Using ship-borne observations of methane isotopic ratio in the Arctic Ocean to understand methane sources in the Arctic

Abstract

Abstract. Characterizing methane sources in the Arctic remains challenging due to the remoteness, heterogeneity and variety of such emissions. In situ campaigns provide valuable datasets to reduce these uncertainties. Here we analyse data from the summer 2014 SWERUS-C3 campaign in the eastern Arctic Ocean, off the shore of Siberia and Alaska. Total concentrations of methane, as well as relative concentrations of 12CH4 and 13CH4, were measured continuously during this campaign for 35 d in July and August. Using a chemistry-transport model, we link observed concentrations and isotopic ratios to regional emissions and hemispheric transport structures. A simple inversion system helped constrain source signatures from wetlands in Siberia and Alaska, and oceanic sources, as well as the isotopic composition of lower-stratosphere air masses. The variation in the signature of lower-stratosphere air masses, due to strongly fractionating chemical reactions in the stratosphere, was suggested to explain a large share of the observed variability in isotopic ratios. These results point towards necessary efforts to better simulate large-scale transport and chemistry patterns to make relevant use of isotopic data in remote areas. It is also found that constant and homogeneous source signatures for each type of emission in a given region (mostly wetlands and oil and gas industry in our case at high latitudes) are not compatible with the strong synoptic isotopic signal observed in the Arctic. A regional gradient in source signatures is highlighted between Siberian and Alaskan wetlands, the latter having lighter signatures (more depleted in 13C). Finally, our results suggest that marine emissions of methane from Arctic continental-shelf sources are dominated by thermogenic-origin methane, with a secondary biogenic source as well.

Shipborne measurements of methane and carbon dioxide in the Middle East and Mediterranean areas and contribution from oil and gas emissions

“…Since our study is focusing on anthropogenic emissions and given low natural emissions in the Eastern Mediterranean and Middle East area (Saunois et al, 2020) we used only anthropogenic inventories in our study. However, some level of sensitivity to natural fluxes can be expected.…”

“…Methane (CH4) and carbon dioxide (CO2) are potent anthropogenic greenhouse gases (GHG). The CH4 atmospheric mole fraction has increased by 150% since the pre-industrial era (Saunois et al, 2020). Roughly half of CH4 sources are of natural origin (mainly from wetlands, with contributions from fires and geologic sources).…”

“…Fossil fuel production and use is responsible for the release of 112 Mt CH4 yr -1 to the atmosphere, representing 33% of the total anthropogenic emission of CH4 (Saunois et al, 2020). Considering fossil fuel emissions alone, 68% of the emissions is linked to oil and gas (O&G) while the rest is associated to coal mining.…”

“…Large uncertainties remain associated to the magnitude and spatial and temporal distribution of CH4 sources (Saunois et al, 2020). After a pause between 2000 and 2007, CH4 in the atmosphere has resumed its increase.…”

“…After a pause between 2000 and 2007, CH4 in the atmosphere has resumed its increase. Constraints from current available observations on the respective contribution to its sources and sinks do not allow a definitive explanation of this pattern (Saunois et al, 2017;Saunois et al, 2020;Turner et al, 2019, Nisbet et al, 2020. Schwietzke et al (2016) highlighted the strong underestimation of O&G emissions in current inventories (see also Saunois et al, 2020 and references therein).…”