150 
JJ. Haapala et al. 
8.3 Ice Duration 
Jevrejeva et al. (2004) has compiled the most extensive 
analysis of ice season changes in the Baltic Sea to date, 
using time series at 37 coastal stations from the Bothnian 
Bay to the Arkona Basin. In general, observations showed a 
tendency towards milder ice conditions. For the variables 
studied, the greatest change occurred in ice season length, 
which has decreased by 14-44 days over the past 100 years, 
mostly due to earlier ice break-up. 
A recent study by Ronkainen (2013) confirmed the trend 
towards a shorter ice-covered period in the Finnish coastal 
zone (Fig. 8.6 and Table 8.2). In the Bothnian Bay (Kemi), 
where the ice season is clearly longer than in the eastern Gulf 
of Finland (Loviisa), the trend is -18 d 100 year -1 . In the 
eastern Gulf of Finland, where sea ice is also formed every 
winter, larger changes have been observed. The length of the 
ice season has decreased by 41 d 100 year \ while the trend 
based on the last 50 observations is -62 d 100 year \ 
In the southern Baltic Sea, the first sea ice is formed in the 
coastal lagoons between 10 and 20 December, near the coast 
around 1 January, in the sea area of the western Baltic and 
off the Polish coast in mid-January, and in the German 
coastal areas at the beginning of January (Schmelzer et al. 
2008, 2012; Giijatowicz 2011). Comparing the different 
periods analysed, there has been no major change in the 
average date at which ice formation begins (Fig. 8.7). In 
terms of ice break-up, on average, the last ice in the inner 
coastal waters has disappeared by late February or early 
March, while residual ice has been observed in offshore 
waters until mid-March. The longest duration of ice in the 
coastal waters was observed in the 1961-1990 winters. In 
the past 30-year period, ice disappeared from the eastern 
coastal waters up to ten days earlier. In the Baltic Sea off 
shore waters, no change has been observed in this parameter, 
and in the western Baltic Sea, there has even been a weak 
trend towards a later break-up date. 
Ice duration decreases from east to west, and from the 
inner waters towards the sea areas. A weak trend towards 
shorter ice duration was found for the 1981-2010 period. 
The maximum ice duration occurs in particularly cold and 
long winter periods. There has been at least one cold winter 
with strong ice formation in each of the 30-year periods 
analysed, for example 1962/63, 1978/79 and 1995/96. A 
significant decrease (30 days) has only been observed in the 
sea area west and east of Bornholm Island in the period 
1971-2000. For the period 1981-2010, ice duration during 
very cold winters is nearly identical to that in 1971-2000. 
8.4 Ice Thickness 
For climate studies, the sea ice thickness or, preferably, the 
large-scale sea ice thickness distribution should be the main 
indicator of change in sea ice, since it is essentially the same 
as the mass of the ice pack. However, interpreting the causes 
of change in sea ice thickness is not straightforward, since in 
addition to the atmosphere-ocean energy balance, sea ice 
thickness depends on snow thickness and ice dynamics. 
Monitoring activity in the Baltic Sea is limited to the land- 
fast ice regions, where the sea ice could be thinner than in the 
drift ice regions. According to Jevrejeva et al. (2004), the ice 
thickness time series around the Baltic Sea coast does not 
show any consistent trend: both decreasing and increasing 
trends were reported. A recent study by Ronkainen (2013) 
supported these conclusions. In the Bothnian Bay (station 
Fig. 8.6 Observed changes in 
a length of ice season in Kemi 
and b in Loviisa, and c ice 
thickness in Kemi and d in 
Loviisa (Ronkainen 2013) 
19» 19» 19» 19« 19» 1940 19» 19» 19» 2000 20» 
Year 
19» 19» 19» 1940 19» 19» 19» 19» 19» 20» 20» 
Year