Detectability of Arctic methane sources at six sites performing continuous atmospheric measurements

Abstract

Abstract. Understanding the recent evolution of methane emissions in the Arctic is necessary to interpret the global methane cycle. Emissions are affected by significant uncertainties and are sensitive to climate change, leading to potential feedbacks. A polar version of the CHIMERE chemistrytransport model is used to simulate the evolution of tropospheric methane in the Arctic during 2012, including all known regional anthropogenic and natural sources, in particular freshwater emissions which are often overlooked in methane modelling. CHIMERE simulations are compared to atmospheric continuous observations at six measurement sites in the Arctic region. In winter, the Arctic is dominated by anthropogenic emissions; emissions from continental seepages and oceans, including from the East Siberian Arctic Shelf, can contribute significantly in more limited areas. In summer, emissions from wetland and freshwater sources dominate across the whole region. The model is able to reproduce the seasonality and synoptic variations of methane measured at the different sites. We find that all methane sources significantly affect the measurements at all stations at least at the synoptic scale, except for biomass burning. In particular, freshwater systems play a decisive part in summer, representing on average between 11 and 26 % of the simulated Arctic methane signal at the sites. This indicates the relevance of continuous observations to gain a mechanistic understanding of Arctic methane sources. Sensitivity tests reveal that the choice of the land-surface model used to prescribe wetland emissions can be critical in correctly representing methane mixing ratios. The closest agreement with the observations is reached when using the two wetland models which have emissions peaking in AugustSeptember, while all others reach their maximum in JuneJuly. Such phasing provides an interesting constraint on wetland models which still have large uncertainties at present. Also testing different freshwater emission inventories leads to large differences in modelled methane. Attempts to include methane sinks (OH oxidation and soil uptake) reduced the model bias relative to observed atmospheric methane. The study illustrates how multiple sources, having different spatiotemporal dynamics and magnitudes, jointly influencePublished by Copernicus Publications on behalf of the European Geosciences Union. 8372T. Thonat et al.: Detectability of Arctic methane sources the overall Arctic methane budget, and highlights ways towards further improved assessments.

Assessment of the theoretical limit in instrumental detectability of northern high-latitude methane sources using <i>δ</i><sup>13</sup>C<sub>CH4</sub> atmospheric signals

“…Emissions from geological sources stem from the GLOGOS database (Etiope, 2015) and amount to 4.0 Tg CH 4 yr −1 in our domain. ESAS emissions are prescribed to 2 Tg CH 4 yr −1 in agreement with the estimate made by Thornton et al (2016a) based on a ship measurement campaign and with the estimate made by Berchet et al (2016) based on atmospheric observations at surface stations. The temporal and geographic variability of the ESAS emissions is based on the description by Shakhova et al (2010), following the modelling framework of Berchet et al (2016).…”

“…Most major source types for methane are present in the northern high latitudes: natural wetlands, oil and gas industry, and peat and forest burnings. There are also other sources that have received increasing attention over the past decade: freshwater systems (Walter et al, 2007;Bastviken et al, 2011;Tan and Zhuang, 2015;Wik et al, 2016), subsea permafrost and hydrates in the East Siberian Arctic Shelf (ESAS, in the Laptev and East Siberian seas; Shakhova et al, 2010;Berchet et al, 2016;Thornton et al, 2016a), and terrestrial thermokarst (Wik et al, 2016).…”

Monthly Gridded Data Product of Northern Wetland Methane Emissions Based on Upscaling Eddy Covariance Observations

“…9). Warwick 10 et al (2016) and Thonat et al (2017) showed that the northern wetland CH4 emissions should peak in August-September in order to explain correctly the seasonality of atmospheric CH4 mixing ratios and isotopes measured across the Arctic. Hence the wetland CH4 emissions presented here are peaking approximately one month too early to perfectly match with their findings.…”

“…Bloom et al, 2017aZhang et al, 2016) and 2) uncertainties related to the key CH4 emission drivers and responses to these drivers (e.g. Bloom et al, 2017a;Saunois et al, 2017). Evaluation of the emission estimates is 5 thus urgently needed, and the results will feed on improvements in process models.…”

“…The Arctic is projected to warm during the next century at a faster rate than any other region, which will likely significantly impact the carbon cycling of wetland ecosystems (Tarnocai, 2009;Zhang et al, 2017b) and permafrost areas of the Arctic-Boreal Region (Schuur et al, 2015). To date, CH4 emission estimates for northern wetlands are typically based on process models Bloom et al, 2017a;Chen et al, 2015;Melton et al, 2013;Stocker et al, 2013;Wania et al, 2010;Watts et al, 2014;Zhang et al, 2016) or inversion modelling Bruhwiler et al, 2014;10 Spahni et al, 2011;Thompson et al, 2017;Thonat et al, 2017;Warwick et al, 2016), yet scaling of existing chamber measurements to northern wetland area has also been published (Zhu et al, 2013). However, the first two are not completely independent since the attribution of CH4 emissions derived using inversion models to different emission sources (e.g.…”