Linking urban population exposure to heatwaves with land cover change across the UK

It is projected that up to 90% of the UK population will live in urban areas by the 2050s, alongside concurrent changes in land cover and increasing heat risks driven by climate warming. However, existing methods remain insufficiently robust for investigating urban extreme heat at fine spatial scales, and numerical models in particular struggle to accurately represent land cover changes. This study aims to characterise historical urban heatwaves and assess urban population exposure to these events across the UK from a temporal perspective. We ran high-resolution (0.05°, ~5 km) physical-process-based simulations from 2000 to 2014 using the Community Terrestrial Systems Model (CTSM), incorporating transient urban land cover representation over the years. Model reliability was validated against the HadUK-Grid (5 km) gridded observational dataset, with an RMSE of 0.5°C for the 95th percentile of simulated UK-average summertime daily maximum air temperatures compared to those in HadUK-Grid. Results showed increasing trends of urban heatwave impacts over time, characterised by four urban-area-aggregated indicators, including frequency, duration, season length, and intensity. Urban population exposure to heatwaves reached 367.2 million person-days in 2003, with 46.1 million urban population experiencing an average of 8.0 heatwave days. In 2013, exposure increased to 412.1 million person-days, with urban land cover change contributing an additional 8.6 million person-days. This work informs urban heatwave exposure assessment and adaptation planning by accounting for the dynamic influences of climate, population, and land cover changes.