Global Change Biology, FS 2017
Marius Hodel
In 2006 atmospheric CH4 concentrations had
plateaued, however, since then they have increased
rapidly – Why?
Date: 10.04.2017
Supervisor: Susanne Burri
Introduction
Methane (CH4) is an important greenhouse gas the warming potential of which is 25 times higher than
the one of CO2 (Solomon, 2007). Its concentration in the global atmosphere has more than doubled in
the last 250 years and reached 1852.3 ppb (parts per billion) by the end of 2016 (Dlugokencky et al.,
2011; Dlugokencky and NOAA/ESRL, 2017). Therefore, the organic gas is a relevant contributor to
global warming. Because of the relatively short life cycle of methane reducing methane emissions
could quickly have a positive influence on the climate and mitigate climate change (Dlugokencky et
al., 2011).
The methane cycle including its sources and sinks is well known. Bacteria under anaerobic conditions
produce methane in wet environments (e.g. wetlands, rice fields, swamps) and in the stomachs of
ruminants (Wahlen, 1993). Other sources occur from coal mining, leakage of natural gas and biomass
burning (Wahlen, 1993). The main sink for methane is found in the troposphere: Methane is broken
down to CO2 and H2O through multiple oxidation reactions initiated by OH radicals (Wahlen, 1993).
A minor sink occurs in the soil where methanotrophic bacteria oxidize CH4 and hereby remove it from
the atmosphere (Dlugokencky et al., 2011).
Total global emissions can be estimated reasonably well and the main sources and sinks are well
known (Kirschke et al., 2013). However, the emissions from individual sources are fairly uncertain
(Dlugokencky et al., 2011; Kirschke et al., 2013). If we want to make use of the big mitigation
potential, impacts of single sources have to be known better. Moreover recent developments in the
concentration of atmospheric methane raise further questions about the global methane cycle and the
extent of the different sources.
Questions
Between 1999 and 2006 there was nearly no increase in atmospheric methane concentration. It had
plateaued. However, since 2007 the atmospheric methane concentration is increasing again, in the last
three years even faster than before (Dlugokencky and NOAA/ESRL, 2017). This fact raises the
following questions: (i) Why did the concentration of atmospheric methane stop growing between
1999 and 2006? (ii) What are the reasons for the reappearing growth of methane concentration after
2007? (iii) Which sources or sinks are responsible for these variations?
Results
Before the atmospheric methane concentration had plateaued in the early 2000s, growth rate already
started to decline in the 1990s (Dlugokencky and NOAA/ESRL, 2017). While the large inter-annual
variations can be explained through fluctuating wetland emissions and fire events during El-Niño in
1997/98, the reason for the long-term decline in growth of atmospheric methane is thought to be a
decline in anthropogenic emissions in consequence of the economic collapse of the former Soviet
Union (Bousquet et al., 2006). However, since these emissions have risen again after 1999 there must
be another explanation for the plateaued atmospheric methane concentration after the year 2000.
Bousquet et al. (2006) assumed that a significant drop in northern wetland emissions led to a stop of
the atmospheric methane growth. This drop, caused by dryness that lasted for four years (Hoerling and
Kumar, 2003), seems to have offset the increasing anthropogenic emissions. On the contrary Dalsoren
et al. (2016) assumed, dry conditions in the tropics led to decreasing wetland emissions. They further
stated that additionally increasing CH4-loss through enhanced OH-concentration in the atmosphere
contributed to the stagnation of atmospheric methane concentration. There seems to be no agreement
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Global Change Biology, FS 2017
Marius Hodel
on the reasons for the stabilization of atmospheric methane between 1999 and 2006. The responsible
factors remain unclear. Because of uncertainties in emission trends scientists are cautious at drawing
conclusions, nevertheless Kirschke et al. (2013) suggest that the stabilization “can potentially be
explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial
emissions.”
After about seven years of more or less stable atmospheric methane concentration it started to increase
again in 2007 (Rigby et al., 2008) and the growth rate even increased further in the last few years
(Dlugokencky and NOAA/ESRL, 2017). Isotopic analyses can be used to differ between biogenic
(wetlands and agriculture) and thermogenic sources (biomass and fossil fuel burning). Recent
measurements showed a shift in the isotopic ratio 13C/12C in atmospheric methane (Nisbet et al., 2016).
This leads to the assumption that mainly biogenic sources are responsible for the observed increase
(Nisbet et al., 2016). Because of the abrupt increase in 2007 and the large inter-annual variations a
(fluctuating) natural source is more likely than an anthropogenic source. Although increasing
agricultural emissions are thought to be a contributor to the recent increase, it is assumed that
emissions from tropical wetlands are the prevalent source (Nisbet et al., 2016). In addition to wet
conditions in the tropics Bousquet et al. (2011) assume that high temperature at high latitudes and
resulting enhanced wetland emissions in the Northern Hemisphere were another cause for the abrupt
increase of atmospheric methane in 2007. However, in a recent study Turner et al. (2016) suggest that
increasing U.S. anthropogenic methane emissions in the last decade are possibly responsible for 30 to
60% of the global increase. They further state that this increase goes along with an increase in U.S. oil
and shale gas production, but it cannot be clearly attributed to this source. Another contributor to the
continuing growth of atmospheric methane could be the stabilization of CH4-loss. Due to
meteorological variability methane-loss stabilized and this could have been responsible for the
increase in CH4 concentration after high methane emissions occurred in 2007 and 2008 (Dalsoren et
al., 2016).
Nevertheless there is, like for the plateau of atmospheric methane concentration between 1999 and
2006, no consistent explanation for the rapid increase beginning in 2007 (Kirschke et al., 2013). Big
uncertainties regarding methane emissions make it impossible to draw consistent conclusions. CH4emissions from natural wetlands are “highly uncertain” (Saunois et al., 2016b). Estimations of
methane emissions also depend on the methodology: The results of top-down inversions differ largely
from results of bottom-up models (Saunois et al., 2016a). In a recent review paper Saunois et al.
(2016a) state that the reason for the changes in atmospheric CH4 are still not known. If changes of
methane concentration cannot be explained, predictions of future CH4 emissions and developments in
atmospheric concentrations are hardly reliable. As this is important information needed to deal with
climate change, scientific breakthroughs are essential and these are only to achieve by great
collaboration between researchers (Saunois et al., 2016b).
Conclusion
Despite the fact that there are many studies that tempt to explain the recent developments in
atmospheric methane, there is no consistent explanation. Changes in natural wetland emissions and
biomass burning are known to cause inter-annual variations but their influence on the longer-term
developments in the last two decades is unclear. Different anthropogenic sources (agricultural sources,
burning of fossil fuels) also contribute to methane emissions. However, emissions of different (natural
and anthropogenic) sources are uncertain and changes can either offset each other or add up. Isotopic
analyses can give information about the source but it does only reveal if the source is biogenic or
thermogenic and not if the source is natural or anthropogenic. Additionally uncertainties exist in the
isotopic ratio of different sources. Without a better knowledge about the contribution of different
sources and the changes in CH4-lifetime it is not possible to explain the developments of atmospheric
methane concentration after 1999. However, the understanding of past events is essential to make
reliable predictions for future developments of atmospheric methane concentrations.
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Global Change Biology, FS 2017
Marius Hodel
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