Impact attribution of European floods: towards an operational system

Floods are an ever-present risk to society and economy in Europe, influenced by both climatic and socioeconomic drivers. An accurate and timely attribution of impacts is important for risk management, “loss and damage” debate and public communication in context of climate change. Here, we discuss the opportunities and challenges of operationalizing attribution for European flood impacts in the framework of Horizon Europe project “Compound extremes attribution of climate change: towards an operational service” (COMPASS). The prospective operational service would build upon the framework for attribution of historical flood impacts for 42 European countries. The work so far includes an extensive modelling chain covering both riverine and coastal floods that can reconstruct temporal changes in hazard, exposure and vulnerability to quantify their influence on the observed flood impacts. It considers drivers such as climate change, catchment alteration, population and economic growth, land use change, and evolution of flood precaution and adaptation. High-resolution datasets with long time series are used to first reconstruct each flood event under the factual (historical) scenario, and then under counterfactual scenarios in which a particular climatic or socioeconomic driver is set to 1950 conditions. In this way, the role of each driver can be quantified relative to a common temporal benchmark. In total, 1729 impactful floods occurring between 1950 and 2020 were attributed to the various drivers, highlighting the role of not only climate change (hazard), but particularly population growth (increase in exposure) and adaptation (decrease in vulnerability). Further integration with available operational services, primarily the Copernicus Climate Change Service, would enable timely input data processing for the hydrological and hydrodynamic modelling of riverine and coastal flooding. The approach will be extended to multihazard events, which will be showcased through the use case of extra-tropical cyclone Xynthia, which resulted in major impacts from both coastal flooding and extreme wind speeds in France in 2010.