Escalating Hydroclimatic Extremes and Volatility in the UK Under 2°C and 4°C Warming

Hydrological extremes, including both floods and droughts, are projected to intensify under global warming, yet their joint evolution and variability across temperate catchments remain underexplored. This study uses bias-corrected regional climate projections from the UK Climate Projections 2018 (UKCP18) and a distributed HBV-TYN hydrological model to simulate future river flows across 698 UK catchments under 2 °C and 4 °C global warming scenarios. Compared to previous UK-wide assessments, this study provides the most extensive high-resolution, bias-corrected, ensemble-based analysis of river flow responses under warming-level-specific timeframes. A comprehensive suite of hydrometeorological metrics is applied, including the maximum number of consecutive dry days, extreme precipitation totals, high and low flow percentiles, flood return levels, and the Standardised Precipitation and Streamflow Indices. Results reveal a regionally divergent intensification of extremes of extremes: (1) precipitation extremes intensify under warming, with daily and multi-day rainfall maxima increasing in western and northern regions, while dry spells lengthen in the south and east; and (2) high flows and flood magnitudes increase especially in western and northern regions, while persistent low flows and drought frequency intensify in the south and east. Projections also indicate a growing incidence of hydroclimatic whiplash, abrupt shifts between wet and dry extremes, posing challenges for water resources management. This is the first national-scale UK study to quantify hydrological whiplash across a full climate ensemble. These findings highlight the urgent need for regionally tailored, climate-resilient water management strategies to address the compounding risks of flooding, drought, and increased hydroclimatic volatility under future warming.