River flow amplification under climate change: attribution and climate-driven storylines of the winter 2023/24 UK floods

Climate change is expected to alter the magnitude and frequency of river floods. Post-event analyses often assess the rarity of events from historical observations and a growing number of studies attempt to attribute the event’s severity to anthropogenic warming. Recent studies also advocate for the creation of “what-if” event storylines to explore consequences if an observed event turned out worse. However, few studies have harmonised these different approaches when conducting retrospective analyses of hydrological extremes. Climate change attribution including river flows also remains rare. Here, a framework for post-event analyses of hydrological extremes is demonstrated using the winter half-year 2023/24 UK river flooding as a case study. Persistent high river flows were observed across the UK and Western Europe, following on from notable winter floods in 2013/14, 2015/16 and 2019/20. The ‘ClimaMeter’ analogue-based attribution suggests that a 6-month period with similar atmospheric circulation patterns to the observed winter half-year 2023/24 has become warmer and wetter (by an average 8.8%) in 1945-2021 compared 1850-1925. Monthly river flow reconstructions extended back to 1850 show that river flows during the analogue periods in the recent past have also become 13.5% higher. Applying the UNSEEN approach by pooling seasonal hindcasts show the potential for river flows to be 46% higher than the baseline in a worst-case storyline. Finally, hydrological simulations driven by a single model initial-condition large ensemble suggest that when accounting for internal variability, a robust climate signal in winter half-year river flows have emerged for some areas but may remain concealed until mid-21st century and beyond. Our results contribute to the use of storyline approaches in post-event analysis and highlights the changing risk of winter UK flooding. This framework can be applied to future hydrological extremes both in the UK and elsewhere to inform long-term planning for climate adaptation.