Climate Change in the Baltic Sea Region: A Summary
Authors: Meier H. E. Markus , Kniebusch Madline , Dieterich Christian , Gröger Matthias , Zorita Eduardo , Elmgren Ragnar , Myrberg Kai , Ahola Markus , Bartosova Alena , Bonsdorff Erik , Börgel Florian , Capell Rene , Carlén Ida , Carlund Thomas , Carstensen Jacob , Christensen Ole B. , Dierschke Volker , Frauen Claudia , Frederiksen Morten , Gaget Elie , Galatius Anders , Haapala Jari J. , Halkka Antti , Hugelius Gustaf , Hünicke Birgit , Jaagus Jaak , Jüssi Mart , Käyhkö Jukka , Kirchner Nina , Kjellström Erik , Kulinski Karol , Lehmann Andreas , Lindström Göran , May Wilhelm , Miller Paul , Mohrholz Volker , Müller-Karulis Bärbel , Pavón-Jordán Diego , Quante Markus , Reckermann Marcus , Rutgersson Anna , Savchuk Oleg P. , Stendel Martin , Tuomi Laura , Viitasalo Markku , Weisse Ralf , Zhang Wenyan 
Published: August 2021

Abstract. Based on the Baltic Earth Assessment Reports of this thematic issue in Earth System Dynamics and recent peer-reviewed literature, current knowledge about the effects of global warming on past and future changes in climate of the Baltic Sea region is summarized and assessed. The study is an update of the Second Assessment of Climate Change (BACC II) published in 2015 and focusses on the atmosphere, land, cryosphere, ocean, sediments and the terrestrial and marine biosphere. Based on the summaries of the recent knowledge gained in paleo-, historical and future regional climate research, we find that the main conclusions from earlier assessments remain still valid. However, new long-term, homogenous observational records, e.g. for Scandinavian glacier inventories, sea-level driven saltwater inflows, so-called Major Baltic Inflows, and phytoplankton species distribution and new scenario simulations with improved models, e.g. for glaciers, lake ice and marine food web, have become available. In many cases, uncertainties can now be better estimated than before, because more models can be included in the ensembles, especially for the Baltic Sea. With the help of coupled models, feedbacks between several components of the Earth System have been studied and multiple driver studies were performed, e.g. projections of the food web that include fisheries, eutrophication and climate change. New data sets and projections have led to a revised understanding of changes in some variables such as salinity. Furthermore, it has become evident that natural variability, in particular for the ocean on multidecadal time scales, is greater than previously estimated, challenging our ability to detect observed and projected changes in climate. In this context, the first paleoclimate simulations regionalized for the Baltic Sea region are instructive. Hence, estimated uncertainties for the projections of many variables increased. In addition to the well-known influence of the North Atlantic Oscillation, it was found that also other low-frequency modes of internal variability, such as the Atlantic Multidecadal Variability, have profound effects on the climate of the Baltic Sea region. Challenges were also identified, such as the systematic discrepancy between future cloudiness trends in global and regional models and the difficulty of confidently attributing large observed changes in marine ecosystems to climate change. Finally, we compare our results with other coastal sea assessments, such as the North Sea Region Climate Change Assessment (NOSCCA) and find that the effects of climate change on the Baltic Sea differ from those on the North Sea, since Baltic Sea oceanography and ecosystems are very different from other coastal seas such as the North Sea. While the North Sea dynamics is dominated by tides, the Baltic Sea is characterized by brackish water, a perennial vertical stratification in the southern sub-basins and a seasonal sea ice cover in the northern sub-basins.