Climate change and heat stress in Malawi: A multi-index assessment with implications for health, labor, and cooling energy demand.

Malawi's population, predominantly engaged in rain-fed agriculture and outdoor labor, faces increasing heat stress risks under climate change. This study characterizes historical heat stress at 22 stations (2000-2023) using ERA5 reanalysis and seven complementary thermo-physiological indices, establishing an evidence-based baseline for climate-health adaptation planning. Validation against station observations confirms ERA5 reliability with a conservative cool bias. Bangula (Nsanje) in the Lower Shire Valley emerges as the extreme heat hotspot, with maximum UTCI reaching 48.44°C and "very strong heat stress" (>38°C) persisting for 139 days per year (38% of the year). Critically, highland stations (>1000 m) are warming three times faster than lowlands (0.64 vs 0.22 °C/decade), threatening traditional climatic refugia. Nocturnal heat stress, quantified by tropical nights and diurnal temperature range, reveals emerging recovery limitations in lakeshore districts. The findings identify five priority districts requiring immediate intervention: Nsanje (extreme daytime heat, 74 events >46°C), Kasungu/Mchinji (rapid highland warming, +0.93-0.94°C/decade), Salima (emerging nighttime stress), Blantyre (urban heat), and Mangochi (reduced nocturnal recovery). These results provide physiologically relevant thresholds for public health advisories, occupational safety guidelines, agricultural planning, and energy demand management.