Wetter or drier? Paleohydrological evidence from a 30-year resolution Holocene biomarker δD record from Moossee, Switzerland

The hydrological cycle intensifies under global warming, causing humid areas to become wetter. However, rising temperatures also amplify seasonal ecosystem dryness, complicating the link between temperature and hydroclimate. Such divergent mechanisms challenge generalizations like 'warm and wet' in paleoclimatology on a global scale. On a regional scale, knowledge about evapotranspiration in response to past warming and cooling is still limited, but highly relevant to understand future hydroclimate. Here, we analyse the hydrogen isotope composition (δD) of aquatic and terrestrial biomarkers in varved sediments from Moossee, Switzerland, covering the past 7300 years at a temporal resolution of 30 years. Based on a dual biomarker approach, we reconstruct evapotranspiration at Moossee. Our data suggests that cool and wet conditions repeatedly favored rising lake levels and advancing glaciers in the Alps but lowered treelines, e.g. at the onset of the Neoglacial, dated to ∼5.5 cal ka BP. In contrast, warmer periods like the Mid Holocene or the Roman Warm Period were associated with dryness. Short-term hydrological fluctuations are partly explained by volcanic and solar forcings. Aside from an increased risk of strong convective rainfall and floods, paleohydrology provides robust evidence that an intensified hydrological cycle under global warming will substantially favor summer drought in Central Europe.