Abstract
Methane stored in seabed reservoirs such as methane hydrates can reach the atmosphere in the form of bubbles or dissolved in water. Hydrates could destabilize with rising temperature further increasing greenhouse gas emissions in a warming climate. To assess the impact of oceanic emissions from the area west of Svalbard, where methane hydrates are abundant, we used measurements collected with a research aircraft (Facility for Airborne Atmospheric Measurements) and a ship (Helmer Hansen) during the Summer 2014 and for Zeppelin Observatory for the full year. We present a model‐supported analysis of the atmospheric CH4 mixing ratios measured by the different platforms. To address uncertainty about where CH4 emissions actually occur, we explored three scenarios: areas with known seeps, a hydrate stability model, and an ocean depth criterion. We then used a budget analysis and a Lagrangian particle dispersion model to compare measurements taken upwind and downwind of the potential CH4 emission areas. We found small differences between the CH4 mixing ratios measured upwind and downwind of the potential emission areas during the campaign. By taking into account measurement and sampling uncertainties and by determining the sensitivity of the measured mixing ratios to potential oceanic emissions, we provide upper limits for the CH4 fluxes. The CH4 flux during the campaign was small, with an upper limit of 2.5 nmol m−2 s−1 in the stability model scenario. The Zeppelin Observatory data for 2014 suggest CH4 fluxes from the Svalbard continental platform below 0.2 Tg yr−1. All estimates are in the lower range of values previously reported.
Highlights
Natural and anthropogenic greenhouse gases (GHGs) warm Earths climate
Methane stored in seabed reservoirs such as methane hydrates can reach the atmosphere in the form of bubbles or dissolved in water
We find that the downwind mixing ratio increases correspond to a flux of 2.50 nmol mÀ2 sÀ1 using the stability model between 20 June 2014 and 31 July 2014
Summary
Natural and anthropogenic greenhouse gases (GHGs) warm Earths climate. Methane (CH4) is a major GHG, with both anthropogenic and natural emissions. Methane hydrates (or methane clathrate hydrates) are crystalline compounds in which CH4 is trapped in a lattice of water ice (e.g., 4CH4 · 23H2O) [Sloan and Koh, 2008]. They are stable in solid form within marine sediments at low temperature and high pressure and occur at all continental margins and in permafrost regions of the Earth [Collett et al, 2009]. An estimated 1.55 × 105 Tg of CH4 (total CH4 mass) may be stored in hydrate reservoirs in the sea bed under the shallow waters of the Arctic Ocean alone [Kretschmer et al, 2015]. Dissolved CH4 can be oxidized by methanotrophic bacteria [Gentz et al, 2014; Steinle et al, 2015] but may be transported to PISSO ET AL
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