Atmos. Chem. Phys., 18, 17207–17224, 2018 https://doi.org/10.5194/acp-18-17207-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Methane at Svalbard and over the European Arctic Ocean Stephen M. Platt1, Sabine Eckhardt1, Benedicte Ferré2, Rebecca E. Fisher3, Ove Hermansen1, Pär Jansson2, David Lowry3, Euan G. Nisbet3, Ignacio Pisso1, Norbert Schmidbauer1, Anna Silyakova2, Andreas Stohl1, Tove M. Svendby1, Sunil Vadakkepuliyambatta2, Jürgen Mienert2, and Cathrine Lund Myhre1 1NILU – Norwegian Institute for Air Research, P.O. Box 100, 2027 Kjeller, Norway 2CAGE-Centre for Arctic Gas Hydrate, Environment and Climate, Department of Geosciences, UiT The Arctic University of Norway, 9037 Tromsø, Norway 3Department of Earth Sciences, Royal Holloway, University of London, Egham, UK Correspondence: Stephen M. Platt ([email protected]) Received: 15 June 2018 – Discussion started: 5 July 2018 Revised: 12 October 2018 – Accepted: 31 October 2018 – Published: 5 December 2018 Abstract. Methane (CH4) is a powerful greenhouse gas. long-range transport from land-based sources, lending confi- Its atmospheric mixing ratios have been increasing since dence to the present inventories for high-latitude CH4 emis- 2005. Therefore, quantification of CH4 sources is essential sions. However, we also identify a potential hotspot region ◦ for effective climate change mitigation. Here we report ob- with ocean–atmosphere CH4 flux north of Svalbard (80.4 N, ◦ −2 −1 servations of the CH4 mixing ratios measured at the Zep- 12.8 E) of up to 26 nmol m s from a large mixing ra- pelin Observatory (Svalbard) in the Arctic and aboard the re- tio increase at the location of 30 ppb. Since this flux is con- search vessel (RV) Helmer Hanssen over the Arctic Ocean sistent with previous constraints (both spatially and tempo- from June 2014 to December 2016, as well as the long- rally), there is no evidence that the area of interest north of term CH4 trend measured at the Zeppelin Observatory from Svalbard is unique in the context of the wider Arctic. Rather, 2001 to 2017. We investigated areas over the European Arc- because the meteorology at the time of the observation was tic Ocean to identify possible hotspot regions emitting CH4 unique in the context of the measurement time series, we from the ocean to the atmosphere, and used state-of-the-art obtained over the short course of the episode measurements modelling (FLEXPART) combined with updated emission highly sensitive to emissions over an active seep site, without inventories to identify CH4 sources. Furthermore, we col- sensitivity to land-based emissions. lected air samples in the region as well as samples of gas hydrates, obtained from the sea floor, which we analysed us- ing a new technique whereby hydrate gases are sampled di- rectly into evacuated canisters. Using this new methodology, 1 Introduction we evaluated the suitability of ethane and isotopic signatures 13 The atmospheric mixing ratio of methane (CH ), a powerful (δ C in CH4) as tracers for ocean-to-atmosphere CH4 emis- 4 sion. We found that the average methane / light hydrocarbon greenhouse gas with global warming potential ∼ 32 times (ethane and propane) ratio is an order of magnitude higher higher than carbon dioxide (CO2) (Etminan et al., 2016), for the same sediment samples using our new methodology has increased by over 150 % since pre-industrial times (Hart- compared to previously reported values, 2379.95 vs. 460.06, mann et al., 2013; IPCC, 2013). The CH4 mixing ratio in- respectively. Meanwhile, we show that the mean atmospheric creased significantly during the 20th century, and then sta- bilized from 1998 to 2005. This brief hiatus ended in 2005 CH4 mixing ratio in the Arctic increased by 5:9 ± 0:38 parts per billion by volume (ppb) per year (yr−1) from 2001 to and the mixing ratio has been increasing rapidly ever since 2017 and ∼ 8 pbb yr−1 since 2008, similar to the global trend (Hartmann et al., 2013; IPCC, 2013). For example, the global of ∼ 7–8 ppb yr−1. Most large excursions from the baseline mean CH4 mixing ratio was 1953 ppb in 2016, an increase of 9.0 ppb compared to the previous year (WMO, 2017). An CH4 mixing ratio over the European Arctic Ocean are due to ∼ 8–9 ppb increase per year in atmospheric CH4 is equivalent Published by Copernicus Publications on behalf of the European Geosciences Union. 17208 S. M. Platt et al.: Methane at Svalbard and over the European Arctic Ocean to a net emissions increase of ∼ 25 Tg CH4 per year (Worden Arctic temperatures in 2007 to elevated global CH4 mixing et al., 2017). ratios in the same year due to increased high-latitude wetland The reasons for the observed increases in atmospheric emissions. Other Arctic CH4 sources sensitive to tempera- CH4 are unclear. A probable explanation, identified via shifts ture include forest and tundra wildfires, likely to increase in 13 in the atmospheric δ C in CH4 isotopic ratio compared to frequency and intensity with warmer temperatures and more 13 the Vienna Pee Dee Belemnite standard (δ C in CH4 vs. V- frequent droughts (Hu et al., 2015), and thawing permafrost PDB) is increased CH4 emissions from wetlands, both in the and tundra (Saunois et al., 2016). −1 tropics (Nisbet et al., 2016) as well as in the Arctic (Fisher Oceanic CH4 sources, are small globally (2–40 Tg yr ) et al., 2011). For example, Nisbet et al, 2016 report that the compared to terrestrial sources such as wetlands (153– −1 −1 increases in CH4 concentrations since 2005 coincided with 227 Tg yr ) and agriculture (178–206 Tg yr ) (Kirschke et 13 a negative shift in δ C in CH4. Because fossil fuels have al., 2013; Saunois et al., 2016). However, oceanic CH4 fluxes 13 δ C in CH4 above the atmospheric background, this nega- are highly uncertain and may be particularly important in tive shift implies changes in the balance of sources and sinks. the Arctic due to the extremely large reservoirs of CH4 un- I.e. even if fossil fuel emissions are partly responsible for the der the seabed, and the potential for climate feedbacks. For increases in the CH4 atmospheric mixing ratio since 2005, example, gas hydrates (GHs), an ice-like substance formed their relative contribution has decreased. This suggests a role in marine sediments, can store large amounts of CH4 under for emissions from methanogenic bacteria in wetland soils low-temperature and high-pressure conditions within the gas and/or ruminants, since these do have strongly negative δ13C hydrate stability zone (GHSZ) (Kvenvolden, 1988). Around in CH4 compared to ambient values and fossil sources, or Svalbard the GHSZ retreated from 360 to 396 m over a period changes in the sink strength (reaction with hydroxyl radicals, of around 30 years, possibly due to increasing water tempera- OH). ture (Westbrook et al., 2009), though numerous other sources There is also evidence that the fraction of CH4 emitted by dispute this: for example, Wallmann et al. (2018) suggest that fossil fuels is higher than previously thought, based on mix- the retreating GHSZ is due to geologic rebound since the ing ratios of co-emitted ethane (Worden et al., 2017; Dal- regional ice sheets melted (isostatic shift). The climate im- søren et al., 2018), suggesting that current emission inven- pact of decomposing GHs is poorly constrained, in part due tories need revaluating. As well as increases in the average to large uncertainties in their extent (Marín-Moreno et al., global CH4 mixing ratio, ethane, often co-emitted with an- 2016). Though Kretschmer et al. (2015) give a recent esti- thropogenic CH4 has also increased. However, this ethane mate of 116 Gt carbon stored in hydrates under the Arctic increase is weaker and less consistent than that of CH4 itself Ocean, other estimates vary widely, from 0.28 to 512 Gt car- (Helmig et al., 2016), indicating another source than fossil bon (Marín-Moreno et al., 2016, and references therein). fuel emissions contibutes to recent CH4 increases, as well Presently, little of the CH4 entering the water column over as a lack of consensus as to which sources are predominantly active geologic seep sites and at the edge of the GHSZ around responsible for the increase in the CH4 mixing ratio. Accord- Svalbard reaches the atmosphere. CH4 fluxes to the atmo- −2 −1 ingly, it is clear that although a total net CH4 flux to the at- sphere were below 2:4±1:4 nmol m s in summer 2014 at −1 mosphere of ∼ 550 Tg CH4 yr is well constrained via ob- a shallow seep site (50–120 m depth) off Prins Karls Forland servations (Kirschke et al., 2013), the relative contribution of (Myhre et al., 2016) and below 0.54 nmol m−2 s−1 for all the individual sources and sinks responsible for the rapid in- waters less than 400 m deep around Svalbard in 2014–2016 creases since 2005 is uncertain (Dalsøren et al., 2016; Nisbet (Pisso et al., 2016). Such low ocean–atmosphere CH4 fluxes, et al., 2016; Saunois et al., 2016). This makes future warming even over strong sub-sea sources, may be due to the efficient due to CH4 emissions difficult to predict. Therefore, the re- consumption of CH4 by methanotrophic bacteria (Reeburgh, cent observed increase in the atmospheric CH4 mixing ratio 2007). However, the extent to which microbiology or any has led to enhanced focus and intensified research to improve other factor mitigates the climate impact of sub-sea seep sites our understanding of CH4 sources, particularly in response to across the wider Arctic region, and whether it will continue global and regional climate change.