Methane in the Baltic and North Seas and a reassessment of the marine emissions of methane

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Abstract
During three measurement campaigns on the Baltic and North Seas, atmospheric and dissolved methane was determined with an automated gas chromatographic system. Area‐weighted mean saturation values in the sea surface waters were 113 ± 5% and 395 ± 82% (Baltic Sea, February and July 1992) and 126 ± 8% (south central North Sea, September 1992). On the bases of our data and a compilation of literature data the global oceanic emissions of methane were reassessed by introducing a concept of regional gas transfer coefficients. Our estimates computed with two different air‐sea exchange models lie in the range of 11–18 Tg CH4 yr−1. Despite the fact that shelf areas and estuaries only represent a small part of the world's ocean they contribute about 75% to the global oceanic emissions. We applied a simple, coupled, three‐layer model to evaluate the time dependent variation of the oceanic flux to the atmosphere. The model calculations indicate that even with increasing tropospheric methane concentration, the ocean will remain a source of atmospheric methane.