Leaf wax isotopes reveal enhanced humidity and earlier growing season during Dansgaard-Oeschger warming events in Europe

The study of past abrupt warming events provides knowledge of climate dynamics that are critical for future projections. During the last glacial period, Dansgaard-Oeschger (D/O) cycles caused rapid temperature increases across the North Atlantic at rates comparable to contemporary climate change. However, the hydroclimate response in Europe during these events remains poorly constrained due to scarce continuous paleohydrological records. Here, we present a continuous record of leaf wax hydrogen isotopes (δDwax) from a 60,000-year lake sediment record in Germany. δDwax is depleted during warm interstadials, contrasting with model simulations and published precipitation isotope proxies from central Europe. Using proxy system models combined with an isotope-enabled transient simulation (iTRACE), we demonstrate that this discrepancy arises from shifts in growing season timing and relative humidity, which modify the δDwax signal. Compared to cold stadials, warmer interstadials featured earlier growing season onset and increased relative humidity. These findings align with projections of intensified precipitation in this region under warming due to enhanced atmospheric moisture. Our results highlight how seasonality and humidity obscure precipitation δD signals in plant wax isotopes, demonstrating that incorporating these factors into proxy system models improves model-data comparisons and enables more robust paleoclimate reconstructions.