Elucidating diverse population exposure to compound drought and heatwave events from two meteorological drought indices (SPI and SPEI)

Abstract Anthropogenic climate change has significantly exacerbated the frequency and severity of Compound Drought and Heatwave (CDHW) events, increasing risks such as water shortages, wildfires, and heat-related mortality. Previous studies often use a single drought index, such as the Standardized Precipitation Index (SPI) or the Standardized Precipitation Evapotranspiration Index (SPEI), while our study uses both SPI and SPEI to elucidate the effect of different drought indices on the quantification of population exposure to CDHW events. Six General Circulation Models under four future Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, SSP5-8.5) are considered. Exposure is defined as the product of CDHW Magnitude (°C) and the population in each region (million people), providing a quantitative measure of how CDHW events affect populations. The role of potential evapotranspiration (PET) in CDHW population exposure projections is examined by comparing SPI, which considers only precipitation, with SPEI, which accounts for both precipitation and PET in drought measurements. Results show that after 2050, CDHW Magnitude population exposure diverges significantly across scenarios, with SSP3-7.0 exhibiting the largest increase, reaching 0.72 (SPI) and 1.78 (SPEI) million person-°C by the end of the century. Regions such as Western Africa (WAF), Southeastern Africa, and South Asia (SAS) experience the largest increases in population exposure under SSP3-7.0 with SPEI, reaching 6.93, 6.77, and 5.56 million person-°C, respectively. Additionally, regions like Western & Central Europe, the Mediterranean, WAF, Western Central Africa, Eastern Asia, and SAS display heightened sensitivity to PET, with discrepancies between SPEI and SPI projections exceeding 1 million person-°C. Attribution analysis reveals that climate change, particularly when drought is calculated using PET by SPEI, is the primary factor, followed by interaction change and population change. These findings emphasize the critical role of PET in CDHW projections and the need for region-specific adaptation strategies to manage escalating risks in highly vulnerable areas.