Methane emissions in the southern North Sea: From single cruise tracks to an aerial German Bight extrapolation

In the context of the project "Integrated greenhouse gas monitoring system for Germany", a central objective is to facilitate the availability of marine cruise-based greenhouse gas data for the land-based or atmospheric end-user community.

During annual ship cruises in the southern German Bight since 2019, the concentrations of dissolved methane were continuously determined. Based on wind speed and atmospheric methane concentration, the diffusive methane flux was calculated. Since the configuration of cruise tracks is generally limited to single lines between different ports, spatial extrapolation to wider areas is needed for regional flux estimates.

Therefore, we utilised water body categories provided by the Federal Environment Agency, which included euhaline, polyhaline tidal flats, and coastal waters, to extrapolate methane emissions to the entire region. Six regional categories were defined based on salinity, tidal flat exposure, tidal range, current velocity and other factors. For the years 2019-2023 the six categories had significantly different methane fluxes. The combined fluxes ranged from 3.2 g CH₄ / m² in 2019 to 20.3 g/m² in 2023. By combining the methane recorded during cruise track with the shape files of the different water body categories, we can provide methane emissions estimates and variability from the whole southern North Sea which are suited for use by local authorities and stakeholders.