Recent Change—Sea Ice 
8 
Jan J. Haapala, lina Ronkainen, Natalija Schmelzer, 
and Marzenna Sztobryn 
Abstract 
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. 
Keywords 
Sea ice • Extent • Duration • Thickness • Climate variations and change • Baltic Sea 
8.1 Introduction 
for shipping (Jevrejeva et al. 2004), but later for scientific 
analyses of ice conditions (Speerschneider 1927; Jurva 1937; 
Palosuo 1953). In the first Baltic Sea assessment (BACC 
Author Team 2008), Schmelzer et al. (2008) and Vihma and 
Haapala (2009) gave extensive summaries of the existing 
literature. This chapter updates those reviews but does not 
repeat the extensive discussions already published. 
The importance of understanding the variability and changes 
in sea ice has been recognised for centuries. The first written 
accounts of ice conditions in the Baltic Sea were documented 
in harbour logbooks of the Fourteenth century (Tarand and 
Nordli 2001; Schmelzer and Holfort 2011). A regular obser 
vational network was established in the nineteenth century 
among the Baltic Sea countries, mainly to provide guidance 
The state of sea ice is determined by its extent, thickness 
and drift. Among these variables, ice extent is the most 
reliable measured quantity. In the past, ice extent was 
determined by visual observations from the coast, ships and 
aircraft, but since the start of the satellite era in the 1970s, 
accurate daily measurements are possible. In the Baltic Sea, 
regular ice thickness measurements are limited to the land- 
fast ice sites. These measurements, conducted by drilling, 
although accurate represent variations in ice thickness for 
coastal areas only. Sea ice drift can be determined by posi 
tion logging buoys (Lepparanta et al. 2001) or by detecting 
sea ice displacement using pairs of satellite images (Lep 
paranta et al. 1998; Karvonen 2012), but so far, no studies 
have been conducted focusing on long-term variability and 
J.J. Haapala (E3) 
Finnish Meteorological Institute, Helsinki, Finland 
e-mail: jari.haapala@fmi.fi 
I. Ronkainen 
University of Helsinki, Helsinki, Finland 
N. Schmelzer 
Bundesamt Für Seeschifffahrt Und Hydrographie (BSH), 
Hamburg, Germany 
M. Sztobryn 
Hydrological Forecast Office, Institute of Meteorology and Water 
Management, Gdynia, Poland 
© The Author(s) 2015 
The BACC II Author Team, Second Assessment of Climate Change for the Baltic Sea Basin, Regional Climate Studies, 
145 
DOI 10.1007/978-3-319-16006-1_8