Item

Abstract

Summertime compound hot extremes combining sequential daytime-nighttime heat are expected to be more impactful than singular hot days or nights. The overwhelmingly-adopted univariate definition, however, leaves a stark knowledge gap about impacts, past changes, drivers and future risks of this potentially high-impact type of hot extremes. <p>Based on a series of multi-disciplinary analysis, we first quantitatively verify that compared to daytime-only and nighttime-only hot extremes, compound events cause higher mortality and morbidity, consume more electricity for cooling demand, and favor more widespread and prolonged wildfires. From urban- to hemispheric-scales, the dangerous yet previously overlooked type of hot extremes are increasing faster in frequency and intensity as opposed to conventional hot days and nights. Attribution analysis points to major contributions from anthropogenic forcings in the observed increases, mainly greenhouse gas emissions, aerosols and urbanization at different spatial scales. Future accelerated increases in compound hot extremes would make the event the most common type as early as in the 2030s, and result in three- to eight-fold increases in population exposure, dependent on the spatial scale, demographic and climate scenarios being considered. This might translate into even ~10-fold increases in heat-related mortality amongst vulnerable people in some urban agglomerations.<p>Our results highlight the urgent need of adaptation planning tailored to rapidly-emerging hazard of summertime compound hot extremes.