Atmospheric methane (CH4), a relatively long-lived atmospheric species with a lifetime of 8–10 years (Kirschke et al., 2013), is the second most abundant anthropogenic greenhouse gas, with a radiative forcing (RF) of 0.97 ± 0.23 W m-2 (including indirect radiative forcing associated with the production of tropospheric ozone and stratospheric water vapour; Stocker et al., 2013) after CO2 (RF in 2011: 1.68 ± 0.35 W m-2, Stocker et al., 2013). Approximately one-fifth of the increase in radiative forcing by human-linked greenhouse gases since 1750 is due to methane (Nisbet et al., 2014). Identified emission sources include anthropogenic and natural contributions. Human activities associated with the agricultural and the energy sectors are the main sources of anthropogenic methane through enteric fermentation of livestock (17 %), rice cultivation (7 %), for the former, and coal mining (7 %), oil and gas exploitation (12 %), and waste management (11 %), for the latter. On the other hand, natural sources of methane include wetlands (34 %), termites (4 %), methane hydrates and ocean (3 %) along with biomass burning (4 %), a source of atmospheric methane that is both natural and anthropogenic. The above-mentioned estimated contributions to the atmospheric content of methane are based on Chen and Prinn (2006), Fung et al. (1991), Kirschke et al. (2013) and on emission inventories used for the GEOS-Chem v9-02 methane simulation (Turner et al., 2015), although it is worth noting that the global budget of methane remains insufficiently understood.

Methane is depleted at the surface by consumption by soil bacteria, in the marine boundary layer by reaction with chlorine atoms, in the troposphere by oxidation with the hydroxyl radical (OH), and in the stratosphere by reaction with chlorine atoms, O(1D), OH, and by photodissociation (Kirschke et al., 2013). Due to its sinks, methane has important chemical impacts on the atmospheric composition. In the troposphere, oxidation of methane is a major regulator of OH (Lelieveld, 2002) and is a source of hydrogen and of tropospheric ozone precursors such as formaldehyde and carbon monoxide (Montzka et al., 2011). In the stratosphere, methane plays a central role as a sink for chlorine atoms and as a source of stratospheric water vapour, an important driver of decadal global surface climate change (Solomon et al., 2010). Given its atmospheric lifetime, and its impact on radiative forcing and on atmospheric chemistry, methane is one of the primary targets for regulation of greenhouse gas emissions and climate change mitigation.

As a result of growing anthropogenic emissions, atmospheric methane showed prolonged periods of increase over the past 3 decades (World Meteorological Organization, 2014). From the 1980s until the beginning of the 1990s, atmospheric methane was rising sharply by about ∼ 0.7 % year-1 (Nisbet et al., 2014) but stabilized during the 1999–2006 time period (Dlugokencky, 2003). Many studies were dedicated to the analysis of methane trends, in particular the stabilization of methane concentrations between 1999 and 2006, and various scenarios have been suggested. They include reduced global fossil-fuel-related emissions (Aydin et al., 2011; Chen and Prinn, 2006; Simpson et al., 2012; Wang et al., 2004), a compensation between increasing anthropogenic emissions and decreasing wetland emissions (Bousquet et al., 2006), and/or significant (Rigby et al., 2008) to small (Montzka et al., 2011) changes in OH concentrations. However, Pison et al. (2013) emphasized the need for a comprehensive and precisely quantified methane budget for its proper closure and the development of realistic future climate scenarios.

Since 2005–2006, a renewed increase of atmospheric methane has been observed and widely discussed in many studies (Bloom et al., 2010; Dlugokencky et al., 2009; Frankenberg et al., 2011; Hausmann et al., 2016; Helmig et al., 2016; Montzka et al., 2011; Rigby et al., 2008; Schaefer et al., 2016; Spahni et al., 2011; Sussmann et al., 2012; van der Werf et al., 2010), leading to various hypotheses. In this work, for the first time, we report of an increase in methane observed since 2005 at a suite of NDACC sites distributed worldwide, operating Fourier transform infrared (FTIR) spectrometers. The paper is organized as follows: Sect. 2 includes a brief description of the 10 participating sites, and the retrieval strategy and degrees of freedom and vertical sensitivity range of the FTIR measurements. Section 3 focuses on the methane changes since 2005 as derived from the NDACC FTIR measurements and the GEOS-Chem model, along with comparisons between both model and observations. This section also provides a source-oriented analysis of the recent increase of methane using the GEOS-Chem-tagged simulation. Finally, Sect. 4 discusses the potential source(s) responsible for the observed increase of methane since the mid-2000s.

The international Network for the Detection of Atmospheric Composition Change (NDACC) is dedicated to observing and understanding the physical and chemical state of the stratosphere and troposphere. Its priorities include the detection of trends in atmospheric composition, understanding their impacts on the stratosphere and troposphere, and establishing links between climate change and atmospheric composition.

We characterize the global increase of methane total column from 10 NDACC stations since 2005 and over 10 years' worth of observations, with a mean annual growth ranging from 0.26 ± 0.02 (Wollongong, 2σ level of uncertainty) to 0.39 ± 0.09 % year-1 (Toronto). Observational methane time series anomalies and their changes (along with their associated uncertainties) since 2005.0, illustrated in green in Fig. 4 and detailed in Table 2, have been analysed for all 10 sites using the statistical bootstrap resampling tool. They account for a linear component and a Fourier series, taking into account the intra-annual variability of the data set (Gardiner et al., 2008). As in Mahieu et al. (2014), the order of the Fourier series is adapted to each data set depending on its sampling, i.e. limiting the order for the polar sites for which only a partial representation of the seasonality is available. Anomalies of methane total column time series, illustrated in Figs. 2 and 5, have been computed using the methane total column computed by the linear component of the statistical bootstrap tool on 1 January 2005, as a reference. Table 2 shows trends of methane total column computed from FTIR observations over the 2005–2014 and 2005–2012 time periods as well as from a tagged GEOS-Chem simulation between 2005 and 2012. The latter is further discussed in Sect. 3.1.2.

On a regional scale, we compared our results with annual changes of methane as computed over the 2005–2014 time period from surface GC-MD observations (Gas Chromatography – MultiDetector) carried out in the framework of the AGAGE programme (Advanced Global Atmospheric Gases Experiment, Prinn et al., 2000) and from in situ surface measurements taken in the framework of the NOAA (National Oceanic and Atmospheric Administration) ESRL (Earth System Research Laboratory) carbon cycle air sampling network (Dlugokencky et al., 2015). Five representative observation sites have been considered: Alert (Nunavut, Canada, 82.45∘ N, -62.51∘ E, 200.00 m a.s.l., Dlugokencky et al., 2015), Mace Head (Ireland, 53.33∘ N, -9.90∘ E, 5.00 m a.s.l., Prinn et al., 2000), Izaña (28.29∘ N, 16.48∘ W, 2372.90 m a.s.l., Dlugokencky et al., 2015), Cape Grim (Australia, 40.68∘ S, 144.69∘ E, 94.00 m a.s.l., Prinn et al., 2000), and Halley (United Kingdom, 75.61∘ S, 26.21∘ W, 30.00 m a.s.l., Dlugokencky et al., 2015).

Firstly, in situ measurements collected at Alert, representative of the northern polar region, show an increase of methane of 0.29 ± 0.02 % year-1 (or 5.40 ± 0.41 ppb year-1) since 2006, which is in agreement with our FTIR observations at Eureka with a mean annual change of 0.28 ± 0.05 % year-1. For the northern midlatitudes, we find an agreement between changes of methane as computed from surface measurements at Mace Head with an increase of 0.30 ± 0.02 % year-1 (or 5.58 ± 0.32 ppb year-1) and from our FTIR observations. Indeed, we observe consistent increases of methane of 0.32 ± 0.03, 0.27 ± 0.03, and 0.29 ± 0.08 % year-1 since 2005 at Zugspitze, Jungfraujoch, and Toronto. Comparisons between changes of methane from FTIR and in situ surface measurements have also been taken for the Izaña station and show a close to statistical agreement with a mean annual increase of 0.33 ± 0.01 and 0.28 ± 0.02 % year-1 respectively. In the Southern Hemisphere, AGAGE GC-MD measurements of methane at Cape Grim, representative of the midlatitudes, shows a mean annual increase of 0.31 ± 0.01 % year-1 (or 5.40 ± 0.16 ppb year-1) which is in agreement with FTIR changes at Lauder of 0.29 ± 0.03 % year-1. However, we should note the slightly larger mean annual changes of methane of Cape Grim in situ observations with respect to Wollongong FTIR measurements. Indeed, it needs to be mentioned that FTIR measurements before the instrument change in 2007 (Bomem DA8 vs. Bruker IFS 125HR; see Table 1) show noisier results. These noisier observations at the beginning of the time period under investigation may affect the relatively small annual changes of methane overall. As a result, the 2005–2007 time series shows no changes of methane while the 2007–2014 time period shows a mean annual change of 0.32 ± 0.03 % year-1 (or 11.94±1.03×1016 molecules cm-2 year-1) with respect to 2007.0, which is in agreement with both Lauder FTIR and Cape Grim GC-MD methane changes since 2005. Finally, we computed a mean annual change of methane of 0.32 ± 0.01 % year-1 (or 5.45 ± 0.14 ppb year-1) from in situ surface measurements taken at Halley, which is in good agreement with the mean annual change of methane computed from FTIR Arrival Heights retrievals that amounts to 0.32 ± 0.07 % year-1.

In summary, we observe from NDACC FTIR measurements a global average annual change of methane of 0.31 ± 0.03 % year-1 (averaged over 10 stations, 2σ level of uncertainty) which is in agreement with a mean annual change of 0.31 ± 0.01 % year-1 (or 5.51 ± 0.17 ppb year-1), as computed from the monthly global means of baseline data derived from AGAGE measurements (Prinn et al., 2000).

In addition, analyses of tropospheric and stratospheric partial columns changes show tropospheric mean annual changes of methane that are statistically in agreement (at the 2σ level) with changes of total column over the 2005–2014 time period. Mean annual changes from the Atmospheric Chemistry Experiment Fourier transform spectrometer methane research product (ACE-FTS, Bernath et al., 2005) have also been examined. For consistent comparison, ACE-FTS stratospheric columns of methane have been defined in the same way as the stratospheric FTIR product, i.e. from the average tropopause height of the station to 30 km. Changes of stratospheric methane according to ACE-FTS retrievals are statistically in agreement with our NDACC FTIR changes of stratospheric columns and show small to non-significant changes of methane in the stratosphere. Indeed, changes of stratospheric methane according to the ACE-FTS methane research product (Buzan et al., 2016) are not significant and amount to -0.12 ± 0.13 % year-1 for the northern high latitudes, 0.10 ± 0.30 for northern midlatitudes, 0.08 ± 0.24 for the tropical region, -0.10 ± 0.31 for the southern midlatitudes, and -0.04 ± 0.14 % year-1 for the southern high latitudes.

The cause of the methane increase since the mid-2000s has often been discussed and still has not been completely resolved (Aydin et al., 2011; Bloom et al., 2010; Dlugokencky et al., 2009; Hausmann et al., 2016; Kirschke et al., 2013; Nisbet et al., 2014; Rigby et al., 2008; Ringeval et al., 2010; Schaefer et al., 2016; Sussmann et al., 2012). On the sink side, Rigby et al. (2008) identified a decrease of OH radicals with a large uncertainty (-4 ± 14 %) from 2006 to 2007 while Montzka et al. (2011) found a small drop of ∼ 1 % year-1, which might have contributed to the enhanced methane in the atmosphere. On the other hand, Bousquet et al. (2011) reported that the changes in OH remain small (< 1 % over the 2006–2008 time period). Nevertheless, observations of small interannual variations are in agreement with the understanding that perturbations in the atmospheric composition generally buffer the global OH concentrations (Dentener, 2003; Montzka et al., 2011).

The small to non-significant changes of methane in the stratosphere, as reported from the analysis of the ACE-FTS methane research product, confirm that the increase of methane takes place in the troposphere. It is indeed driven by increasing sources emitted from the ground (Bousquet et al., 2011; Nisbet et al., 2014; Rigby et al., 2008), primarily affecting its tropospheric abundance and justifying the need for a source-oriented analysis of this recent increase.

Our analysis of the GEOS-Chem-tagged simulation determines that secondary contributors to the global budget of methane, such as coal mining and gas and oil transport and exploitation, have played a major role in the increase of atmospheric methane observed since 2005. However, while the simulation we used comprises the best emission inventories available so far, it has its limitations. Firstly, Schwietzke et al. (2014), Bergamaschi et al. (2013) and Bruhwiler et al. (2014) reported that the EDGAR v4.2 emission inventory overestimates the recent emission growth in Asia. Indeed, Turner et al. (2015) reported from a global GOSAT (Greenhouse gases Observing SATellite) inversion that Chinese methane emissions from coal mining are too large by a factor of 2. Other regional discrepancies between the EDGAR v4.2 inventory and the GOSAT inversion such as an increase in wetland emissions in South America and an increase in rice emissions in South-east Asia, have been pointed out by Turner et al. (2015) as well. On the other hand, it has been shown that the current emissions inventories, including EDGAR v4.2, underestimate the emissions of methane associated with the gas and oil use and exploitation, as well as livestock emissions (Franco et al., 2015, 2016; Turner et al., 2015, 2016). Furthermore, Lyon et al. (2016) pointed out that emissions from oil and gas well pads may be missing from most bottom-up emission inventories. The problem of the source identification clearly resides in the need for a better characterization of anthropogenic emissions and especially in emissions of methane from the oil and gas and livestock sectors.

Concerning the oil and gas emissions, ethane has shown a sharp increase since 2009 of ∼ 5 % year-1 at midlatitudes and of ∼ 3 % year-1 at remote sites (Franco et al., 2016) which is attributed to the recent massive growth of oil and gas exploitation in the North American continent, with the geographical origin of these additional emissions confirmed by Helmig et al. (2016). Since ethane shares an anthropogenic source of methane, i.e. the production, transport and use of natural gas and the leakage associated to it (at 62 %; Logan et al., 1981; Rudolph, 1995), Franco et al. (2016) were able to estimate an increase of oil and gas methane emissions ranging from 20 Tg year-1 in 2008 to 35 Tg year-1 in 2014, using the C2H6/ CH4 ratio derived from GOSAT measurements as a proxy, confirming the influence of fossil fuel and gas production emissions impact on the observed methane increase. Moreover, Hausmann et al. (2016) reported an oil and gas contribution to the renewed methane in Zugspitze of 39 % over the 2007–2014 time period based on a C2H6/ CH4 ratio derived from an atmospheric two-box model. However, as Kort et al. (2016) and Peischl et al. (2016) pointed out, the variability in the C2H6/ CH4 ratio associated to oil and gas production needs to be taken into account in a more rigorous manner as the strength of the C2H6/ CH4 relationship strongly depends on the studied region and/or production basin.

In conclusion, we report changes of atmospheric methane between 2005 and 2014 from FTIR measurements taken at 10 ground-based NDACC observation sites for the first time. From the 10 NDACC methane time series, we computed a mean global annual increase of total column methane of 0.31 ± 0.03 % year-1 (averaged over 10 stations, 2σ level of uncertainty), using 2005.0 as reference, which is consistent with methane changes computed from in situ measurements. From the GEOS-Chem-tagged simulation, accounting for 11 tracers (10 emission sources and one sink) and covering the 2005–2012 time period, we computed a mean annual change of methane of 0.35 ± 0.03 % year-1 since 2005, which is globally in good agreement with the FTIR mean annual changes. In addition, we presented a detailed analysis of the GEOS-Chem tracer changes on both global and local scales over the 2005–2012 time period. To this end, we considered relative year-to-year changes in order to quantify the contribution of each tracer to the global methane change since 2005. According to the GEOS-Chem tagged simulation, wetland methane contributes mostly to the interannual variability while sources that contribute the most to the observed increase of methane since 2005 are mainly anthropogenic: coal mining, gas and oil exploitation, and livestock (from largest to smallest contribution). While we showed that GEOS-Chem agrees with our observations and consequently with the in situ measurements, the repartition between the different sources of methane would greatly benefit from an improvement of the global emission inventories. As an example, Turner et al. (2015) suggested that EDGAR v4.2 underestimates the US oil and gas and livestock emissions while overestimating methane emissions associated to coal mining. From the emission source shared by both ethane and methane and from various ethane studies, it is clear that further attention has to be given to improved anthropogenic methane inventories, such as emission inventories associated with fossil fuel and natural gas production. This is essential in a context of the energy transition that includes the development of shale gas exploitation.

Finally, it is worth mentioning that Schaefer et al. (2016) argue with the fact that thermogenic emissions of methane are responsible for the renewed increase of methane during the mid-2000s. Indeed, from methane isotopologue observations and a one-box model deriving global emission strength and isotopic source signature, Schaefer et al. (2016) reports that the recent methane increase is predominantly due to biogenic emission sources such as agriculture and climate-sensitive natural emissions. These results contrast with the context of a booming natural gas production and the resumption of coal mining in Asia. However, it is also worth noting that the 13C / 12C and D / H ratio of atmospheric methane show distinctive isotope signature depending on the source type (Bergamaschi, 1997; Bergamaschi et al., 1998; Quay et al., 1999; Snover et al., 2000; Whiticar and Schaefer, 2007). In the same way, isotopic fractionation occurs during sink processes with specific ratios depending on the removal pathway (Gierczak et al., 1997; Irion et al., 1996; Saueressig et al., 2001; Snover and Quay, 2000; Tyler et al., 2000). Therefore, the underexploited analysis of the recent methane increase through trend analysis of methane isotopologues, such as 13CH4 and CH3D, is an innovative way of addressing the question of the source(s) responsible for the recent methane increase.

Most of the data used in this publication were obtained as part of the Network for the Detection of Atmospheric Composition Change (NDACC) and are publicly available (see http://www.ndacc.org). Time series used to produce Fig. 5, as well as GEOS-Chem-tagged simulation time series, can be found on the University of Liège's repository (see http://orbi.ulg.ac.be/handle/2268/207090). In situ surface measurements taken in the framework of the NOAA ESRL carbon cycle air sampling network (Dlugokencky et al., 2015; version: 2015-08-03; date accessed: 9 May 2016) are available at ftp://aftp.cmdl.noaa.gov/data/trace_gases/ch4/flask/surface/. Surface GC-MD observations carried out in the framework of the AGAGE programme (Prinn et al., 2000; date accessed: 9 May 2016) are available at http://agage.eas.gatech.edu/data_archive/agage/gc-md/.