In mid-July, a persistent downpour hit Germany, Luxembourg, Belgium and the Netherlands. Severe flooding occurred in the German states North Rhine-Westphalia and Rhineland-Palatinate, in Luxembourg, and along the river Meuse in Belgium and the Netherlands. Scientists from the World Weather Attribution (WWA) service have asserted that the additional 1.2 degrees by which humans have warmed the planet since the Industrial Revolution have made floods of such magnitude more likely. Between 20 percent and 900 percent, is the number they calculated.
Scientists from Germany, Belgium, the Netherlands, Switzerland, France, Luxembourg, the US and the UK, collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the heavy rainfall causing the severe flooding.
When the rain came, soils in those regions were already saturated. Some valley sections are very narrow with steep slopes leading to funnel-like effects in the event of extreme floods. These factors were modified locally also by differences in land cover, infrastructure and water management alleviating or increasing the devastation of the extreme flooding. At the Ahr river the flood is estimated to be a 500 year event or rarer according to preliminary data.
The flooding resulted in at least 184 fatalities in Germany and 38 in Belgium and considerable damage to infrastructure, including houses, motorways and railway lines and bridges and key income sources. Road closures left some places inaccessible for days, cutting off some villages from evacuation routes and emergency response. The worst affected areas were around the rivers Ahr, Erft and Meuse.
The WWA service analyzed how human-induced climate change affected maximum 1-day and 2-day rainfall events in the summer season (April-September) in two small regions where the recent flooding has been most severe in the Ahr-Erft region (Germany) and on the Meuse (Belgium) and anywhere over a larger region including Germany, Belgium and the Netherlands.
Some of the main findings include:
- The severe flooding was caused by very heavy rainfall over a period of 1-2 days, wet conditions already before the event and local hydrological factors. While river discharge and water levels are the physical components most directly linked to the impacts of the event, we focus our assessment on the main meteorological driver, the heavy rainfall event. This is due to the fact that some hydrological monitoring systems were destroyed during the flood and data of sufficiently high quality and quantity is not currently available.
- The observed rainfall amounts in the Ahr/Erft and the Belgian part of the Meuse catchment broke historically observed rainfall records by large margins. In regions of that size the robust estimation of return values and the detection and attribution of trends is challenging and thus pushes the limits of what current methods of extreme event attribution are designed for.
- Therefore we broadened the analysis by assessing the influence of climate change on similar types of events that could occur anywhere in Western Europe in a large region between the north of the Alps and the Netherlands. We found that in the current climate, for a given location within this larger region, we can on average expect one such event every 400 years. That also means we expect such events to occur more frequently than once in 400 years within the larger Western European region.
- Climate change increased the intensity of the maximum 1-day rainfall event in the summer season in this large region by about 3 – 19% compared to a global climate 1.2 °C cooler than today. The increase is similar for the 2-day event.
- The likelihood of such an event to occur today compared to a 1.2 °C cooler climate has increased by a factor between 1.2 and 9 for the 1-day event in the large region. The increase is again similar for the 2-day event.
The results found by the WWA service highlight that at local scale detection of extreme precipitation trends is hindered by variability, but when considering such events occurring over the larger Western Europe region, significant trends attributable to human-induced climate change are evident even if it cannot be predicted where exactly these events occur.
All available evidence taken together, including physical understanding, observations over a larger region and different regional climate models give high confidence that human-induced climate change has increased the likelihood and intensity of such an event to occur and these changes will continue in a rapidly warming climate.