A simple approach for developing storylines of flood impacts under various global warming levels

The past few years have seen increasingly frequent and intense floods, culminating in 2024 with a year characterized by widespread and devastating inundations worldwide. In Europe despite major advancements in flood forecasting and flood protection measures undertaken in the past, in2024 heavy rainfall events resulted in severe flood impacts and massive socio-economic losses, claiming over three hundred lives. Extreme precipitation, breaking observed records, is expected to have an increased likelihood under global warming all the more we should question and scrutinize our knowledge about the frequency and severity of floods. Reliable estimates of these are challenging even for the current climate conditions, and disaster gaps can lead us to an underestimation of the risks. The use of the UNSEEN method (Thompson et al. 2017) has been proved to be very valuable in estimating both, unprecedented but plausible extreme floods and droughts.

Here we present a simple method to expand the UNSEEN method to develop storylines under various global warning levels. We selected precipitation scenarios with different spatial patterns for estimated return periods between 100 and 1000 years from pooled re-forecasts from ECMWF (ENSext and SEAS5), providing 8400 years of plausible weather sequences. The selected climate scenarios are perturbed by increasing the precipitation intensity according to the Clausius-Clapeyron relation for five different global warming levels, and used to run coupled hydrologic-hydraulic simulations. The results show that record-breaking, high-impact river floods are possible under the current atmospheric conditions, and climate change substantially aggravates flood impacts, as the relative increase in peak discharge can be significantly larger than the increase in precipitation, leading to a disproportionally high flood impact increase. The development of storylines of extreme flood events with a high spatial and temporal resolution are a valuable tool to explore, describe, and communicate extreme events and their dynamics. Such instruments are key for developing an informed vision and comprehensive protective measures in terms of flood risk management and emergency response.

 

References

Thompson, V., Dunstone, N. J., Scaife, A.A., Smith, D. M., Slingo, J. M., Brown, S. and Belcher, S.E.: High risk of unprecedented UK rainfall in the current climate. Nat Commun 8, 107, https://doi.org/10.1038/s41467-017-00275-3, 2017.