Evolution of Heat wave characteristics across South Asia and identification of the most affected cities in the recent decade

Heat wave (HW) is a hazardous climate extreme that can lead to serious impacts on human health, posing challenges to the UN Sustainable Development Goals #3, #11, and #13. This study examines the heat wave characteristics across South Asia and surrounding regions during a 45-year period (1980-2024) with a particular focus on recent intensification and increasing population exposure. Daily 2-m air temperature data of European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis 5 (ERA5) was used for identification of hot days and resultant heat waves. The annual count of hot days (DH) is the number of days the daily maximum temperature surpasses 90^th percentile daily maximum temperature, whereas hot nights (NH) refers to exceedance of 90^th percentile daily minimum temperature, calculated over a 15-day moving window representing long term climatology of the time of the year being considered. A minimum of three consecutive compound hot day and nights was identified as a HW event. Three HW indices were computed annually: the number of HW events (HW_n), the number of HW participating days (HW_p), and HW magnitude (HW_m), which accounts for combined daytime and nighttime temperature departures. For each grid location, these indices were aggregated at decadal scales from the 1980s to the 2020s to examine the evolution of the HW characteristics across the study domain.

The study revealed that globally, NH has increased substantially (266.19%) from 1980s to 2020s leading to more frequent HWs (44,907 events/year in 1980s to 333,424 events/year in 2020s) during the study period. In Peninsular India, HW_n was found to be as high as 98 events and HW_p was as high as 533 days in the 2020s. HW_m was even more than 500 °C² in some locations in Eastern Asia during the same decade, indicating that both day-time and night-time temperatures showed large anomalies with respect to the long-term climatology.

To quantify the impact of rising heatwaves on rising population, gridded population data from WorldPop of University of Southampton was used to determine the change in population exposure to the HW indices over the recent decade (2015–2024). The major cities with marked increase in population exposure to HW occurrences (i.e., HW_n) were identified as Zhengzhou (China), Chengdu (China), Dhaka (Bangladesh), Faridabad (India) and Lahore (Pakistan) with exposure changes ranging from 2.92 × 10⁷ person-events to 6.34 × 10⁷ person-events. The maximum change in population exposure to DH is in Istanbul, Turkey (1.57 × 10⁸ person-days) whereas the same for NH is in Ho Chi Minh City, Vietnam (5.61 × 10⁸ person-days). The rising exposure to NH indicates that many cities are losing the ability of natural night-time cooling and require targeted intervention. Thus, this study offers valuable insights on the spatial and temporal evolution of heat wave characteristics across the most densely populated regions of the world and is expected to be useful for developing policies on climate-resilient urban infrastructure planning.