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Study region: Pannonian Ecoregion, Hungary. Study focus: Bridging the gap between atmospheric influences and aquatic environments, this study embarked on a comprehensive reconstruction of daily surface water temperatures across lakes and rivers within the Pannonian Ecoregion over an extensive period of 150 years (1870–2021). New hydrological insights for the region: The analysis revealed a clear warming trend in waters over the past 150 years, and majority of this warming occurred in recent three to four decades (average warming rate: 0.317 °C/decade). Seasonal patterns indicated that winter and spring exhibited faster warming rates, followed by autumn and summer. There has been a significant increase in the number, duration, and intensity of heatwaves in both lakes and rivers, particularly pronounced in the last 30–40 years, and with the rise of air temperatures, river and lake heatwaves tend to intensity. These findings underscore the escalating impact of climate change on freshwater systems in the Pannonian Ecoregion, emphasizing the urgent need for mitigation measures. As the first study on river and lake heatwaves in Hungary and one of the few studies on river heatwaves worldwide, this study will provide reference for analysing extreme thermal events in aquatic systems.

Here we track the water-table position and temperature of the Mount San Giovanni aquifer (Iglesiente-Sulcis mining district, SW Sardinia, Italy) during the past 600 ka by determining the ages (U/Th dating) and stable isotope compositions (δ 18 O, δ 13 C and Δ 47 ) of a variety of subaqueous carbonate speleothems (e.g. calcite spars, dogtooth calcite crystals and calcite coatings). Clumped isotopes ( Δ 47 ) provide quantitative estimates of carbonate formation temperatures (and thus water temperatures) that are independent of the oxygen isotope composition of water (δ 18 O w ). Then, the δ 18 O w of the paleo-water has been reconstructed from the clumped isotope temperature (T Δ 47 ) and the δ 18 O of the carbonate (δ 18 O c ). We find that some high-temperature calcite spars formed prior to 600 ka at temperatures above ~120 °C. Lower-temperature spars (~70 °C) precipitated at ~400 ka, and cold-water subaqueous speleothems (~10–20 °C) formed in perched ponds at different levels of the karst systems between 410 ka and 110 ka, while coeval precipitation of subaerial flowstones occurred in the upper level of the shallower caves until 82 ka. We infer that the groundwater level dropped by ~120 m from ~400 to ~250 ka, with a relatively rapid rate of ~0.8 mm/yr. Considering the tectonic stability of Sardinia during the Quaternary, this high rate derives from climate driven geomorphological processes at the surface rather than tectonic uplift. The δ 18 O values of the paleo-aquifer water range from −6.0 ± 0.7‰ during MIS 5c, and similar to modern cave water values (−5.1‰), to −7.7± 0.4‰ during the colder MIS 8. These values indicate that the groundwater reflected the δ 18 O signal of meteoric water, with no significant contributions from metasomatic and metamorphic waters. The observed δ 18 O w variability can be explained by glacial/interglacial paleoclimate changes affecting rainfall δ 18 O (and thus groundwater δ 18 O). We conclude that clumped isotope thermometry on subaqueous carbonate speleothems is a useful tool for tracking paleo-aquifer temperatures and δ 18 O w reconstructions. [ABSTRACT FROM AUTHOR]