Climate Change is a major challenge in the Baltic Sea Region. The report “climate change in the Baltic Sea Area HELCOM thematic assessment” (1) provides an overview:

Surface air temperatures “have overall shown a significant increase in the Baltic Sea region over the past 140 years. Since 1871, the annual mean temperature trends show an increase of 0.11 °C per decade north of 60°N and 0.08 °C south of 60°N, while the trend of the global mean temperature was about 0.05 °C per decade for the period 1861 to 2000. The daily temperature cycle is also changing and there has been an increase in temperature extremes. These changes are resulting in changes in the seasons: the length of the growing season has increased, whereas the length of the cold season has decreased. In future, the largest warming was projected for the north in winter. The increase in winter daily mean temperatures will be most pronounced in the coldest periods, while warm extremes in summer are also expected to become more pronounced than at present” (1).

Surface water temperatures “in the Baltic Sea increased in all seasons since 1985. The annual mean sea-surface temperature has been estimated to have increased by up to 1 °C/decade from 1990 to 2008, with the greatest increase in the northern Bothnian Bay. The annual maximum ice extent of the Baltic Sea decreased by 20% and the length of the ice season in the Bothnian Bay, decreased by 18 days during the last century. In future, the summer sea-surface temperature increase is likely to be about 2 °C in the southern parts of the Baltic Sea and about 4 °C in the northern parts near the end of this century” (1).

Precipitation in the “Baltic Sea area during the past century has varied between regions and seasons, with both increasing and decreasing precipitation. Model projections indicate that precipitation will increase in the entire Baltic Sea runoff region during winter, while in the summer increases in precipitation are mainly projected only for the northern half of the basin. In a future warmer climate, extremes of precipitation are projected to increase, implying a greater risk of urban flooding, among other impacts” (1).

Sea-surface salinity shows a decrease. This “may be largest in the region of the Danish straits, especially in the Belt Sea, and small in the northern and eastern Baltic, with the smallest change in the Bothnian Bay” (1).

Sea-level rise is projected to increase between 0.6 m and 1.1 m sea-level rise for the Baltic Sea over the 21st century. In some areas this is compensated by vertical land movement which varies between 0 m/century in Denmark and 0.8 m/century in the Bothnian Bay (1).

Climate change has and will have multiple consequences on the Baltic Sea ecosystem, examples are:

“A temperature increase is expected to result in a change in the species composition and length of the spring phytoplankton bloom season. Changes in the composition of the spring bloom community will also influence the benthos. The projected changes in temperature and particularly salinity are likely to influence the zooplankton community composition, with potential negative consequences for the food conditions and growth of the main plankton-eating fish, Baltic herring and sprat” (1).

“A projected continued decrease in salinity in the future will have a major effect on the distribution of benthic species, with a continued retreat of marine species towards the south.” “Oxygen deficiency is the single most important environmental factor causing habitat loss and reducing the biodiversity of benthic invertebrates….The ‘business-as-usual’ nutrient input scenario yielded an increase of the anoxic area by more than a factor of two, while a moderate increase of about 30% was obtained for the hypoxic area” (1).

“In the littoral zone, the potential decrease in salinity may affect key species such as seagrasses. Some species such as the eelgrass Zostera marina may disappear from areas such as the Gulf of Finland”. Blue mussel communities may face a decrease in growth rate and a spatial shift further to the south-western Baltic Sea (1). “Climate change may have positive effects on littoral vegetation. Milder winters with less coastal sea ice will reduce the scraping of ice in the uppermost part of the algal belt that removes key species” (1).