Emerging changes in lake temperature extremes and variability in South America

South America contains some of the world’s most ecologically and hydrologically diverse freshwater systems, which are increasingly vulnerable to climate change and human pressures. Despite their importance, the diurnal and interannual variability of lake surface water temperature (LSWT) across the continent remains poorly understood. In this study, we analyze thermal patterns in 2,406 South American lakes, spanning both historical (1981–2020) and future (2021–2099) periods. We assess LSWT trends, lake heatwave dynamics, and the influence of key meteorological drivers on lake thermal dynamics. Our results show that 97.0% of lakes (n = 2,333) experienced significant warming over the past four decades (+ 0.11 K decade^− 1), with 86.2% (n = 2,074) also exhibiting rising diurnal temperature variability (+ 0.02 K decade^− 1). Air temperature was the dominant driver in the northern and southern regions, while shortwave radiation played a greater role in shaping diurnal dynamics. LSWT anomalies are projected to rise, with heatwave events increasing in frequency, duration and intensity by the end of the 21st century. Additionally, we introduce a novel thermal typology based on diurnal and seasonal temperature ranges, identifying distinct lake response types, such as Thermally Extreme and Thermally Buffered, highlighting differential sensitivities to climate forcing. These findings provide new insight into lake thermal behaviour under climate change and underscore the need for targeted adaptation and conservation strategies to protect freshwater ecosystems in South America.