Global Trends in Day‐Night Compound Thermal Extremes in Lakes

Plain Language Summary: Lakes thermal conditions are essential for the normal functioning of aquatic ecosystems. While thermal extremes in lakes are well‐studied at a daily average scale, the combined effects of extreme temperature events over day‐night cycles remain poorly understood. Using 40 years of hourly global lake temperature data, we provide the first comprehensive analysis of these events' patterns and trends across 81,295 globally distributed lakes. Our results reveal that day‐night sustained extremes are more frequent, lasted longer, and are more intense compared to univariate daytime or nighttime extremes, and have undergone greater changes. These changes align closely with the overall warming of lakes worldwide. Comparisons with air compound thermal extremes show that lake compound thermal extremes exhibit lower intensity but longer durations and a temporal lag due to the heat storage capacity of lakes. These insights are critical for developing effective strategies to mitigate the overlooked impacts of compound day‐night thermal extremes on aquatic species and ecosystem health.