| Summary: | 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 km 2 (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.
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