Direct Impacts of Climate Change on Freshwater Ecosystems - Climate Change Impacts on Freshwater Ecosystems - page 44

precipitation, but the expected magnitude of the change strongly depends on the
climate model employed.
Future climate scenarios and Euro-limpacs
Global circulation models (GCMs), as mathematical representations of the climate
system, based on well-established physical principles and on observations of the
atmosphere, ocean, cryosphere and land surface, provide credible quantitative
estimates of future climate change, particularly at larger scales (e.g. Räisänen 2007;
IPCC 2007). A comparison of observed and simulated present-day climate generally
shows good agreement for many basic variables and thus provides considerable
confidence in the ability of climate models to deliver reliable future climate
projections, although individual models can differ in their simulations. Climate pro-
jections from model runs depend on which assumptions for future greenhouse gas
emissions are used. The standard approach is to use the SRES (Special Report on
Emission Scenarios) emission scenarios A1, A2, B1 and B2 based on storylines of
'how the world will develop until the end of this century'. They comprise distinct
potential future scenarios of greenhouse gas emissions, population growth and
economic development (Naki´enovic´ et al . 2000) (Fig. 3.6).
Physical impacts
Long-term data from surface waters already show changes associated with climate
warming. Rising air temperatures are reflected in increasing surface temperatures
in lakes and streams, in higher thermal lake stability and in a longer ice-free
season in lakes, with a later freezing in autumn or winter and an earlier melt in
spring or summer. Increasing hypolimnetic temperatures in lakes may lead to a
higher risk of deep-water anoxia. Changing wind patterns may alter the input of
mixing energy to lakes, and hence affect their overall heat balance and internal
heat distribution. Changes in wind and air temperature will be reflected in
changes in the physical behaviour of lakes, which may go hand in hand with a
modification of the chemical and biological characteristics of surface waters.
Changing precipitation patterns, like changes in the total amount, seasonality or
intensity, may alter hydrological cycles including river runoff regimes. Wetlands, in
particular, may be affected by changes in flooding. A change in the amplitude,
frequency, duration or timing of floods may affect biogeochemical processes,
plant nutrient dynamics and plant communities.
Regional climate variability is often related to recurrent patterns of atmospheric
circulation such as the North Atlantic Oscillation (NAO), the Northern Annular
Mode or the El Niño-Southern Oscillation. For Europe, the NAO, as pointed out
above, is the most prominent pattern of atmospheric variability. It corresponds to
changes in the westerly winds, and the NAO index is a measure of the strength
of the meridional sea-level pressure gradient between the Icelandic Low and the
Azores High. Potentially, the NAO can have an impact on temperature and
precipitation over large areas of Western and Northern Europe, and freshwater
ecosystems have been shown to be sensitive to changes in the NAO.