Attribution of the July 2021 flood event in the Ahr region to anthropogenic climate change

Flood event attribution, including the analysis of extreme precipitation and flood peaks, is crucial for understanding how anthropogenic climate change influences these events. This study employs an unconditional attribution approach to quantify changes in the likelihood of the July 2021 flood in the Ahr region, western Germany, in a factual world representing the current climate compared to a pre-industrial counterfactual world without anthropogenic greenhouse gas emissions.

To achieve this, the non-stationary weather generator nsRWG, conditioned on large-scale circulation patterns (CPs) and regional mean daily temperature (t2m), is used to generate 100 realizations of synthetic precipitation and temperature data over a 30-year period for both worlds. The CPs, derived from the classification of mean sea level pressure, and t2m are obtained from the ERA5 reanalysis dataset for the factual world and from natural historic simulations of several CMIP6 GCMs for the counterfactual world. The nsRWG-generated data are further disaggregated to an hourly resolution and fed into the hydrological model mHM, set up for the Ahr basin, to simulate streamflow and derive hourly peak flow. The simulated extreme precipitation and peak flows are analyzed to estimate the likelihood of the July 2021 flood event in each climate state, forming the basis for calculating the probability ratio between the two worlds.

Our model-based results indicate that the likelihood of 1-day and 2-day extreme precipitation of the Ahr event is on average 1.28 and 1.63 times higher, respectively, in the current climate. The flood peak appears to be 1.07 times more likely in the present climate compared to the counterfactual world. These findings suggest that anthropogenic climate change has notably increased the likelihood of events like the July 2021 flood. The use of a weather generator in combination with a hydrological model paves the way towards hydrologic event attribution and sets the stage for further research into attribution of flood impacts.