. However, interpreting the various isotopic

ad

13 CH 4 values is difficult and so,
as yet (without other corroborating evidence), conclusions should be viewed with
caution. Yet the suggestion is that both human activities (such as land clearance and
use of wood as a fuel) and natural climate change have influenced the pre-industrial
carbon cycle and that, up to the early 21st century, researchers have understated the
human influence on the late pre-industrial Holocene methane budget (Ferretti et al.,
2005).
Having said this - that we may perhaps have underestimated human influence on
the global carbon cycle the past few centuries (or even a score of centuries) - this
is a long way off from saying that humans were discernibly influencing the global
carbon cycle 5000 years ago or earlier (see early in section 4.4). Looking at the
agricultural methane argument, let's assume that 10% of humans on the planet at the
timehada1hapaddy field. This is surely an exaggeration because agriculture was
then only in its infancy and productivity (yield per unit area) was low, but rice and
farm ruminants have emissions in the same order of magnitude and so this might be
considered an average. A back-of-the-envelope calculation (even using present, high
paddy field fluxes) reveals that we are talking about less than a 1% change in the pre-
industrial natural methane flux (both of sources and of sinks) and which is equivalent
to even less of current anthropogenic (let alone combined with the natural) methane
flux. So, this is within the natural variability of the system. (Conversely, for loose
comparison, modern fossil fuel carbon and land-use emissions are several per cent of
the natural terrestrial flux.) Consider also that very early human rice cultivation would
more likely have taken place in existing wetlands (and not artificially created ones)
and so not significantly add to overall methane fluxes, and similarly early ruminant
grazing simply replaced existing wild ruminant grazing. Would there be a significant
additional early human anthropogenic methane flux? It is therefore unlikely that pre-
Iron Age society had much of an impact on the global carbon cycle as has been
proposed.
Of course, back-of-the-envelope calculations are not good enough for anything
other than as an indication of a likelihood, and the debate as to whether humans
were discernibly affecting the global cycle 8000-5000 years ago continued for a few
years. The computer model analysis by Joy Singarayer, Paul Valdes and colleagues in
2011 was particularly welcome (as an addition to other previous counters to the early-
Holocene start of the Anthropocene, interested readers may see the Quaternary review
by Wanner et al., 2008). The simulations by Singarayer and colleagues captured the
declining trend in real methane concentrations ascertained from ice cores that dated
from the end of the last interglacial period (115 000-130 000 years ago) to recent
times, which had been used to diagnose the Holocene methane rise as unique, and
hence arguably signalling the start of the Anthropocene. Their findings suggested
that no early agricultural sources are required to account for the increase in methane
concentrations in the 5000 years before the industrial era and additionally that the
methane rise was most likely natural, from wetlands, with a probable significant
contribution from South American ecosystems.

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