Climate change has already influenced aspects of the Baltic Sea ecosystem. A warming trend in sea-surface waters has been clearly demonstrated. Remote sensing data for the period 1990–2008 indicate that the annual mean sea-surface temperature increased by up to 1°C per decade, with the greatest increase in the northern Bothnian Bay and large increases in the Gulf of Finland, the Gulf of Riga, and the northern central Baltic.
A change towards milder ice winters has been observed over the past one hundred years. In particular, the annual maximum ice extent has decreased and the length of the ice season has become shorter.
Based on projections from regionally downscaled climate models, we are likely to expect large-scale alterations in the hydrography, biogeochemistry, and physical properties of the Baltic Sea during this century, including long-term changes in temperature, ice cover, salinity, oxygen, nutrient concentrations, and primary production. There are likely to be consequent direct and indirect responses of phytoplankton, zooplankton, benthos, fish, seabirds, and habitats.
At the global level, current greenhouse gas emissions are most closely following the IPCC Regional Concentration Pathway (RCP) 8.5 scenario. Within the Baltic Sea, this scenario projects a 2.5°C to 4.0°C sea surface warming above mean conditions for the years 2050–2099, with the highest increases seen in coastal areas (Figure 12).
Figure 12: Ensemble mean sea surface temperature from the 5th Coupled Model Intercomparison Project (CMIP5) interpolated on a 1×1 grid for the entire year in the Baltic Sea ecoregion. (Left) Historical SST for the 1956–2005. (Right) Difference in the mean climate in the future time period (RCP8.5: 2050–2099) compared to the historical reference period.