Fourth Study Conference on BALTEX Scala Cinema Gudhjem

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Impact of Climate Change Effects on Sea-Level Rise in Combination with
an Altered River Flow in the Lake Mälar Region
Gunn Persson, L. Phil Graham, Johan Andréasson and H.E. Markus Meier
Swedish Meteorological and Hydrological Institute, SE 601 76 Norrköping, Sweden, E-mail: gunn.persson@smhi.se
1. Introduction
Changes to the climate in the Baltic Basin are expected to
result in an elevated sea level in a future perspective. The
precipitation patterns as well as evaporation is also
expected to alter. This, in turn, will affect the total
amount of freshwater flowing into the sea, as well as the
distribution of these flows. Both sea level rise and
changed runoff may lead to major flooding events having
severe impacts on the spatial development of cities and
regions as well as sustainable development of the entire
Baltic Sea region.
2. Lake Mälaren
Many interests are focussed on Lake Mälaren. The
Stockholm-Mälar region consists of five counties and
more than 3 million inhabitants. There is an increase of
15.00-20.00 inhabitants per year. Basic needs such as
supply of drinking water and the disposal of sewage
water is linked to the Lake Mälaren but also actions like
transportation, fishing, recreation and tourism. The water
has also an important role from an ecological perspective.
The Stockholm region has a shortage of fresh water and it
is crucial for the development as a whole that the water
supply can be maintained in the long term. The region is
growing rapidly and now has 1.85 million inhabitants. A
shortage of housing facilities has led to that many
summer cottages, on islands in Lake Mälaren but also in
the archipelago, have become permanent houses. In this
respect there is already today problems with saltwater
intrusion and houses dangerously close to the shorelines.
In the eastern part of Stockholm a new housing area is
constructed, Hammarby waterfront, the biggest building
project in modern time within the region which will
house 20.000 inhabitants in future.
3. Underdimensioned discharge capacity
Recent years flooding problems combined with the
increasing vulnerability of the modern society raised the
question on how to better manage the outflow of water
from Lake Mälaren to the Baltic Sea. Calculations show a
need for rebuilding the outlet points and increase the
discharge capacity. The proposed measures are calculated
to increase the capacity from 710 m 3 /s to 1370 m 3 /s.
The difference in height between the Baltic Sea level and
the Lake Mälaren water level is low. The mean
conditions are 60-70 cm difference and at times the
brackish water from the Baltic Sea runs into the Lake
which is experienced as an increased content of chloride
in the raw water at the waterworks.
4. SEAREG − Sea Level Change Affecting the
Spatial Development in the Baltic Sea
Region
The INTERREG IIIB-project SEAREG focuses on the
socio-economic and environmental assessment of the
effects of climate change on sea level rise and river
runoff in the Baltic Sea region. The project addresses the
region as a whole regarding sea level but also looks into
more local areas, of which the Stockholm-Mälar region is
one.
Figure 1. The Lake Mälaren drainage basin.
5. Methods
The future increased sea level and changed runoff pattern
is studied in combination with the ongoing land uplift.
The future climate scenarios are from the Rossby Centre
at SMHI (Räisänen et al. 2003, 2004) and represent the
period 2071-2100. They are based on the HadAM3H and
ECHAM4/OPYC3 global models downscaled by the
regional RCAO model developed at the Rossby Centre.
Two emission scenarios A2 and B2 are used in the
scenarios. The control period is 1961-1990.
To estimate uncertainties of the global and regional
models three sea level scenarios for the Baltic sea were
compiled considering global average sea level rises
between 0.09 and 0.88 m together with land uplift and the
impact of regional wind from the time slice experiments
(Meier et al, 2004).
The hydrological modelling was performed with the socalled
− Sweden model − based on the HBV-model and
with which runoff from basically all Swedish river basins
can be calculated. The model is run in a “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).