Spatiotemporal trends in heat stress across Bangladesh from 1981 to 2023 using ERA5 temperature, relative humidity, and Heat Index

Global climate change has intensified the frequency and severity of extreme temperature events, particularly in climate-vulnerable regions such as Bangladesh. Heat waves have become an increasingly important climate hazard in recent decades. This study examines the spatiotemporal patterns of temperature, relative humidity, and the Heat Index (HI) across Bangladesh using ERA5 data from 1981 to 2023. An independent point-based comparison of ERA5 maximum temperature with Bangladesh Meteorological Department (BMD) observations at Dhaka for 2021 showed strong agreement ( r = 0.922). Results reveal a consistent nationwide warming trend. Relative humidity increased significantly in northwestern Bangladesh but declined in the southeast and coastal areas. Consequently, HI shows a strong positive trend, reaching approximately 0.67 °C decade⁻¹, in 14 districts of Rajshahi and Rangpur divisions within the Köppen-Geiger Cwa climate zone. Bivariate analysis highlights distinct regional patterns: the northwest shows concurrent increases in temperature, relative humidity, and HI, indicating intensified thermal stress, whereas lower HI trends in eastern and coastal regions coincide with declining humidity. The relationship between HI and precipitation trends is weak ( r ≈ 0.35). The frequency and intensity of hot, very hot, and extremely hot days have increased, with 2023 recording notable extremes consistent with El Niño conditions. These findings identify northwestern Bangladesh as a key heat-stress hotspot and emphasize the role of humidity in amplifying thermal stress.