Intensifying summer-monsoon human thermal stress across eastern China's major urban agglomerations

Source abstract

Extreme heat is among the deadliest climate hazards, and urbanization concentrates its burden in cities, yet human heat stress remains poorly quantified where exposure is rising fastest. We analyzed 45 summer-monsoon seasons (June–September 1980–2024) of the daily Universal Thermal Climate Index (UTCI) at 0.1° resolution across China's three largest urban agglomerations: Beijing–Tianjin–Hebei (BTH), the Yangtze River Delta (YRD) and the Pearl River Delta (PRD). Seasonal-mean UTCI rose significantly in every grid cell (regional trends +0.32 to +0.41 °C decade −1 ); Pettitt change points fall progressively later from north to south (1997–2006) but mark where a largely gradual rise is concentrated rather than abrupt regime shifts. Heat-stress events became more frequent (+1.9 to +4.0 event-days decade −1 ), near-permanent event hotspots nearly tripled, and very-strong-stress days increased fastest in the PRD (+4.8 days decade −1 ). The rarest joint duration–severity events in the YRD and PRD occurred in 2024, with return periods of 124 and 279 years under a stationary fit (bootstrap lower bounds above 45 years), or 34 and 113 years under 2024-conditioned margins. Air temperature and downward longwave radiation explain at least half of the interannual UTCI variance and trend significantly, whereas humidity contributes more in the south. Elevation-matched urban–rural contrasts correspond to about one-tenth of urban warming in BTH and PRD but a larger share of peak-event growth; because 0.1° reanalysis fields smooth canopy-scale extremes, this grid-scale share likely understates the urban effect. These findings support physiologically based heat surveillance and heat-resilient urban design.