Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Use of Hydrological Data and Climate Scenarios for Climate Change
Detection in the Baltic Basin
Sten Bergström, Johan Andréasson, L. Phil Graham and Göran Lindström
Swedish Meteorological and Hydrological Institute, SE 601 76 Norrköping, Sweden, e-mail: sten.bergstrom@smhi.se
1. Introduction
The last decades of the 20 th century and the first years of
the 21 th were extraordinary in the Baltic Basin. Winters
were generally much warmer than normal for the 20 th
century and precipitation and river runoff were high
above average values (Lindström and Bergström, 2004;
Lindström and Alexandersson, 2004). There were also a
number of extreme floods in Northern and Central
Europe, like in 1997 in River Odra, in 2000 in Central
Sweden and in 2002 in Central Europe. The hot summers
of 1997, 2002 and 2003 are also outstanding and to some
degree linked to severe flood events. Hydropower
production peaked in Scandinavia at the end of the
century, with strong impacts on prices and consumption
of electricity. This resulted in very unstable prices in
2003 when a dry year unexpectedly appeared with its
beginning in the summer of 2002.
In the light of the recent climate developments in the
Baltic Basin it is natural to ask questions about climate
variability and climate change, but also about societal
vulnerability and physical planning. The most immediate
question is whether this is a confirmation of global
warming or just an effect of natural variability, and how
climate anomalies match different scenarios of
development for the climate in the area. Thanks to long
climatological records, hydrological databases and the
availability of regional climate scenarios from the
Swedish regional climate modelling programme,
SWECLIM, there are new opportunities to approach the
answer. One important source is also the hydrological
database on river runoff to the Baltic Sea which has been
collected within the BALTEX research programme.
2. Two studies on detection of climate change
At SMHI two studies have been carried out to compare
temperatures and runoff in the climate scenarios with
observed anomalies. The hypothesis is that the time
period 1961-1990 constitutes the reference period and
that a changing climate may show up in the anomalies of
the following years. So far analyses have been carried out
for the different sub-basins of the Baltic Sea (Graham,
2004) as well as for hydrological conditions in Sweden
(Lindström and Aleaxandersson, 2004). As some of the
last years of river runoff are not yet available for parts of
the Baltic Sea, observations have been supplemented by
modelled river flow, based on the HBV hydrological
model (Graham, 1999) and meteorological observations,
for some years.
The climate scenarios are from the Rossby Centre at
SMHI (Räisänen et al. 2003a,b). They are based on the,
HadAM3H and ECHAM4/OPYC3 global models
dynamically downscaled by the regional RCAO model
developed within SWECLIM. Two emission scenarios,
A2 and B2 according to (Nakićenović et al. 2000) are
used in these scenarios. These two global models show a
quite different regional response for the Baltic area
concerning changes in precipitation, mainly because they
respond differently in terms of atmospheric circulation.
They therefore span a quite large range of variation of
river flow. The control period is 1961-1990 and the
scenarios represent the period 2071-2100.
The hydrological model is the Swedish HBV model
(Bergström and Forsman, 1973) which is run in a so
called “delta-change mode”, which means that the
relative changes between the climate control runs and the
scenarios are superimposed upon an existing climate
database and then used as input to the hydrological model
(Bergström et al., 2001).
3. Results
In Fig. 1 observed anomalies in river flow during 1991-
2002 are compared to changes in river runoff according
to the scenarios for the Baltic Basin and its sub-basins for
the period 2071-2100. Fig 2 shows more detailed studies
from northern and southern Sweden. Northern Sweden is
in this case defined as the drainage area from River
Dalälven and northwards. The data in Fig. 2 is further
grouped into 12-year periods to make them comparable to
the 12 observed years since 1990.
4. Conclusions
It is obvious from the two figures that the two main
climate scenarios represent a wide range of outcomes.
This reflects the importance of the global circulation on
regional hydrological effects. But it is also clear that the
tendency of the observed anomalies for the period 1991-
2002 are roughly in the direction of the scenarios even
though inter-annual variabilities are great. The general
tendency towards warmer and wetter conditions in the
north is there and so is the warming in the south. A closer
regional analysis (not shown here) also confirms the
tendency towards dryer conditions in the south-east of
Sweden. Analyses of extremes and changes of the
seasonal cycle are more inconclusive. This is mainly due
to the great variability in the observed records.