Attribution of extreme river flow conditions: A new framework using global climate model-driven river discharge simulations

River flow is projected to change under global warming with impacts on both societies and ecosystems. In particular, shifts in the intensity and likelihood of extremely dry or wet conditions pose significant risks, including increased flooding, water scarcity, and disruptions to shipping, transportation, and aquatic habitats. However, while several extreme event attribution studies have investigated the role of climate change in the generation of selected hydrological extremes, a comprehensive and systematic assessment is lacking. One key hurdle for hydrological extreme event attribution is the limited availability of river flow simulations from climate model ensembles. The common practice is to deploy a case-study-specific modelling chain that relies on post-processed climate model output to drive a local hydrological model. While this ensures a high degree of adaptation to regional hydrological conditions, it implies a large methodological overhead and delays rapid assessment of hydrological extreme events.

To address this challenge, we investigate a novel approach that trades local precision for global coverage. We test if global climate model-driven river discharge simulations provide a suitable alternative when assessing changing probabilities of hydrological extremes in an extreme event attribution framework. Focusing on case studies to allow for the necessary attention to detail, we examine to what extent the probability of recent hydrological extremes has changed in both the observed record and an ensemble of climate model-driven river discharge simulations. To this end, we capitalize on a newly developed dataset of river discharge simulations derived by routing runoff from 18 CMIP6 and 25 ISIMIP3b simulations along the global river network. Both strengths and limitations of the proposed approach will be explored, especially given open questions on the role of anthropogenic climate change in recent disastrous hydrological extreme events worldwide.