Extreme rainfall is set to become more intense and more frequent by 2100 in Belgium

A study by researchers from the Climatology and HECE labs at the University of Liège, using the regional climate MAR model, projects a significant increase in the intensity and frequency of extreme rainfall events in Belgium, with peaks that could exceed 100 mm of rain per day every twenty years. These results reinforce the urgent need to adapt our infrastructures to the increased risk of flooding.

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n July 2021, Belgium suffered devastating floods as a result of torrential rains never before seen in the region. Was this an exceptional event, or the harbinger of a new climate? A team of climatologists and hydrologists from the University of Liège and the Royal Meteorological Institute (KMI) answer this question in a study published in the Journal of Hydrology: Regional Studies. Their conclusion is clear: extreme rainfall will become more frequent and more intense in Belgium, even in the most optimistic global warming scenarios.

A high-resolution regional model

To assess future risks, the researchers used the MAR (Regional Atmospheric Model) regional climate model developed at the Laboratory of Climatology to simulate Belgium's climate at a spatial resolution of five kilometres. The model was forced by several global climate models used notably in the latest IPCC report (CMIP6 database), according to four greenhouse gas emission trajectories (from the most moderate SSP126 to the most alarming SSP585), up to the year 2100.

The scientists applied a method of statistical analysis of extreme values to estimate the evolution of rare and violent rainfall, i.e. with a return period of twenty years. They then corrected the model's biases using observation data from the Royal Meteorological Institute (KMI), guaranteeing results that were closer to reality.

Results: Global warming is fuelling extreme precipitation events

Three major findings stand out:

1. The intensity of extreme rainfall increases by an average of 7% per degree of global warming, in accordance with the Clausius-Clapeyron* law of physics.
2. The frequency of rare events is increasing significantly: a current 100-year rainfall (which occurs statistically every 100 years) could recur every 20 years in certain areas, particularly in the Ardennes region.
3. The worst affected regions, where extreme precipitation levels could reach up to 120 mm per day every 20 years by 2100, will be the Ardennes relief, particularly the Vesdre and Semois valleys, and to a lesser extent the coastal plains (Figure 1). In the Semois valley, for example, if the Paris Agreements are complied with (+1.5°C), 85 mm per day would be expected every 20 years, which is already +25% compared with today. In the event of climate inaction, this figure would rise to 100 mm/day every 20 years.

Two climatic mechanisms explain these trends. On the one hand, warmer air can contain more moisture, naturally increasing the potential for intense precipitation. Secondly, the increased frequency of cut-off lows - cold upper-air lows that destabilise the underlying air masses - is linked to the disruption of the jet stream. This upper air current is slowed by the differential warming between the poles and the equator. However, climate models have difficulty in accurately representing this phenomenon, which could further underestimate the risks mentioned in this study.

The extreme example of July 2021

On 14 July 2021, an average of 100 mm of rain fell in twenty-four hours in the Vesdre catchment area. Such an exceptional event in today's climate would become recurrent (every twenty years) in a +3°C world, where more extreme rainfall could even be observed. Finally, as the authors point out, nature does not always follow the probabilities, and a repetition of this episode remains entirely possible in the short term.

According to Josip Brajkovic, lead author of the study: "Even in low-emission scenarios such as the Paris Agreement (which aims to limit warming to +1.5°C), episodes like the one in July 2021 will become more likely. We now need to integrate this reality into land-use planning and hydrological risk management.

The study does not directly assess the hydrological consequences (such as flooding), but its results provide a valuable basis for future modelling, particularly in relation to urban drainage systems or catchment management.

*Clausius-Clapeyron's law describes how the vapour pressure of a substance varies with temperature during a change of state (such as boiling or melting). It links this variation to the latent heat and the volumes of the phases in equilibrium. It can be used, for example, to predict how the boiling temperature changes with pressure.

Scientific reference

Brajkovic J.,  Fettweis X., Noël B., Ghilain N., Archambeau P., Pirotton M. & Doutreloup S., Increased intensity and frequency of extreme precipitation events in Belgium as simulated by the regional climate model MAR, Journal of Hydrology: Regional Studies, 2025, 59, p. 102399

Contact

Josip Brajkovic