The recent increase of atmospheric methane from 10 years of ground-based NDACC FTIR observations since 2005

Abstract

Abstract. Changes of atmospheric methane total columns (CH 4 ) since 2005 have been evaluated using Fourier transform infrared (FTIR) solar observations carried out at 10 ground-based sites, affiliated to the Network for Detection of Atmospheric Composition Change (NDACC). From this, we find an increase of atmospheric methane total columns of 0.31 ± 0.03 % year −1 (2σ level of uncertainty) for the 2005-2014 period. Comparisons with in situ methane measurements at both local and global scales show good agreement. We used the GEOS-Chem chemical transport model tagged simulation, which accounts for the contribution of each emission source and one sink in the total methane, simulated over 2005-2012. After regridding according to NDACC vertical layering using a conservative regridding scheme and smoothing by convolving with respective FTIR seasonal averaging kernels, the GEOS-Chem simulation shows an increase of atmospheric methane total columns of 0.35 ± 0.03 % year −1 between 2005 and 2012, which is in agreement with NDACC measurements over the same time period (0.30 ± 0.04 % year −1 , averaged over 10 stations). Analysis of the GEOS-Chem-tagged simulation allows us to quantify the contribution of each tracer to the global methane change since 2005. We find that natural sources such as wetlands and biomass burning contribute to the interannual variability of methane. However, anthropogenic emissions, such as coal mining, and gas and oil transport and exploration, which are mainly emitted in the Northern Hemisphere and act as secondary contributors to the global budget of methane, have played a major role in the increase of atmospheric methane observed since 2005. Based on the GEOSPublished by Copernicus Publications on behalf of the European Geosciences Union. W. Bader et al.: The recent increase of atmospheric methaneChem-tagged simulation, we discuss possible cause(s) for the increase of methane since 2005, which is still unexplained.

On the Causes and Consequences of Recent Trends in Atmospheric Methane

“…This is consistent with, and possibly due to, prior information from wetland process models. Other studies find wetland contributions to the renewed rise that range from minor [66,67] to dominant, i.e. 40% or 70% of the increase [68,69].…”

“…There is no clear evidence that its strong isotopic fractionation influenced δ 13 CH 4 trends [66]. A strong role of stratospheric-tropospheric exchange and stratospheric CH 4 removal is not evident in total atmospheric column measurements of [CH 4 ] [67]. Back mixing of CH 4 -depleted stratospheric air changes tropospheric δ 13 CH 4 by ≤ 0.5‰ [95], so the impact on recent trends is likely small.…”

“…Transport of boreal CH 4 is also shallower, increasing surface observations more than rapidly convected tropical sources [2••]. Stratospheric CH 4 does not increase significantly during the renewed rise [67] and the rate of increase in the mid-upper troposphere is lower than at the surface [101], pointing to surface emissions as the cause of the renewed rise.…”

Comparison of the GOSAT TANSO-FTS TIR CH<sub>4</sub> volume mixing ratio vertical profiles with those measured by ACE-FTS, ESA MIPAS, IMK-IAA MIPAS, and 16 NDACC stations

“…Data used herein are from the NDACC database. A summary of retrieval settings is provided by Bader et al (2017). Lauder and Arrival Heights, at 45.0 and 77.8 • S, respectively, use a retrieval strategy that adheres to that defined in Sussmann et al (2011), with a relaxed Tikhonov regularization constraint at Arrival Heights due to the characteristic atmospheric dynamics over Antarctica.…”

Atmospheric CO and CH<sub>4</sub> time series and seasonal variations on Reunion Island from ground-based in-situ and FTIR (NDACC and TCCON) measurements

“…The representativeness of both measurements depends on the location of the site, the air transport, and the lifetime of the target species (Folini et al, 2009;Dils et al, 2011;Sepúlveda et al, 2014). In addition, the representativeness of FTIR retrievals, and their vertical sensitivity is also related to the spectral range and retrieval strategy, which has to be taken into account when comparing the 5 Total Carbon Column Observing Network (TCCON) and the Detection of Atmospheric Composition Change (NDACC) data (Ostler et al, 2014;Kiel et al, 2016).…”