We examine the evolution of ice seasons in the Baltic Sea during the 20th century based on a set of 37 time series from the coastal observation stations. The statistical question of combining data from sites with different ice probabilities is solved by using fractiles of the distributions. These 100 yr long time series, including date of freezing, ice break-up, number of days with ice, and maximum annual ice thickness, provide evidence of a general trend toward easier ice conditions; the largest change is in the length of ice season, which is decreasing by 14 to 44 d per century. The trends of a reduction of about 8 to 20 d per century to earliest ice break-up are in a good agreement with a warming trend in winter air temperature over Europe. A statistically significant decreasing trend in probability of ice occurrence in the southern part of the Baltic Sea was detected; however, there is no change in probability of ice occurrence in the northern part.

Sea ice conditions in the Baltic Sea have been systematically monitored for more than 100 years. All sea ice-related parameters display large interannual variability, but a change towards milder ice winters has been observed over the past 100 years: in particular, the annual maximum ice extent has decreased and the length of the ice season has become shorter. There is no correlation between consecutive ice seasons because the thermal memory of the Baltic Sea is only 2-3 months. Interannual variability in sea ice conditions is principally driven by the large-scale atmospheric circulation, described by the North Atlantic Oscillation. In addition to a tendency towards milder winters, the occurrence of severe ice winters has also decreased considerably over the past 25 years.

This chapter discusses past and ongoing change in the following physical variables within the North Sea: temperature, salinity and stratification; currents and circulation; mean sea level; and extreme sea levels. Also considered are carbon dioxide; pH and nutrients; oxygen; suspended particulate matter and turbidity; coastal erosion, sedimentation and morphology; and sea ice. The distinctive character of the Wadden Sea is addressed, with a particular focus on nutrients and sediments. This chapter covers the past 200 years and focuses on the historical development of evidence (measurements, process understanding and models), the form, duration and accuracy of the evidence available, and what the evidence shows in terms of the state and trends in the respective variables. Much work has focused on detecting long-term change in the North Sea region, either from measurements or with models. Attempts to attribute such changes to, for example, anthropogenic forcing are still missing for the North Sea. Studies are urgently needed to assess consistency between observed changes and current expectations, in order to increase the level of confidence in projections of expected future conditions.

ABSTRACT:A 700-year time series of ice winter severity index (referred to as Ice Index in this contribution) which additionally includes the period from 1301 to 1500 is now available for the region of the western Baltic. It is compared with air temperature time series, NAO, and other relevant indices. As expected for a regional time series, agreement with larger-scale northern hemisphere temperature variations is found only on longer time scales. Very good correspondence, even on shorter time scales, is obtained using a geographically close time series of annual mean air temperatures for the Low Countries (The Netherlands and Belgium). Although the data basis for the additionally included 1301-1500 period is quite sparse, there is also very good correspondence in this period. Correlation with the NAO Winter Index is generally negative, but the correlation is not significant for earlier periods.

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