• Energy Research

Abstract. Methane (CH4) has a 28-fold greater global warming potential than CO2 over one hundred years. Atmospheric CH4 concentration has tripled since 1750. Anthropogenic CH4 emissions from China has been growing rapidly in the past decades, and contributes more than 10 % of global anthropogenic CH4 emissions with large uncertainties in existing global inventories, generally limited to country-scale statistics. To date, a long-term CH4 emissions inventory including the major sources sectors and based on province-level emission factors is still lacking. In this study, we produced a detailed bottom-up inventory of anthropogenic CH4 emissions from the eight major source sectors in China for the period 1980–2010. In the past three decades, the total CH4 emissions increased from 22.2 [16.6–28.2] Tg CH4 yr−1 (mean [minimum-maximum of 95 % confidence interval]) to 45.0 [36.4–58.3] Tg CH4 yr−1, and most of this increase took place in the 2000s. This fast increase of the total CH4 emissions after 2000 is mainly driven by CH4 emissions from coal exploitation. The largest contribution to total CH4 emissions also shifted from rice cultivation in 1980 to coal exploitation in 2010. The total emissions inferred in this work compare well with the EPA inventory but appear to be 38 % lower than EDGAR4.2 inventory. The uncertainty of our inventory is investigated using emissions factors collected from published literatures. We also distributed province-scale emissions into 0.5º × 0.5º maps using social-economic activity data. This new inventory could help understanding CH4 budgets at regional scale and guiding CH4 mitigation policies in China.

Abstract. Globally, terrestrial ecosystems have absorbed about 30% of anthropogenic emissions over the period 20007–2007 and inter-hemispheric gradients indicate that a significant fraction of terrestrial carbon sequestration must be north of the Equator. We present a compilation of the CO2, CO, CH4 and N2O balance of Europe following a dual constraint approach in which (1) a land-based balance derived mainly from ecosystem carbon inventories and (2) a land-based balance derived from flux measurements are confronted with (3) the atmospheric-based balance derived from inversion informed by measurements of atmospheric GHG concentrations. Good agreement between the GHG balances based on fluxes (1249 ± 545 Tg C in CO2-eq y−1), inventories (1299 ± 200 Tg C in CO2-eq y−1) and inversions (1210 ± 405 Tg C in CO2-eq y−1) increases our confidence that current European GHG balances are accurate. However, the uncertainty remains large and largely lacks formal estimates. Given that European net land-atmosphere balances are determined by a few dominant fluxes, the uncertainty of these key components needs to be formally estimated before efforts could be made to reduce the overall uncertainty. The dual-constraint approach confirmed that the European land surface, including inland waters and urban areas, is a net source for CO2, CO, CH4 and N2O. However, for all ecosystems except croplands, C uptake exceeds C release and us such 210 ± 70 Tg C y−1 from fossil fuel burning is removed from the atmosphere and sequestered in both terrestrial and inland aquatic ecosystems. If the C cost for ecosystem management is taken into account, the net uptake of ecosystems was estimated to decrease by 45% but still indicates substantial C-sequestration. Also, when the balance is extended from CO2 towards the main GHGs, C-uptake by terrestrial and aquatic ecosystems is compensated for by emissions of GHGs. As such the European ecosystems are unlikely to contribute to mitigating the effects of climate change.

Abstract. 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 top-down 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 top-down 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.