Cycling of the greenhouse gas methane in sea ice and sea water - Besides carbon dioxide methane is one of the most important greenhouse gases in the atmosphere which contribute to climate warming. Most of the methane which is released to the atmosphere is produced in biological processes under the strict absence of oxygen. In the last years methane was observed in unexpectedly high concentrations in the Arctic Ocean surface waters despite the presence of oxygen. This phenomenon is called “methane paradox” and was first discovered in 1977 by scientists investigating methane in the subtropical North-Atlantic Ocean (Scranton and Brewer, 1977). It is likely that the methane is produced in the oxygen rich water by microorganisms, but the exact processes remain yet to be understood. This article explains the concept for methane production in oxygen rich waters in the sea ice covered central Arctic and highlights the ongoing research we performed during the research expedition TRANSARC II with the German Research icebreaker Polarstern in summer 2015.