Baltic Sea ice conditions will change
Global warming reduces the extent of ice cover on the Baltic Sea, and shortens the ice season. However, variation between winters will remain as a characteristic of ice conditions.
Ice area will diminish
In the Baltic Sea the ice cover is most extensive in March. The average extent of ice is 204,000 km², with the southern edge of ice in northern Baltic Sea. In harsh winters, ice covers the entire Baltic Sea (total area 420,000 km²), while in mildest winters, only the Gulf of Bothnia and the eastern parts of the Gulf of Finland have ice (total area approx. 50,000 km²).
Model estimates indicate approximately three degrees higher annual mean temperatures after mid-century than at present, and this would decrease the average ice cover by 100,000–140,000 km2. The extremes, too, will change significally: harsh ice seasons with all of the Baltic Sea covered with ice, would perhaps not occur at all, and the mildest winters would see ice only at the bottom of Bothnian Bay.
Shorter ice season
At present, the ice season lasts 130–200 days in the Bothnian Bay, 80-100 days in the eastern part of the Gulf of Finland, and 0–60 days in northern Baltic Sea. A three degree increase in the mean temperature of the Baltic Sea area shortens the ice cover period by 40–70 days.
The ice season will become shorter at both ends, but the time of freezing over will change more than the date of ice break-up. This is due to the fact that freezing depends not only on the coldness of winter but also the heat content of sea water that has accumulated mainly during the summer. Hence warmer autumns will delay sea ice formation.
Thawing of the ice depends on its thickness, spring temperatures and the amount of solar radiation arriving on the surface of the ice. Warmer winters make the ice thinner. So it thaws faster, even though solar radiation will not increase in any way in future.
Projecting ice conditions in the next few decades
In lack of precise estimates based on sea ice modelling it can be presumed that if warming progresses in a linear fashion, Baltic Sea area climate would be about 0.5–1 °C warmer in 2030 than today. In these circumstances, ice conditions in the Baltic Sea would be slightly milder than at present: the largest extent of ice in winter would be 30,000–50,000 km² smaller on average, and the ice season 10–20 days shorter than in the present climatic conditions. The mildest winters would see ice only in the Bothnian Bay, Archipelago Sea and eastern Gulf of Finland. On the other hand, severe ice seasons may still occur due to natural climatic variation.
In addition to the extent of sea ice, its thickness must be taken into account
In addition to winter temperature conditions, snowfall, and in open sea ice areas, the movement of ice, influence the increase in sea ice thickness. In the Bothnian Bay, the ice may reach a maximum thickness of 70–120 centimetres in fast ice areas near the coast. In ridged ice areas, even the average thickness of ice may reach several metres, with individual walls of ridged ice even 20–30 metres thick.
Long series of ice thickness measurements in the Baltic Sea are only available for fast ice areas. As no uniform trend is discernible in time series, it is probable that variations in the quantity of snowfall are reflected more on the thickness of fast ice than changes in air temperature.