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07/09/2025 16:20

Climate change on the Rhine: water temperature expected to rise by up to 4.2 °C by the end of the century

Dominik Rösch Referat C – Controlling, Öffentlichkeitsarbeit
Bundesanstalt für Gewässerkunde

The water temperatures of the Rhine are rising. This is shown by an analysis carried out in 2024 by the German Federal Institute of Hydrology (BfG) and the Dutch research institute Deltares as part of their work for the International Commission for the Protec-tion of the Rhine (ICPR). Effects on ecology and the economy are already being ob-served and will intensify in the future.

In spring 2025, the water level of the Rhine was significantly below the long-term average. ‘Such conditions are a foretaste of what we can expect more frequently for the Rhine in spring and summer in the future,’ says ICPR President Miriam Haritz.

Low water levels are often accompanied by high water temperatures. The study published to-day uses model results to show a clear trend: the Rhine is warming significantly as a result of climate change.

Review: 45 years of proven temperature rise

The evaluation of historical measurement series for the period 1978-2023 has already shown a clear warming trend: the Rhine has warmed significantly since 1978, especially in the area south of Karlsruhe. For example, the average annual water temperature in Basel rose by 0.4 °C per decade during the study period. Although anthropogenic heat discharges – such as those from closed nuclear power plants – have declined, they have not been able to mitigate the rise in water temperatures, which is directly linked to the rise in air temperatures due to climate change.

Looking to the future: models up to the year 2100

The new simulations of water temperature development carried out as part of this study are based on the high CO2 emission scenario of the Intergovernmental Panel on Climate Change (IPCC). Experts from BfG, Deltares and the riparian states have carried out model calculations to quantify the expected rise in Rhine temperatures. They expect the Rhine water to warm by be-tween +1.1 and +1.8 °C by the middle of the century. By 2100, the annual average water tem-perature could even rise by 2.9 to 4.2 °C compared to the period 1990-2010, which was used as a reference for all calculations. The study results show that the southern sections of the Rhine from Switzerland to Karlsruhe will be particularly affected.

Specifically, this means that, on average, the number of days with temperatures below 10 °C in the main stream of the Rhine will fall from the current 170 days to 104 days by the end of the century. Conversely, the number of days with temperatures above 21.5 °C will rise significantly, from the current average of 32 days to 106 days per year (corresponding to almost one third of the year). On these 106 days a year with temperatures above 21.5 °C, almost 50 days a year will see temperatures above 25 °C and up to 28 °C. The modelled water temperatures could also be exceeded due to factors not taken into account in the study, in particular anthropogenic heat discharges, which are difficult to predict.
Species and floodplains under pressure
Climate change is causing average air temperatures to rise, putting pressure on all natural habi-tats. Aquatic habitats and wetlands are particularly affected by climate change due to the in-creasing frequency and severity of low water levels and rising water temperatures, which worsen the effects of anthropogenic pressures from the various uses of the river and its habitats.

‘Rising water temperatures are having a significant impact on living conditions in the Rhine. If critical temperature thresholds are exceeded over long periods of time, ecological damage can occur. Aquatic organisms then suffer from heat stress, for example, which weakens them and makes them more vulnerable to disease,’ says Tanja Bergfeld-Wiedemann, a BfG scientist who collaborated on the study. ‘At higher temperatures, the solubility of gases in water also de-creases, which means that less oxygen is available to the animals,’ claims the biologist.

The heating of the water in winter also provides favourable conditions for the spread of certain invasive alien species. In the long term, species such as the grayling, which prefers cold water, could suffer population declines or disappear from parts of their current range in the Rhine sys-tem. Predatory fish that are not sensitive to temperature, such as catfish, are strengthened and exert additional pressure on aquatic species, although some of them die at temperatures around 25 °C.

The rise in water temperature can be limited in small tributaries by creating shaded refuges, restoring riparian forests and restoring healthy floodplain ecosystems. In large tributaries and in the main channel of the Rhine, measures are more difficult to implement..
Restrictions on water use threaten

If the water temperature rises above a threshold value, the competent authorities may be forced to impose restrictions on water use (e.g. industrial or agricultural withdrawals), which can lead to conflicts over use and economic difficulties. The rise in temperature will result in less capacity being available for existing and new cooling water consumers.
It is therefore essential to intensify the work already underway in the countries in order to strive for economical and sensible water use in consultation with the users who depend on this shared resource.

Modelling across national borders

The study was the first to test a cross-border model approach that combines national model results from Switzerland, Germany and the Netherlands along a uniform climate signal. ‘Close cross-border cooperation was crucial to obtaining a consistent picture of future water tempera-tures along the entire Rhine,’ says Pascal Boderie, research scientist at Deltares.

Adaptation as a shared responsibility

As part of the ongoing update of their strategy for adapting to climate change, the countries in the Rhine catchment area are working under the umbrella of the ICPR on concrete measures that take into account the various uses of the river. The aim is to preserve and strengthen the ecological functions of the Rhine. The results of the temperature analyses are fed directly into this process.

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The Rhine near Cologne, photographed in April 2025
Source: Marc Daniel Heintz
Copyright: IKSR

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Criteria of this press release:
Business and commerce, Journalists
Environment / ecology
transregional, national
Research results, Scientific Publications
English

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