Climate Change and the European Water Dimension - Agri ...

Box # 1: The North Sea
The North Sea is a large (ca. 7.105 km2) semi-enclosed, epi-continental sea
located between mainland Europe (from Norway in the north to France in the
south) and the British Isles forming its western boundary. The water budget
and circulation in the North Sea is driven by a combination of tides, wind, and
density gradients, leading to a cyclonic circulation with Atlantic salt water
entering from the north (Norwegian Sea) along the British coast, and warmer
waters from the Channel flowing along the French/Dutch/Belgian coasts. In
addition, it receives low-salinity water from the Baltic through the Kattegat and
the Skagerrak.
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Climate variability has strong influence on the hydrodynamics and hydrology of the
North Sea and the way such a coastal system will respond to future climate change
is of major concern to the ca. 190 million people living in the adjacent countries. The
North Sea originates from a major climatic change that occurred 12.000 years ago
causing post-glacial sea level rise of at a speed of 80 cm.century -1 . The trend is still
persisting today although at much lower speed. In the western North Sea,
Woodworth and Player (2003) have recorded changes in sea level that varies from
1.1 to 2.4 mm.yr -1 depending on the location along the coast of Britain.
In northern Europe, sea level is positively related to higher North Atlantic Oscillation
(NAO) index, particularly for the winter season (Yan et al. 2004). This large-scale
atmospheric circulation pattern is also responsible for an intensification of the
westerly winds over the North Atlantic and the North Sea as observed during the last
40 years (Figure IV.C.5, Siegismund and Schrum 2001), with a possible
consequence of an increase in the North Sea salt content resulting from an
intensification of the water exchange with the North Atlantic (Schrum 2001).
Coincidentally, the present-day climate conditions acting simultaneously on the tides,
wind-driven circulation, and on the meridional density distribution in the N.-E Atlantic
Ocean are most favourable to the renewal of water masses in the North Sea (Blaas
et al. 2001).
Related climatological phenomena occurring in northern countries include increased
precipitation and, consequently, higher river discharge, and increased storm events,
acting on the hydrography of the coastal systems at different scales. Discharges of
the Rhine river vary strongly on an annual and inter-annual time scale but a positive
trend is observed since 1950 with an increase of the discharge by 4.4 m 3 s -1 per year
(de Jonge and de Jong 2002) which would be responsible for a 10% increase of
suspended material in the adjacent coastal area. Similarly, an increase in freshwater
runoff from the Schelde River has also been observed over the last 10 years (Struyf
et al. 2004), particularly over the winter season in correlation with a larger increase in
rainfall during that period of the year.
The total catchment area of the North Sea is about 8.4 10 5 km 2 with a total input of
about 300 to 350 km3 of freshwater distributed through major rivers, e.g. Thames,
Rhine, Meuse, Elbe, and melt water from Scandinavia (Ducrotoy et al. 2000). In
spite a major dry periods during the last 20 years, a large part of the Thames
catchment has experienced an increasing rate of flood events also related to winter
precipitation increase. A global climate scenario would indicate an increase of 15-25
% of the Thames runoff by 2050 (http://firma.cfpm.org/regions/regThames.htm).
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