The physics of the ice season in the Baltic Sea is presented for its research history and present state of understanding. Knowledge has been accumulated since the 1800s, first in connection of operational ice charting; deeper physics came into the picture in the 1960s along with sea ice structure and pressure ridges. Then the drift of ice and ice forecasting formed the leading line for 20 years, and over to the present century, ice climate modeling and satellite remote sensing have been the primary research topics. The physics of the Baltic Sea ice season is quite well understood, and toward future ice conditions realistic scenarios can be constructed from hypothetical regional climate scenarios. The key factor in climate scenarios is the air temperature in the Baltic Sea region. The local freezing and breakup dates show sensitivity of 5–8 days’ change to climate warming by 1 °C, while this sensitivity of sea ice thickness is 5–10 cm. However, sea ice thickness and breakup date show sensitivity also to snow accumulation: More snow gives later breakup, but the thickness of ice may decrease due to better insulation or increase due to more snow-ice. The annual probability of freezing decreases with climate warming, and the sensitivity of maximum annual ice extent is 35,000–40,000 km2 (8.3%–9.5% of the Baltic Sea area) for 1 °C climate warming. Due to the large sensitivity to air temperature, the severity of the Baltic Sea ice season is closely related to the North Atlantic Oscillation.
History and Future of Snow and Sea Ice in the Baltic Sea
Projected Oceanographical Changes in the Baltic Sea until 2100
H.E. Markus Meier and Sofia Saraiva
In this article, the concepts and background of regional climate modeling of the future Baltic Sea are summarized and state-of-the-art projections, climate change impact studies, and challenges are discussed. The focus is on projected oceanographic changes in future climate. However, as these changes may have a significant impact on biogeochemical cycling, nutrient load scenario simulations in future climates are briefly discussed as well. The Baltic Sea is special compared to other coastal seas as it is a tideless, semi-enclosed sea with large freshwater and nutrient supply from a partly heavily populated catchment area and a long response time of about 30 years, and as it is, in the early 21st century, warming faster than any other coastal sea in the world. Hence, policymakers request the development of nutrient load abatement strategies in future climate. For this purpose, large ensembles of coupled climate–environmental scenario simulations based upon high-resolution circulation models were developed to estimate changes in water temperature, salinity, sea-ice cover, sea level, oxygen, nutrient, and phytoplankton concentrations, and water transparency, together with uncertainty ranges. Uncertainties in scenario simulations of the Baltic Sea are considerable. Sources of uncertainties are global and regional climate model biases, natural variability, and unknown greenhouse gas emission and nutrient load scenarios. Unknown early 21st-century and future bioavailable nutrient loads from land and atmosphere and the experimental setup of the dynamical downscaling technique are perhaps the largest sources of uncertainties for marine biogeochemistry projections. The high uncertainties might potentially be reducible through investments in new multi-model ensemble simulations that are built on better experimental setups, improved models, and more plausible nutrient loads. The development of community models for the Baltic Sea region with improved performance and common coordinated experiments of scenario simulations is recommended.