Controls on methane cycling in the Baltic Sea

The Baltic Sea is an ideal natural laboratory for the study of climate change impacts on the ocean, where the greatest rate of change is now being found in semi-enclosed seas. The recent IPCC AR5 assessment reports that under RCP 8.5 Baltic sea surface temperatures will increase by 4.3°C by the end of the century, with the changes in stratification and ice coverage very likely to affect fisheries and the spreading of hypoxia. I will discuss the recent work on the cycle of methane in the Baltic, and its relationship to historical conditions, geological development, present day stability of the water column, and the presence of hypoxia in the deep basins. I will show that methane turnover is closely related to the hydrodynamic conditions, and that although high methane in the anoxic sediments is present, leakage to the atmosphere currently is and will likely remain small. The Baltic comprises of several sub-basins facing different degrees and variability of hypoxia, and thus can serve as an early indicator of aquatic methane dynamics worldwide. It is also a good example to illustrate that globally predicted responses of enclosed seas to global change, such as itensified hypoxia and steepening of the redoxcline might be misleading on a regional scale.

Speaker: Gregor Rehder