![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Methane- Shifting Atmospheric Sources
Concentrations of atmospheric methane, a potent greenhouse gas, have
been steadily rising since the Industrial Revolution. Between 1999 and
2006 there was an interruption in this steady rise, a period during
which the rise plateaued. As the concentration of atmospheric methane is
controlled by both the magnitude of methane being emitted and the
availability of the reactants that break down the methane, the cause of
this plateau is the subject of debate. For the period of this plateau a
full account of the methane budget requires an understanding of volumes
of methane that was being emitted to the atmosphere by various sources,
as well as the availability of the chemicals in the atmosphere that
decompose the atmospheric methane.
According to Newton the carbon isotope composition of the methane goes
some way to identifying the sources; largest sources, the burning of
fossil fuel, burning biomass and agriculture and emissions from
wetlands, all produce methane that has a different isotopic composition.
The measurements of the isotopic composition of methane found in ice
cores and air samples that have been archived, as well as the
composition that has been recorded by monitoring stations were utilised
(Schaefer et al., 2016) in
order to isolate the factors that contributed to the methane plateau (Science,
352, 80-84; 2106).
Atmospheric concentrations of methane and isotopic composition rose in
tandem before 1999, indicating increasing emissions of methane with a
relatively higher ratio of 13C to 12C. This
isotopic ratio is consistent with a fossil fuel-like source. During the
plateau pereiod this trend broke down and after 2006, methane
concentrations started rising again, though the isotopic composition
fell. It is indicated by this that the emitted methane had less 13C,
which is consistent with methane being emitted from a biogenic source.
It has been confirmed by box-modelling that the trend that has been
observed is best explained by fossil fuel-related emissions being
reduced beginning between 1992 and 1993, with possible contributions
being from changes in atmospheric reactants, namely hydroxyls, whereas
the methane concentrations rise after 2006 is best explained by an
increase in biogenic emissions.
It is not possible to readily discriminate between different sources of
biogenic emissions, such as ruminant livestock, cultivation of rice,
tropical wetlands and Arctic permafrost. Satellite measurements of
methane do, however, rule out a substantial increase in methane
emissions from the high latitudes; over this time tropical climate
changes would be expected to increase wetland emissions, though not in
the areas in which the highest levels of emissions have been observed.
Contrasting with this, livestock inventory increases and the expansion
of cultivation of rice have been reported to have occurred over this
interval, and it has been recorded by methane inventories that emissions
of methane from agriculture were increasing over the period 2000-2006.
Therefore it probably isn’t surprising that such a large influence on
the concentrations of methane in atmosphere has been attributed
agriculture.
However, it is surprising that fossil fuels appear to have contributed
less, especially as there has been recent exploitation of both coal and
unconventional gas reserves. It is suggested by these findings that
carbon budgeting, and efforts at mitigation, must consider closely the
expansion of agriculture, especially if the global trend towards more
meat consumption continues.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |