Variability and quasi-decadal changes in the methane budget overthe period 2000–2012

Following the recent Global Carbon Project (GCP)
synthesis of the decadal methane (CH4/ budget over 2000–
2012 (Saunois et al., 2016), we analyse here the same dataset
with a focus on quasi-decadal and inter-annual variability in
CH4 emissions. The GCP dataset integrates results from topdown
studies (exploiting atmospheric observations within an
atmospheric inverse-modelling framework) and bottom-up
models (including process-based models for estimating land
surface emissions and atmospheric chemistry), inventories of
anthropogenic emissions, and data-driven approaches.The annual global methane emissions from top-down studies,
which by construction match the observed methane
growth rate within their uncertainties, all show an increase in
total methane emissions over the period 2000–2012, but this
increase is not linear over the 13 years. Despite differences
between individual studies, the mean emission anomaly of the top-down ensemble shows no significant trend in total
methane emissions over the period 2000–2006, during
the plateau of atmospheric methane mole fractions, and also
over the period 2008–2012, during the renewed atmospheric
methane increase. However, the top-down ensemble mean
produces an emission shift between 2006 and 2008, leading
to 22 [16–32] Tg CH4 yr􀀀1 higher methane emissions
over the period 2008–2012 compared to 2002–2006. This
emission increase mostly originated from the tropics, with
a smaller contribution from mid-latitudes and no significant
change from boreal regions.
The regional contributions remain uncertain in top-down
studies. Tropical South America and South and East Asia
seem to contribute the most to the emission increase in the
tropics. However, these two regions have only limited atmospheric
measurements and remain therefore poorly constrained.
The sectorial partitioning of this emission increase between
the periods 2002–2006 and 2008–2012 differs from
one atmospheric inversion study to another. However, all topdown
studies suggest smaller changes in fossil fuel emissions
(from oil, gas, and coal industries) compared to the
mean of the bottom-up inventories included in this study.
This difference is partly driven by a smaller emission change
in China from the top-down studies compared to the estimate
in the Emission Database for Global Atmospheric Research
(EDGARv4.2) inventory, which should be revised to smaller
values in a near future. We apply isotopic signatures to the
emission changes estimated for individual studies based on
five emission sectors and find that for six individual top-down
studies (out of eight) the average isotopic signature of the
emission changes is not consistent with the observed change
in atmospheric 13CH4. However, the partitioning in emission
change derived from the ensemble mean is consistent with
this isotopic constraint. At the global scale, the top-down ensemble
mean suggests that the dominant contribution to the
resumed atmospheric CH4 growth after 2006 comes from microbial
sources (more from agriculture and waste sectors than
from natural wetlands), with an uncertain but smaller contribution
from fossil CH4 emissions. In addition, a decrease in
biomass burning emissions (in agreement with the biomass
burning emission databases) makes the balance of sources
consistent with atmospheric 13CH4 observations.
In most of the top-down studies included here, OH concentrations
are considered constant over the years (seasonal variations
but without any inter-annual variability). As a result,
the methane loss (in particular through OH oxidation) varies
mainly through the change in methane concentrations and not
its oxidants. For these reasons, changes in the methane loss
could not be properly investigated in this study, although it
may play a significant role in the recent atmospheric methane
changes as briefly discussed at the end of the paper.