The ICPR report 209 describes the developments of Rhine water temperature during the past 30 years. Eventual effects of changes in water temperature on the aquatic fauna are described in the <link file:4321 _blank>ICPR report 204.
Investigations into the longstanding development of the temperatures of the Rhine indicate a clear correlation between the water temperature and the development of air temperatures in the Rhine catchment. Water temperatures reflect particularly high air temperatures, such as during the summers of 2003 and 2006 as well as in April 2007.
On average, between 1978 and 2011, water temperatures rise by about 1°C to 1.5°C. However, during this period, there is no continuous rise in temperature, but above all a rise during the years 1987-1989. This is explained by the particularly pronounced so called North Atlantic Oscillation (NAO) during this period. These are great variations in barometric pressure in the North Atlantic, as a result of which and particularly in winter, warm maritime air increasingly flows to Western Europe.
On a regional scale, thermal discharges from power plants further contribute to an increase beyond the natural water temperature. Until 2010, approximately 60 % of the <link file:2957 _blank download den einer>major authorized thermal discharges into the Rhine concerned a short section of the Upper Rhine between Karlsruhe and Worms. The rise in temperature until Mainz is largely caused by thermal discharges into this section, to a minor degree by natural warming. The measured average rise in temperature at Mainz is of about 1.4 °C and falls to about 1 °C at Koblenz.
While the major tributaries Neckar, Main and Moselle hardly contribute to lowering Rhine water temperatures during the fall and winter months, no such contribution is measurable during the summer months.
During the past decade, the number of days with water temperatures in excess of 22°C or 25°C considerably increased compared to the two preceding decades.