Comment on the results
It is hard to see a clear continuous trend in the sea ice extent as the yearly variations are large. For the years 1957-1987 the average level was approximately 200 000 square kilometers, while the average maximum level extension the past 30 years has been approximately 130 000 square kilometers. Thus, it seems to be a shift in the ice climate around 1990.
How is maximum sea ice defined?
The indicator shows the largest area measured during an ice season for the Baltic Sea and Kattegat. The area of this entire area is approximately 420,000 square kilometers. Skagerrak is not included, and neither are lakes such as Vänern, Vättern and Mälaren.
Why is this indicator important?
Mapping of sea ice has been done for a long time and is linked to shipping in the Baltic Sea area. From the 1970s, there has been an ambition to be able to keep ship traffic running to all important ports along the Swedish coast all year round, and the need for accurate mapping of the ice situation increased as a result. With the increase in winter shipping, the need for a modern icebreaker operation also grew to be able to assist the ships to the Swedish ports in the north.
The indicator is important for shipping but also for understanding the impact on the Baltic Sea as an ecosystem. There is an interest in following the spread of sea ice from a recreational point of view, or for planning other types of transport in coastal areas and archipelagos, for example via ice roads.
How has the indicator been calculated?
Ice maps, from which calculations of the ice extent is made, is produced daily at SMHI. Ice maps have been digitized by SMHI from 1957 until present day, which is why the measurement series starts in 1957. One can find information for earlier years, but comprehensive maps for the entire Baltic are difficult to find so far back in time.
Nowadays there are satellite pictures, but previously a number of ice maps were drawn by pilots flying regular routes over the Baltic. What we do know is that just about the entire Baltic was covered with ice during the cold war winters of 1939/1940, 1940/1941 and 1941/1942. Another trio of cold winters with significant ice extent is 1984/1985, 1985/1986 and 1986/1987, again when almost the whole of the Baltic was covered with ice.
SMHI’s ice maps are made up of small boxes in different sizes, and for each box the percentage of ice cover is specified. The ice extent is defined as the sum of the areas of all the boxes that are at least 15% covered by ice. If a box only has 12% it is not included, while the whole of a box with 18% ice cover is included in the total sum. There is no difference whether a box has 20% or 100% ice coverage, as long as the limit of 15% has been reached (or exceeded).
Sea ice in the future
In a warmer future climate, the average maximum ice extent will decrease. The length of the ice winter will be shortened and the average thickness of the ice will also decrease. The changes are expected to be greatest in the south, while the Gulf of Bothnia and the northern Bothnian Sea are least affected. However, there is no indication the sea ice will completely disappear from the Baltic Sea region during the current century. The variations from year to year will also be large in the future.