Stable atmospheric methane in the 2000s: key-role of emissions from natural wetlands

Abstract

Two atmospheric inversions (one fine-resolved and one process-discriminating) and a process-based model for land surface exchanges are brought together to analyze 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 two inversions agree on the time-phasing of global emission anomalies. The process-discriminating inversion further indicates that wetlands dominate the time-variability of methane emissions with 90% of the total variability. Top-down and bottom-up methods are qualitatively in good agreement regarding the global emission anomalies. The contribution of tropical wetlands on these anomalies is found to be large, especially during the post-Pinatubo years (global negative anomalies with minima between −41 and −19 Tg y−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 y−1 for the inversions and anomalies due to tropical wetlands between +12 and +17 Tg y−1 for the process-based model). Between 2000 and 2006, during the stagnation of methane concentrations in the atmosphere, total methane emissions found by the two inversions on the one hand and wetland emissions found by the process-discriminating-inversion and the process model on the other hand are not fully consistent. A regional analysis shows that differences in the trend of tropical South American wetland emissions in the Amazon region are mostly responsible for these discrepancies. A negative trend (−3.9 ± 1.3 Tg y−1) is inferred by the process-discriminating inversion whereas a positive trend (+1.3 ± 0.3 Tg y−1) is found by the process model. Since a positive trend is consistent with satellite-derived extent of inundated areas, this inconsistency points at the difficulty for atmospheric inversions using surface observations to properly constrain tropical regions with few available observations. A consequence is the need to revisit the large increase in anthropogenic emissions computed at the global scale by some inventories for the early 2000s, although process-based models have also their own caveats and may not take into account all processes.