METHANE IN THE BALTIC AND NORTH SEAS AND A REASSESSMENT OF THE MARINEEMISSIONS OF METHANE

During three measurement campaigns on the Baltic and North Seas, atmos pheric and dissolved methane was determined with an automated gas chro matographic system. Area-weighted mean saturation values in the sea su rface 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 glob al oceanic emissions of methane were reassessed by introducing a conce pt 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 represen t a small part of the world's ocean they contribute about 75% to the g lobal oceanic emissions. We applied a simple, coupled, three-layer mod el 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.