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The Baltic Sea is highly impacted by global warming and other anthropogenic changes and is one of the fastest‐warming marginal seas in the world. To detect trends in water temperature and to attribute them to atmospheric parameters, the results of two different ocean circulation models driven by reconstructed atmospheric forcing fields for the period 1850–2008 were analyzed. The model simulations were analyzed at temporal and spatial scales from seasonal to centennial and from intrabasin to basin, respectively. The strongest 150‐year trends were found in the annual mean bottom temperature of the Bornholm Deep (0.15 K/decade) and in summer mean sea surface temperature (SST) in Bothnian Bay (0.09–0.12 K/decade). A comparison of the time periods 1856–2005 and 1978–2007 revealed that the SST trends strengthened tenfold. An attribution analysis showed that most of the SST variability could be explained by the surface air temperature (i.e., sensible heat flux) and the latent heat flux. Wind parallel to the coast and cloudiness additionally explained SST variability in the coastal zone affected by the variations in upwelling and in offshore areas affected by the variations in solar radiation, respectively. In contrast, the high variability in stratification caused by freshwater and saltwater inflows does not impact the long‐term variability in the SST averaged over the Baltic Sea. The strongest SST trends since the 1980s can be explained by the superposition of global warming and a shift from the cold to the warm phase of the Atlantic Multidecadal Oscillation.

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Abstract. Based on the Baltic Earth Assessment Reports of this thematic issue in Earth System Dynamics and recent peer-reviewed literature, current knowledge of the effects of global warming on past and future changes in climate of the Baltic Sea region is summarised and assessed. The study is an update of the Second Assessment of Climate Change (BACC II) published in 2015 and focuses on the atmosphere, land, cryosphere, ocean, sediments, and the terrestrial and marine biosphere. Based on the summaries of the recent knowledge gained in palaeo-, historical, and future regional climate research, we find that the main conclusions from earlier assessments still remain valid. However, new long-term, homogenous observational records, for example, 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, for example, 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 were 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 datasets 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 timescales, is greater than previously estimated, challenging our ability to detect observed and projected changes in climate. In this context, the first palaeoclimate simulations regionalised 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 are dominated by tides, the Baltic Sea is characterised by brackish water, a perennial vertical stratification in the southern subbasins, and a seasonal sea ice cover in the northern subbasins.