Abrupt changes in heatwave evolution in Mediterranean Chile and California

Abstract Heatwaves are extreme weather events with significant impacts on natural ecosystems, human health, and socio-economic systems. While global trends indicate an increase in the frequency, duration, and intensity of heatwaves due to climate change, regional studies are essential to understand local variations and impacts. This study aims to address the gap in knowledge regarding the evolution of heatwaves in Central Chile and California, two regions with similar Mediterranean climates but located in different hemispheres. We analyzed temperature data from meteorological stations in Central Chile and California over the past five decades. Using various heatwave definitions and indices, we quantified trends in heatwave frequency, duration, and intensity. The number of heatwave events per year in Central Chile increased by approximately one event every 15 years, while in California we register an average increase of one event per decade, although regional variations exist. These trends have accelerated over the last two decades, with abrupt changes in heatwave behavior detected around 2010 in both regions. We suggest that strong reductions in anthropogenic aerosol emissions around this time removed atmospheric aerosol cooling and may have abruptly amplified heatwaves post-2010. These findings highlight the accelerating impact of changing climate conditions on heatwave characteristics in Central Chile and California, and indicate possible unintended consequences of air quality improvements. Our results underscore the urgent need for adaptive measures to mitigate the adverse effects of increasing heatwaves.