Abstract
Abstract. Two atmospheric inversions (one fine-resolved and one process-discriminating) and a process-based model for land surface exchanges are brought together to analyse the variations of methane emissions from 1990 to 2009. A focus is put on the role of natural wetlands and on the years 2000–2006, a period of stable atmospheric concentrations. From 1990 to 2000, the top-down and bottom-up visions agree on the time-phasing of global total and wetland emission anomalies. The process-discriminating inversion indicates that wetlands dominate the time-variability of methane emissions (90% of the total variability). The contribution of tropical wetlands to the anomalies is found to be large, especially during the post-Pinatubo years (global negative anomalies with minima between −41 and −19 Tg yr−1 in 1992) and during the alternate 1997–1998 El-Niño/1998–1999 La-Niña (maximal anomalies in tropical regions between +16 and +22 Tg yr−1 for the inversions and anomalies due to tropical wetlands between +12 and +17 Tg yr−1 for the process-based model). Between 2000 and 2006, during the stagnation of methane concentrations in the atmosphere, the top-down and bottom-up approaches agree on the fact that South America is the main region contributing to anomalies in natural wetland emissions, but they disagree on the sign and magnitude of the flux trend in the Amazon basin. A negative trend (−3.9 ± 1.3 Tg yr−1) is inferred by the process-discriminating inversion whereas a positive trend (+1.3 ± 0.3 Tg yr−1) is found by the process model. Although processed-based models have their own caveats and may not take into account all processes, the positive trend found by the B-U approach is considered more likely because it is a robust feature of the process-based model, consistent with analysed precipitations and the satellite-derived extent of inundated areas. On the contrary, the surface-data based inversions lack constraints for South America. This result suggests the need for a re-interpretation of the large increase found in anthropogenic methane inventories after 2000.
Highlights
The growth rate of atmospheric methane (CH4) has experienced large variations since the early 1990s: after a decade of decrease, interrupted by a peak in 1997–1998 (Dlugokencky et al, 1998; Cunnold et al, 2002; Wang et al, 2004; Bousquet et al, 2006), the growth rate of atmospheric methane remained small from 1999 to 2006, except a peak in 2002– 2003, only to increase again since 2007
The standard deviation for wetlands in INVANA is 16 Tg yr−1, which indicates that wetlands explain about 90 % of the variability of total methane emissions inferred with INVANA
The two different atmospheric inversions (T-D) and B-U visions of methane emissions agree on two key-points at the global scale:
Summary
The growth rate of atmospheric methane (CH4) has experienced large variations since the early 1990s: after a decade of decrease, interrupted by a peak in 1997–1998 (Dlugokencky et al, 1998; Cunnold et al, 2002; Wang et al, 2004; Bousquet et al, 2006), the growth rate of atmospheric methane remained small from 1999 to 2006, except a peak in 2002– 2003, only to increase again since 2007 As methane is emitted by a large variety of sources, the explanations for the observed atmospheric variations have generally implied changes in several source or sink types. The 1991–93 growth rate anomaly is linked to the Pinatubo eruption, which led to a decrease in methane loss, due to reduced tropospheric hydroxyl radical (OH) concentrations and stratospheric chemistry changes (Bândaet al., 2013), followed by a decrease in natural wetland emissions. The collapse of the economy of the former USSR and Eastern Europe has led to decreased anthropogenic emissions starting in 1991 and spanning most of the 1990s (Dlugokencky et al, 2003)
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