8 Recent Change—Sea Ice 147 
Table 8.1 Trends in the annual 
updated summary table based on 
maximum sea ice extent (MIB), annual maximum sea ice thickness (MIT) and duration of the ice season (DIS), 
i Vihma and Haapala (2009) 
Region 
Period 
Trend 
Statistical significance 
Source 
MIB 
Baltic Sea 
1901-1995 
- 
No «90 %) 
Haapala and Leppâranta (1997) 
MIB 
Baltic Sea 
1720-1995 
- 
Yes (97 %) 
Haapala and Leppâranta (1997) 
DIS 
Polish coast 
1896-1993 
- 
Yes 
Sztobryn (1994) 
DIS 
Szczecin Lagoon 
1888-1995 
- 
Yes 
Girjatowicz and Kozuchowski 
(1999) 
DIS 
Finnish coast 
1889-1995 
- 
Yes (99 %) 
Haapala and Leppâranta (1997) 
DIS 
Finnish coast 
1950-2010 
- 
NR 
Ronkainen (2013) 
DIS 
Port of Tallinn 
1500-2000 
- 
NR but evident since mid-1800s 
Tarand and Nordli (2001) 
DIS 
West Estonian 
archipelago 
1949-2004 
- 
Yes 
Jaagus (2006) 
DIS 
Southern coast of the 
Gulf of Finland 
1949-2004 
- 
No 
Jaagus (2006) 
DIS 
Gulf of Finland and 
Gulf of Riga 
1900-1990 
- 
NR 
Jevrejeva (2000) 
DIS 
Port of Riga 
1529-1990 
- 
Yes (99.9 %) for severe winters; no 
for mild and average winters 
Jevrejeva (2001) 
DIS 
Baltic Sea coasts 
1900-2000 
- and + 
Depends on station 
Jevrejeva et al. (2004) 
MIT 
Baltic Sea coasts 
1900-2000 
- and + 
No 
Jevrejeva et al. (2004) 
MIT 
Gulf of Bothnia 
1899-1995 
+ (Kemi) 
Yes (Kemi) 
Haapala and Leppâranta (1997), 
- (other) 
No (other) 
Launiainen et al. (2002) 
MIT 
Finnish coast 
1950-2010 
+ (Kemi) 
NR 
Ronkainen (2013) 
- (Loviisa) 
MIT 
Gulf of Bothnia 
1980-2000 
- 
NR but evident 
Launiainen et al. (2002) 
MIT 
Northern Gulf of 
Finland 
early 1900- 
1990s - 
NR 
Alenius et al. (2003) 
- decreasing trend; + increasing trend; NR statistical significance of the trend not reported 
The MIB displays large year-to-year natural variability 
due to the large-scale variation in atmospheric circulation, 
commonly described by the North Atlantic Oscillation 
(NAO, see Chap. 4, Box 4.1). Vihma and Haapala (2009) 
used the mean January-March NAO index in their analysis 
and showed that during a negative NAO phase (NAO < -0. 
5), the mean MIB is 259,000 km 2 (range: 150,000- 
405,000 km 2 ), while during a positive NAO phase 
(NAO > +0.5), the mean MIB is 121,000 km 2 (range: 
45,000-337,000 km 2 ) (Fig. 8.2). 
In spite of the obvious correlation between the NAO and 
MIB, in some NAO-positive winters, sea ice extent has been 
greater than average. In the middle of winter, the heat con 
tent of the Baltic Sea is already low, and a long-lasting 
blocking situation could cause an anomalously cold period 
and extensive ice growth. Interestingly, the relationship 
between the NAO index and MIB has not remained constant 
over time (Omstedt and Chen 2001; Janssen 2002; Schrum 
and Janssen 2002; Chen and Li 2004). Meier and Kauker 
(2002) found that dining two periods, around 1926 and 
1966, the correlation increased simultaneously with 
improvements in the observation methods for MIB. Changes 
in the NAO-MIB relationship can, however, also be due to 
changes in the location of the atmospheric pressure patterns 
(Koslowski and Loewe 1994; Chen and Li 2004). 
All previous studies have reported a significant decreas 
ing trend in MIB (BACC Author Team 2008; Vihma and 
Haapala 2009). Including observations until 2011, the trend 
in MIB for the past 100 years is -3400 km 2 10 year -1 or 
~2 % 10 year 1 (Fig. 8.3). Another apparent change is a fall 
in the frequency of severe winters over the past 20 years. 
Figure 8.4 shows that the modal probability of the MIB has 
remained the same whether the period considered is the past 
100, 30 or 20 years, but mild ice seasons have become more 
common, and years when the Baltic Sea is almost com 
pletely ice-covered have not occurred over the past 25 years. 
Schmelzer et al. (2012) examined changes in the proba 
bility of sea ice occurrence in the southern Baltic 
Sea. Figure 8.5 compares results for two 30-year periods, 
1961-1990 and 1981-2010, and shows a general tendency