Climate change in the Baltic Sea region: a summary

Abstract

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.

The importance of considering the duration of extreme temperatures when investigating responses to climate change

“…Stickleback migrate to the bay from the open sea in the spring to spawn, and leave it at the end of the summer when the breeding season ends. Temperature fluctuates in the bay across days (Granroth‐Wilding & Candolin, 2022 ) and years (Meier et al, 2022 ), depending on climate conditions, because of its shallow topography; max depth is about 1.5 m. Thus, the population has experienced fluctuations in water temperature during the breeding season in its evolutionary past. We transported the fish to Tvärminne Zoological station, University of Helsinki, within 20 min.…”

Intraspecific Trait Variability of a Diatom and a Dinoflagellate Along a Salinity Gradient

“…The strains of S. marinoi (C1406, C1407, C1412, C1418, and C1428) were isolated from different locations in the Bothnian Sea, while the strains of A. ostenfeldii were isolated from different locations in the Åland Sea (AOF0913, AOF0936, AOF0940, and AOK1024) and Gotland (AOVA0930). The Bothnian Sea and Åland Sea are sub-divisions of the Gulf of Bothnia with a 4-5 psu salinity range, while Gotland is located in the Baltic Proper characterized by salinities of 7-8 psu (Meier et al, 2022). All strains were cultivated in F/2 medium (NO 3 = 888 μM, PO 4 = 36 μM, Si = 106 μM, trace metals and vitamins), and acclimated to the experimental conditions (temperature = 16°C, photosynthetic photon flux density = 150 μmol photon m -2 s -1 , light-dark cycle = 16:8 h, sunlight spectrum Aquarius plant LED, AB Aqua Medic GmbH).…”

Assessing the Radioactivity Level of Seawater and Sand Samples from the Baltic Sea Region

“…Sample 3 is also the sand sample, but it was collected at the top of the approximately 10 m high dune, almost 46 m from the coast. This sand was not in direct contact with seawater, but the same as in the seawater sample and the foreshore sand sample, radionuclides in Sample 3 can be collected from atmospheric precipitates, caused by rainfall (most rainfall is around 650 mm per year) and fog (most often during winter-fall periods) 15 . Therefore, Sample 3 was used in our experiments for reference purposes.…”